Asteroid may have had impact on Mars

Three studies posit a celestial collision

This artist's rendition shows an impact on the surface of Mars. Scientists say new evidence supports the theory that a monster impact took place on the red planet 4 billion years ago. (ASSOCIATED PRESS VIA JEFFERY ANDREWS-HALE)

NEW YORK - The lopsided shape of Mars might well be a result of a cataclysmic impact of a Pluto-size meteor billions of years ago, three teams of scientists are reporting. This would suggest that the lowlands of Mars's northern hemisphere are a single gigantic impact crater, the largest crater in the solar system.

"The early solar system was a pretty exciting place," said Francis Nimmo, an associate professor of earth and planetary sciences at the University of California, Santa Cruz, and the lead author of one of three scientific papers to appear in today's issue of Nature.

NASA's Viking orbiters observed in the 1970s that the bottom two-thirds of Mars was about 2 miles higher in altitude than its top third. Since then, planetary scientists have offered two hypotheses to explain the dichotomy: Either some strangeness with the internal dynamics of Mars generated a thicker planetary crust in the south, or the northern surface was stripped away by a megameteor impact.

The impact idea, first proposed in the 1980s by Steven W. Squyres, now an astronomy professor at Cornell, and Don Wilhelms of the US Geological Survey, ran into several objections. The boundary between lowlands and highlands does not have a simple round shape like most craters, there is no crater rim, and an impact that large should have, in theory, melted the entire surface.

The three Nature papers "have removed significant objections to the impact model," said Walter S. Kiefer, a staff scientist at the Lunar and Planetary Institute in Houston, who wrote an accompanying commentary in Nature.

Squyres said the new findings did not prove that his idea was right, but "they've really gone and made some new observations, which make a strong case that the idea really makes sense."