The Underwater Ocean of Enceladus

Scientists with the Cassini team have found more evidence that the geysers of Saturn's moon Enceladus come from an ocean of liquid water underneath the planet's icy crust. Cassini flew very close to erupting geysers in 2008 and 2009, close enough for its cosmic dust analyzer to get actual samples of the plumes:

Data from Cassini's cosmic dust analyzer show the grains expelled from fissures, known as tiger stripes, are relatively small and predominantly low in salt far away from the moon. But closer to the moon's surface, Cassini found that relatively large grains rich with sodium and potassium dominate the plumes. The salt-rich particles have an "ocean-like" composition and indicate that most, if not all, of the expelled ice and water vapor comes from the evaporation of liquid salt water. . . .

"There currently is no plausible way to produce a steady outflow of salt-rich grains from solid ice across all the tiger stripes other than salt water under Enceladus's icy surface," said Frank Postberg, a Cassini team scientist at the University of Heidelberg, Germany, and the lead author on the paper. When water freezes, the salt is squeezed out, leaving pure water ice behind. If the plumes emanated from ice, they should have very little salt in them.

Yet another reason to send a probe to Enceladus instead of another rover to Mars.