— 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, the U.S. space agency announced on Wednesday.
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 suggests that 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, which must have been at least 3.9 billion years ago.
This 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 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 are 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 researcher at the Massachusetts Institute of Technology. He and other co-authors reported the new findings in the journal Nature this week.
At about 8,500 km across, the Borealis basin is about four times wider than the next-biggest impact basin known, the Hellas basin on southern Mars.
The Borealis basin on the northern hemisphere is also one of the smoothest surfaces found in the solar system. The southern hemisphere is largely high, rough, heavily cratered terrain, which ranges from 4 km to 8 km higher in elevation than the basin floor.
An accompanying report calculates that the impacting object that produced the Borealis basin must have been about 1,930 km across, which is larger than the size of 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.
