New theory of how Mars was shaped.

The rugged face of Mars may have been shaped by plate tectonics, the same geological process that is responsible for earthquakes, ocean basins and continental drift on Earth, according to Norman H. Sleep, professor of geophysics, Stanford University. This could explain why the northern third of Mars is covered by a lowland with an elevation that averages about two mile slower than the rest of the planet. Previous explanations for the lowland's creation have invoked the impact of a large meteor or multiple impacts by a number of smaller ones. Other theories have postulated as-yet-unknown geological processes within Mars' interior.

Until now, scientists have assumed that Mars' crust never broke into a number of smaller rigid plates, like those on Earth, but remained whole throughout the planet's life. According to Sleep's reconstruction, 3-4,000,000,000 years ago, the Martian crust split into two plates. A single spreading center where new crust was formed extended in a 5,000-mile arc that girdled the planet's equator, its initial position marked today by a distinct escarpment. On the opposite side of the north pole, two subduction zones--deep trenches where old crust is forced back into the planet's interior--were established. One was situated in the Tharsis region, where the planet's largest volcanoes now are located.

Over a period of approximately 200,000,000 years, the spreading center created about 5,000 miles of new, thinner crust, Sleep calculates. He explains the striking group of three volcanoes that form a line along the Tharsis Ridge as extinct island arc volcanoes, comparable to the Japanese island chain. The hypothesis also explains Mars' other two huge volcanoes--Olympus Mons and Alba Patera--as a residual effect of the subduction of new hydrothermally altered crust that took place in that area of the planet.

Much of Sleep's efforts involve figuring out how the lower gravity...