True, but I’ll counter that surface liquid isn’t necessary. Galilean moons Europa and Ganymede and Saturn’s moon Enceladus are all believed to have liquid water (warmed by tidal heating) beneath their surface.
Correct me if I’m wrong, but aren’t all those bodies large enough to retain an atmosphere?
Actually, I guess those moons ARE large enough to have an atmosphere, they just don’t for whatever reason. They have trace atmospheres, much like our own moon, but their surface pressures are measured in fractions of a micro Pascal, whereas 1 atm is about 100,000 pascals. So their “atmosphere” is on the order of one trillionth to one hundred billionth as dense as our own - similar to the very hardest laboratory vacuums. Titan is the only moon that has a dense atmosphere. Titan is similar in size to the moons I mentioned, and its atmosphere is more dense and more massive than Earth’s despite Titan’s small size/mass (compared to Earth). So there are factors besides size that play a role in the formation and retention of an atmosphere.
A pretty good online source concerning these factors is Origins of Atmospheres by Kevin Zahnle. A couple of interesting articles by Catling, Kasting and Zahnle, published in Science, deal with the possible influence of life on the depletion of certain gases.
In case someone is interested in the new field of “astrobiology” in general, I’ve enjoyed the book Planets and Life - The Emerging Science of Astrobiology, edited by Woodruff T. Sullivan III and John Baross.
Thanks a lot! It’s a small world; John Baross is actually my grand-advisor (he was my MS advisor’s advisor). I didn’t realize he was so involved in astrobiology, I know him as a marine microbial ecologist.