Yesterday’s big news in astronomy: the positing of a lake under the south pole of Saturn’s moon Enceladus. Nobody actually saw a lake. Admittedly, it’s a best guess given the data from Cassini’s flights by the small moon. It’s not really surprising to me. I blogged several years ago on outer solar system moons that were likely candidates for surprises, along with moons that have already surprised observers.
Why does Enceladus have geysers and Mimas–closer to Saturn and an orbit more likely to produce slushiness–not? Maybe Mimas has dried out, so to speak. If the water below has long ago gushed into space what might be left are semi-collapsed caverns and interior rubble. Mimas has a significantly lower density than Enceladus. That might mean that the moon had much less stone originally. Or that the interior water was long lost, and now the interior rubble just floats around a bit instead of layers rubbing to create friction and heating. Low density in small bodies like asteroids is usually attributed to empty spaces within.
Among Saturn’s larger moons (Mimas, Enceladus, Tethys, Dione, and Rhea), Enceladus has the highest density among them. A greater ratio of stone to ice is what scientists think. But as for that lake, 4.5 billion years is a long time to be holding on to a body of water on a moon that small. Definitely worth taking a closer look … someday.