Crater Dater Deflator: Impactors Can Be Recycled

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first_imgThey came from outer space – that was the old paradigm about impactors that made craters on planetary bodies.  Then, we learned how secondary craters can confuse a surface’s history (06/08/2006, 09/25/2007).  Now, two papers in Icarus show that moons can do a lateral pass.    Alvarellos et al,1 showed that Jupiter’s moon Io can send high-speed impactors to Europa and beyond.  Material erupted or blasted off Io by a comet can drift in orbit for a median time of 56 years (quickest 179 days, average 146 years).  Then, it can either return to Io again, find a new target on Europa, or (to a lesser extent) hit the further-out moons.  Most of this occurs within just tens or hundreds of years.  Each impact, in turn, can generate secondary craters, reported Zahnle et al.2  Presumably, some sufficiently accelerated material could escape Jupiter or Saturn altogether and hit the inner planets.    Apparently few scientists had seriously considered this source of crater creators.  Some wrote about it but considered the amount of mass transfer to be trivial.  “However,” said Alvarellos et al., “our work has shown that far from being uninteresting, a non-negligible amount of matter can be transferred between these moons in the form of impact ejecta,”  The Zahnle et al paper agreed: “the model predicts that a significant fraction of the 200-500 m diameter craters on Europa are not traditional secondary craters but are instead sesquinary craters3 caused by impact ejecta from Io that had gone into orbit about Jupiter.”  The amount of mass delivered appears to exceed the micrometeoroid flux.1.  Alvarellos et al, “Transfer of mass from Io to Europa and beyond due to cometary impacts,” Icarus, Volume 194, Issue 2, April 2008, pages 636-646, doi:10.1016/j.icarus.2007.09.025.2.  Zahnle et al, “Secondary and sesquinary craters on Europa,” Icarus, Volume 194, Issue 2, April 2008, pages 660-674, doi:10.1016/j.icarus.2007.10.024.3.  They defined the term, quote: “Sesquinary” stems from the Latin root “sesqui-” meaning one-and-a-half; its most familiar use in English is in “sesquicentennial.”  We use sesquinary to describe craters by impact ejecta that went into orbit about the central planet…. Sesquinary craters have a character intermediate between primary craters and conventional secondary craters. [end quote]Since this new special-delivery mechanism can, in principle, apply to Uranus, Mars and other bodies, the crater-count dating conundrum just got worse.  How many of the thousands of craters on pockmarked moons resulted from “planetocentric” material within the system?  If ejected material can be recycled, what is the potential crater count from one sufficiently large impact?  How long would it take for an average moon to become saturated with craters?  They may look old, but crater-covered moons might have gotten their scarred faces in far less time than previously imagined.(Visited 86 times, 1 visits today)FacebookTwitterPinterestSave分享0last_img

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