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| The Department Graduate Students Spacecraft Missions |
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Jay Melosh, on the SNC meteorites When I came to the University of Arizona, a big problem had come up that I had been working on or considering. Round about that time, 1980, it was recognized that some very strange meteorites—the so-called SNC meteorites, shergottite, nakhlite, and chassigny—had come from a larger body. The proposal had been made that perhaps they came from Mars. At that time, the best understanding of how impact craters worked said that there was no way to eject a rock at anything like Martian escape velocity and have it survive its flight. Gene Shoemaker said in no uncertain terms—I can still remember his voice echoing through the room—that it was absolutely impossible to get a rock off of Mars without it either melting or vaporizing. Yet these rocks had neither melted not vaporized, and it became more and more clear as 1980 turned to ’81, and ’81 to ’82, that they in fact really had come from Mars. ’83 was the clincher—dissolved atmospheric gases were found in one of the Martian meteorites that were an absolute dead-ringer for the Martian atmosphere. Basically all scientific resistance collapsed. |
I was attracted to this problem where the best theory and the observations disagree, and started working on mechanisms by which the rocks could be ejected. In the end I recognized that it was the interaction of the shockwave in the rock with the surface of the planet that actually was responsible for ejecting the material into the upper atmosphere, a process I call spallation. I worked on that pretty exclusively for the first couple of years I was here, 1982 to ’86 or so. I was working pretty hard on that mechanism by which rocks get ejected from their parent bodies. That’s now pretty much the accepted mechanism; we have a lot of direct observational evidence that that process is, in fact, what happens. |
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