Magazine article Science News

Plate Tectonics ... on Mars

Magazine article Science News

Plate Tectonics ... on Mars

Article excerpt

Magnetic map reveals ancient activity on the Red Planet

Slipping, sliding, sinking, rising: Earth's surface is in constant motion. Fragmented into giant sheets of solid rock that glide atop a layer of hotter, more pliable material, the globe's appearance is forever changing. Where two sheets meet, violent activity ensues. When they crash head-on, mountains arise; where one sheet dives under another, earthquakes may erupt.

Scientists first proposed this movement, now known as plate tectonics, in 1912, but confirmation didn't come until the 1960s. Since then, this model has revolutionized understanding of the forces that shape our planet.

Now astronomers have found evidence that another planet may have undergone a similar series of facelifts. New measurements of Mars' magnetic field suggest that plate tectonics reigned supreme on the Red Planet--for at least the first half billion years or so of its 4.5-billion-year existence.

The movement of sheets of crust could have been every bit as important on the Mars of long ago as it is on Earth today, says Jack E.P. Connerney of NASA's Goddard Space Flight Center in Greenbelt, Md. In the April 30 SCIENCE, Connerney, Mario H. Acuna of Goddard, and their colleagues describe magnetic activity on Mars observed by the Mars Global Surveyor spacecraft.

Planetary scientists have contended for more than 25 years that water was once abundant on Mars. Early images had revealed dried-up channels scarring the planet's surface.

"If one believes early Mars was wet, then it makes plate tectonics more reasonable," says Gerald Schubert of the University of California, Los Angeles. "Alternatively, one could turn this around... accept plate tectonics, and use this as independent support for a lot of water on early Mars."

On Earth, water serves as a natural lubricant that helps keep the sheets of crust in motion. Connerney suggests that water may have played the same role on ancient Mars.

As the movement of plates dredged up rock from the depths of Mars and brought it back down again, it could have transported both water and carbon dioxide. The recycled carbon dioxide may have generated, or at least helped sustain, a dense, carbon-rich atmosphere early in the history of Mars. This blanket of greenhouse gas could have warmed the planet. Plate tectonics thus would add support to the view that the Red Planet was once a warmer, wetter place with a climate hospitable for life.

An end of plate tectonics on the Red Planet several billion years ago could explain an enduring mystery: If the surface of Mars once had an abundant supply of water, where did it all go?

Geologist Norman H. Sleep of Stanford University argues that water continually transported to and from the surface as a result of plate tectonics would have been trapped in the planet's crust after tectonic activity ceased. Little water would have been left on the surface once plate tectonics stopped, he says.

Five years ago, Sleep proposed that plate tectonics could have created the vast lowlands on the northern half of Mars, just as it has formed ocean basins on Earth.

If plate tectonics proves correct, it "explains and unifies the entire geological history [of the Red Planet]. It also will provide a second example to help understand the physics of plate tectonics on Earth," Sleep says.

Adds Maria T. Zuber of the Massachusetts Institute of Technology, "It's going to tell us much more about how the core of the planet dumped its heat." That's vital, she says, because heat loss from the planet's interior might explain why the ancient planet was warm enough and its atmosphere thick enough for water to exist on the surface.

"The thermal state of the interior of the planet is something that you really need to understand to get to the early climate history of Mars. It's a very critical piece of the puzzle," Zuber says. …

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