[meteorite-list] Martian Meteorites Made Easy
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:17 2004
Martian Meteorites Made Easy
By J. Kelly Beatty
Sky & Telescope
November 25, 2002
For two decades, planetary geologists have been painting their dynamicist
colleagues into an ever-tighter corner. Several dozen meteorites with
Marslike compositions argue that asteroidal impacts occasionally blast
rocks off the Red Planet, and that some of these rocks eventually find
their way to Earth. Yet dynamicists initially scoffed at the notion that
chunks of Mars could ever be accelerated to escape velocity (5 kilometers
per second) without having them shocked to smithereens.
After much number crunching, however, impact modelers eventually deduced
that it could be done - if the impact event were powerful enough to leave
behind a crater at least 10 km across. The timing seemed plausible; the
youngest Martian meteorites are volcanic basalts only 180 million years
old, and collisions big enough to make 10-km craters occur on Mars about
once every 200 million years on average. But such an impact should have
left a sizable, fresh-looking scar on one of the planet's lava-covered
plains, and nothing so obvious has turned up. Moreover, the evidence in
hand suggests that at least six separate ejections have taken place.
Fortunately, computer impact simulations now suggest that such big bangs
aren't needed after all. In the November 7th edition of Science Express,
three researchers conclude that collisions yielding craters only 3 km
across are energetic enough to eject millions of small Martian rocks into
interplanetary space. Collisions of this size should happen on Mars every
200,000 years or so, and consequently chunks of the Red Planet should be
plunking down on Earth several times each year.
James N. Head (Raytheon Missile Systems), who performed the computer
modeling for his doctoral thesis at the University of Arizona, also
managed to solve another Martian-meteorite quandary. Most of these stones
crystallized within the last few hundred million years, yet roughly half
of the Red Planet's surface is a good 4 billion years old. So why haven't
more "old Martians" been found? The key, as Head and his colleagues
explain, is that the meteorites must have originally been buried in the
layer of regolith, or crushed rock, that covers the planet's exterior.
Younger regions, like the lava plains, have relatively thin crush zones,
but the most ancient terrains are covered to depths of hundreds of meters.
Because the presence of a thick regolith reduces ejection speeds, only
very energetic impacts can excavate material from these regions - and
since big impacts are infrequent on Mars, ancient samples of Mars should
reach Earth only rarely.
Received on Tue 26 Nov 2002 12:01:27 PM PST