Anyone
who wants a piece of Mars can imagine sending an automated spacecraft to land
on Mars, grab a sample, and blast off for return to Earth. No problem; just pay a billion dollars or so
and you can have it! Such a deal! As they tell everyone in the bazaar in
Istanbul, “Just for you!”
But asteroid
and comet impacts on Mars happen from time to time, blasting fragments of Mars’
crust into orbit around the Sun. Some of
these fragments drift into Earth-crossing orbits; a few enter Earth’s
atmosphere and a very lucky few survive entry heating to fall on the ground and
be recovered as meteorites. Most of them
will never be recognized as meteorites, especially once their fusion crust
weathers off. A few decades ago we knew
of only about two dozen such Martian meteorites, but since then we have reaped
the benefits of meteorite searches in two locations where meteorites are so
different from the local country rocks that they virtually stand up and say
“look at me”! One of these is the vast
sand seas (ergs) of the Sahara Desert; another is the snowy wastes of
Antarctica. Rocks in either place are
rare, and the overwhelming majority of them are meteorites. In turn, the large majority of these Saharan
and Antarctic meteorites are fragments of asteroids—but some are pieces of the
Moon or Mars, and possibly other bodies as well.
To date
some 200 meteorites found on Earth have been traced back to an origin on
Mars. Virtually all are members of three meteorite classes, shergottites, nakhlites, and chassignites, collectively called SNC meteorites. They do not sample Mars’ surface
either widely or democratically; they may indeed represent only three or four
places on Mars where recent and unusually violent impacts occurred. But they are real pieces of Mars, and they
are vastly cheaper than a billion dollars!
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