--- On Thu, 9/3/09, Alan Rubin <[email protected]> wrote: <<snip... The metal liquid sank to the crater floor, incorporated some rapidly melted silicate debris andcooled. This is a controversial model and not universally accepted.>>
I think this theory has a potential fatal flaw if what we think we know about taenite/kamacite growth is valid. Without an insulating blanket the molten pool will not exist in a molten state long enough to permit crystallization aka Widmanstatten patterns. Be it remembered that Widmanstatten pattern/crystal growth is very very slow on the order of 10's of degrees cooling per million years. It is difficult to develop a scenario that integrates a large crater on an Goldilocks Asteroid which works. Goldilocks: Not too small as escape velocity is so low there is no fall back/re-accretion to bury the melt; Not too large as the asteroid would have already differentiated into a metallic core...so it has to be just right, at the threshold of the larger size with sufficient gravitational acceleration to not just recapture ejecta but to do it rapidly enough to insulate the molten metal. I envision a steeply conical deep crater which could minimize the amount of fall back ejecta to cover the surface. keep the pool--if in fact, such one exists. This scenario also requires to nearly identical impacts; one down the throat of another, millions of years apart. This tends to disfavor the crater floor theory on just the statistics. It would be interesting to locate a crater on an asteroid that fits the definition of Dewar flask. Popigai, here on earth had the depth and fall back to insulate a 600m melt on the crater floor and it only stayed molten for "a few thousand years" Not millions! This was a scenario that was given all benefit of favorable condition and still could not stay molten long enough. I can see why this theory has some doubters. Were we able to find a rapidly quenched FeNi meteorite without the Widmanstatten marker than I could see a scenario for this theory, but to my meager knowledge of irons I can't recall one. One caveat, I can not positively confirm any silicated iron (e.g. Miles) shows or doesn't show a pattern when etched. Ergo, I may have made the case for validating or invalidating the theory. As far as impact-induced melting and melt pockets scattered around the interior, meeting the insulation demands, I find much more reasonable. A vignette example would be Portales Valley as it proves a process on a micro level indicating the possibility that it has operated on a macro level. Elton ______________________________________________ http://www.meteoritecentral.com Meteorite-list mailing list [email protected] http://six.pairlist.net/mailman/listinfo/meteorite-list
