[meteorite-list] NASA Spacecraft Reveal Largest Crater in Solar System

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 26 Jun 2008 16:26:59 -0700 (PDT)
Message-ID: <200806262326.QAA05542_at_zagami.jpl.nasa.gov>

June 25, 2008

Steve Cole
Headquarters, Washington
202-358-0918
stephen.e.cole at nasa.gov

Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster at jpl.nasa.gov

David Chandler
Massachusetts Institute of Technology
617-253-2704
dlc1 at mit.edu

RELEASE: 08-159

NASA SPACECRAFT REVEAL LARGEST CRATER IN SOLAR SYSTEM

PASADENA, Calif. -- New analysis of Mars' terrain using NASA
spacecraft observations reveals what appears to be by far the largest
impact crater ever found in the solar system.

NASA's Mars Reconnaissance Orbiter and Mars Global Surveyor have
provided detailed information about the elevations and gravity of the
Red Planet's northern and southern hemispheres. A new study using
this information may solve one of the biggest remaining mysteries in
the solar system: why does Mars have two strikingly different kinds
of terrain in its northern and southern hemispheres? The huge crater
is creating intense scientific interest.

The mystery of the two-faced nature of Mars has perplexed scientists
since the first comprehensive images of the surface were beamed home
by NASA spacecraft in the 1970s. The main hypotheses have been an
ancient impact or some internal process related to the planet's
molten subsurface layers. The impact idea, proposed in 1984, fell
into disfavor because the basin's shape didn't seem to fit the
expected round shape for a crater.

The newer data is convincing some experts who doubted the impact
scenario.

"We haven't proved the giant-impact hypothesis, but I think we've
shifted the tide," said Jeffrey Andrews-Hanna, a postdoctoral
researcher at the Massachusetts Institute of Technology in Cambridge.


Andrews-Hanna and co-authors Maria Zuber of MIT and Bruce Banerdt of
NASA's Jet Propulsion Laboratory in Pasadena, Calif., report the new
findings in the journal Nature this week.

A giant northern basin that covers about 40 percent of Mars' surface,
sometimes called the Borealis basin, is the remains of a colossal
impact early in the solar system's formation, the new analysis
suggests. At 5,300 miles across, it is about four times wider than
the next-biggest impact basin known, the Hellas basin on southern
Mars. An accompanying report calculates that the impacting object
that produced the Borealis basin must have been about 1,200 miles
across. That's larger than Pluto.

"This is an impressive result that has implications not only for the
evolution of early Mars, but also for early Earth's formation," said
Michael Meyer, the Mars chief scientist at NASA Headquarters in
Washington.

This northern-hemisphere basin on Mars is one of the smoothest
surfaces found in the solar system. The southern hemisphere is high,
rough, heavily cratered terrain, which ranges from 2.5 to 5 miles
higher in elevation than the basin floor.

Other giant impact basins have been discovered that are elliptical
rather than circular. But it took a complex analysis of the Martian
surface from NASA's two Mars orbiters to reveal the clear elliptical
shape of Borealis basin, which is consistent with being an impact
crater.

One complicating factor in revealing the elliptical shape of the basin
was that after the time of the impact, which must have been at least
3.9 billion years ago, giant volcanoes formed along one part of the
basin rim and created a huge region of high, rough terrain that
obscures the basin's outlines. It took a combination of gravity data,
which tend to reveal underlying structure, with data on current
surface elevations to reconstruct a map of Mars elevations as they
existed before the volcanoes erupted.

"In addition to the elliptical boundary of the basin, there are signs
of a possible second, outer ring - a typical characteristic of large
impact basins," Banerdt said.

JPL manages the Mars Reconnaissance Orbiter for NASA's Science Mission
Directorate, Washington. For more information about the mission,
visit:

http://www.nasa.gov/mro

        
-end-
Received on Thu 26 Jun 2008 07:26:59 PM PDT