October 7, 2010 | David F. Coppedge

Keeping Saturn Old

Keeping a planet like Saturn going for billions of years has been a problem lately, especially when evidences show that what we see today of its rings and moons could not have lasted that long.

  1. Ringside gambling:  The rings of Saturn are majestic, colorful, and young-looking.  Their ices are too clean, and the forces acting on them too pervasive, to have lasted 4.5 billion years.  The old idea that they formed along with Saturn has, therefore, fallen into disfavor.  Robin Canup came up with a “new alternative,” according to the BBC News: a body the size of giant moon Titan (larger than Mercury) floated within range of Saturn and dove in.
        “Just how these icy rings came about has always been a mystery,” the BBC said.  Earlier theories envisioned an icy comet, not a minor planet, forming the rings, leading to its 90 to 95% icy composition.  To keep the rock out, Canup proposed that the impacting body’s surface ices got stripped off on the way in and became the rings, then the rocky part smacked into the planet.  Canup’s story requires a body 10 times the size of previously-supposed comets.  She even thinks there was enough material left over to form icy moons like Enceladus, Dione and Tethys.  Whether or not such an explanation is likely, Carl Murray thought it was “a clever way to explain the peculiarly icy nature of the rings.”
  2. Titan’s vanishing oceans:  An article on Science Daily was primarily devoted to allowing Akiva Bar-Nun of Tel Aviv University to say “I told you so” about Titan’s oily lakes and 6,000-foot mountains.  One of the things he had predicted in 1979, though, was that there would be enough hydrocarbons formed on the surface over its age to cover the large moon 43 meters deep.  Later estimates were around ten times that high.  Bodies of liquid the Cassini spacecraft actually found in 2005 are restricted to scattered lakes in the polar regions.
        Bar-Nun agreed that the liquids accumulate from precipitation of compounds formed in the atmosphere by solar radiation.  He disagreed with astrobiologists and researchers like Sarah Horst (see 10/08/2010) who find Titan a tempting target in the search for life.  “The chemical processes on Titan are different than those on Earth because there is no water vapor in Titan’s air, leading to hydrocarbon-based lakes unlike those seen on our planet.  Because of this, the frequent claims that Titan could be a laboratory for the investigation of life’s emergence on Earth are unfounded, he says.”
  3. Enceladus: bubbly or wobbly?  The discovery in 2005 of active geysers at the south pole of little moon Enceladus “jolted many astronomers,” according to a story by Mike Wall on Space.com echoed on Live Science.  How could this “small, frozen and presumably dead moon” be “geologically active” after four and half billion years?  One “new way of thinking” that former Cassini Project Scientist Dennis Matson came up with to account for its “unique properties” is fizz.  A subsurface ocean picks up ions in the rock that bubble upward and explode out the south polar cracks.  According to his computer models, it doesn’t take much fizz to produce the effect.  Mike Wall got excited about this “Perrier ocean” model without asking too many questions, like how the ocean survived for billions of years in a moon just 500 miles across, why they erupt at the south pole, and why other moons don’t do this.  Matson admitted, “Until now, how you got so much heat out was a big, big problem.
        Some of the details of that big, big problem were stated more overtly in an article on PhysOrg, echoing a news feature from Jet Propulsion Laboratory, that took another “new way of thinking” to explain the puzzle.  No one expected little Enceladus to be “one of the most promising places in our solar system to look for extraterrestrial life” (because of its water); “Instead, it should have frozen solid billions of years ago.”  Whereas larger astrobiological targets like Mars (4,200 miles across) and Europa (2,000 miles across) “harbor hints” of subsurface water, Enceladus (500 miles across) “just doesn’t have the bulk needed for its interior to stay warm enough to maintain liquid water underground.”  Internal heating from radioactive decay is woefully inadequate: “in smaller moons like Enceladus, the cache of radioactive elements usually is not massive enough to produce significant heat for long, and the moon should have soon cooled and solidified.  So, unless another process within Enceladus somehow generated heat, any liquid formed by the melting of its interior would have frozen long ago.
        The JPL story calls for friction between the sides of subsurface cracks to keep the interior warm.  As Enceladus wobbles in its orbit due to a tiny bit of libration (non-uniform rotation) from its slightly-out-of-round shape, it may gain the added increment of tidal stress to cause the friction – perhaps up to five times as much, according to computer models.  The hypothesis was made by trying to match the computer model with known hot spots at the south pole.  They didn’t line up without adding libration.  Libration has not been observed, but if it occurs, must be less than 2%.  Terry Hurford, author of the model, believes that’s enough: “the extra heat makes it likely that Enceladus’ ocean could be long-lived, according to Hurford.”  The L-word life was not far behind: “This is significant in the search for life, because life requires a stable environment to develop.”
        Similar questions arise, however, with the wobble model as with the bubble model.  Why does this happen only at Enceladus, and not nearby Mimas or Tethys?  What makes this unique to this one moon?  Don’t other moons librate?  Are all others perfect spheres?  Have they no tidal stresses?  Invoking ad hoc conditions after the fact is generally frowned upon in science.

Another article on PhysOrg shows how Cassini scientist Paul Schenk (see his interesting blog with 3-D flyovers of planets and moons) has detected Enceladus “spray paint” on Mimas, Tethys, Dione and Rhea.  The JPL article contained this amazing factoid: “Scientists estimate from the Cassini data that the south polar heating is equivalent to a continuous release of about 13 billion watts of energy.” The Space.com article added that this energy is “five times more heat per unit area than flows through Earth’s geologic hot spot, Yellowstone National Park”. 
Photos.  A new clear picture of the geysers was posted Oct 6 at JPL.  The PhysOrg article included two Cassini photos; the first one, taken 11/21/2009, also at JPL, shows over 30 individual jets (see large at Planetary Photojournal).

A reader calculated that Enceladus could power 2.3 cities the size of Las Vegas with 13 billion watts.  After all, Las Vegas is at the “south pole” of Nevada, a state about as big across as Enceladus.  Isn’t it wonderful that this little tiny moon has been putting out this power, and this water-ice paint, for four and a half billion years?  Science says so.  You must believe.

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Categories: Solar System

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