[meteorite-list] Crater Count Led Mars Historians Astray

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Mar 22 18:13:53 2005
Message-ID: <200503222313.j2MNDR417365_at_zagami.jpl.nasa.gov>

http://www.newscientist.com/channel/space/mg18524925.100

Crater count led Mars historians astray
David L. Chandler
New Scientist
26 March 2005

THE method used by planetary scientists to estimate the ages of various
regions of Mars is flawed.

"This really changes things," says Nadine Barlow of Northern Arizona
University in Flagstaff. For instance, the findings will significantly
change our understanding of when Mars may have been volcanically active.

To estimate the age of any region on Mars, geologists count the number
of meteor craters they can see in images of the area. The idea is that
the older a surface, the more craters should have accumulated over time.
Crater counts give an indication of the relative age of different
Martian regions.

To determine absolute ages, these counts are then compared with crater
counts from regions of the moon, some of which have been precisely dated
thanks to the rocks brought back by the Apollo astronauts. Assuming that
the rate of meteor impacts at any given time was roughly uniform
throughout the inner solar system, those ages should translate directly
to Mars.

Now it seems there is a fatal flaw in this method, at least when it is
based on counts of very small craters. According to Barlow, recent
infrared images taken by NASA's Mars Odyssey spacecraft have
unexpectedly revealed very long ray patterns around even relatively
small craters, showing that much more material was ejected by meteor
impacts than had been calculated. Massive plumes of ejected rock would
have rained down to produce, in some cases, millions of secondary craters.

It means the vast majority of craters smaller than 2 kilometres in
diameter may be secondary craters, making them virtually useless for
dating surfaces, says Al McEwen of the University of Arizona in Tucson.
For example, McEwen studied a 10-kilometre Martian crater called Zunil.
The impact of the meteor that produced it could have created millions of
secondary craters thousands of kilometres away from the main impact.

William Bottke of the Southwest Research Institute in Boulder, Colorado,
who is an expert on the dynamics of the asteroid belt, also concludes
that small craters are predominantly secondaries. He says there are just
not enough small objects in the asteroid belt to account for the
observed rates of small impacts on Mars. "Small craters may not be
telling you much," Bottke says.

Some crater-free areas on Mars have been estimated to be only about
150,000 years old. But based on the new findings, McEwen concludes that
the age of such regions can only be pinned down to within 10 million
years. And such uncertainty will remain until actual rock samples from
Mars are dated. Bottke agrees: "We probably can only set rough limits."

>From issue 2492 of New Scientist magazine, 26 March 2005, page 18
Received on Tue 22 Mar 2005 06:13:27 PM PST


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