Washington, June 26 : New analysis of Mars' terrain using NASA spacecraft observations has revealed 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.
But, scientists have been puzzled by the strikingly different kinds of terrain in the northern and southern hemispheres of the Red Planet.
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.
But, 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.
The new analysis has suggested that a giant northern basin that covers about 40 percent of the surface of Mars, sometimes called the Borealis basin, is the remains of a colossal impact early in the solar system's formation.
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 even 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.
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.
According to Bruce Banerdt of NASA's Jet Propulsion Laboratory in Pasadena, California, "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."