Saturn's moon Enceladus is a mystery. From Earth it looks tiny and cold, and yet it's not a dead hunk of rock. Passing spacecraft see trenches and ridges, similar to Earth's, and in 2005 NASA's Cassini mission spotted ice geysers streaming from its south pole.
"The moon is actually alive in a sense," says Sean Hsu with the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder.
Beneath the surface, most researchers believe it even has a liquid ocean. Now Hsu and his colleagues have found new evidence that it's a downright balmy ocean.
A new analysis suggests that Enceladus' ocean is being heated from the bottom up. That could explain plumes of ice seen at its south pole.
The team used the Cassini spacecraft, which orbits Saturn, to detect tiny particles of silica floating in space. It's not sand exactly, but researchers think the particles did come from the bottom of Enceladus' ocean.
The silica particles could only be made if that ocean were hot. "We think that the temperature at least in some part of the ocean must be higher than 190 degrees Fahrenheit," Hsu says. "If you could swim a little bit further from the really hot part then it could be comfy."
In fact, 190°F is cooler than many hydrothermal vents at the bottom of Earth's oceans. Hsu says experiments on Earth also suggest the ocean is similar in salinity and pH to oceans here.
The evidence, published in the journal Nature, is somewhat circumstantial. The theory is that the silica formed and then dissolved in seawater beneath Enceladus' icy crust. It then left the moon through geysers, and filled Saturn's E-ring. From the E-ring, the silica eventually wound up in the giant planet's magnetosphere, which is where Cassini saw it..