DISCOVER Vol. 22 No. 5 (May 2001)
Table of Contents

No Rain on Mars Plain
The Red Planet's "coastlines" turn out to be evidence for violent volcanoes or an asteroid impact
By Eric Powell

The northern plains of Mars contain the most featureless piece of real estate in the solar system, besting even Kansas in terms of sheer flatness. But just why the area is so flat has puzzled scientists ever since the 1970s, when Viking missions first beamed back images of the oddly smooth surface.

To those who believe Mars was once wet and Earth-like, the unusual terrain, which covers roughly a quarter of the Red Planet, looks an awful lot like the floor of a dried-up ocean. Even a 1999 paper -- which looked at recent maps that were more detailed than the Viking images -- identified a series of ridge formations on the plains as ancient coastlines. But now a study by planetary scientists Paul Withers, a graduate student at the University of Arizona at Tucson, and Gregory Neumann, of the Massachusetts Institute of Technology, challenges the notion that these ridges are ancient Martian beaches. They think the culprit was tectonic stress in Mars' upper, brittle crust, caused either by volcanic activity or an asteroid impact.

Images from the Viking missions to Mars (left) showed flat expanses that were taken to be evidence of ancient oceans. But new pictures from the Mars Global Surveyor spacecraft, at much higher resolution (right), show ridges more likely created by tectonic activity than water. Courtesy of MOLA Science Team and reprinted by permission from Nature Volume 410, Number 6829, Page 651, copyright 2001 Macmillan Magazines Ltd.

Withers, along with the Martian sea proponents before him, were able to evaluate the putative shorelines in extraordinary detail thanks to the Mars Orbiter Laser Altimeter on the Mars Global Surveyor spacecraft now circling the Red Planet. This instrument generates high-resolution topographic maps by bouncing a laser beam off the planet's surface and recording the time it takes for the beam to return to the craft in orbit. The maps have a vertical resolution measured in terms of meters, a huge improvement over the kilometer-scale resolution of the old Viking maps.

After reexamining the ridges, Withers began to question that they had been created by ancient oceans. "We think that these candidate shorelines were formed by tectonic processes," he says. Withers and Neumann assumed that if the ridges had been part of an ancient coastline they should run at a constant altitude, which would indicate the old martian sea level. After detailed inspection, however, Withers found otherwise: "The ridges don't have a relationship to topographic contours. They go straight uphill, sometimes go along at a constant altitude, and mostly go somewhere in between." So, no ancient sea level.

If the ridges weren't caused by an ocean, where did they come from? "They have a tendency to form near major tectonic features like the Utopia impact basin or the Tharsis volcanoes," says Withers. A close look at the 300-foot-high ridges reveals telltale wrinkles in their formation, which on Earth were typically caused by compressive tectonic forces. Withers and Neumann surmise that when the 10-mile-high Tharsis volcanoes formed- or when an asteroid suddenly carved out the Utopia basin- the stress generated was enough to distort the crust of the northern plains, resulting in ridges that look uncannily like a shoreline to human eyes.

"This research doesn't definitively answer the question of whether there was an ocean," says Withers. "We've just looked at these shoreline candidates." So the theory that the flat, northern plains were once the bottom of an ocean is still alive. But it does go to show that placid-looking locales can't be judged by appearance alone: The ridges testify that even the flattest place in the solar system gets shaken up every once and a while.

- Posted 04/10/01

"Enigmatic Northern Plains of Mars," Paul Withers, Gregory A. Neumann, Nature, Volume 410, Number 6829, Page 651 (2001). See www.nature.com.

The paper originally identifying the ridges as ancient shorelines: "Possible Ancient Oceans on Mars: Evidence from Mars Orbiter Laser Altimeter Data." James W. Head III, Harald Hiesinger, Mikhail A. Ivanov, Mikhail A. Kreslavsky, Stephen Pratt, and Bradley J. Thomson, Science, December 10, 1999; 286: 2134-2137. See www.sciencemag.org.

"Rethinking Water on Mars and the Origin of Life." Richard A. Kerr, Science, April 6, 2001. See www.sciencemag.org.

Wither's web page is http://www.lpl.arizona.edu/~withers/

The Mars Orbiter Laser Altimeter page is http://ltpwww.gsfc.nasa.gov/tharsis/mola.html