Every time we get new information about Enceladus, I keep thinking about how the original Orion team would have felt if they really had made the trip to Saturn they once discussed for their fabled atomic rocket. Enceladus, thought Freeman Dyson, looked to be a logical place to refuel because it was believed to be rich in ice and hydrocarbons. But no one in those pre-Cassini days could have imagined what Dennis Matson (Jet Propulsion Laboratory) is now talking about:

“Deep inside Enceladus, our model indicates we’ve got an organic brew, a heat source and liquid water, all key ingredients for life. And while no one is claiming that we have found life by any means, we probably have evidence for a place that might be hospitable to life.”

All of which falls into the ‘never in my wildest dreams’ category, for Enceladus has hardly led the list when one discusses life’s possible venues in the Solar System. But Cassini found geysers ejecting water vapor and ice from the moon’s south polar region back in 2005. And that raised the whole issue of what was producing the needed heat to cause the eruptions. Add to that the fact that some of the molecules found in Enceladus’ plumes require elevated temperatures to form.

One way into the problem is to consider these temperatures as the result of radioactive decay. Let radioactive isotopes of aluminum and iron decay over a period of about seven million years and you generate enough heat to rearrange the mix of ice and rock into a rocky core surrounded by ice. Billions of years later, you should still see a relatively warm core fueled by this radioactive decay and given a boost by Saturn’s tidal pull.

As to those molecules in the plumes, Matson’s new study of same notes the Cassini spectrometer findings: water vapor, methane, carbon dioxide, propane, acetylene and gaseous nitrogen. Matson believes the nitrogen comes from the thermal decomposition of ammonia down where the warm core and liquid water meet. The temperatures involved may go as high as 577 degrees Celsius (1070 degrees Fahrenheit).

So is there an organic-rich mixture below the surface of Enceladus even today? Cassini found hydrocarbon chains in the moon’s plume, which means we can’t rule this scenario out. If so, that hardly means life is present, but a new measurement of the plume’s chemistry during a March 2008 flyby may help to refine the model, and tell us whether this tiny, surprising object is ready to yield still more of its secrets.

The paper is Matson et al., “Enceladus’ plume: Compositional evidence for a hot interior,” Icarus Vol. 187 Issue 2 (April 2007), pp. 569-573, with abstract available here. Bear in mind the contrasting clathrate model, which comes up with an alternative explanation for the Enceladus plumes that does not rely on the presence of liquid water. We now have two solid hypotheses about something quite unusual, and rely on Cassini’s next pass to help us sort out which of these is the most likely explanation.