Iapetus Is Losing Its Dry Ice

first_imgHow long can a moon afford to leak?  Iapetus is losing its dry ice (carbon dioxide) through sublimation at a prodigious rate, say scientists in a paper in Icarus this month.1  “One can see that the long-term stability of CO2 is problematic.”    Solving the mystery of the dark side on this mystery moon has been a priority since Voyager days.  After the Sept. 10, 2007 flyby of Iapetus by Cassini (09/13/2007), a strange new theory was proposed: a runaway process makes the ice sublimate and migrate to the bright side or the poles (see Cassini press release from Oct 8, 2007).  This apparently happens to both water ice and carbon dioxide ice (dry ice); see 10/16/2007, bullet 3.    The new Icarus paper modeled what happens to the carbon dioxide.  Because it is so volatile, it sublimates to gas and starts moving.  The warmth of the dark side makes it leave the leading hemisphere permanently.  It migrates to cold traps at the poles – for awhile.  During Saturn’s 29.5 year orbit, the angle of sunlight shifts.  The carbon dioxide molecules leave one pole and migrate to the other in a series of 350 hops.  Thus, the CO2 moves from pole to pole each Saturnian year.    Not all of the gas makes it back.  Palmer and Brown calculated that 12% of it reaches escape velocity and leaves the moon each orbit, never to return.  Yet carbon dioxide was detected in abundance on the surface.  How could any be left after billions of years?  If Iapetus had started with a layer of carbon dioxide ice 5 km thick, they estimated, it could only last for 1.6 billion years.  That’s just about a third of the assumed age of the solar system (4.5 billion years).    One way to save the old age would be to add in more carbon dioxide over time.  The authors ruled out comets as a source, or vents from the interior, like on Enceladus (02/10/2007).  All they could suggest was that carbon dioxide is continually produced by photolysis of the unknown dark material.  Yet the dark layer is known to be thin, because fresh meteor impacts show bright ice underneath.    In any case, they think, the carbon dioxide could not be primordial (as old as the moon); “due to the large escape rate of CO2 from the surface, any free CO2 ice found on Iapetus implies active production, such as photochemical generation, liberation during an impact, or by an active vent.”1.  Eric E. Palmer and Robert H. Brown, “The stability and transport of carbon dioxide on Iapetus,” Icarus, Volume 195, Issue 1, May 2008, Pages 434-446, doi:10.1016/j.icarus.2007.11.020.Do they have any evidence for photochemical generation, liberation during an impact, or active vents?  None whatsoever.  They have been forced into hand-waving to explain away their debt of billions of years.  They thought they could just drop in the Bank of Time again and make another one of their reckless drafts (07/02/2007).  This time, the teller Japheth (the root name of Iapetus) is refusing to heed their song and dance, saying they can’t withdraw any more billions without showing some collateral.(Visited 14 times, 1 visits today)FacebookTwitterPinterestSave分享0last_img

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