12/06/93
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STANFORD -- The rugged face of Mars may have been shaped by plate tectonics, the same geological process that is responsible for ocean basins, continental drift and earthquakes here on Earth.
That is the contention of Norman H. Sleep, professor of geophysics at Stanford University. Speaking Wednesday morning, Dec. 8, at the American Geophysical Union meeting in San Francisco, Sleep described the results of his efforts to apply what is known about terrestrial plate tectonics to the much different conditions on the Red Planet.
"What I am arguing is that plate tectonics fits what we know about Mars as well as anything else. It also has the advantage that we know it can happen," Sleep said.
Sleep first suggested his hypothesis informally about four years ago to explain why the northern third of Mars is covered by a lowland with an elevation that averages about two miles lower 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 meteors. Other theories have postulated as-yet-unknown geological processes within the planet's interior.
If accepted by fellow planetologists, Sleep's proposal would require a major rethinking of the geological history of Mars. Until now, scientists have assumed that the planet's 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, some time between 3 and 4 billion 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 an 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 are now located.
Over a period of about 200 million 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 the area.
Much of Sleep's efforts have involved figuring out how the lower gravity on Mars -- it is about two-fifths that of Earth -- alters tectonic processes. Among other things, he has calculated that the process of creating new crust must have proceeded at a rate comparable to that on Earth (about 8 centimeters a year on Mars compared with 2 to 20 centimeters per year on Earth). He also estimates that it was much harder to subduct new crust on Mars than it is on Earth. (Terrestrial crust that is 10 million years old is relatively easy to subduct, but Martian crust must cool for about 60 million years before it can be readily subducted.) And, Sleep said, 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.
Finally, 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 could easily 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 was rapidly 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 gradually muted by wind erosion.
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