[meteorite-list] Mars-- megawhack?

[Darren Garrison]

http://www.cnn.com/2008/TECH/space/06/25/twofaced.mars.ap/index.html

Ancient impact may explain Mars mystery

LOS ANGELES, California (AP) -- Why is Mars two-faced? Scientists say fresh
evidence supports the theory that a monster impact punched the red planet,
leaving behind perhaps the largest gash on any heavenly body in the solar
system.

 Today, the Martian surface has a split personality.

The southern hemisphere of Mars is pockmarked and filled with ancient rugged
highlands.

By contrast, the northern hemisphere is smoother and covered by low-lying
plains.

Three papers in Thursday's journal Nature provide the most convincing evidence
yet that an outside force was responsible.

According to the researchers, an asteroid or comet whacked a young Mars some 4
billion years ago, blasting away much of its northern crust and creating a giant
hole over 40 percent of the surface.

New calculations reveal the crater known as the Borealis basin measures 5,300
miles across and 6,600 miles long -- the size of Asia, Europe and Australia
combined.

It's believed to be four times bigger than the current titleholder, the South
Pole-Aitken basin on Earth's moon.

 Astronomers have long puzzled over Mars' landscape ever since images beamed
back in the 1970s showed different-looking halves. An orbiting spacecraft later
observed the northern lowlands were on average 2 miles lower than the southern
highlands and had a thinner crust.

Scientists who had no role in the studies said the latest research strengthens
the case for a colossal Martian impact, but it does not rule out the other
theory that hot rock from inside the planet could have welled up and formed the
different crusts.

"The betting odds have gone up a lot in favor of the impact model," said Walter
Kiefer, a staff scientist at the Lunar and Planetary Institute in Houston.

The idea of an ancient impact was first advanced by Steve Squyres of Cornell
University and Don Wilhelms of the U.S. Geological Survey in the 1980s. Squyres,
currently the lead scientist for the twin Mars rovers, had always hoped other
scientists would "pick that ball up and run with it."

"It wasn't a totally nutty idea that there could have been an impact," Squyres
said.

But finding evidence of one proved difficult because part of the basin rim is
now covered by a bulging volcanic range.

For one study, a team of scientists from the Massachusetts Institute of
Technology and NASA's Jet Propulsion Laboratory recreated what the Martian
surface would have looked like before the volcanoes formed using gravity and
surface measurements from spacecraft. They determined the impact basin is
oval-shaped, similar to what would be expected if a space object had hit at an
angle.

"The shape is really one of the key pieces of evidence that it was probably
formed in a giant impact," said MIT postdoctoral researcher Jeffrey
Andrews-Hanna, whose original "gut feeling" favored the other theory.

A separate group led by the California Institute of Technology developed 3-D
simulations to determine the "sweet spot" of conditions that would form the
basin.

According to their calculations, a 1,000-mile-wide object traveling at more than
13,000 miles per hour -- or 24 times faster than a jetliner -- would hit Mars at
an angle between 30 and 60 degrees. The collision would be equal to an explosion
of 75 to 150 trillion megatons of TNT.

In the third study, a team of researchers led by the University of California,
Santa Cruz, found that shock waves from such an impact would disrupt the
southern crust.

All three teams believe there was a single giant blow and not several small hits
because there's no evidence of other basins.