March 13, 2007 | David F. Coppedge

The Enceladus Problem Heats Up

How can a small icy moon produce hot-water geysers?  That is the Enceladus problem: for a small moon assumed to be 4.5 billion years old to be forcefully gushing out water from its south pole was a great surprise when the Cassini spacecraft first detected the geysers in 2005 (11/28/2005).  Ever since, scientists have been puzzling how to get the plumes to last so long (03/01/2006).  A new paper in Icarus1 concedes this is a bigger problem than thought; it would require tremendous heat at depth to keep the system going.
    The existence of nitrogen in the plumes, assumed to result from decomposition of ammonia, leads to estimates of 500K to 800K (440-980°F) in the moon’s interior: “a very hot environment” for unusual chemistry and explosive physics.

A major question is how is it possible to get high temperatures inside Enceladus, while it is already surprising to observe ongoing geological activity at this satellite.  We prefer to approach the problem the following way.  Pre-Cassini geophysical models have failed to account for the young surface of Enceladus observed by Voyager.  There is a need to revisit the assumptions and initial conditions used for these models.
    Several other lines of evidence now point to the existence of high temperature material (well above the melting point of water) in at least some portions of Enceladus’ interior.  The plumes themselves and the thermal power radiated by the south polar anomaly suggest higher heating levels than currently provided by long-lived radiogenic species and current tidal dissipation combined (Porco et al., 2006).
    Supplying the heat required to explain the observed phenomena is a major challenge for theoretical models of the satellite’s thermal history.

The only suggestions they can come up with are (1) maybe tidal heating is more efficient than thought, or (2) maybe Enceladus formed early when short-lived radionuclides were able to melt the interior, then subsequent long-lived radioactive decay maintained the heat for all this time.  They referred to other papers on these ideas but did not speculate on how long such heating could persist, and why it did not affect similar moons, like Mimas, in the same way.
    At the end they speculated briefly about how inner heat might cook up a “rich, warm, aqueous, organic ‘soup’ below the surface” up to and including “hydrocarbons, nitriles, methanol and possibly amino acids” not yet observed.  Sniffing some of the ingredients might be possible if Cassini is able to sweep through the plume a year from now (March 12, 2008).  Though the authors did not mention the L word life (except for the indirect word prebiotic), some of the popular news media were quick to pick up on this thought (see BBC News and  The JPL press release, though stating that “all key ingredients for life” are present, was careful to qualify that “no one is claiming that we found life by any means” – only that “we probably have evidence for a place that might be hospitable to life.”  National Geographic, however, made the L word its centerpiece, announcing, “Saturn’s Icy Moon May Have Been Hot Enough for Life, Study Finds.”

1Matson, Castillo, Lunine and Johnson, “Enceladus’ plume: Compositional evidence for a hot interior,” Icarus, Volume 187, Issue 2, April 2007, pages 569573, doi:10.1016/j.icarus.2006.10.016.

Enceladus has nothing to do with life, OK? (time to review 02/15/2007).  Scientists and reporters should stop throwing out this distraction from the real issue: an observational fact is at clear variance with assumptions about the age of the solar system.  The researchers admit that the known heat sources are inadequate separately and in combination.  Two years ago, it was admitted Enceladus lacks a rocky core that could store enough radioactive sources (see 04/08/2005), and last year another scientist admitted that short-lived radionuclides are just a knob one can twiddle in the models without being able to make a hard statement (see 03/01/2006).  Meanwhile, the geysers are real.  They continue to erupt enough material to replenish the E-ring and spray-paint neighboring moons with ice (02/10/2007).
    If this were the only anomaly in the evolutionary dating scheme, perhaps they could do some special pleading, but then there is Titan’s surface (05/04/2006, 06/09/2005) and atmosphere (10/18/2006, 09/14/2006), Saturn’s rings (09/06/2005), Io’s volcanoes, the surface of Venus, our moon (11/09/2006), comets (06/10/2006) and much more in the solar system arguing against long ages.  The earth-based dating methods that give long ages are not without their own problems (e.g., 08/08/2006, 05/08/2006); without the need for geological epochs for evolution to occur, the difficulties with all dating methods would be apparent.  The E-ring is young (07/11/2006), the geysers have finite energy sources, and the surface of Enceladus is young (08/30/2005).  Why not accept the obvious inference that the whole moon is young?  If Darwin didn’t need the time, it would be obvious to everybody.  This “major challenge” must be faced (02/06/2006).  We agree it is time to “revisit the assumptions and initial conditions” that have “failed to account for the young surface of Enceladus observed by Voyager.”  Stating the inference to the best explanation – that Enceladus is young – will require courage on the part of a planetary scientist willing to go where the evidence leads.

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Categories: Physics, Solar System

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