Last week, we looked at the possibility that Saturn’s moon Tethys (which is, in effect, a giant ice ball) could harbor extraterrestrial life. It’s fairly rare to hear Tethys discussed on its own; it’s more often discussed along with its sister moon Dione, which is similar in appearance and nearly identical in size.
But despite their cosmetic similarities, Dione is very different on the inside—and much more likely to sustain life than Tethys.
You may remember reading that Tethys’ composition, which is mostly water ice, could harbor the kind of life we’re familiar with if some of that ice were in liquid form. But something has to warm it up to keep it liquid, and Tethys is too far from the Sun to be warmed by sunlight. What’s more, Tethys’ light composition does not lend itself well to geological activity.
This is where geological activity in Dione’s high-density core could pay off, and there is indeed considerable evidence that Dione once had both geological activity and a liquid underground ocean. Whether it still has either is an open question, and it will probably be a long time before we’ll be in a position to know for certain, but the fact that Dione had a liquid ocean at one time certainly increases the possibility that it has one now.
The story of Tethys and Dione is the story of two moons that look very similar, but tell very different stories—and this further reinforces what we’ve already come to know in the early stages of exobiology, which is that the appearance of an object tells us very little about whether or not it can sustain life. As my colleague Paul Seaburn pointed out, a recent experiment demonstrated that the way we look for life on exoplanets wouldn’t have necessarily told us that even Earth, which is thriving with surface and atmospheric life, is a habitable planet; it’s possible that if we were outsiders looking at this solar system with our current state of technology, we might expect to find life on methane-abundant Neptune instead. When we don’t know what lives inside of our own planet’s mantle, we can’t expect our relatively limited tools to give us a definitive answer on the possibility of subterranean life on alien worlds. That said, Dione is certainly a possible candidate for extraterrestrial life—and a much stronger candidate, by the standards we have, than most of the other objects in our solar system.