[meteorite-list] Did A Giant Impact Create The Two Faces of Mars?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 16 Mar 2007 18:13:51 -0700 (PDT)
Message-ID: <200703170113.l2H1DpP27737_at_zagami.jpl.nasa.gov>

http://space.newscientist.com/article/dn11387-did-a-giant-impact-create-the-two-faces-of-mars.html

Did a giant impact create the two faces of Mars?
David Shiga
New Scientist
15 March 2007

The impact of a giant asteroid could explain why Mars has two very
different faces - but only if it struck the planet with a glancing blow,
computer simulations suggest.

A longstanding puzzle about Mars is why its northern and southern
hemispheres are so different. The northern hemisphere is much flatter
and lies lower than the southern hemisphere, with a difference in
elevation between the two of about 5 kilometres.

In the 1980s, scientists suggested a giant impact by an asteroid about
300 kilometres across in Mars's early history could have led to a
permanent depression in the planet's northern hemisphere.

Now, two teams of scientists have created the first computer simulations
testing whether such an impact could have produced the observed
differences.

Shawn Hart of the University of California in Santa Cruz, US, led one of
the simulation teams. His team found that such an impactor would produce
huge amounts of lava - enough to cover the planet in an ocean of molten
rock somewhere between 14 and 48 kilometres thick. That would have ended
up erasing any record that an impact happened in the first place.

"We therefore consider it unlikely that you'll create the Martian
crustal dichotomy utilising a single giant impact," Hart says.

But simulations carried out by the second team, led by Margarita
Marinova of Caltech in Pasadena, US, suggest the giant impact hypothesis
could still do the job if it struck only a glancing blow.

Less lava

In simulations of grazing collisions, the team found the impactor still
makes a depression, but produces less heat, melting only a relatively
small area around the impact site. Watch an animation
<http://media.newscientist.com/data/images/ns/av/dn11387V1.gif> showing
the result of a giant impact (805 kB).

In this scenario, the total amount of lava produced is equivalent to a
5-kilometre-deep layer distributed over the whole planet. This would be
small enough to avoid erasing the depression, the researchers say.

"You can have a very large impact and not melt the whole planet and
preserve some signature of this impact," Marinova told New Scientist.

William McKinnon of Washington University in St Louis, Missouri, US,
says a giant impact may explain the crustal dichotomy. But he cautions
that it is too early to draw hard and fast conclusions from the
simulations. "I think it's exciting work, but we don't have the whole
picture yet," he told New Scientist.

An unresolved question is exactly how the lava would arrange itself
after such an impact, says Walter Keifer of the Lunar and Planetary
Institute in Houston, Texas, US. "Where does the melt go and how much of
the basin does it fill is a key issue," he told New Scientist.

The results were presented on Wednesday at the Lunar and Planetary
Science Conference in Houston, Texas, US.
Received on Fri 16 Mar 2007 09:13:51 PM PDT