Impact may have scarred Mars

Science News, July 19, 2008 by Ashley Yeager

[ILLUSTRATION OMITTED]

Mars has two faces: a northern hemisphere with smooth, low ground and a southern hemisphere with a high elevation and many craters. "It's one of the really striking things about the planet," says Steven Squyres of Cornell University. Now, three papers in the June 26 Nature offer an explanation. "Something big smacked into Mars and stripped half the crust off the planet," says Francis Nimmo of the University of California Santa Cruz, lead author of one of the studies. In 1984, Squyres and astrogeologist Don E. Wilhelms suggested that a single impact caused the dichotomy in the Martian surface. The new studies support this idea. By mapping surface elevations, crustal thickness and variations in gravitational pull, MIT's Jeffrey Andrews-Hanna and colleagues uncovered a gigantic scar--a pockmark covering about 40 percent of the planet's surface. "Finding this elliptical boundary is a smoking gun," Andrews-Hanna says. Only an impact could make such a shape, he adds. Working independently, two modeling groups simulated slamming different-sized objects into Mars (image shown) to pinpoint the "sweet spot" conditions that could gouge out such a scar but not melt the planet's entire crust. Their findings fit well with the MIT report.