[meteorite-list] Martian Meteorite Mysteries Resolved

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:14 2004
Message-ID: <200211080552.VAA23496_at_zagami.jpl.nasa.gov>

http://www.newscientist.com/news/news.jsp?id=ns99993030

Martian meteorite mysteries resolved
Hazel Muir
New Scientist
November 7, 2002
  
Nagging doubts about a family of Martian rocks that planet-hopped to
Earth have finally been laid to rest after two-decades of investigation.
Scientists say they can now explain why the rocks from Mars are so
unexpectedly young and common.

In the 1980s, scientists found convincing evidence that several meteorites
found on Earth came from Mars. Analysis of gases trapped inside the rocks
matched measurements of the Martian atmosphere by the Viking landers.

These rocks must have been blasted off the Red Planet by asteroid impacts,
then wandered the Solar System for a few million years, before
crash-landing on Earth.

But the vast majority of the 26 Martian meteorites known today are
relatively young - about 200 million years old. Yet 90 per cent of the
Martian surface is much older than that.

"If we had a random sampling of the Martian surface, most of these
meteorites should be ancient, about 3.9 to 4.5 billion years old," says
James Head of the University of Arizona in Tucson.

Colossal craters

Also, studies of the meteorites suggest they left Mars during six or
seven different impacts. Computer simulations have suggested these
impacts must have been enormous, leaving behind craters 12 kilometres
wide or more.

Such huge impacts are extremely rare - so rare in fact that scientists
would not expect to find any Martian rocks on Earth at all. But Head
and his colleagues say they have now resolved these contradictions. The
availability of greater computing power allowed them to simulate impacts
on Mars on a far finer scale than before, making them more physically
realistic.

The team found that relatively frequent small impacts, leaving craters
only three kilometres across, can launch millions of small rocks into
space. What is more, young exposed rock appears far easier to chip off
Mars than older rock covered in debris, explaining the young age of the
Martian rocks found.

But the new simulation is bad news for those seeking extra-terrestrial
life. It suggests researchers will be lucky to find more ancient
meteorites like ALHA84001, the one that some scientists think contains
remains of Martian bacteria. This meteorite is 4.5 billion years old,
and probably began its journey to Earth when a huge, rare impact shook
the Red Planet.

Journal reference: Science (DOI 10.1126/1077483)
 
Received on Fri 08 Nov 2002 12:52:24 AM PST