New analysis of Mars' terrain using observations of two of NASA’s Mars-bound spacecrafts reveals what appears to be by far the largest impact crater in the Solar System.

NASA's Mars Reconnaissance Orbiter (MRO) and Mars Global Surveyor (MGS) space probes have provided detailed information about the elevations and gravity of the northern and southern hemispheres of Mars.

A new study using this information may crack one of the biggest riddles in the Solar System: Why does Mars have two strikingly different kinds of terrain in its northern and southern hemispheres? The huge crater has stimulated intense scientific interest.

The enigmatic two-faced nature of Mars has perplexed scientists since the first comprehensive images of the Martian surface were relayed to Earth 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 theory, proposed in 1984, fell into disfavor because the basin's shape did not match the expected round shape for a crater. The recent 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.

Andrews-Hanna and co-authors Maria Zuber of the Massachusetts Institute of Technology, and Bruce Banerdt of NASA's Jet Propulsion Laboratory, report the intriguing findings in Nature this week.

A giant northern basin that covers about 40% of Mars' surface, sometimes called the Borealis basin, is the remains of a colossal impact early in the Solar System's evolution, the new analysis proposes. At 8,500 km across, it is about four times wider than the next-biggest impact basin known, the Hellas basin in Martian southern hemisphere. A related report calculates that the impacting object that produced the Borealis basin must have been about 2,000 km across. That is similar in size to 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, rugged, densely cratered terrain, which ranges from 4 to 8 km higher in elevation than the basin floor.

Other giant impact basins have been discovered that are elliptical rather than circular. However, a complex analysis of the Martian surface from the two Mars orbiters revealed the distinct 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 demanded 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.

Further Reading

MRO
http://www.nasa.gov/mission_pages/MRO/main/index.html

Aymen Mohamed Ibrahem