There is no doubt in my mind that the Apollo mission was one of the greatest scientific adventures of all time. People without a scientific background may not see why this was so. The Apollo mission was the culmination of the US space effort, stated clearly by John Kennedy: to put a man on the moon and return him by the end of the 1960's. It is too bad that we cannot claim the effort was motivated by science. Actually, it was motivated by the Cold War, and the fright experienced in the US when the Soviets successfully launched their Sputnik, the first artificial satellite, in 1957.
An enormous amount of new information about the Moon was learned prior to the Apollo landings. Among other things, we really learned from orbiters what its mass was!
I recall vividly a discussion that took place in one of my astronomy courses when I was a student in the 1950's. We were discussing the phenomena known as precession of the equinoxes. It takes place because the Moon and the sun pull on the bulge of the earth and cause it to wobble like a top. Of course, this makes it seem to us as though the entire celestial sphere were wobbling. Among other things, precession makes it difficult to determine the positions of stars with extreme accuracy. I had to give a report on stellar positions, and I naively said that we could calculate precession from Newtonian mechanics.
My professor was Alexander Vyssotsky, whose specialty was the positions and motions of stars. When he heard me say one could calculate precession, he chuckled, and said that you could do this if you knew the mass of the Moon. ``Andt nobody knowss mass off Moon,'' he said, in his thick accent. It struck me as extraordinary that the mass of a body so mundane as the Moon wouldn't be very accurately known. Of course in those years, many astronomers thought of the Moon as quite uninteresting, so naturally we had to know everything there was to know about it.
It turns out that by various techniques, we did know the mass of the Moon, but only to two-figure accuracy. The space program has added three more figures. Precession is still best determined empirically, but uncertainties in the structure of the earth now hinder the theoretical calculation more than the problem of the mass of the Moon.
The greatest scientific harvest of the Apollo program was the returned lunar samples. It is difficult to realize that the last of these were returned in 1972--24 years ago, as these words are written! I wouldn't even attempt to estimate the number of scientific books and papers written on some aspect of the lunar rocks--and soils. It must number in tens of thousands. But aside from occupying scientists for many, many man hours, what global information do we now have that was missing prior to the Apollo samples? We shall try to answer this question in the next few sections. Before doing this, we must lay the groundwork for the study of solid materials. Rocks are composed of minerals.
Minerals are defined as naturally occurring solids. We may think of them as having a definite crystalline structure and chemical composition. There are several thousand minerals listed in references of one kind or another, but we can get good insight into the chemistry of igneous rocks as well as meteorites from about two dozen of the most common minerals.
Minerals come in families. There will be a sort of generic name, which can stand for a variety of names that will specify the chemistry or structure of the mineral more specifically. We shall first consider three family names, all for silicate minerals: olivines, pyroxenes, and feldspars.
There are two subcategories of olivines called forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivines may have compositions ranging from one of these so-called end members to the other.
In the simplest classification of the pyroxenes, there are four end members. It is easiest to see this with the help of a diagram, shown in figure 10-1.
Figure 10-1: The Pyroxene quadrilateral. Each corner of the figure represents a specific composition, while each point within the quadrilateral represents a composition that is a mixture of the four end members. It is convenient to give special names to areas within the pyroxene field, such as augite. A few pyroxenes have compositions that fall outside the field shown.
One of the most common pyroxenes in terrestrial rocks is augite. A mnemonic for remembering the chemical formula for the pyroxenes is to add SiO2 to an olivine. We then get two of the end members of the pyroxene field, enstatite (Mg2Si2O6) and ferrosilite (Fe2Si2O6). It is a mnemonic, and not a formula, so it must simply be remembered that the other positive ion in the pyroxenes belongs to calcium. The other end members are diopside (MgCaSi2O6) and hedenbergite (FeCaSi2O6).
It is common to write the chemical formulae for these minerals in another way. Enstatite, for example, could be written MgSiO3. A modified notation could be used for diopside: (MgCa)SiO3. Here the two elements given in parentheses can enter in different relative amounts, such that there is always one doubly charged ion on the average, per SiO3.
The most common minerals in surficial rocks of the earth are the feldspars. We can display this family with the help of a ternary (or triangular) diagram, as shown in Figure 10-2. A mnemonic that is useful is to think of adding SiO2 to a pyroxene. This leaves some details to be memorized. For example, in the calcium-rich plagioclase feldspar known as anorthite (CaAl2Si2O8), does not have the ending (Si3O8) that characterizes the potassium feldspar orthoclase (often, K-feldspar, KAlSi3O8) or albite (NaAlSi3O8). In anorthite, there is a double substitution relative to albite: The Ca substitutes for the Na, but the calcium ion is doubly charged. Therefore one of the silicon's (+4) is also exchanged for an aluminum (+3).
Figure 10-2: The Feldspars. In this schematic diagram, the shaded region indicates compositions that do not occur naturally. Feldspars with compositions between albite and anorthite are called plagioclase.
Again there are special names given to minerals that fall in special regions of the feldspar triangle. The most important is plagioclase, which is basically any mixture of albite and anorthite (with very minor potassium). When plagioclase feldspar is the principal constituent of a rock, it is called and anorthosite.
In order to fill our our short list of cosmochemically important minerals, we need to mention the two nickel-iron alloys found in iron meteorites, and the refractory oxides. The alloys were listed at the bottom, in Table 2-1. They represented the ``metal'' of our four elements (metal, rock, ice, and the SAD) of Chapters 2 and 3. We discussed the refractory oxides in the previous chapter. It is important also to mention corundum (Al2O3), and perovskite (CaTiO3).
Perovskite is very important in two completely different cosmochemical contexts. It is one of the main minerals in meteorites that show (presolar) isotopic anomalies. Perovskite could be an important constituent of interstellar grains. Certainly, some calcium-rich mineral must be, because calcium is one of the most strongly depleted elements from the interstellar gas. Perovskite also plays an important role in the structure of the earth's mantle. In this case, it is not the mineral itself but its crystalline structure that is important. This structure turns out to be assumed by closely packed silicate minerals. We shall return to this point later in the present chapter.
There is an old joke that if you don't know what a rock is, you may take it for granite. We need to do a little better than this if we want to understand moonrocks. Fortunately, we do not need the elaborate vocabulary necessary to describe all of the rocks that can be found in the earth's crust. Of the three major divisions of terrestrial rock types, igneous, metamorphic, and sedimentary, we may concentrate on the first.
The lunar rocks are igneous, which means they were formed from a melt. As far as we know, all of the Moon was molten at some point in the past, otherwise the rocks wouldn't be igneous. Geologists can recognize igneous rocks in a variety of ways. Most professionals can tell an igneous rock from a hand specimen. It helps if the rock has a freshly-broken surface.
In my collecting days as an amateur geologist, I took many samples to geologists to find out what I had, and if my guesses were right. Often they would use a hand lens to examine the crystalline structure. A common trick was to put some weak hydrochloric acid on the rock. A fiz indicated a limestone or a carbonate, and therefore, not an igneous rock. A typical comment that I would get if I asked too many questions would be ``well, to answer that, I'd have to look at a thin section.''
For more than a century geologists have ground and polished thin slices of rocks, and looked at them through microscopes. The power of the technique is enormously enhanced if the light that passes through the thin section can be polarized, and the plane of polarization rotated during the examination. What is then seen is not only informative, but it has a kaleidoscopic beauty. The technique is a part of optical mineralogy and it is still the most useful technique to get the mineralogical content of a rock.
Petrology is the study of rocks, from the Greek petra, or rock. Rocks are made of minerals. Perhaps a helpful mnemonic is that minerals are to rocks as atoms are to molecules.
Petrologists use various terms to describe the texture and the composition of rocks, and it is well for us to recognize this distinction. Still other words describe the history of a rock. Naturally, these terms are related. Rocks have the texture and composition they do because of their history. It is useful to think about the terms that follow keeping in mind the concepts of, texture, composition, and history. It will also be good to ask how and when the terms are related. Consider, for example, the size of mineral grains in rocks.
An easy way to describe the texture of rocks is by the size of the grains. We may speak of fine grain and coarse grain rocks. There are fancier words, but we don't need them here. The size of grains or mineral crystals in a rock depend on how slowly the rock cooled. The slower the cooling, the larger the grain size of rocks. In igneous rocks, the grains are usually mineral fragments or crystals. However, when large crystals occur in a background matrix made of much finer grains, the rock is still considered to be fine grained.
Fine-grained rocks with larger crystals are called porphyries. (The singular is porphyry.) If there is a mineral with a high melting (or freezing) temperature in a melt that is cooling, the resulting rock is likely to be a porphyry.
Another term that describes rock texture is breccia. A breccia is made of cemented, broken rock fragments. The fragments themselves are called clasts. Breccias are important because almost all of the lunar rocks are breccias. Even though they once crystallized from a melt, these rocks have been subject to extensive shocks from meteoroid impacts. Moonrocks are generally called igneous rather than metamorphic, but experts do speak of shock metamorphism.
We can give a kind of two-dimensional classification of terrestrial rocks, in which we use the x-axis to indicate composition and the y-axis for grain size. In this case, we shall only use two categories of grain size, fine and coarse. ``Fine grained'' means that one cannot see the grains, even with the help of a hand lens. The classification is shown in Figure 10-3. Rock types on the left are called mafic because they are rich in the olivines and pyroxenes that contain magnesium (ma) and iron (Fe). The types on the right are called felsic because of their higher content of feldspars and quartz (or silica). Note that the calcic feldspar, anorthite, associates more typically with the olivines and pyroxenes.
Figure 10-3: A Simple Classification of Rock Types
A common type of lunar rock that does not appear in Figure 10-3 is the anorthosite, that we mentioned above. Anorthosites contain 90% or more plagioclase feldspar. The lunar anorthosites are dominated by the mineral anorthite. Terrestrial anorthosites are typically much richer in albite, the sodic feldspar.
This anorthite has played a significant role in the history of lunar chemistry. We shall come to it shortly.
The major elements of the earth's crust are oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, and sodium. All other elements are less than 1% by weight. These elements occur in the minerals that we have discussed: the olivines, the pyroxenes, and the feldspars. And the elemental abundances of the crust arise because of the properties of these minerals. The odd-Z elements sodium, aluminum, and potassium are overrepresented in the crust because they occur in the ubiquitous feldspars. Geologists speak of an upper and a lower crust. The upper crust is sometimes called the `sial' because of the silica (SiO2) and aluminum from the feldspars. The lower crust is called the `sima' because magnesium-rich minerals are more important than the feldspars.
The earth is still a geologically active body, and was probably equally active in the past. A variety of processes may cause mantle rocks to be melted locally. When this happens, sodic and potassium feldspars work their way upward. This is first because these minerals are less dense than the olivines and pyroxenes that dominate the composition of the mantle. Second, these feldspars are more easily melted than mantle rocks--it takes less energy to melt them.
So the earth's crustal chemistry is the result of the properties of its constituent minerals. This adds a dimension to our main theme that the history of matter is written in its abundance patterns. We must look at the abundances of minerals as well as chemical elements. When solid materials are involved, we can't understand the elemental abundances without considering the properties of minerals.
We mentioned that the calcic feldspar anorthite was an exception among the feldspars in having a high melting point. Interestingly, it still has a density that is lower than typical olivines and pyroxenes. So what happened when the Moon was molten was that the calcic (plagioclase) feldspar crystallized out of the magma ocean, and floated up.
Thus it turns out that substantial portions of both the terrestrial and lunar crusts are dominated by feldspars, but the feldspars are different chemically, and the processes that led to their dominance are also very different. The lunar feldspars are calcic, rich in anorthite, while the terrestrial, crustal feldspars are have more sodium or potassium. The more complicated the history of rocks, that is, the more melted and remelted the material, the richer the feldspars are in potassium. Orthoclase, KAlSi3O8, is the least dense of the feldspars.
It is useful to distinguish between the earth's oceanic and continental crust. Much of the oceanic crust has been formed in the process known as sea floor spreading. The most discussed example of this takes place in the mid-Atlantic ridge, where lava is extruded, forming new oceanic crust. This lava is basaltic in composition, and is therefore rather poor in feldspar.
It is thought that the material of the continental crust has been formed from material that has been melted and remelted many times. As a result, the lighter and more easily melted minerals work their way upward. The upper part of the continental crust is very rich in feldspars. In the lower part, magnesium-rich minerals become more prevalent. These are not necessarily olivines and pyroxenes, but derivatives that can form from them in the presence of water.
The Moon has its areas where basaltic material predominates also. These regions were called maria by Galileo, who first observed them with his homemade telescope. The word means seas.
Physical geographers often divide large areas into what they call physiographic provinces. In the US, for example, coastal plains, the Appalachian plateau, and the Rocky Mountains are different physiographic provinces. There are about two dozen of them altogether. In the case of the Moon, there are only two, the highlands (or terra) and the lowlands, or maria.
We know that the maria were formed after the terra because of radioactive dating of the returned lunar samples. After the original magma ocean had mostly solidified, at least at the surface, the Moon was again subject to heating from the long-lived radioactive elements, potassium, uranium and thorium. This heating melted some of the material that had been denuded in anorthite when the highlands were first formed.
We know that the Moon was still being pounded by meteoroid impact after the crust solidified, because the record is there to see in the craters. Some of the larger craters are called basins, and it was into these basins that much of the basalt flowed when the Moon was reheated. The kind of basalt flooding that took place on the Moon happened also at various places on the earth. In eastern Washington state, for example, vast areas were flooded by basalt. One may stop along roadcuts near Interstate 90, and pick up samples.
Flood basalts can also be seen in nearby Idaho, in the Snake River Plain, adjoining Craters of the Moon National Monument. Collecting is illegal in the Monument, of course, but just beyond the Monument border, one may collect some of the basalt. Figure 10-4 shows a hand sample obtained by the author. Plagioclase phenocrysts may be seen in addition to the little holes or vesicles caused by gas bubbles as the rock was freezing. This rock resembles some lunar samples.
Figure 10-4: Flood Basalt from Snake River Plain in Idaho. The white crystals are plagioclase phenocrysts. Note the holes, or vesicles, caused by gas bubbles.
The chemistry and mineralogy of the lunar and terrestrial flood basalts are different, but they were formed in very similar manners. Melt extruded through fissures or faults in the overlying crust, and then flowed out horizontally. Mafic lavas, rich in olivines or pyroxenes, behave this way. Felsic melts are much more viscous, and are the cause of explosive vulcanism.
Geologists speak of four eras in the history of the earth. The oldest is called the Precambrian. It's duration is much greater than that of the other three put together. Although the earth is thought to be about 4.5 billion years old, the record of the last half-billion years is much more elaborate than that of the first four billion.
Until the discovery and application of radioactive dating, the best indications for the ages of geological layers came from included fossils. However, the fossil record is very sparse prior to the Cambrian period. Much of the early history of the earth has been erased by erosion, and the phenomena known as plate tectonics or continental drift. The great minds of 19th century geology could develop the history of the earth following the Cambrian period, and the emergence of an extensive fossil record. Earlier times could all be lumped into the Precambrian. These men had no idea of the relative durations of Pre- and Postcambrian times---indeed, they though the Precambrian might simply extend back in time, with no limit.
Since the introduction of radioactive dating, modern work has added a great deal to our understanding of the Precambrian. Textbooks now commonly divide it into the Hadean, Archean, and Proterozoic eons.
It is well established that continental regions on the surface of the earth have moved relative to one another, forming supercontinents, and then breaking up again. Oceanic crust has been destroyed by these motions. It has been driven down into the mantle in what are called subduction zones. Because of their lower density, some continental regions have survived for a considerable fraction of the earth's history. The oldest terrestrial rocks that can be dated have ages of about four billion years.
If we try to push beyond the record that can be read from the oldest rocks, we must rely heavily on theory. The age of the solar system itself derives from dates of meteorites. The oldest values are about 4.6 billion years. We think that the sun, the earth, and the other planets formed some tenths of a billion years after this.
The earth formed by the accumulation of solid material. Solid presolar (q.v.) grains were present in the solar nebula, and never melted, but the majority of matter from which the planets formed was gaseous at the earlies times. It is likely that some solids condensed from the nebula on these presolar grains, in much the same way that rain droplets form on dust particles in the earth's atmosphere today. As more solids formed, the condensation centers collided, and in some cases stuck together, eventually forming planetesimals. These continued to sweep up solids, forming the terrestrial planets.
A major feature of the earth's structure is its nickel-iron core. We are not sure how this formed. Possibly the core formed first, and the mantle accreted about it. However, this is not the favored theory today. Most experts think that the original material from which the earth formed was a jumble of solid matter, not unlike that which occurs in many chondritic meteorites--rocky minerals interspersed with metallic, or reduced nickel-iron. If this is the case, the core had to form after the earth had accumulated to the point that heating was important. One source of heating is radioactive decay.
We think, from the fact that there are iron meteorites, relatively small bodies could be heated sufficiently to form cores. The asteroids are often discussed as possible meteoritic parent bodies. The largest of these, Ceres, is only about 0.3 times the size of the Moon. Small bodies cool quickly, indeed, one can show that the cooling time goes as the square of the size. If these bodies generated sufficient heat to form a core, more powerful sources of radioactive heating would be required than the 40K, uranium, and thorium that are active today.
There is good evidence that the unstable nuclide 26Al was present in the early solar system, and it is often mentioned as a possible heat source for smaller bodies. Its half-life is 7 x 105 years, so it provides energy at a much higher rate than 40K, uranium, and thorium. Their relevant half-lives are of the order of a billion years.
If the earth's core formed from little specks of iron that melted and then sank, this process itself is a source of considerable energy. One may readily calculate the difference in energy of a homogeneous body the size and density of the earth. This energy may be compared with a earth-like sphere with a metallic core and rocky mantle. This energy would be released during core formation, and should be more than sufficient to totally melt the earth. However, since we do not know how the core formed, we cannot say how rapidly this energy was made available.
The heat released by the radioactivities or by core formation could have readily cooked out volatile materials within the body of the planet. We are not really sure how the earth got its volatiles, the air and water so necessary for life. The earth's crustal abundances show a marked depletion of the noble gases, and the heavier of these could not have simply ``boiled away.'' It is most probable these and other volatiles were never present as a part of an early atmosphere.
There are two possible explanations of the current source of the earth's volatiles. They were either cooked out of the interior, or they were brought in by volatile-rich comets. Both mechanisms must have acted to some degree, but we cannot be sure which dominated.
If volatiles were cooked from the interior, they had to be brought in by the planetesimals that formed the bulk of the planet. We can be relatively certain that water, CO2, and N2 were gaseous at the time of the accretion of the earth. Some gaseous material could have been attached to the surfaces of the accreting solids by the weak chemical bonds. The process is called adsorption. Other gaseous molecules might have been enclosed inside porous solids.
Comets, on the other hand were probably formed beyond the snow line where CO2 and certainly water had condensed as solids. The main problem with using them to bring in the volatiles is that it is hard to predict their infall rate. One theory of comets supposes that most of them were originally in the inner solar system, and they were thrown out by perturbations mostly from Jupiter. If this is the case, most comets were headed away from the earth, with a small residual left to bring in the volatiles. This residual is difficult to calculate.
The earth is a large body, and it certainly seems to be flat. It's only when there is some way to examine enough of it that becomes clear it's round (spherical). Many aspects of the physical universe become fundamentally different when it is possible to view them from a broad perspective. Our ideas about the nature of living things changed fundamentally with the use of the microscope. Similarly, powerful telescopes have enabled us to see that our universe extends well beyond our solar system and Galaxy.
There is an interesting chapter in the history of geology that illustrates this. It concerns speculations on the age of the earth. During the Middle Ages, most educated people tried to discern information about the age of the earth from the scriptures. The famous Irish cleric James Ussher concluded in the mid 1600's that the world began in 4004BC. Some 200 years later geological wisdom was that there was no beginning to the world at all!
The exorcism of the idea of a beginning of the world is often attributed to two British geologists, James Hutton (1726-1797) and Charles Lyell (1797-1875). Lyell's Principles of Geology was the definitive work for many years, and echos of it remain in geology texts today. Those scholars whose outlook was based largely on notions of the ``creation'' and the ``flood'' came to be known as catastrophists. They thought there was a time when the world was different in most ways from what it was in their time. At one point, for example, it was ``without form and void.''
Hutton and Lyell were uniformitarianists. The uniformitarian point of view is probably best expressed in Hutton's words. As far as the history of the earth was concerned, he saw no sign of a beginning, and no prospect of an end. Lyell was somewhat more cautious. He wrote that any time when the earth was fundamentally different from it's present state was outside the bounds of what he considered to be legitimate science.
There is an irony in this. Hutton and Lyell may be considered true scientists, who laid the foundations of modern geology. Nevertheless, from the modern point of view, they were very wrong about beginnings and endings. Why?
When we review the kinds of information available to geologists in the eighteenth and nineteenth century, we find it severely limited. Radioactive dating of rocks did not occur until the early 1900's. Tectonic activity and erosion erased most of the evidence of the early earth. What Hutton and Lyell saw, then, were erasures superimposed upon erasures. They had no basis to conclude that the manuscript had once been empty.
Most of Lyell's geology deals with the most recent 0.6 billion years of the earth's history. That time represents the interval in which bones and shells could be found in the form of fossils. Prior to this period, soft-bodied life left few traces. Prior to the use of radioactive dating methods, fossils were the most common tools used to give relative sequences for layered structures of the earth. It is therefore not surprising that geology concentrated on the rather short time interval for which this tool was available.
It is quite clear that the uniformitarians simply had too restricted a view to be able to see back to the birth of the earth. They were in some ways like the people who looked around and concluded the earth must be flat. This is what we mean by a flat-earth view of earth history.
We are now confident that the earth formed some 4.6 billion years ago. Because we know how stars evolve, we also know the earth will be burned to a crisp in another 5 billion years or so. The early part of the earth's history is largely gone. Other bodies, notably our Moon have preserved records extending much further into the past.
The Moon's mean density is 3.3 times that of water. This is significantly less than the density of the earth, 5.5, so it has been known for a century or so that the compositions of the earth and Moon are significantly different. It is rather easy to account for the Moon's low density by assuming that it lacks an iron core. Why might this be?
A theory of the origin of the Moon that was popular in Gamow's time was that the Moon was torn from the mantle of the earth after the core had formed. This might account for the density, but the mechanism by which tidal forces deformed the earth, and separated it into two masses is no longer considered tenable. In addition, there are important differences in the chemistry of the earth and Moon that were revealed by the Apollo program.
Most discussions of lunar rocks point out that they lack the hydrated minerals common in terrestrial crustal rocks. There is no water on the Moon from which such minerals might have formed, and almost certainly never has been. Whatever processes brought the earth its volatiles were not efficient for the Moon.
Another aspect of the lunar rocks that differentiates them from the earth's crustal rocks, or even our estimates of the composition of its mantle rocks is the fraction of metallic iron in them. While native or metallic iron is rarely found in terrestrial rocks, it is a ubiquitous minor phase of lunar materials. If the Moon had been torn from the earth's mantle, we would expect less metallic iron.
There is, however, and intriguing similarity in the lunar and terrestrial rocks, and that is a depletion of the so-called siderophile elements. These are the elements that may be seen in troughs in a plot of crustal abundances, such as in Figure 10-5. The best defined of these troughs is that of rhenium (Re) through gold (Au). There is a comparable trough involving ruthenium (Ru) through silver (Ag). These elements fall in analogous positions in the periodic table (Table 1-2).
Figure 10-5: Elemental Abundances in the Earth's Crust. Siderophile elements (see text) are depleted in the crust. Note also the very large depletions of the noble gases.
The Australian geochemist A. E. Ringwood described a model for the formation of the Moon that would account for its chemistry. In his picture late stages of bombardment of the earth by meteoroids would actually vaporize some fraction of the mantle, boiling off a ring of material which could later recondense and form the Moon.
In Ringwood's picture, lunar material would have thus been subject to two condensation epochs, one when the earth was formed, and a second epoch leading to the formation of the Moon itself. In the second epoch, additional volatiles could have been driven away by the solar wind. In addition, free oxygen is presumed less prevalent during the second condensation than when the bulk earth accreted, which would account for the free iron in lunar rocks.
It is generally thought that mantle rocks are predominately olivine, mostly Mg2SiO4. In lunar rocks, the dominant minerals are pyroxenes, e.g. Mg2Si2O6, richer in SiO. Ringwood's picture accounted for this by having SiO boil off preferentially when bombardment had raised the temperature of the earth's surface to some 1500K.
This picture was designed to account for the lunar chemistry, not surprisingly, because Ringwood is a geochemist. He dealt less convincingly with a problem connected with the dynamics of the Moon's orbit.
The Moon's orbit does not lie in the equatorial plane of the earth. In fact, it is nearer to the plane of the earth's orbit, the ecliptic plane, although it is slightly inclined to that as well. If the Moon formed from recondensed materials that had boiled off the earth, its orbit would almost surely have been in the earth's equatorial plane.
Ringwood in fact suggested that one or more large impacting bodies, in the latest stages of the overall process of lunar formation might have knocked the accreting Moon from an equatorial orbit. Modern ideas take this one step further, and postulate that the entire process of lunar formation was dominated by a late impact on the earth of a body with the approximate size of the planet Mars.
The theory of lunar formation by a giant impact was proposed about 1976 by A. G. W. Cameron and others. This is the same versatile scientist who independently proposed the processes of nucleosynthesis that we discussed in Chapter 6. The giant impact theory is now widely accepted, and is described in considerable detail in elementary astronomy texts. There is a 1996 semipopular book about the Moon by Paul Spudis, of the Lunar and Planetary Institute in Huston, in which this theory is called the ``Big Whack.'' Perhaps this view of the formation of the Moon has been accepted without due caution.
The Big Whack theory of the Moon's origin can readily account for the fact that the orbital plane does not coincide with that of the earth's equator. Presumably, the impactor had an orbit near to the plane of the ecliptic, and the present Moon's orbit still ``remembers'' this.
In the scenario often illustrated in textbooks, the impact took place very early in the earth's history, but after both objects had formed cores. Most of the moon comes from the mantle of the impactor. The core of the impactor eventually merges with that of the earth.
The Moon then forms from the mantles of the impactor and the earth. Opinions differ about how much of the current Moon came from each possible source. Most of the Moon probably was once a part of the impactor. This helps to account for geochemical differences between the Moon and earth. The mantle of the impactor, plus some fraction of the earth's mantle become vaporized as a result of the collision, and recondense. During the condensation phase, volatiles may be driven off in a recapitulation of the mechanism of formation of the terrestrial planets. This picture is much the same as in the earlier theory discussed by Ringwood, and accounts for the dryness of the lunar rocks and soil.
While there are significant geochemical differences between the terrestrial and lunar rocks, there is one intriguing similarity. Ratios of the isotopes of oxygen show no differences. Oxygen has three stable isotopes, 18O, 17O, and the dominant isotope, 16O. For reasons that are not well understood, the relative abundances of these isotopes appear to be characteristic of different regions within the solar system. The atmospheres of Venus, earth, and Mars, as well as different kinds of meteorites all have distinct ``signatures'' of oxygen isotopic abundances.
Lunar and terrestrial rocks have the same isotopic signature. How can this be if the Moon formed out of material from a foreign body? Advocates of the Big Whack hypothesis usually say that the impactor must have formed near the earth. This is neither probable nor impossible. It is not probable because the earth could have readily swept up materials that would have formed the other body. It is not impossible because we do not know the precise conditions of the accumulation of the earth, and cannot say how improbable assembly of the putative impactor near one astronomical unit really was.
For the present, the Big Whack hypothesis is the best idea to date for the origin of the Moon.
When we look at the surface of the Moon, even with the naked eye, we see light and dark areas. These markings were mostly in place as early as three billion years ago. A view of the earth from space shows precious little that looked as it did as recently as a few tenths of a billion years ago.
Lunar rocks are primarily calcium rich feldspars called anorthosites, or they are basalts that flooded the floors of the maria. Terrestrial rocks are enormously more varied. Complicated histories of melting or sedimentation can be read in their texture and mineralogy.
The earth formed from solids that condensed from a flattened gas cloud called the solar nebula. Initially, most of this nebula was gaseous, but as the material cooled, solid grains formed. These grains later coagulated into larger and larger masses. Most of the mass that was in the region of the solar nebula near the terrestrial planets was swept away by a strong solar wind.
Our Moon was most probably formed when a large planetoid struck the earth. Silicate debris from this collision was ejected into orbit about the earth. Eventually this material collected to form the Moon.
When I first became interested in astronomy in the 1950's there wasn't the variety of popular books that there is today. Books by the famous British astronomers Sir Arthur Eddington and Sir James Jeans were fascinating, but somewhat old. We have mentioned George Gamow's books, which were available as Mentor paperbacks for 35 cents.
There was another 35-cent book by the Astronomer Royal of Britain, Sir Harold Spencer Jones, called Life on Other Worlds. Even though it had a 1952 edition, rather recent, I never read it. I had asked my old professor, Alexander Vyssotsky, what he thought of it. Dr. Vyssotsky said that Jones had written a nice book on astronomy, and had put in the section on life to make it interesting to general readers.
Now, after some forty years, I read in Jones's preface: The question whether life exists on other worlds is one that still excites curiosity, and to which astronomers are expected to give an answer. This seems still to be true. The American astronomer Carl Sagan made a very good living writing and lecturing on the topic of life on other worlds. Sagan did some very fine research work in planetary astronomy, but many of his colleagues thought of him as a ``sensationalist.'' In spite of his notoriety, or perhaps because of it, he was never elected to the U. S. National Academy of Sciences.
At the time of Jones's book, many astronomers believed that seasonal variations in the coloration of Mars was caused by vegetation. We now know that this is not the case. Modern writers on the subject of life beyond our earth have less to believe in than Jones.
On the other hand, our understanding of life on earth has progressed exponentially since Jones's time. One year after the second edition of his book, Crick and Watson announced the discovery of the structure of DNA.
We are still far from a complete understanding of the processes that take place in living cells. Nevertheless, there is now a rather broad, general understanding, and new results come thick and fast. Modern biology is surely an example of an exploding science.
It is appropriately the astronomer's job to find possible sites beyond our world where life might arise. Spencer Jones posed this problem in a forceful way. If we were to locate a planet with precisely the conditions of the earth, can we be sure that life would arise there. While we know much more about life than we did 50 years ago, we still have only a vague notion of how it arose on our own planet.
Astronomers and laymen are very excited over the discovery of planets circulating other, nearby stars. Numerous announcements of such planets had been made over the last half century. The older ones were never widely accepted. The situation is different now. Extraterrestrial planets are accepted by most astronomers, with a few reservations. Thus far, no really earthlike planets have been found, but this is probably just a matter of time.
Spencer Jones raised another interesting question. Is it possible that living material might exist on what we would consider hostile worlds? There was once considerable speculation about life based on silicon, which has an atomic structure similar to that of carbon. Based on what we know now, we can surely say that life as we know it requires the liquid phase of matter. Given the overwhelming abundance of hydrogen in the universe, the most likely liquid is water. This would seem to rule out silicon-based life, since silicon forms preferentially compounds in the solid state.
Could silicon, or some other element form the basis of life through some process that we simply haven't thought of yet? It's hard to know what to do with a question like this. Until there is some specific mechanism or experiment, all we can do is shrug our shoulders. Who knows? Certainly microbial life has been discovered in hostile environments. Bacteria grow near vents in the ocean floor where the temperatures are greater than 100 Celsius. These conditions are still far from the 450 Celsius at the surface of Venus. Any life forms on Venus would have to be fundamentally different from those we are now beginning to understand.
What does it mean to ``understand'' life?
Surely there is no universally accepted answer to this question. Experts in the sciences have opinions that vary in fundamental ways. In this section, I shall offer a strictly personal view. It will parallel those of many scientists, and conflict with those of others. I shall try to point out areas of controversy. Let us begin with some general remarks about the physical sciences, where the ``understanding'' is generally accepted to be more advanced.
There are four physical sciences, physics, chemistry, astronomy and geology. The first and latter two differ from one another in a fundamental way. Physics and chemistry deal with laws that are believed to be unchanging. As far as we know, protons, electrons, and neutrons are the same now as when they were first created in the Big Bang. Neither their charges, nor their masses, nor, in the case of the neutron, the decay lifetime has changed since that time. Boyle's law, and the pH of pure liquid water are unchanging.
One might wonder how it would be possible to do science at all if these laws did change with time. The ability to confirm hypotheses by repeating experiments lies at the core of science. If laws were to change with time, one might not be able to do an experiment today that would confirm one done yesterday. There have been suggestions that perhaps the laws slowly change, but at the present time we have no reason to believe this might be so.
Astronomy and geology use physics and chemistry as tools. In these sciences, as has been illustrated in this book, history plays an important role. While no physicist or chemist is interested in the history of an electron or a dissociation constant, geologists and astronomers are keenly interested in the history of the earth and of stars. Certainly there are books written about the history of physics and chemistry, but these are concerned with the development of ideas. There is also a history of geological or astronomical ideas. In this book, when we have talked about the history of matter, we meant outlined things that had happened to that matter.
Generally speaking, astronomers and geologists attempt to understand their subjects with the help of physics and chemistry. We may conclude that the earth's core formed as the result of the high density of iron, and the law of gravitation. In that way, we try to ``understand'' core formation.
Now comes the hard part. Can we ``understand'' biology by applying the laws of physics and chemistry in a similar way. It is at this point that opinions of experts diverge substantially. The general notion that there is a hierarchy of sciences, with physics at its base, is often called reductionism. Reductionists believe that all of chemistry is in principle explicable in terms of physics. Similarly, all biology may one day be explained in terms of the laws of chemistry and physics.
A great deal of the controversy over reductionism comes from the phrase ``in principle.'' In practice, there isn't much about the physical world that physics can explain completely. There are a few very simple systems for which complete, closed solutions are possible. These solutions would allow the prediction of the behavior of macroscopic systems for all time. A famous example is the two-body problem, for example, the sun and one planet.
For microscopic systems, the laws of quantum mechanics allow precise prediction of properties of atomic hydrogen--again, a two-body problem.
More complicated physical systems are treated by various methods of approximation. The real solar system, or atoms with multiple electrons, are treated by what are called perturbation methods. These are not intended to give exact answers. They are, however, expected to give increasingly accurate answers, depending on the effort expended. When a physicists say that a problem is solved in principle, they mean the perturbation technique would eventually give them an answer as precisely as they might want it.
Scientists come in a great variety. Many simply observe and catalogue data. Others try to organize the data, and still others try to find some underlying regularity that will allow the specific results to be predicted from general laws. These are the theoreticians. Theoreticians by their nature proceed as though underlying laws existed, waiting to be found. It seems pointless to speculate on whether this will always be the case.
James Watson, the codiscoverer of the structure of DNA, must be a reductionist. Chapter 2 of his book Molecular Biology of the Gene has the title: Cells obey the laws of chemistry. Equally famous life scientists assert reductionism isn't possible.
The author of this book is a reductionist--to a point. If it is also necessary to assert that all observational phenomena will ultimately be reduced to physical laws, I part company with the reductionists. It seems equally pointless, and frankly, silly, to assert absolute reductionism as it is to deny it. All that one can say is that great progress has been made by those who behave as though underlying laws would always emerge.
There is a lot of snobbishness among scientists. Physical chemists sometimes look down on the organic chemists, and physicists sometimes look down on astronomers. Perhaps physical scientists looked down on biologists, although it is hard to see how this might happen these days. I suspect a lot of the hostility to the reductionist view is simply a kind of psychological backlash to the notion that: my science is more fundamental than your science. Certainly it's childish, but there is a little bit of kid in the best of us.
Apart from backlash, there is probably some residual mysticism that is threatened by reductionism. The reductionist view is antithetical to the notion that life, and especially human life, is fundamentally different from inorganic matter. Until the synthesis of urea in 1828, it was thought that organic molecules could only be made by living organisms. The Aristotelian notion of a vital life force, unique to living matter, persisted at least the end of the nineteenth century. It may linger today, in the form of antireductionism. A life scientist once confided to me that he and his colleagues were all ``closet vitalists''!
In the sections that follow, we shall take the reductionist point of view. Many important questions about life have been explained by the laws of chemistry and physics, and more are likely to be.
Living matter is mostly a solution of organic chemicals, along with some harder material making bone and teeth. By definition, organic chemicals involve carbon. The variety of possible compounds is almost limitless, as any student of organic chemistry soon learns.
Molecules are held together by several different kinds of bonds. Ionic bonds, which are typical of minerals, involve the exchange of electrons. There is then one positive and one negative ion which can attract one another, and this is the source of the bonding. Organic molecules are mostly formed by what are called covalent bonds, where the atoms ``share'' electrons. The origin of the covalent force is quantum mechanical, and a little harder to understand than for ionic bonds. Elementary explanations usually say the sharing of electrons brings about a noble gas structure, with 8 outer electrons.
Ionic and covalent bonds are relatively strong.
There are a variety of weaker bonds, several of which play key roles in the chemistry of living matter. The hydrogen bond is an important example. It is roughly a factor of ten weaker than ionic or covalent bonds, and acts mostly between atoms of hydrogen, and oxygen or nitrogen. We mentioned weak bonding in connection with adsorption in Section 10.5. We shall not attempt to distinguish among other kinds of weak chemical bonds. The significant point about such bonds is that they are readily broken. In life's cycles, weak bonds are constantly broken and repaired.
Much living matter is protein. Proteins are generally large molecules which may be formed by chemical reactions among a simpler class of organic compounds called amino acids. Two amino acids can react chemically to give a water molecule and a ``peptide.'' The term is related to a Greek word meaning digestion. Polypeptides, or many amino acids bound together after the extraction of water molecules, may be called proteins if they are large enough. One of the purposes of digestion is to break up the proteins into smaller molecules.
Proteins are one example of polymers--chains of simpler molecules bonded together by covalent forces. The proteins are polymers of amino acids. They may be broken into their constituents by addition of water, a process called hydrolysis by the chemists. They call the opposite process condensation; we have used the same word to describe the appearance of solids from a cooling gas. We should not confuse the two meanings. In the present case, a long molecule is broken into two with the release of a water molecule. We discuss condensation later in this chapter.
The number of amino acids that can be synthesized in the laboratory is quite large, but life forms seem to need only 20 of them.
Living cells manufacture many different kinds of proteins, according to a recipe written in the chromosomes, or DNA molecules. These macromolecules are polymers of structures called nucleotides. The nucleotides have three components, an organic base, a sugar, and a phosphate group. The polymer linkage is between the sugar and phosphates. The terminology is not made any easier by the fact that the base-sugar pair is called a nucleoside.
Only four different bases occur in DNA, A, T, G, and C. The letters stand for organic bases, illustrated in Figure 11-1. The three members of the nucleosides are held together by normal covalent bonds. The bases bind the two DNA strands together with weak hydrogen bonds-A always couples to T and C to G, so if you know which bases are along one strand, you automatically know the one on the other.
Figure 11-1:
Schematic of the two strands of the DNA molecule,
showing the ``zipper'' molecules A, T, G, and C. These
zipper teeth are one and two ring molecules, shown in
more detail on the right. It is the shape of these molecules
and the fact that they are weakly (hydrogen) bonding that
makes them match in the specific pairs A-T and G-C.
(Adapted from a figure by Frank Shu.)
The celebrated genetic code consists of sequences of three bases. If we think of these bases as letters, then there can be 43 = 64 words in the genetic alphabet. This means that there are more than enough words to specify the 20 amino acids that are put together in the cells to make proteins. Some of the amino acids can be specified by more than one word. Individual proteins are coded in the DNA base sequences as ``sentences.'' Grammar checkers would not approve of these sentences, since those specifying proteins can be hundreds of words long.
The genetic sentences of the DNA are transcribed, or copied, and translated by special machinery in the cell involving several forms of RNA molecules. These molecules resemble shorter, single strands of DNA, with very minor changes in the nature of the sugars on the strand. RNA has one more oxygen. In addition, RNA uses a base called U rather than T. The end result of a translation of the code by machinery within the cell is a protein.
We shall not give details of the transcription and translation process here. Instead, we turn to some general remarks about the nature of the chemical processes and the molecules that take part in them. These remarks will be directed specifically to the conditions necessary for life as we know it.
The molecular weight of water is only 18. Oxygen and Nitrogen molecules that make up the air have molecular weights of 32 and 28. Why are these molecules gaseous at room temperature while the much lighter water molecules are still liquid? The answer to this lies in the hydrogen bonds. These are illustrated in Figure 11-2.
Figure 11-2: A Cluster of H2O molecules. Hydrogen bonds are indicated by the red regions between the large (O) and small (H) spheres. Similar bonding occurs, between hydrogen and nitrogen atoms.
We can get a rough estimate of the effective size of a complex of water molecules from boiling points. Simple hydrocarbons, with the general formula CH3(CH2)nCH3, have boiling temperatures lower than that of water until n is reasonably large. The boiling temperature of heptane (n=5) is 98C, and that of octane is 126C. Methane, CH4 boils at -57C. If we divide the molecular weight of heptane, 100 by that for water, 18, we conclude that between 5 and 6 H2O's tend to be stuck together in water.
This affinity of water molecules for one another accounts for the solubility of many organic molecules. Molecules where the positive and negative charges separate--polar molecules--tend to be soluble in water. An example is alcohol. Alcohols are hydrocarbons with an OH group attached in the place of one of the hydrogens. When in solution, this oxygen strongly attracts one of the hydrogens of the water molecules, while the hydrogen at the end of the OH group attracts oxygen in a water molecule. The net result is that water molecules tend to surround those of alcohol. So the two fluids mix. We say the molecules are hydrophilic.
The series of hydrocarbons, CH3(CH2)nCH3, have no marked concentration of charge, and for this reason, oil, or gasoline and water do not mix. These molecules are hydrophobic.
Many of the organic molecules have one end that is hydrophobic while the other is hydrophilic. This would be the case for an alcohol with a long chain of CH2's prior to the OH on the end carbon. A more extreme case happens with molecules of soap. Here, the neutral or hydrophobic end of the molecule will dissolve in an oil or grease (also hydrophobic). The hydrophilic end will dissolve in water. Washing gets rid of both the soap and the oil or grease.
Molecules with one end hydrophobic and the other hydrophilic are called amphiphilic (amphi- means both). A group of amphiphilic molecules in a water environment will try to stick their hydrophilic ends into the water and their hydrophobic ends away from the water (Figure 11-3).
Figure 11-3: A molecular bilayer enclosure, or vesicle. The configuration is a natural for amphiphilic molecules, whose hydrophilic ends (dark balls) point toward the water, while the hydrophobic ends are protected within the bilayer. Most organic membranes are constructed of such bilayers. Typically, the hydrophobic ends have two tails, rather than the single one pictured.
Many proteins are found in the rich solutions (soup) making up most life forms. The enzymes that make possible the chemical reactions within cells are folded protein chains with several hundred or so amino acids. The folding greatly reduces the overall size of the molecules. What causes them to fold?
The precise sequence of folding steps is not known, although a the folded structure of a number of proteins is known. As a result, we can say the structures are held together mostly by weak bonds. These weak bonds may be broken by high temperatures, and the proteins denatured. This is what happens to egg whites when eggs are boiled. Some folds of proteins involve genuine covalent bonds. One of the amino acids, cysteine, ends in an SH group. In solution, the H's may be lost, and if the chain folds back so that two cysteine structures come close, a strong, covalent, disulfide (S-S) bond may form.
Enzymes fold due to hydrogen bonding, sometimes disulfide bonding, and typically to shield the hydrophobic ends of amino acids. The outer parts of these structures have the charged, hydrophilic ends sticking out. Their specific purposes is to catalyze chemical reactions--to make them go faster. Enzymes do this in very complicated ways, but weak chemical (including polar) bonding is nearly always involved.
Plants convert carbon dioxide and water to starches and sugars plus oxygen. A simple way of writing this overall reaction is:
This chemical reaction is thermodynamically ``uphill'' and would not proceed unless energy were supplied to drive it. The energy is supplied by sunlight. Plants and animals use the energy stored by photosynthesis. Life's overall processes are endothermic. They require an energy supply that comes (exclusively?) from the sun.
When I took biology in the 1940's, the teacher would stress the notion that the sun was the source of all energy. We know today that this is not completely true, even for our planet. The earth, has internal energy that derives from gravitation as well as radioactive decay. Chemical changes also release energy. In a few exceptional cases, one of these sources may supply the original energy to life forms. Certainly the energy for conventional life is derived from sunlight.
Plants provide chemical energy, both for themselves and for animals that eat them. But the natures of living cells is sufficiently delicate that reactions such as the one shown above could not proceed directly. Let us look at the reaction above in the reverse direction, C6H12O6 + 6O2 --> 6CO2 + 6H2O, used in animal metabolism. The amount of energy released by burning one molecule of glucose, C6H12O6, is some 750 times the typical kinetic energy of translation of molecules at body temperature. Fortunately cells have a way of extracting the energy in a series of steps.
A key role in most energy exchanges is played by phosphates of an organic compound known as adenosine. It is usually just called `A'. The base A of DNA, adenine, occurs in adenosine, coupled to a sugar molecule. The combination is called a nucleoside. Add a phosphate group, and we have a nucleotide. Adenosine may add one, two, or three phosphate groups, and the resultant molecule is then adenosine monophosphate (AMP, a nucleotide), diphosphate (ADP), or triphosphate (ATP). It takes energy to add a phosphate group, and energy may be released by extracting the phosphate group.
The amount of energy involved in adding or subtracting a phosphate group is nearly forty times less--per molecule--than burning glucose. Cells get the energy from sugars by a roundabout way, using ATP. If all of the energy from oxidization of a glucose molecule were released at one point in the cell, critical complex molecules would be dissociated.
The amount of energy released by removing a phosphate from ATP is also of the same order as the energy of a hydrogen bond. It is easy to see why this molecule plays a ubiquitous role in the chemistry of life.
In the course of storing the energy from the sun's rays plants make ATP; animals use it to metabolize the food from plants or other animals.
It has become traditional to refer to the chemical bonds holding the phosphate groups to A as high-energy bonds. It's quite misleading, but there is probably nothing we can do about it. There is a legitimate, technical sense in which these weak bonds may be described as having a high energy. It's hardly worth the effort to clarify the point. As we have seen, it is fortunate that the bonds are rather weak, so when they are broken or formed large amounts of energy are not involved. If this were not the case, the cells would find some other weak bonds to use in degrading the energy when carbohydrates are oxidized.
Oxygen is not necessary for life. Certain bacteria known as anaerobes do not require it, and a subcategory called obligate anaerobes cannot even survive in its presence. When life first appeared on the earth, it is highly likely free oxygen was absent. Calculations of condensation, such as those discussed in Chapter 9, show this for the time when solids first began to form near the earth. There was no source of oxygen from that point until plant photosynthesis.
We therefore think the first life forms were anaerobic.
The building blocks of life, the hydrocarbons, the amino acids, the phosphates, and water were available or could have been, when the earth was very young. Some of the oldest terrestrial rocks are sedimentary, and we infer from this that the oceans are of the order of four billion years old.
A famous experiment was performed in the 1950's by Stanley Miller and Harold Urey. They heated a mixture of mostly methane, water, and ammonia, and subjected it to an electrical spark to simulate lightning. After several days, the mixture was found to darken, and to contain more complicated organic compounds, including some amino acids.
This particular approach to the origin of life has not been significantly improved. We may have lost ground. Modern planetary scientists think there may have been less CH4 and NH3 and more CO2 and N2 in the earth's early atmosphere than in the Miller-Urey experiment. Experiments done with the latter composition were much less successful in making amino acids than the original one.
We should not be discouraged. Long ago, Carl Sagan pointed out that we were unlikely to see an experiment that began with primitive gases, and ended up with an animalcule crawling up the side of the beaker. We know from the fact that certain meteorites contain amino acids that these compounds could be made in the early solar nebula. It may not be important whether these compounds were synthesized on the earth, or brought in by late meteoroid or comet bombardment.
What is known is that the basic chemicals from which living forms are constructed were present on the primitive earth. Moreover, the critical solvent, water, with its weak bonds, was present in abundance. Temperatures existed that were sufficient to break these bonds occasionally, but not continually, were also present. From the point of view of the astronomer, these are the conditions necessary for life as we know it.
Planetary scientists often call this notion the Goldilocks theory. Just like Baby Bear's porridge, the earth was not too hot and not too cold for liquid water solutions within which weak bonds could play their key roles.
Until the time we learned the general structure of DNA and RNA, and ``cracked'' the genetic code, there was much speculation on possible life forms based on silicon. We have already noted this in our discussion of Spencer Jones's book. Silicon is below carbon in the periodic table, and has the same outer electron structure. But silicon is less abundant cosmically (at least in the SAD) than carbon, and the pure silicon hydrides or silanes are far more limited in number and variety than the hydrocarbons.
Ubiquitous silicon compounds abound in rocky materials in the form of minerals. By definition, minerals are all solids, and the silicon is typically bound to oxygen rather than hydrogen. The complex chemistry of life seems inconceivable in solids, because the atoms are essentially frozen in place. Some migration does occur but it is excruciatingly slow. If life forms were to be based on silicon, it would have to be in a liquid phase, and this immediately conjures images of volcanos and magmas.
At the temperatures of silicate magmas, there are relatively weak bonds that play a key role in the viscosity of mafic vs. felsic lavas. But it would seem that only in comic books are we likely to see a silicon man emerge from a volcanic vent, and take a few strides before becoming permanently frozen. Anyone care to speculate about the Easter Island statues?
Molecules with the same chemical formulae have shapes that resemble ``handedness.'' Molecules may have identical numbers and kinds of atoms, but one looks like the other as seen in a mirror, just as a right hand looks like a left hand if you look at its reflection. Liquid solutions of right-handed or left-handed molecules will rotate the plane of polarized light if it is passed through them. The property is called optical activity.
The explanation of the phenomena was found by the great French biochemist Louis Pasteur in 1844. Tartaric acid, a product of winemaking was found to have the same chemical formula as racemic acid, yet the former was optically active and the latter was not. Pasteur crystallized a salt of tartaric acid, and with the help of a microscope and tweezers, physically separated crystals that were mirror images of one another.
When these crystals were dissolved in water it was found that one set rotated the plane of polarized light in one direction, and the other in the other. Racemic acid was found to have equal numbers of both left- and right-handed molecules.
Equal mixtures of left- and right-handed molecules have come to be called racemic mixtures-optically inactive. The interesting point is that all molecules manufactured by life forms are of one kind. It isn't known why this is so. It is clear that molecules of the ``other'' handedness would not work in the chemistry of current life. Shape is critical for so many of life's processes. We shall say more about this in the next section. For the present, suffice it to say that left and right-handed molecules have different shapes, so that a process that would work for one handedness might not work for another.
Why one handedness and not the other? In a charming comment in his General Chemistry text, Linus Pauling speculates that life had to start out with one handedness or the other. Once the choice had been made, subsequent living matter had to keep the pattern. He says: Perhaps a better explanation than this can be found--but I do not know what it is.
Chemists have known about catalysts for many years. Traditionally, they are defined as substances that speed a chemical reaction. They are not, themselves, chemically changed by the reaction, and this makes them reusable.
Virtually all of the biochemical reactions are catalyzed by enzymes. Enzymes are proteins whose shapes and distribution of electrical charges enable them to speed up specific reactions. In order for two molecules to react, it is necessary that the energy balance--before and after the reaction. Add the energies stored in the molecules before and after the reactions. Any difference must be either supplied as heat to the reactants, or removed after the reaction has taken place. Chemists call the first kind of reaction endothermic, heat is required, and the second, exothermic, heat is given off.
When proteins are built up according to the genetic code, the reactions are endothermic, and energy (heat) is supplied, typically by the phosphate ATP. But even with the required energy, it is possible the reaction would not proceed.
Proteins are built up from amino acids. In these reactions, a hydrogen atom from one molecule and an OH group from the other form a water molecule that goes into the general solution within the cell. The remaining molecules are then joined by a bond--the famous peptide bond discussed above. This reaction is called condensation (see above), because water comes out of the process.
In order for condensation to take place, two things must happen. First, the two reacting molecules must come together in the proper orientation for the H of one and the OH of the other to form the water molecule. Second, the existing bonds connecting the H and the OH to their parent molecules must somehow be broken. It takes energy to do these two things--an additional energy to that necessary to balance the overall reaction. This additional energy is called a reaction barrier.
Chemists have a symbolic way of representing that energy. Suppose
we consider a symbolic reaction of the form 
. We can represent the forces that draw A to B
by a potential plot similar to the one of Figure 9.1. In
Figure 11-4, the potential energy is again plotted
vertically. The horizontal (x-) axis is often called the reaction
coordinate. For a simple system, such as the CN molecule, it is
proper to think of it as the separation of the atoms. In general,
we may take it as a measure of the extent of the reaction.
The reaction is complete when the coordinate has the value indicated on the diagram by ``AB.'' Part (a) of the figure illustrates an endothermic reaction, while part (b) shows an exothermic one. In the first instance the total energy when the molecule AB is less than when they are unbound, so heat must be added to achieve a balance. In the latter, the situation is reversed. Part (a) shows the system with the molecule in a local energy minimum.
Figure 11-4: The Reaction Barrier. Molecules A and B react to form the molecule AB. The potential energy is plotted vertically. The x-axis is a symbolic measure of the extent of the reaction. The reaction has taken place where `AB' is written on the plot. `A + B' implies the molecules have not reacted. In (a), the reaction is endothermic while in (b) it is exothermic. Note that in both instances, more energy is required to get to the configuration AB than the mere difference between AB and A + B. This extra difference is called the reaction barrier.
The energy necessary to get over these barriers might come from the ordinary motions of A and B due to heat. But typically this energy falls short in biochemical reactions. The enzymes, with their special shapes and charge distributions, can grab one or both reactants, and line them up in just the right way. At the same time, the interaction between the enzymes and the reactants will be such that the reaction barrier is substantially lowered. This lowering may be the result of a subtle redistribution of charge over the reacting species and the enzyme with which they are in contact.
Thus, the enzymes work their magic through various manifestations of weak chemical bonding. Their attraction for the reactants is typically very specific, and depends on the shape of both the enzyme and the reactants. The enzyme is a long, folded molecule, with hydrophilic branches sticking outward. There is usually one particular site that has the right shape to catalyze the reaction. For a long time, the matching of the shapes of organic molecules in reactions has been compared to that of a ``lock and key.''
Enzymes are proteins, and they catalyze the reactions involving other proteins made by the recipe of the genetic code. Biologists know an enormous amount about these procedures. In spite of the awesome complexity of the DNA molecule, textbooks describe in detail how the code is transcribed and translated into the structure of proteins. Since the reactions that produce proteins are catalyzed by enzymes, we have a problem. How did the first proteins manage to get made?
Much is also known about how DNA is replicated during cell division. Even popular accounts, such as the description in Boyce Rensbergen's Life Itself contain bewildering detail. But there is no account of how the DNA molecule itself came to have its present structure. It is easy to see why. We may observe the replication of DNA in the laboratory, just as we may observe the construction of proteins. We were not been present to observe the evolution of the DNA structure.
Astronomers were not present at the birth of the sun, yet they think they know many of the details of its birth. What is it that is available to them that appears lacking to the biologist?
The astronomers have been able to observe the birth of stars other than the sun. We discussed that in Chapter 8. By observing clusters of stars in various parts of the Galaxy, astronomers have put together a reasonably complete picture of stellar evolution. The biologist has not had a comparable perspective.
What would the biological analogue be of the astronomer investigating giant clouds where stars are being born? It would be the study of living forms stretching from those minimally capable of reproduction through current single-celled organisms to higher life forms. The beginning of this sequence is missing. Even simple life forms have DNA with molecular weights in the millions.
The missing life or proto-life forms may no longer exist. They probably originated at a time when the earth, and especially its atmosphere were quite different from their present states. It is also possible that no fossil record was left of the early life forms, although it would be most exciting to discover some.
Life scientists have suggested that the original templates for reproduction were not the double helices of DNA, but the simpler RNA molecules. In the process of transcription and translation, the entire genetic code is written picewise to RNA molecules. It just doesn't happen all at once. So DNA seems unnecessary for the earliest life forms. This line of reasoning gets us to the level of viruses, which are usually RNA strands, with some protein covering. There is now good reason to think that RNA could play the role of enzymes and catalyze reactions. Specific examples of catalytically active RNA strands were discovered in 1981, and are called ribozymes. The earliest systems might do without both DNA and proteins.
Viruses have been studied in considerable detail, mostly from the viewpoint of thwarting their activity. Usually, they are not considered to be alive, because they cannot reproduce without invading the cells of some host, and taking over its genetic machinery. Many organic molecules are able to organize themselves and produce longer structures by polymerization. This kind of reproduction is not considered an indication of life. In order for reproduction to be considered part of the process of life, it must be carried out by a specific recipe that is a part of the parent molecules.
Suppose you had a freight yard where the tracks doubled back so the cars could run around and bang into one another (assuming they each had little engines). Each car has a coupling at both ends, so after a while all of the cars could be hooked up. The difference between life and polymerization is that in the former, the coupling is only made if the two cars fit a recipe. In the end, instead of a random assembly of cars, there would be (at least part of) a train! The molecules of living cells assemble trains, so to speak, and don't merely hitch fragments.
It is common to think the processes that began life are no longer functioning today? We mentioned that the ancient earth was quite different from the present one. But researchers are finding more and more evidence of microbial life forms in bizarre and hostile environments. Is it possible that the first steps from non-living to living matter are still being taken, under our very noses?
Modern versions of the Miller-Urey experiment are being carried on in laboratories today. Wouldn't it be exciting to discover nature performing similar experiments? Perhaps we haven't yet looked hard enough. Any such natural experiment would involve molecules that are too small to be seen with ordinary microscopes. Even for viruses, with their macromolecular RNA, we need electron microscopes. And these are typically used for the study of disease.
One of the great triumphs of historical biochemistry was to show that life does not arise spontaneously from non-living matter. Theories of the origin of life assert just the opposite. What Spallanzani and Pasteur were able to show was that life does noe arise from non-living material within the limited time and spatial frames of their laboratory experiments. In the historical sciences of astronomy and geology, we know that nature is not so severely limited.
When DNA strands are pounded by the surrounding molecules of the cell plasma, about one bump in 100 is enough to open a single hydrogen bond. With the help of this buffeting certain enzymes can position themselves in such as way that the transcription of the genetic code by RNA can proceed.
If the strands opened too easily, the giant molecules would be too unstable. If they were too tightly bound, replication would not be possible. The frequency of collisions between molecules depends on the temperature. For a given density of material, collisions are more frequent the higher the temperature. They are also more violent. But the right amount of buffeting of these molecules is necessary. The temperature can not be too high or too low.
Life's processes require a severely restricted set of conditions. Liquid water is essential, and in the astronomical domain, this state is rare. That is why there is such interest in extraterrestrial locations where liquid water might exist. Recently, considerable attention has been devoted to a possible ocean just below the ice crust on the Jovian satellite Europa. People have discussed possible subsurface liquid water on Mars, and even at the centers of comets. In the case of comets, heat could have been supplied early in the history of the solar system by radioactive decays, most of which are now exhausted.
It is true that life forms can survive far more extreme conditions than those under which it may flourish. So it is not impossible that dormant, microbial life could have survived a journey in interplanetary space.
Some have thought that first steps toward life so enormously improbable that it is arrogance to think they occurred here on earth. Francis Crick, codiscoverer of the DNA structure, argued this point in a 198? book Life Itself. People who take this point of view think it likely that life originated ``somewhere'' among the 1011 stellar systems of our Galaxy, and managed to seed the rest of the system. This notion is generally called the panspermia hypothesis. It requires the survival of life forms in interstellar rather than interplanetary space.
The immense distances mean long travel times for the floating ``spores.'' At a typical speed for stars moving among one another in our region of the Galaxy, it would take some tens of thousands of years to cover the distance from one to the nearest neighbor.
Most life scientists favor a local origin for life. But this does not rule out the possibility of a separate origin on another earthlike planet elsewhere in the Galaxy. For many years astronomers have speculated on the fraction of stars that might possess a planet similar to the earth. When Spencer Jones wrote on this topic at midcentury, it was thought probable that life existed on the planet Mars in the form of vegetation. Jones nevertheless thought earth like conditions might be very rare among stars.
Modern discoveries of planetary masses orbiting nearby stars have changed this picture. Even before these discoveries, many astronomers would have--guessed--the probability of a planet with life about some star was relatively large.
We often read that the climate of our planet is precariously balanced. A tad larger hole in the ozone layer, or a degree or two more global warming could be disastrous. This is certainly a problem for our current civilizations. But if we just ask for a temperature range where water would remain liquid, the balance is less critical. A rough calculation shows that we might move our planet in or out by some 20 to 40% of its current distance from the sun, without boiling or freezing most of its water. Thus a planet orbiting some other sun would not have to be at precisely the earth-sun distance.
The prospects seem as bright as ever that there might be life on other worlds.
The most basic building blocks for life are water and simple molecules containing carbon, hydrogen, nitrogen, and oxygen, along with sulfur and phosphorus. These were certainly present very early in the earth's history. We are unsure of the relative molecular abundances as well as the origin of volatile species-outgassing from the interior, or from comets. It is generally considered most probable that life originated on earth. Biologists have learned in great detail how life processes sustain themselves with the help of the genetic code. But we have very little insight into the earliest self-replicating life processes. Undoubtedly this is because we have been able to study the former process, but not the latter. It is not impossible that these earliest steps are going on today, undiscovered or unrecognized. A search seems worthwhile.
We have emphasized the significance of weak chemical bonding in life processes. These bonds and their properties are responsible for the singular characteristics of liquid water. Hydrophobic and hydrophilic properties of molecules account for the folding of enzymes and the structure of organic membranes. Liquid water is essential for life as we know it. From the point of view of the astronomer, it is not so unlikely that a planet would exist at the right distance from a typical star like the sun. There could be many such stars within our own Galaxy, with the right chemicals and conditions for life. Until we know more about the earliest steps in the construction of nucleic acids (RNA or DNA) we cannot assign a probability to the existence of life on such planets.
Perhaps it is arrogant to think that we have some idea of what went on in the first minutes of the history of our universe. If this is a problem, not to worry. We pointed out in an earlier chapter that scientists make conceptual simplifications of the real world that they call models. So it is perfectly OK to think of the first few minutes as they occur in our model of the universe.
We have seen in Chapter 7 how hydrogen and helium emerged from the Big Bang, along with trace quantities of other nuclei such as deuterium and 3He. Densities were not believed to be high enough for nuclear reactions to bridge the famous mass 5 and 8 gaps where there are no stable nuclides. For this reason virtually all other nuclei were built up in the interiors of stars.
We also saw in Chapter 7 that the relative abundances of the light nuclides that did emerge from the Big Bang can tell us both the temperature and density of the universe at that time. Then with the help of the universal, 3o, blackbody radiation measured in our present universe, we can find the density of protons and neutrons now. This number would tell us whether we have an open or closed universe, if it were not for the mysterious dark matter.
Astronomers have put a good deal of effort into the determination of the amount of helium that came out of the Big Bang. Most of the helium that is made in stars is burned to carbon, or just locked up inside stars with small masses. But it has been determined that the helium content of the universe does increase slowly by processes that we don't know in detail. Perhaps not all of the helium created in hydrogen burning is destroyed or locked up. Maybe in stellar explosions, some of this helium is returned to the interstellar medium.
We can get a better notion of what the original helium abundance might have been by looking at galaxies that have rather low metallicities. The lower the metallicity in the galaxy, the less likely it will be that stellar processes have added helium to the interstellar gas. The best kinds of galaxies to look at tend to be rather far away, although not as far as the quasars. Astronomers study the light that comes from giant gaseous regions that are lit by hot young stars.
The most recent work shows that the primordial helium represented about 24% of the total mass of protons, neutrons. Electrons are very light and can be neglected as far as the mass is concerned. On the basis of this result our model universes would continue to expand unless there were an amount of dark matter that outweighed the protons and neutrons by a factor of about 20!
We can say precious little about this dark matter beyond the fact that its gravitational effects are needed. They are needed to keep galaxies and clusters of galaxies from flying apart. It is also needed to make them in the first place.
What happened to matter in the universe after the first few minutes? We have precious little in the way of observational constraints. One of the few pieces of information that stretches back toward that time concerns observations of the current universal (3o) radiation. It seems marvelous that it could be detected at all, but over the years following its discovery ever more precise measurements were made upon it.
Eventually it became clear that we could measure the direction of our sun and galaxy with respect to the universe itself. Einstein thought there was no one frame of reference that was any more special than any other. He was wrong on this point. This 3o radiation itself provides a unique frame, in terms of which motion may be measured.
We can tell our direction of motion from subtle changes in the temperature of this radiation. Photons that are coming toward us are just a little more energetic than those we are moving away from. Cosmologists called this a dipole asymmetry in the background radiation. There are two poles, one hot and one cold. The effect is pretty small, only about one part in a thousand, but it has been amply confirmed by several groups since its discovery in the late 1970's.
After the discovery of the asymmetry, it seemed more information might be wrung from this fascinating radiation. It comes to us from a time when the universe became transparent, a time sometimes referred to by cosmologists as the recombination epoch.
Well after the synthesis of the light nuclides, the universe was still much too hot for electrons to combine to make neutral hydrogen and helium atoms. It was more like a million years after the Big Bang that the temperature in the universe was low enough for neutral atoms to form. During all of the hot period, free electrons prevented photons from going very far. Once they recombined, there was little to prevent us from seeing the photons of that epoch. That's what the 3o radiation is, light from the first million years of the history of the universe that is now reaching us. It is red shifted because of the expansion.
In 1989, the Cosmic Background Explorer (COBE) satellite was launched, with the primary purpose of looking for irregularities in the background radiation. After several years of very careful analyses, the COBE team announced detection of fluctuations in the background radiation. What they found were very small, but definite variations of the temperature (of some 10-5K!!) for regions of the sky separated by 7o or more.
These fluctuations did not correspond to the large scale structure now observed. The COBE fluctuations need to be projected from their time toward the present, that is, from a time some 106 years after the big bang to times corresponding to the distances of the large scale structure. When this is done, it is found that the smallest COBE fluctuations correspond to distances an order of magnitude or more greater than the observed large scale structure.
Theoreticians try to bridge this gap with calculations, and an interesting result of their work is that they must add some information about the nature of dark matter in order to get smoothly from the COBE to the large scale structure.
Several guesses have been made about the nature of the dark matter. One guess is that it might be the neutrinos that we discussed in Chapter 1. Neutrinos are fast-moving particles. If they have a rest mass, it must be rather small, and the neutrinos would have velocities just below that of light. If they have no rest mass, the would travel at light speed, and be no help for the dark matter problem. Let us make the first assumption. Then if there were enough neutrinos, they could contribute to the dark matter problem.
In addition to neutrinos, some unknown particles might travel near light speeds, and account for the dark matter. It is also possible that the dark matter could be massive particles, like the proton or neutron. It is less likely that protons and neutrons themselves are responsible for the dark matter than some unknown kinds of particles, although this is not certain. Theoreticians try to model the behavior of the early universe with both kinds of particles, light ones, with velocities near the speed of light, and more massive ones. They call the first kind of particle hot dark matter and the second cold dark matter. Hot dark matter forms larger structures than cold dark matter. The ``clouds'' are puffed up, as though by a high temperature.
It is interesting that the most promising models to date require both hot and cold dark matter. If this result continues to hold, the majority of mass in the universe may not be just dark matter, but comparable amounts of two kinds of dark matter! While the experts build their models, the skeptic may be forgiven for asking if we really understand the behavior of matter over large distance scales.
Clearly the problem of dark matter presents the most fundamental challenge to physics today.
After our explorations of matter at the largest scales in the universe, let us now turn to the neighborhood of the sun. Elements heavier than helium are thought to have originated in the interiors of stars. We do not know all details of how these newly synthesized elements made their way back into the interstellar medium to be used in the formation of new generations of star. But the general picture is quite clear. We see stars in the process of ejecting material. In some cases, as in supernovae or novae, the ejections are violent. We also observe instances of quiescent ejection in the form of stellar winds.
When the theory of nucleosynthesis was worked out in the 1950's a key role was played by the chemical abundances in meteorites. The critical isotopic abundances were thought at that time to be almost universal within the solar system. Plots such as Figure 4.5 were made with the help of relative isotopic abundances from terrestrial materials. At that time it was thought that at an early stage in the history of the solar system, all solids were vaporized and thoroughly mixed with the background gas.
We are now confident that a small fraction of the solids that were present at the ``beginning'' were never volatilized; they are isotopically distinct from average material. In a previous section we discussed how the oxygen isotopes vary from one sample of solar system material to another. Much of the work in this field has been done by the University of Chicago chemist R. N. Clayton.
Figure 12-1: Oxygen isotopes in various materials from the solar system. The ordinate and abscissa are given here as per cent, but the usual unit of measurement is 10 times larger, and is called ``per mil'', or parts per thousand. The range of values covered by this figure is between 1 and 2 per cent, or 10 and 20 per mil. (Figure from Harry McSween's article in The New Solar System).
Changes from Standard Mean Ocean Water or SMOW are measured in parts per thousand, for which there is a special double per cent or ``per mil'' sign: o/oo. The numerical values are given the symbol delta. So if the delta-17 value is +1o/oo, for example, it means that the 17O-isotope is 1 part in a thousand more abundant relative to 16O than in SMOW.
A general rule of the thumb is that normal chemical processes tend bind the heaviest isotope most strongly. Also, chemical reactions that affect the ratio of 17O to 16O by a certain amount will change the 18O to 16O by double that amount, just because 18 - 16 is twice 17 - 16. For this reason, the Earth-Moon (or terrestrial fractionation line) in Figure 12-1 has a 2 to 1 slope.
The Meteorites called C3, as well as a category of meteoritic material called CAI's plot on a line with a slope of 1:1 rather than 1:2. These positions cannot be explained by mass-dependent effects. The most likely explanation for the one-to-one fractionations is an admixture of pure 16O. This would make the fractions of both heavy isotopes small. A sample with pure 16O would plot at (-1000.0, -1000.0).
The CAI's are calcium-aluminum-rich inclusions found within the larger carbonaceous meteorite matrices. Samples of CAI's fall along the 1:1 line with delta-18 values as low as -40 per mil. This could imply a 4% admixture of pure 16O. Where might such material originate?
Just before massive stars have exhausted their nuclear fuels, their structure is like a sequence of shells. Starting from the star's surface, there may be a shell of unburned hydrogen, then one of helium, another of carbon, and yet another of oxygen (Figure 12-2). If such a star ignites as a supernova, it may blow off some nearly isotopically pure material.
Figure 12-2: Onion-Shell Structure of a Massive Star. Not all shells are shown explicitly. At the center, all nuclear fuels are exhausted. This star could detonate as a supernova, and return the nearly pure carbon and oxygen isotopes to the interstellar gas. Some massive stars strip off their surface layers of hydrogen and helium in winds. Later winds could also be rich in 16O or 12C.
Astronomers have observed a variety of supernova remnants, vestiges of these violent stellar explosions. In some cases neutron stars have been left behind. They are now observed as pulsars, rapidly spinning, incredibly dense objects that can concentrate radiation somewhat like a searchlight. In other cases, we only observe the ejected gases, and surrounding interstellar gas that has been heated by shock waves from the explosion.
An interesting example of such a supernova remnant is known as Cas A. This was the name given to a source noted first by radio astronomers in the constellation of Cassiopeia, the W-like constellation that circles the North Star. While the first observations of Cas A were made in the late 1940's it was several years later that optical observers discovered anything unusual in the direction of this powerful radio source. Deep photographs taken with the 200-inch telescope revealed faint nebulosity now identified as material thrown off in the explosion that took place in the late 1600's.
Some of these nebulous knots have been studied in detail. Much of the work was done by the US astronomer Robert Kirshner and his colleagues. They found most unusual abundances. In particular, certain knots are highly oxygen rich. It has never been demonstrated, but we speculate here that the oxygen is nearly pure 16O. This would be the material needed to explain the 1:1 fractionation line in Figure 12-1.
Some of the calcium-aluminum rich fragments found in carbonaceous meteorites have anomalous isotopic abundance patterns of a variety of elements. Often an excess of neutron-rich isotopes of several elements will be found in the same fragment. In these cases, it is thought that these grains formed very near to stars whose material had been subject to neutron addition. They were never completely vaporized during the formation of the solar system, and therefore still carry the isotopic signature of the region in which they formed.
Recently, laboratory techniques have improved to the point where it is possible to study trace species in tiny quantities of meteoritic material. Much attention has focused on the dark, background matrix of carbonaceous meteorites. It turns out that this material contains microscopic diamonds as well as carbide minerals-substances that would only be expected to form in carbon-rich environments.
Most stars have compositions similar to the sun, where the abundance of oxygen exceeds that of carbon. When this is true, then as any gas mixture cools, the very tough diatomic carbon monoxide molecule (CO) will form as much as possible. This means that if oxygen is more abundant than carbon, as is usually the case, all of the carbon will be gobbled up by the CO molecule. There will be virtually no free carbon for the formation of diamonds, graphite, or any of the metallic carbides. Indeed, theory has told us that virtually all of the materials in the matrix of carbonaceous meteorites could not have been formed from material with the SAD abundance mix, where O > C in abundance.
How is this to be reconciled with the new measurements on carbonaceous meteorites. The most obvious possibility is that the material comes from stars where C > O in abundance. Astronomers have known such carbon stars for many decades. Many of these stars are thought to be burning helium to carbon in an inner shell with hydrogen still be burning to helium in an outer shell (Section 6.2). If some instability mixes materials from these outer zones, s-process neutrons can be generated. In addition, carbon may be mixed to the stellar surface, where it can be detected spectroscopically.
In addition to the optical spectra of carbon stars, infrared measurements have indicated the presence of carbides in the circumstellar matter associated with carbon stars. So seems that the sort of material observed in carbonaceous meteorites is generally present in our Galaxy.
Apparently some of this material was injected into the interstellar medium, and found its way into the clouds which collapsed to eventually form a cluster of stars, including the sun. Some of these microscopic bits of carbonaceous material did not vaporize and survived as pre solar grains in the meteorites. We know this because of non-SAD isotopic ratios. If this material had been vaporized and mixed with all of the other gas of the solar nebula, the special signature from any individual star would be lost.
The carbonaceous material of the meteoritic matrices has been found to have a range of values for 12C to 13C. Some of these values are much larger than have been found in any carbon stars. Indeed, the findings are almost consonant with an admixture of pure 12C, a situation reminiscent of that with 16O. We might invoke a similar scenario. Perhaps some presolar grains formed from gas with freshly synthesized carbon and oxygen, where 12C, and 16O would be expected to be made in nearly their pure form.
We know that when hydrogen burns by the CNO cycle the ratio of 12C to 13C typically ends up having a value around 4. In cool carbon stars, we have been able to measure this isotopic ratio from molecular absorption lines, and values range from about 4 to the SAD value of 90. Presolar grains with measured 12C to 13C ratios up to 7200 cannot have come from typical carbon stars.
In Chapter 3 we described a broad picture in which interstellar gas was processed through stars. As these stars evolved, some of them returned their gas to the interstellar medium, enriched in the products of nucleosynthesis. In Chapters 5 and 6 we considered some of the processes by which the measured pattern of the SAD arose. It is now time to examine the evidence that stellar nucleosynthesis has actually been taking place in our part of the Galaxy.
If the galaxy originated with mostly pure hydrogen and helium, the oldest stars began their lives with no heavy elements. Astronomers have sought such stars. We know from very basic considerations that if some of these first generation stars had masses of about 0.8 times the solar mass or less, they would not have evolved in the lifetime of the Galaxy. Indeed, all stars in this mass range would still be on the main sequence (Section 3.4), and their atmospheres should have the same composition they were born with.
We can identify low mass (cool) stars from their spectra, so that it is possible to select just these objects, and not worry about changes that have taken place during the lifetime of an individual star. If we restrict our study to low mass stars, can study the star formation and chemical history of the gas over the entire age of the Galaxy.
The chemical history of the Galaxy turns out to be a census problem. Clearly we cannot observe all low mass stars near the sun. We must get a sample. As any pollster knows, it is easy to get a sample, but it is not easy to get a sample that tells you what the entire population would tell you. How is this done with stars?
A great many stars have had their abundances determined but it would be most unwise to begin with a list of such stars. Abundance workers like to analyze unusual stars, so any list of analyzed stars would be heavily biases toward these unusual types. It would not be representative.
Astronomers have chosen two ways of getting an unbiased sample of stellar abundances. In the first, all stars are analyzed, down to some limiting brightness. Fainter stars are excluded. In a second method, a sample of stars within a fixed distance of the sun are studied. In either case, the stars were not chosen because of their abundances, and we may hope they may be representative of the stellar population as a whole.
Stars chosen in this way rarely have their abundances determined in the most basic way, by spectroscopic analyses. On the other hand, the colors of cool main sequence stars can be closely related to their atmospheric abundances. In most of these stars, the spectra are blanketed with metallic absorption lines, primarily from neutral iron. These lines are not distributed uniformly over the spectral range, but tend to crowd in the ultraviolet. Stars with high metal abundances have much of their ultraviolet radiation absorbed, and other things being equal, are redder.
Astronomers measure the colors of stars in a variety of bands distributed over the spectrum. From these measurements, they can tell both the temperature and the metal abundances of the stars. Careful measurements can also reveal if cool stars are giants are dwarfs, although the best indications here come from spectroscopy.
Traditionally, colors have been generally available for samples of low mass stars, and have been used to extract abundances. The first step in such a study is to make a histogram of the abundances. These give the fraction of stars in various abundance ranges.
Figure 12-3: Abundances of Late Dwarfs in the Solar Neighborhood compared with predictions from ``the'' simple model of galactic chemical evolution.
The data of Figure 12-3 was taken from a 1996 study by the Brazilian astronomers Rocha-Pinto and Maciel. They chose to work with cool dwarfs in a fixed volume of space surrounding the sun. The histogram is compared with a simple, but appealing model of galactic chemical evolution, starting with a pure hydrogen gas which is slowly converted to stars. As the fraction of the gas in the volume diminishes, the abundance in the gas increases as a result of stellar nucleosynthesis.
The simplest models of galactic chemical evolution have been known for a long time to predict too many metal-poor stars relative to the metal-rich ones. The scarcity of metal poor stars is indicated in the figure, and has been called the ``G-dwarf'' problem, because it was first investigated with a sample of mostly G dwarf stars.
The theory of the curve shown in Figure 12-3 is remarkably ``simple.'' Stars are assumed either to have negligible lifetimes, or infinite ones. The first kinds of stars return their processed matter to the interstellar medium instantaneously. The remaining stars lock up the gas in dwarfs with lifetimes equal to that of the Galaxy. In addition, it is assumed that all abundances of heavy elements may be described by one parameter, Z.
There are several ways of resolving the G-dwarf problem, even within the context of such simplified models of stellar evolution. But modern computer programs make it possible to follow the evolution of a realistic distribution of stellar masses. These programs can account for the G-dwarf problem in a number of ways. At the present time, the most likely possibility seems to be that gas has been added to the star-forming region from the galactic halo. The added gas has the effect of delaying the formation of stars at the lowest Z.
The modern computer codes allow one to track individual abundances. This is of particular value today, because we know that different kinds of supernova have been significant throughout the history of the Galaxy. These supernovae are called Types II and Ia for reasons that make sense only to those who have followed this field for some time. The Type II's prevailed in the early history of the Galaxy, and made mostly heavy elements with nuclei composed of alpha particles: 12C, 16O, 20Ca, 24Mg, and 28Si. These supernovae made some iron, but most of the iron now present in the Galaxy was made by Type Ia supernovae.
The realization that different kinds of supernovae were relevant to galactic chemical evolution came slowly over the years following B2FH (Chapters 6 and 7). The results of abundance surveys done in the 1950's and 60's were rather disappointing. A very small number of stars were found to have abundances of heavy elements about 1% of that of the sun. This was thought to be related to the G-dwarf problem. These early studies also (seemed to) show that within the errors of the observations, the heavy elements all seemed to go up and down together. For example, if a star had an abundance of iron that was 10% of the sun's, then its chromium abundance was also down by 10%. Astronomers said that the heavy elemental abundances varied in lockstep.
Early abundance work was done using photographic plates, and the spectral resolution was often only a tenth of that used regularly today. Nevertheless, some of the studies of the 1950's and 60's were carried out with great care, and their basic results have been confirmed by modern work.
We now know that the ratio of alpha-elements to iron has been slowly decreasing over the age of the Galaxy, as the iron built up with the advent of Type Ia supernovae. Similarly, we know that nuclides made primarily by the r-process were more abundant relative to their s-process congeners in the early history of the Galaxy. For reasons that are not entirely clear, s-processing became more prevalent as the Galaxy aged.
These variations from lockstep fascinate astronomers today, because there is so much information written in the abundance patterns. Younger astronomers often seem unaware of the decades when many of us failed to acknowledge these variations. Some have told me there are ``large'' deviations from lockstep, which makes me wonder why it was so hard to demonstrate them originally!
Astronomers are cautious, and it seemed more prudent to say that our errors were large-they were, of course-than to believe trends, hovering on the threshold of credibility. Modern work has mostly not made these trends any larger. It has lowered the errors, so we believe what we see.
We now know that the evolution of iron to alpha-element as well as the s-to-r-process ratio has been generally steady.
Among the very oldest stars, some irregularities appear, but that may be because individual nucleosynthetic events are relevant. A very old star might have gotten gas from a single, older, nearby object that made a lot of barium. After nucleosynthesis has proceeded, these irregularities mostly smooth out.
The chemical elements are made in a variety of processes, some involving explosions, but quiescent nucleosynthesis is also important, coupled with stellar mass loss. All of these processes are a part of the birth and evolution of star clusters, something that we may characterize as a star-forming event. It is probable that none of these events are exactly alike. Nevertheless, the net effect after many such events may be the same as if a series of identical events occurred, each with the average properties of the actual ones.
This assumption makes it possible to build detailed models of galactic chemical evolution. Stars are grouped into mass ranges. The relative number of stars in each mass range--the histogram of stellar masses--is what the astronomer calls the mass function. This mass function is generally known, from counting stars in the solar neighborhood as well as in clusters. At the very low mass end of the function, we are not sure how many stars there are, but for many purposes this is not critical.
The life history of a star is determined by its mass and chemical composition. Once we decide on a mass function we are well on the way to modeling the chemical history of a galaxy. The other parameter we need to know is the star formation rate. Crudely, we can count the number of stars in our Galaxy (1011, and divide by the age of the Galaxy, say 1010 years. This give us an overall rate of 10 stars every year. In principle we could allow for a different rate for each mass range, but this is not generally done because it is difficult enough to get an overall rate for a fixed mass function.
In general, the star formation rate will vary both with time and location in the Galaxy. A standard assumption is to assume that the star formation rate is related to the density of gas from which stars form. This means that if we assume the gas is slowly locked up in dwarf stars, the star formation rate will be highest at the beginning, when the Galaxy was mostly gas, with relatively few stars.
If the gas density is initially highest at the galactic center, that is where the stars formation will be highest. The gas there will be used up most quickly, and the stellar nucleosynthesis should be most advanced.
In spirals, it has long been known that the centers are rather poor in gas content, and that their colors are redder than the arms (See the color online version of M100, also in Figure 3-2). Generally speaking, this is a good indication that the metal abundances are higher there. We mentioned that the colors of metal-rich stars tend to be redder because atomic absorption lines which are most densely packed at shorter wavelengths, cut out much of the blue-violet light. It also turns out that when giant stars evolve, they are typically redder if they are metal rich.
Figure 12-4: Hubble Space Telescope Color Image of the Spiral Galaxy M100.
The theory of stellar evolution is now sufficiently well advanced that one can keep track of the evolution of all stars in a given mass range. If the stars have masses a little less than that of the sun, they will simply lock up the gas. For higher masses, they will evolve, and return processed material to the interstellar medium. Stars with masses above about eight solar masses will do this explosively. Modelers can even adopt rates for the important supernovae types, so that the ratio of alpha-rich nuclei to iron, and r-tos-ratios can be evaluated.
These abundances will be different in general, for each time and location in the Galaxy. How can these models be checked?
Astronomers can only examine the universe as they find it. Although we think we know generally how stars are born and die, we are not in the position of the biologist, who can study many generations of the fruit fly. On the other hand, many relics of the early times survive. These are the low mass stars, and in the case of the planets, we have rocks whose ages unambiguously date to 4.5 billion years ago.
For the low mass stars in the solar neighborhood, we have data such as that illustrated in Figure 12-3. Satisfactory fits to the late-dwarf histogram may be made if gas is added in the early history of the Galaxy, with little arriving at the present epoch.
The gas content of our own and nearby spiral galaxies can be mapped with radio and microwave techniques. Radio telescopes measure the content of neutral hydrogen, while the newer microwave methods are used to map the CO molecule. The molecular hydrogen, we may recall is more difficult to observe than CO because it is a homonuclear molecule. Astronomers have a recipe that allows them to translate the CO intensity into a density of H2. The inference of the amount of H2 from CO takes a little faith, but astronomers have done this kind of thing before: necessity breeds faith.
When the results of the neutral hydrogen and CO maps are combined, they generally show a gas density that is low in the centers of spirals and increases to a maximum at some radius from the center. Eventually it declines toward the edge of the galaxy.
Abundances in galaxies can be measured crudely from the colors of the objects, or from the intensities of emission lines from regions of hot gas--emission nebulae. It is also possible to analyze the strengths of spectral lines of the stars, although in this case, one gets what are called integrated spectra, spectra of many stars together.
All of these measurements generally agree with the notion that the abundances are highest where the residual gas is lowest-at the centers. If one looks far enough from the center, the galaxy starts to end, and the density goes down, of course. Figure 12-5 shows observations of nitrogen and oxygen abundances in the Sc type (see Figure 3.1) spiral M33.
Figure 12-5: Abundance Gradients of Oxygen and Nitrogen in the Sc Spiral M33.
The time dependence of star formation within galaxies is a more controversial topic. The simplest assumption one can make is that stars have formed at the rate of about 10 per year, over the lifetime of spiral galaxies. In these systems, we know that stars must be forming now, because we can observe massive blue giants whose lifetimes are only ten million years or so--astronomically speaking, they were born yesterday.
We don't know much about the rate of star formation. One trick we can use is to examine for the solar neighborhood the relative numbers of stars in different mass ranges. If we count only stars with masses above, let us say 3 solar masses, all objects will have been born within the 240 million years, while those with masses equal to that of the sun will have all been born some 10 billion years ago. If we go to slightly less massive stars, we hit the point where the main sequence lifetime and the age of the Galaxy are the same. All of the stars with those masses, born at any time in the history of the Galaxy, are still on the main sequence.
We count numbers of stars from the highest toward the lower masses. The number of objects in a given mass range changes for two reasons. One is that nature favors certain mass ranges over others. Generally speaking, nature makes more low mass stars than high mass stars. The second reason the counts will change is that the lower in mass we go, the longer an interval of time is available before the stars leave the main sequence. So we expect the number of stars to increase for this reason too-until we hit the range with a main sequence lifetime equal to that of the Galaxy. We may call this the age effect. At that point, if we find stars increasing with decreasing mass it is only nature's proclivity to produce them that counts.
When we hit that mass range, do we find any kind of an indication that the age effect in the star counts? The answer is that we certainly do! This is shown in Figure 12-6, adopted from the work of the US astronomer John Scalo. The solid curve gives the number of stars in the solar neighborhood of the sun in intervals using the logarithm of the mass. The vertical bars indicate uncertainties.
Figure 12-6: The Stellar Mass Function. The number of stars in ``log(mass) bins.'' Here, m = 1 corresponds to 1 solar mass. The plot is for main sequence stars. The counts increase with decreasing mass (see the mass scale inset) as explained in the text. Vertical bars represent uncertainties. The x's show an estimate of how this function would look if the stars were all freshly born.
The x's represent an attempt to show how this function would look if all of the stars were freshly born and none had died away as a result of stellar evolution. This show one of two reasons why the function increases as the mass decreases. First, nature simply makes more low mass stars than high mass stars. The second reason for an increase with decreasing m is that the lifetimes of lower mass stars are longer, so the time interval over which the stars survive increases with decreasing mass. We assume that stars have been produced does not decrease for earlier times than the present.
The plot starts to turn over and flatten just at the masses (0.8 or so) where the lifetimes of the stars equal the assumed age of the Galaxy. After that mass, all stars are still on the main sequence, and no more increase due to the lifetime is expected.
Ironically, the x's show that a turnover is expected for the freshly made stars, so we can't be sure how important the stellar lifetimes are in this plot. What we can say is that there is no evidence of a jump in the overall birth rate for stars. We'll consider such jumps in the next section.
The history of extragalactic research was quite different in different parts of the world. Many astronomers in the US worked to refine the already well-organized scheme of Hubble. Efforts centered around the study of normal spirals and ellipticals. The major emphasis was on the use of these systems to investigate cosmological questions--the age and size of the universe, and whether it would expand forever. Quite early on, astronomers in the Soviet Union emphasized unusual extragalactic systems.
The world's astronomers had no comparable instrument to the Palomar 200-inch optical telescope, but in the area of radio astronomy important contributions were made from England, Australia, and the Netherlands in the years following World War II. Radio telescopes mapped clouds of hydrogen in our Galaxy, and measured bursts of radiation from the sun. There was also keen interest in extragalactic radio sources, many of which turned out to be peculiar in one way or another. Early Soviet extragalactic work emphasized these peculiar systems. Vorontsov-Velyaminov, of the Sternberg Institute in Moscow assembled a catalogue of 355 interacting galactic systems in 1959.
Optical astronomers in the US became interested in interacting extragalactic systems somewhat later. Of course, there was the colorful, prescient, Swiss astronomer Fritz Zwickey who had an interest in these systems very early on. Zwickey, who observed with the Palomar telescopes, inspired the US astronomer Arp to make his own catalogue of peculiar galaxies in 1966. Figure 12-7 shows one of the interacting systems illustrated by Arp. Some three years earlier, the quasars had been identified as extragalactic, and interest in these systems burgeoned.
Figure 12-7: The Interacting Galactic Pair NGC 5257 and 5258 (Arp 240).
We now know that interactions such as that shown in Figure 12-7 can trigger star formation. An isolated spiral galaxy might have a smooth history of star formation, with the parent gas slowly used up. This kind of history is still possible for our own Galaxy, but it is precluded for starburst systems, which became generally recognized after the mission of the Infrared Astronomical Satellite (IRAS) in 1983.
IRAS showed that certain galaxies were emitting huge amounts of energy in the infrared region of the spectrum. We know from studies of our own Galaxy that regions of star formation are very bright in the infrared. What happens is that hot, young stars form toward the centers of giant molecular clouds (GMC's). As we discussed in Chapter 8, much of what goes on in these clouds is obscured by dust. This dust is important for the formation of the H2 molecule, and generally shields the molecules from the ultraviolet radiation of the general interstellar medium.
After stars form inside these clouds, the situation reverses. Now the newly formed stars are the strongest source of ultraviolet radiation. Ultimately, they disrupt the clouds, but for a time before this happens, the dust obscures them. It is at this stage that the GMC's become extremely bright in the infrared. During most of the life of the cloud they are very cold, so when they begin to absorb light from the young stars, they radiate it away in the infrared. This light can escape the dark cloud, because its wavelengths are greater than the size of the dust particles.
We have observed relatively small bursts of star formation in our own Galaxy. These have probably taken place steadily over the last ten billion years or so. In the interacting systems, gravitational forces compress the gas in one or both galaxies. This triggers a burst of star formation much bigger than any we have evidence for in our own Galaxy. If we had had a huge burst of star formation within the last 10 to 100 million years, it should have left a bump on the smooth curve of Figure 12-6.
In the mid 1930's Edwin Hubble summarized his extragalactic research in a book entitled The Realm of the Nebulae. At that time, the word ``galaxy'' still carried strong connotations of the local system that we now distinguish with an upper case `G'- the Galaxy. It is probably fair to say that Hubble's main interest in these systems was to map the extent of the universe, but he was a careful, keen observer, and noted many of the important characteristics of galaxies.
It was already clear in Hubble's time that the ratio of spirals to ellipticals (cf. Figure 3.1) varied with the density of extragalactic systems. Spiral systems prefer the general extragalactic field, or the peripheries of clusters. They avoid the dense, central portions of rich clusters, where ellipticals, and especially giant ellipticals are found.
At one time it was thought that galaxies evolved from ellipticals to spirals. Hubble's classification then showed young systems on the left (of Figure 3.1) and older systems on the right. Hubble even made the provisional suggestion that elliptical galaxies were born in clusters and evolved to spirals. These then evaporated to fill the general field. There was not enough evidence to confirm this suggestion, he said. A writer once said that Hubble called all of his work provisional.
Astronomers still speak of `early' and `late' galaxies but without any evolutionary connotations. It is now clear extensive interactions among galaxies are important for their form or morphology. This is especially true for spirals and ellipticals.
Many clusters of galaxies are embedded in an extremely hot gas. Much of this gas was stripped from the galaxies; some may be part of parent gas cloud from which the system formed. The galaxies follow orbits in the general gravitational field of the cluster itself. Any spiral whose orbit brought it near the high-density center would soon lose its gas.
Large galaxies could capture smaller ones through a process known as dynamical friction. In this process the orbital energy of a small galaxy about a larger one can go to ``heat up'' the merged system. In other words, the stars of both systems go a little faster. This is sufficient to take up the orbital energy of the smaller system so the merger can take place. Near collisions that might not end as a merger could still result in the loss of gas from one or perhaps both galaxies.
Elliptical galaxies have a very wide range in mass, from systems not much bigger than a globular cluster to giants, larger than the more massive spirals. No single scheme is able to account for their history and chemical evolution. There is one common denominator. The elliptical systems lack the cool gas necessary for star formation. Therefore, with a few odd exceptions, ellipticals have no young stars.
There are some 30 galaxies within about 2 megaparsecs of the sun. This Local Group contains 3 large spirals. The rest of the galaxies are dwarfs, both ellipticals and gas-rich irregulars like the Magellanic Clouds. The dwarf ellipticals are essentially gas free with no ongoing star formation. Their chemical histories are nevertheless distinct fom one another. Some are rich in carbon stars, which are rare objects in our Galaxy. For others, there is a spread in stellar ages, even though star formation has now ceased.
The common denominator of all of these dwarf ellipticals is the scarcity of gas. Differing chemical histories are the result of the manner of removal of this gas. In the case of M32, a companion of the giant spiral M31, the gas was probably removed through tidal interactions. Some residual gas and blue stars can be seen in M32, indicating that it may once have had more gas, and resembled the Magellanic Clouds. Supernovae could have ejected the gas from other dwarf ellipticals. This is a powerful mechanism for gas expulsion, that is relevant also for giant systems.
Giant ellipticals live mostly in the inner parts of clusters. For them, the basic mechanisms of interaction, merger, and supernovae expulsion are also valid, but on a vastly greater scale. There is also some residual gas in giant ellipticals; most of it is hot, with temperatures of tens of millions of degrees.
Abundances of the elements in distant spiral galaxies is determined either from the emission regions or the unresolved starlight. The brightest emission regions are those associated with star formation, so these are unavailable for elliptical galaxies. It is possible to analyze the hot gas, and we shall come to this momentarily. Most of the effort in studying the chemical history of elliptical galaxies, though, has come from studies of the spectra of stars.
Individual stars can be seen only in the nearest Galaxies. Even in the local group, main sequence stars are difficult or impossible to observe spectroscopically. But it is possible to analyze the integrated light from many stars, and this is the primary source of our information on abundances in distant ellipticals.
Astronomers make mathematical models of ellipticals and their evolution. To do this they assume a mass function (cf. Figure 12.6) and make calculations of the evolution of stars just as with spiral systems. For the ellipticals, however, it is essential to have some mechanism to eject the processed gas. This is the opposite of the infall used to explain the G-dwarf problem of spirals. Instead of adding a gas with no heavy elements, it must be assumed that the processed material is blown away. The ejection mechanism is often called a ``wind'' by analogy with the strong winds that blow from some stars. However, it is generally acknowledged that the energy for the ejection comes from supernovae explosions as well as winds.
For any time in the history of a model elliptical, one calculates relative numbers of stars of a variety of types. For each of these types one adds the corresponding spectra. Spectra are simply intensities vs. wavelength, so we just add intensities for each wavelength, weighting by the relative numbers of stars of each type. The spectra may be generated purely from the theory of stellar spectra. An alternate way is to use measured spectra of real stars, but to add their spectra according to the recipe giving the relative numbers in the model of the elliptical at any time.
As with spirals, the more gas retained, the higher the abundances can grow. Generally speaking, massive systems will retain more gas, and will have higher abundances. Observations show that dwarf systems have between 1 and 10% of the solar abundances of iron, while giant ellipticals may exceed solar abundances by factors of 2 or more.
It is more probable that mass will be retained in the central regions of the ellipticals than at their edges, and this accounts for abundance gradients that have been now confirmed as a ubiquitous property of these galaxies. Such gradients appear in spirals too, but in this case the explanation is entirely different. In the spirals, the abundances are low away from the centers because there is relatively more unprocessed gas.
Most of the baryonic matter (protons and neutrons) in the universe may be in the form of hot gas in clusters of galaxies. This rather surprising conclusion became evident as a result of satellite surveys of the sky in X-rays beginning in the 1970's. Many of the bright X-ray sources were centered on clusters of galaxies. Giant elliptical galaxies within these clusters also contain a component of hot gas that can be seen in the X-rays. The intracluster gas is thought to be comprised of both the primordial gas from which the cluster formed, and gas ejected from the galaxies due to interactions, winds, and supernovae.
Abundances in the hot, interstellar gas of giant ellipticals appear to be lower than those of the stars. This is a little strange, since that gas should be comprised of the most recently processed material, and therefore at least as rich in ``Z'' as the stars. The intracluster gas has long been known to be metal poor, although not primordial (Z = 0).
Quasars are the most distant objects known. They are so far away that the red shifts from the expansion of the universe are several times greater than the original unshifted wavelengths. Astronomers use a lower case z to indicate the ratio of the shift to the original wavelength. If z = 2, then wavelengths have doubled as a result of the red shift. A useful rule of the thumb is that 1/(1+z) gives the fraction of the present age of the universe since light left a quasar with a red shift of z.
At the time of the present writing, the largest ground-based optical telescope is the 10 meter Keck Telescope on Mona Kea, Hawaii. Astronomers have been using it to obtain spectra of incredibly faint galaxies and distant quasars. Recently they investigated a quasar with a z of 2.56, whose light began its journey to us when the universe was less than a third its present age: 1/(1+2.56) = 0.28. Light from this quasar passes through distant clouds of gas on its way to us. Absorption lines in these clouds can be identified so we can also tell the red shifts of the clouds. In the case under consideration, two large clouds had red shifts of z = 2.08 and 2.56.
Abundances of heavy elements-upper case Z-could be determined from the absorption lines. The abundance of iron in the closer cloud was about 0.2 of the solar value, while that of the more distant one was only 0.0025 that of the sun. This result is in keeping with our expectation. The younger, more distant system, has a lower iron abundance because the stars have had less time to synthesize it.
The relation between z and Z, between red shift and heavy element abundance is not smooth. But a general trend is found with the more distant systems being the more metal poor. It is ironic, however, that the most metal poor objects in the universe are found in our own Galaxy. Thus far none of the distant systems have abundances lower than those in the most metal-poor stars. In these objects heavy elements can have abundances that are 10-4 that of the sun!
The quasars themselves are difficult to analyze for chemical
composition. Their great brilliance is thought to originate from
matter being swallowed by a black hole. Once in, of course,
nothing is seen from the matter, but on the way in the matter
tends to swirl around in a very hot disk. These accretion
disks are tiny by the standards of the dimensions of galaxies.
They fall between the black hole radius, 10 - 100 solar radii
(
7 x 1010 cm) and an inner emission region that
is a tenth of a parsec or so (1 pc cm).
This inner emission region is often called the broad line region (BLR), since broad emission lines originate from it. Quasars also have narrow emission line regions, which resemble those seen from ordinary galaxies. Indeed, quasars are thought to be similar in many ways to nearby active galaxies. Like the quasars, these active galaxies are thought to be powered by a central black hole.
Standard abundance methods had been applied to the narrow line regions of quasars with results rather unremarkable. Abundances scattered from perhaps 0.01 to 1.0 times solar. The broad line regions were more difficult to analyze because their structure was uncertain. Courageous astronomers have analyzed the BLR, however, with the surprising results that the heavy element to hydrogen ratios are one to two orders of magnitude higher than the solar value.
These results are too new to be fit into the general picture of the chemical evolution of the elements. We expected nucleosynthesis in very young objects, of course. There had to be a start somewhere. But until this point we had not found young objects with high abundances. It remains to be seen if the heavy elements from the BLR can somehow escape their narrow confines ( < 1 parsec) and seed the rest of the galaxy. A result of this kind would help to explain why it has been so difficult to find stars with no metals--genuine first generation objects.
There is one general theme governing the abundances of the heavy elements in stars and galaxies. Heavy elements are made from stars which in turn were made from gas. In regions where the gas can be processed through stars many times, abundances are high. When the gas is used up, or expelled, abundances are frozen. Spiral galaxies are systems that can generally process and reprocess their gas. In ellipticals, it is more typical for there to be a burst of star formation and after this the abundances no longer increase. At the centers of some of the larger ellipticals, abundances can reach values several times solar, and this could be accounted for by assuming some of the gas has been retained and reprocessed.
In clusters of galaxies, interactions may cause bursts of star formation, or the merging of one system with another. The intracluster medium is hot and very massive. Abundances in this hot gas are typically 1 to 10% of the solar value.
Quasars sometimes shine through very distant clouds whose abundances can be determined from the absorption lines. In these systems abundances can be as low as 10-3 times solar. Interestingly, the broad line regions of quasars have abundances that exceed the solar value.
The translation was initiated by Charles R. Cowley on 3/11/2000