Enceladus Geysers Still Unexplained

The number of geysers has topped a hundred on Saturn’s overactive moon.  How could they be sustained for billions of years?

A press release from JPL updates what is known about the amazing geyser activity on Enceladus, a moon as small as the diameter of Arizona.  Since the geysers were discovered in 2005, geophysicists and astronomers have wondered how a little body can put out so much heat.  That question remains unanswered.

Multiple passes by the Cassini orbiter have allowed scientists to pinpoint the geyser locations along the south polar “tiger stripes” (fissures).  They have counted 101 geysers so far.  They have also pinpointed their locations at “hot spots” of small extent—too small to be explained by frictional heating along the walls of the fissures.

Individual geysers were found to coincide with small-scale hot spots, only a few dozen feet (or tens of meters) across, which were too small to be produced by frictional heating, but the right size to be the result of condensation of vapor on the near-surface walls of the fractures. This immediately implicated the hot spots as the signature of the geysering process.

This prompted the imaging team lead Carolyn Porco to turn the explanation upside down.  “Once we had these results in hand, we knew right away heat was not causing the geysers, but vice versa,” she said.  “It also told us the geysers are not a near-surface phenomenon, but have much deeper roots.”  Porco co-authored two papers in the Astronomical Journal, one that counts the geysers and associates them with the hot spots, and one that relates brightness variations during the moon’s orbits to possible fissure responses to tidal flexing.

The upshot is that there must be a much larger heat source to keep the deeply-rooted source warm, if it is a liquid ocean.  Tidal flexing might squeeze water out from the deep roots, but apparently it is insufficient to explain all the observations:

They found the simplest model of tidal flexing provides a good match for the brightness variations Cassini observes, but it does not predict the time when the plume begins to brighten. Some other important effect is present and the authors considered several in the course of their work.

Cassini scientists have also been monitoring changes to the plumes at different points in the moon’s orbit.  While tidal flexing might have “something to do with” the geysering, the press release left it unclear if it is sufficient.  If tidal flexing correlates with brightening, that is fine and good, but how long can tidal flexing support the activity?

It seems really odd for Porco to say that the geysers cause the heat, not the reverse.  What, did geysers just sprout up on the moon with no cause, and heat the moon up?  Of course not.  Given all the accumulated data since the geysers were discovered, we’d like to see updated calculations for the total heating capacity of tidal flexing.  (Short-lived radioisotopes should long ago have decayed, so that is an implausible source.)

For years, scientists have been unable to find any plausible heat source that, individually or in combination, could keep Enceladus active for billions of years (e.g., 6/19/08).  We would expect—if tidal heating answered all the questions now—to see the Cassini scientists celebrating, having rescued their belief in 4.5 billion years.  Since they are not celebrating, we can only assume that the age problem remains.  (Search “Enceladus” in our search bar for numerous reports on this issue and its age implications.)