HELLAS IMPACT BASIN. There is ample evidence that some pretty big asteroids have whacked into Earth, but apparently they were small fry compared to one that had a run-in with Mars sometime in the distant past. Researchers have known for some time that Mars has a deep dent in its southern hemisphere. But until the data from the Mars Orbiter Laser Altimeter (MOLA) were analyzed they didn't realize that it was so deep it could swallow Mt. Everest or that the impact hurled debris as far as 2,500 miles across the planet's surface.

The Hellas Basin is six miles deep and 1,400 miles across. It is surrounded by a mile-high rampart of material that was ejected from the crater--enough rock to bury the continental United States under a two-mile-deep layer. "This material contributes significantly to the high elevation of the Martian southern hemisphere and underscores the role of major impacts in shaping the early surfaces of the solid planets," says Maria Zuber of the Massachusetts Institute of Technology, who is a member of the mapping team.


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