Tales of a Death Star

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.

A photograph of Mimas from the 2010 pass by the probe Cassini [Image is in the Public Domain courtesy of NASA/JPL]

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."

I'm always fascinated when we find this sort of thing, because it seems like every time we get new information affecting the terms of the Drake Equation, the estimates are revised upward.  At first, we didn't know if planet formation was at all likely, or if the Solar System was a fluke; now it seems like exoplanets are kind of everywhere we look, and most stars have planetary systems.  Most stars that have been studied have at least one planet in the habitable zone, and the size of the habitable zone is way bigger than we used to think.  Forming the biochemistry of life turns out to be simple; like exoplanets, complex organic molecules turn out to be all over the place.  And so on.

So could Mimas host life?  Entirely possible.  "Not life as we know it, Jim" -- but life nonetheless.  I still think that Europa and Enceladus are more likely (remember the end of the movie 2010?  "All of these worlds are yours except Europa, attempt no landing there") but life could well be common, not just out in the galaxy but right here in our own Solar System.

And maybe I'll live to see confirmation of it.  What a monumental overturning of our self-importance that would be.  It'd be a total game changer.  Proving once and for all that life is abundant in the cosmos... and that we are not alone.

****************************************