Where to Look for Life on Titan

Would fresh impact craters or cryovolcanoes make better targets for astrobiologists?

Cassini's Titan Radar Mapper imaged an eighth impact crater on June 21, 2011.

Saturn’s large moon Titan remains the most exotic planetary body we know of. There is no other place quite like this. Hydrocarbon lakes are common on the surface, replenished by occasional methane downpours from an atmosphere consisting of nitrogen and methane. In many ways, the moon’s thick haze is not very unlike the early atmosphere of Earth.

In a recent paper, Catherine Neish from the University of Western Ontario and colleagues discuss the possibility of biological molecules being present on Titan, and where they might be found. Although life hasn’t yet been discovered there, based on what we know about Titan’s environment, at least some steps toward life should have occurred. Because Neish and colleagues regard liquid water as essential to life, they focus on two main settings where water could exist, at least for short periods, on an otherwise extremely cold surface: cryovolcanic lava flows and fresh impact craters.

Cryovolcanoes are volcanoes that erupt liquid compounds such as water, ammonia, and methane. These flows are seen mostly on the icy moons of the outer Solar System, and are unlike the molten rock flows found on the warmer, terrestrial planets. Despite the cold, Neish and her co-authors find that interesting chemistry could occur near these cryovolcanoes on Titan, but there may not be enough time or energy to produce complex prebiotic molecules.

A more promising target to search for evidence of life should be the moon’s fresh impact craters. The largest of these, called Menrva, has a diameter of several hundred kilometers. Carl Sagan suggested in the 1970s that water could remain liquid in a fresh impact crater for a very long time, perhaps for a million years or more in a crater the size of Menrva. While more recent work lowers these estimates, it is clear that the energy from an impact would liquefy a substantial amount of ground ice and raise temperatures above the melting point of water. The result would be a natural “reactor” in which a variety of chemicals could combine to become biologically interesting molecules.

Neish and her co-authors therefore conclude that deposits of impact melts near fresh large impact craters would be more promising sites for finding biological molecules than would cryovolcanic flows, and that a lander should be sent to one of these craters. This is a different kind of environmental setting than the one David Grinspoon and I have suggested, namely the bottom of Titan’s hydrocarbon lakes, where we expect that in some instances heated water-ammonia slurries from below would mix with a lake’s hydrocarbons to provide a suitable niche for prebiotic chemistry. Places this exotic require out-of-the-box thinking, and to reach the bottom of these hydrocarbon lakes we would need a submarine. On the other hand, the approach suggested by Neish and colleagues has the advantage that it would be easier to accomplish, technologically.

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