New theory of how Mars was shaped

USA Today (Society for the Advancement of Education), June, 1994

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 of Mars--about two-fifths that of Earth--alters tectonic processes. He concluded that creation of new crust must have proceeded at a rate comparable to that on Earth. He also estimates that it was much harder to subduct new crust on Mars. (Terrestrial crust that is 10,000,000 years old is relatively easy to subduct, but Martian crust must cool for around 60,000,000 years before it can be subducted.) Moreover, water must have circulated about twice as deep on Mars as it does on Earth, providing another mechanism to help explain why the planet has become extremely arid.

Sleep estimates that plate tectonism would have played a major role in removing heat from Mars' interior. Because Mars is a much smaller planet than Earth, the amount of plate tectonism that he proposes easily could have cooled the planet's interior until there was no longer enough energy to drive further tectonic activity. If he is right, Mars evolved in several distinct geological stages.

First, the planet formed hot and heat rapidly was removed by melting and widespread eruption. Then, as Mars cooled, a thick crust formed only to be subducted deep into the interior. The next stage began with the onset of plate tectonics, which resurfaced much of the planet. The formation of the northern lowlands was the last event in this process. Although the planet had cooled too much to continue to drive plate movement the crust subducted during the formation of the northern lowlands triggered a period of local volcanism that created the planet's largest mountains. Since then, Mars has been geophysically inactive, its stark features being muted gradually by wind erosion.