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

[Ron Baalke]

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 

	
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