One of the most promising areas of study for astrobiologists -- scientists who are interested in the possibility of life elsewhere in the universe -- is the potential for life on the moons of Jupiter, Saturn, Uranus, and Neptune. We're beginning to develop the technology to detect biosignatures -- chemical traces of living things in the atmospheres of moons or exoplanets -- but it's a hell of a lot easier to find those in our own Solar System than it is around the barely-visible specks of light that are all we can see of most exoplanetary systems.
Despite their distance from the Sun, due to tidal heating there are several of these moons that are thought to have liquid water beneath a frozen crust. Four commonly-discussed possibilities are Europa (Jupiter), Enceladus and Titan (Saturn) and Triton (Neptune); the case is nearly certain for Europa and Enceladus, where fly-bys have detected liquid water geysers erupting from surface cracks in the ice sheet.
What could be down there, I wonder? Single-celled life is the most likely, but with no further information... well, anything's possible. We only have a sample size of one regarding how life forms and evolves, so trying to predict what it would look like somewhere else is going to be speculation at best.
The conventional wisdom has been that the smaller moons are unlikely places to look for life; being smaller, they lose heat faster, so any heat gains they get from the Sun and from tidal compression are far offset by heat loss from their small thermal mass.
That assessment will have to be revised, apparently. A new study -- out this week in Nature -- found that Saturn's moon Mimas, best known for having a huge crater that makes it look like the Death Star from Star Wars, has an ocean of liquid water underneath a crust of ice and frozen methane. It's only four hundred kilometers in diameter, over eight times smaller than our own Moon.
The frozen crust of Mimas is thought to be so thick (something on the order of twenty to thirty kilometers) that it precludes the cracks that cause the geysers on Enceladus and Europa. So the liquid water inside is trapped -- but the effects of tidal heating from the enormous planet it orbits are apparently enough to keep it well above freezing, and therefore very likely to enable the convection currents which overturn nutrients in our own oceans and are essential for the maintenance of ecosystems.
Based on what we know about the formation of moons and their stability in orbit around their host planet, Mimas is estimated to be quite young, something on the order of between five and fifteen million years old. This seems like a very short time even to evolve simple single-celled organisms, but as I said before -- it's not like we have a bunch of test cases from which to draw inferences."Mimas was probably the most unlikely place to look for a global ocean — and liquid water more generally," said study co-author Valéry Lainey, of the Paris Observatory. "So that looks like a potential habitable world. But nobody knows how much time is needed for life to arise."