[meteorite-list] Liftoff for Mars Meteorites

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:14 2004
Message-ID: <200211081820.KAA00774_at_zagami.jpl.nasa.gov>

http://sciencenow.sciencemag.org/cgi/content/full/2002/1107/1

Liftoff for Mars Meteorites
ROBERT IRION
Science Now

A trip from Mars to Earth would be a blast for an astronaut, but
it's no big deal for a rock. That's the conclusion of a new study,
which finds that small asteroids striking the newest, most
pristine parts of Mars are powerful enough to launch millions of
chunks into space. The results may explain why nearly all Mars
rocks discovered on Earth are relatively young objects, rather
than relics of the red planet's ancient past.

Meteorite hunters have recovered 26 rocks that bear chemical
traces of the martian surface and atmosphere.

     [Image of the Los Angeles Meteorite]

     Red planetoid. This meteorite, found in the
     Mojave Desert, is one of 26 known chunks of
     Mars. The cube is 1 centimeter across.the martian surface
     CREDIT: RON BAALKE/NASA JPL

These meteorites--preserved mostly in Antarctic ice fields or Saharan
dunes--must have been ejected from Mars by impacts big enough to
fling them out of the planet's gravitational grasp and send some
of them toward Earth. However, the rocks' ages are odd. All but
one are between 200 million and 1.3 billion years old, even though
about half of Mars' surface is 4 billion years old. Because
asteroids must strike all areas on Mars equally over time,
planetary scientists wondered why Earth hasn't gathered more
samples of ancient Mars.

The answer lies in the varied terrain, according to an online
paper in the 7 November Science Express. Planetary scientist James
N. Head of Raytheon Missile Systems in Tucson, Arizona, and his
colleagues used a computer model to investigate how shock waves
from impacts hurtle martian rocks into space. When the terrain is
young--smooth basalt from recent eruptions, for example--an
asteroid that creates a crater just 3 kilometers wide can propel
millions of fist-size and basketball-size rocks into space. But
older terrain, battered by eons of impacts and laced with
fractures, absorbs much of the impact's energy. As a result, it
takes a 20-kilometer crater--a far more rare event--to expel
swarms of rocks into space. "It's so much easier to launch the
young material that it dominates what we receive on Earth," Head
says.

"This is important work, because it explains an observation that
has puzzled us," says planetary geologist Allan Treiman of the
Lunar and Planetary Institute in Houston. The claim that a
3-kilometer crater is big enough to blast Mars rocks to Earth is
"a very welcome surprise," Treiman says. Ejections in the
Earth-to-Mars direction are harder, he notes, because Earth's
higher gravity and thicker atmosphere keep many impact-launched
rocks from escaping.
Received on Fri 08 Nov 2002 01:20:56 PM PST