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Mars Meteorite origins
JRehling
post Oct 8 2007, 04:57 AM
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Walking the line between wacky and sure to be unoriginal:

The martian meteorites are quirky in the ages, in that about half of them have ages of roughly 175 million years, whereas most of the martian surface is clearly older than that. There must be a selection effect biasing the samples we find.

Is it possible that altitude (on Mars) is a major selection factor? The top of Olympus Mons is above 90% of Mars's atmosphere. Of course, that represents a very small fraction of the surface area of Mars, but when you add in the heights of the five biggest volcanos on Mars, you get a still small but nonzero area of the surface of Mars at high altitude AND likely to be the last places on Mars to get paved over with lava.

When an impactor strikes these areas, it faces much less air resistance (and spalling) on the way in, and then any ejecta flying skyward also faces much less air resistance on the way out.

Additionally, sheets of lava should make for more elastic collisions than the dusty regolith at lower altitudes.

Could it be that these selection effects favor the highest volcanos so greatly that ejecta from these places outnumber ejecta from the rest of the surface combined?
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tty
post Oct 8 2007, 07:43 PM
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Mass ejection from major impacts should not be much affected by the local atmospheric pressure since they literally punch a hole through the whole atmosphere. It might have some effect on smaller impacts.

However I think it likely that the martian meteors we have all come from just a few relatively recent impacts. Most of the stuff in Earth-crossing orbits would be swept up pretty quickly. It could be pure coincidence.

I don't quite buy the argument that an impactor has to hit hard rock to accelerate fragments to 5 kms-1. The Chicxulub impactor landed in limestone/gypsum which is not particularily hard and it still ejected enough material for the re-entering stuff to virtually fry the whole planet, including the antipodes (antipodal fragments must reach more or less orbital speed i. e. 8-11 kms-1 ).

An observation: we have never found any terran meteorites suggesting that most of the stuff ejected by Chicxulub and the group of major impacts at the Eocene/Oligocene border has already been swept up (admittedly a limestone meteorite might not be recognized as such unless it landed in Antarctica).

By the way this might be a good question Emily: just why don't we ever find terran meteorites, one should think that they should be at least as common as martian ones?
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nprev
post Oct 8 2007, 08:00 PM
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QUOTE (tty @ Oct 8 2007, 12:43 PM) *
However I think it likely that the martian meteors we have all come from just a few relatively recent impacts. Most of the stuff in Earth-crossing orbits would be swept up pretty quickly.

By the way this might be a good question Emily: just why don't we ever find terran meteorites, one should think that they should be at least as common as martian ones?


You may have answered your own question here, Tom. There probably hasn't been an impact energetic enough to boost a truly significant amount of terrestrial surface materials to escape velocity since Chicxulub, and 65 My is plenty of time to sweep up the remnants...

Of course, we may find a surprise or two on the Moon someday. smile.gif


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tty
post Oct 8 2007, 08:17 PM
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QUOTE (nprev @ Oct 8 2007, 10:00 PM) *
You may have answered your own question here, Tom. There probably hasn't been an impact energetic enough to boost a truly significant amount of terrestrial surface materials to escape velocity since Chicxulub, and 65 My is plenty of time to sweep up the remnants...

Of course, we may find a surprise or two on the Moon someday. smile.gif


Actually the Eocene/Oligocene impacts (Chesapeake, Popigai etc) ought to have been plenty powerful enough, and so should Ries and Steinheim (Miocene) have been.

You may be right about surprises on the Moon. It has alway seemed to me that the only decent chance we have of ever finding out anything about conditions on Earth before c. 3800 MA BP is to search on the Moon. A fairly substantial proportion of Earths Hadean crust and perhaps mantle ought to have ended up there during the Late Heavy Bombardment. In a way I'm a bit surprised no Terran material turned up in the Apollo samples, though after 3800 million years of impact "gardening" most of it would be buried and/or included in impact breccias. We might even find microfossils of the first Terran life-forms up there....
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