Enceladus and Astrobiology

Enceladus, with its water vapor geysers, may have subenceladian (can’t really be subterranean, can they) chambers of liquid water, perhaps as close as 30 meters to the surface. A special session at the American Geophysical Union fall meeting today featured talks analyzing not just the geological implications of this small moon’s geysers, but also its biological implications. Dr. Chris McKay, a planetary scientist who has become an expert in the young field of astrobiology, discussed Enceladus as a possible home for extraterrestrial life. If the geysers are indeed driven by liquid water (as opposed to sublimation from warm ice, the way a comet or a frozen dinner sheds water vapor when it heats up) then that satisfies the one common element shared by all life on Earth. That’s kind of a big deal, since there aren’t any places other than Earth known to have liquid water at the present time. Europa may have a deep subsurface liquid ocean, and Mars certainly has had at least intermittent periods of warm weather and running water on its surface, but that’s it at the moment. The other ingredients necessary for life are organic material (Carbon and Nitrogen at least, and others in lesser abundance) and some energy. Enceladus may be able to fit both requirements. McKay listed three known examples on Earth of microorganisms that exist in an ecology like one that could exist on Enceladus. These are anaerobic chemoautotrophic organisms that consume molecular Hydrogen (H2) and produce methane (CH4) as a byproduct. Most of the theories for the origin of life on Earth (though not all) do not translate well to Enceladus however. Nevertheless, it’s an interesting place, and while the probability of life existing there may be low, we really don’t know. The implications of extraterrestrial life are so broad that it is certainly worth a look. Enceladus kindly spews its innards into space in the form of its geysers, making getting samples for detailed biological analysis simpler than for Europa.

PIA08199 image of Enceladus, Saturn, and its rings
Image credit: NASA/JPL/Space Science Institute
In this image, Enceladus’s geysers are viewed against the skyscape of Saturn’s nightside and its rings. Saturn is dimly illuminated by light reflecting off the rings.

As for what is ultimately driving the activity on Enceladus, it is stretching of the moon by Saturn’s tides as Enceladus is pushed around by gravitational interactions with other moons. The details, though, including why nearby Mimas is not active, are very much a work in progress. As Dr. Dave Stevenson said in his talk titled “How Does Enceladus Do It?”, “I don’t know.” But the process of answering that question is underway and is illuminating how our distant icy neighbor works.

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