spacetoday.net: Impact may explain differences in Martian hemispheres

Posted: Fri, Jun 27, 2008, 7:21 AM ET (1121 GMT)
Mars giant impact model (UCSC)

The large differences in terrain between the northern and southern hemispheres of Mars can be explained by a giant impact that took place on the Red Planet early in its history, scientists reported this week. The Martian southern hemisphere is primarily composed of rough, heavily cratered terrain, while the northern hemisphere is much lower and smoother, and may have once hosted an ocean of liquid water. In a set of papers published in Nature, scientists reported that this northern basin is most likely the remnant of a crater created when a giant object, between 1,600 and 2,700 kilometers across, collided with the planet at least 3.9 billion years ago. An impact model for the creation of the basin was first proposed many years ago, but discounted because the basin is elliptical, not circular, in shape. Better topographic data from orbiting spacecraft, as well as improved computer models of impacts, led scientists to reconsider the impact model.