for National Geographic News
Early Mars may have been clobbered by a Texas-size asteroid, creating what could be the largest known impact structure in the solar system, according to three new studies.
The huge depression in the planet's northern hemisphere, dubbed the Borealis Basin, offers new evidence that a massive impact was the cause of Mars's so-called crustal dichotomy.
Scientists have known that the crust in the northern hemisphere is much thinner than it is in the south.
But figuring out why has been a challenge, in part because gigantic volcanoes cover 30 percent of the original boundary between the two regions.
Now satellite maps have allowed a team led by Jeffrey Andrews-Hanna of the Massachusetts Institute of Technology to use computer models to peer beneath the volcanoes.
"That allowed us to trace the boundary for the first time," Andrews-Hanna said.
What they found was evidence of a giant ellipse 6,000 miles (10,000 kilometers) long. At first the scientists were puzzled, because most impact craters are circular, not elliptical.
Also, an impact big enough to create such a huge crater could have easily melted the crust of the entire planet, obliterating the southern highlands.
In a second paper, Oded Aharonson of the California Institute of Technology and colleagues used computer simulations to show that very large impacts hitting at just the right angle can create elliptical craters.
The team also found a "sweet spot" of impact angles and asteroid velocities that would melt only the crater floor.
According to the new calculations, the impactor was probably 1,000 to 1,800 miles (1,600 to 2,700 kilometers) wide—two-thirds the size of Earth's moon.
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