Texas-Size Asteroid Slammed Early Mars, Studies Say

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Scientists think it struck the red planet at a 30- to 60-degree angle while traveling at four to six miles (six to ten kilometers) a second.

A third study led by Francis Nimmo of the University of California, Santa Cruz, used a different computer model to confirm that such an impact could thin the northern hemisphere's crust.

That study also predicted that some sort of crustal disruption would have occured where shock waves from the impact met at the opposite side of the planet.

The geology of Mars is not known well enough to identify physical features that might have been created by the event. But there is an abnormal weak spot in the planet's magnetic field at that point.

This is a sign that something, such as a shock wave, might have altered the underlying crust, perhaps by melting it, noted team member Craig Agnor of Queen Mary, University of London.

All three studies will appear in tomorrow's issue of the journal Nature.

The new research hasn't definitively proven that Mars's northern basin was created by a giant impact, said Walter Kiefer of the Lunar and Planetary Institute in Houston, Texas, in a commentary also appearing in Nature.

But, he told National Geographic News, "[the studies] have answered many of the objections that have been around the impact model."

Mark Hammergren, an astronomer at Chicago's Adler Planetarium who was not involved in the research, called the results "really quite striking."

In general, a big impact is not an unreasonable explanation for Mars's crustal dichotomy, he added.

"Early in the formation of the solar system, there was a substantial population of large impactors, and many of them would have shared Mars's orbit," he said by email.

In fact, some studies suggest that around the same time Mars got slammed, an even larger impact may have created Earth's moon. And a large impact on Mercury may have been responsible for that planet's unusually high density.

"These impacts were incredibly devastating," MIT's Andrews-Hanna said, "but without them we wouldn't have the terrestrial planets we see today."

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