Saturn's icy moon Enceladus should not be one of the most promising places in our solar system to look for extraterrestrial life. Instead, it should have frozen solid billions of years ago.
Located in the frigid outer solar system, it's too far from the sun to have oceans of liquid water - a necessary ingredient for known forms of life - on its surface.
Some worlds, like Mars or Jupiter's moon Europa, give hints that they might harbour liquid water beneath their surfaces. Mars is about 4,200 miles across and Europa almost 2,000 miles across.
However, with a diameter only slightly more than 500 miles, Enceladus just doesn't have the bulk needed for its interior to stay warm enough to maintain liquid water underground.
With temperatures around 324 degrees below zero Fahrenheit, the surface of Enceladus is indeed frozen. However, in 2005 NASA's Cassini spacecraft discovered a giant plume of water gushing from cracks in the surface over the moon's south pole, indicating that there was a reservoir of water beneath the ice.
Analysis of the plume by Cassini revealed that the water is salty, indicating the reservoir is large, perhaps even a global subsurface ocean. Scientists estimate from the Cassini data that the south polar heating is equivalent to a continuous release of about 13 billion watts of energy.
To explain this mysterious warmth, some scientists invoke radiation coupled with tidal heating. As it formed, Enceladus (like all solar system objects) incorporated matter from the cloud of gas and dust left over from our sun's formation. In the outer solar system, as Enceladus formed it grew as ice and rock coalesced.
If Enceladus was able to gather greater amounts of rock, which contained radioactive elements, enough heat could have been generated by the decay of the radioactive elements in its interior to melt the body.
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