Mars IMPACT CRATER :The Largest in Solar System Sparks Intense Scientific Interest

Recent analysis of the Red Planet's terrain using NASA's Mars Reconnaissance Orbiter and Global Surveyor spacecraft observations revealed what appeared to be by far the largest impact crater ever found in the solar system.

NASA’s Viking orbiters observed in the 1970s that the bottom two-thirds of Mars was about two miles higher in altitude than its top third. Planetary scientists have since bandied about two hypotheses to explain the dichotomy: either some odd internal dynamics of Mars generated a thicker planetary crust in the south, or the northern surface was blown away by a mega-meteor impact.

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. 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 8,500 kilometers (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 2,000 kilometers (1,200 miles) across.

That's larger than Pluto. The impact gouged out a crater the size of the combined areas of Asia, Europe and Australia, researchers reported in the journal Nature. It appears to have held an ocean in the early days of the planet, before Mars lost so much of its atmosphere and the water either sublimated away or froze beneath the surface.

"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. When the solar system was just maturing 4 billion years ago, big objects often smashed into one another. The formation of the Earth's Moon is attributed to a giant impact on the Earth by a Mars-sized body.

"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 the Massachusetts Institute of Technology, and Bruce Banerdt of NASA's Jet Propulsion Laboratory in Pasadena, Calif., report the new findings in the journal Nature this week.

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 4 to 8 kilometers (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 an 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 in the Tharsis region (visible from Earth to 19th century observers) -a region that is only 2 million years old -very recent in geological terms -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.

In a second report, Margarita Marinova and colleagues at the California Institute of Technology say they made three-dimensional simulations of the impact.

"The impact would have to be big enough to blast the crust off half of the planet, but not so big that it melts everything. We showed that you really can form the dichotomy that way," said Francis Nimmo of the University of California, Santa Cruz.

He said shock waves from the impact would have traveled through the planet and disrupted the crust on the other side, causing changes in the magnetic field. In a third report, Nimmo and colleagues said such magnetic anomalies have been measured in Mars' southern hemisphere.

"We haven't proved the giant-impact hypothesis, but I think we've shifted the tide. The majority of the evidence is now in favor of the giant impact," Andrews-Hanna said in a statement.


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