Evidence that liquid water does or could have existed on Europa?

At a bone-chilling -145 degrees Celsius, the surface of Europa seems an unlikely place for carbon-based life to flourish (1). The only possibility for any cells to survive in such conditions as those that exist on Europa would be underneath the 80 to 170 km thick layer of ice that allows for its smooth surface (2). Some believe that beneath the ice lies a subsurface liquid ocean that is heated by an internal source within the moon's core. The features of Europa allow for speculation of water beneath the surface. These include: Europa's orbit around Jupiter, the "cracks" and faults found on the glassy surface, remnants of geysers and ice-flows, and hydrated minerals, specifically salts, which may indicate a salty subsurface ocean.

Four researchers from the University of Arizona have published their ideas about the "cracks" in Europa's icy surface; they believe that the cracks themselves are unique to Europa and they are caused by tidal stresses from the subsurface ocean as the moon orbits Jupiter. The researchers believe that the cycloidal cracks are caused by an eccentric orbit (the orbit is caused by Jupiter's other moons, Io and Ganymede). When the moon nears Jupiter, the water will rise, and as it moves away, the water will fall. This motion will stress the ice and cause cracks on the surface (3). The motion is considered a "tidal tug of war" between Jupiter and its moons with Europa as the rope (4)!

These cracks, called flexi, are perhaps the best evidence of a subsurface ocean on Europa (Univ. of Arizona) (3)

The discovery of Astypalaea Linea in 1996 by Galileo has allowed for speculation on the possibility of the water beneath the surface concurring with volcanic activity to create ice-filled faults. This strike-slip fault has plates of ice pulling apart from one another where the fault bends. The filling of the gap is formed by the warmer material within the planet. If, in fact, water is welling up to fill the faults, then it is freezing as it reaches the bitter surface (5).

This Galileo image shows the Astypalaea Linea fault (red line) near the moon's south pole. (NASA/JPL/Caltech) (5)

With a solid, smooth surface, it would seem that Europa is solid ice. It is true, however, that there are recent discoveries of iceberg-type formations, and a small crater indentation that would signify a differentiated planet structure. Researchers believe that the area called Chaos Terrain was formed by icy chunks that were once suspended in slush and later frozen to be locked in place. In addition, the Pwyll impact crater seems to be the remnant of a meteorite that crashed into Europa 10-100 million years ago. The theory is that the basin of the crater is so shallow that the subsurface ice must have been warm enough for it to collapse and then the hole was filled in. It also seems that the plates of ice have similar gaps as those of the mid-ocean deep sea ridges where new material is brought up from the interior of the Earth (4).

With the dark markings found on Europa, there seems to be a factor of geysers or ice volcanoes according to Dr. Ronald Greeley of Arizona State University, Tempe. If an internal heat source is present in Europa, then it will be possible for volcanoes and geysers to shoot material from the inner part of the planet to the surface. As research is occurring on Earth to find microbial life near volcanoes, geysers, and other places with extreme conditions, perhaps Europa will become more of a concrete possibility for the same kind of life (6).

The photo was taken from Galileo and shows flows that originated from ice volcanoes or geysers. (NASA/JPL) (6)

Lastly, it must be noted that the surface salts on Europa may have come from within the planet, possibly within the water in the subsurface ocean. This is completely speculation, however, because we have not yet been able to penetrate to the surface of Europa (7).