Mars Once Had Oceans, New Evidence Suggests

Brian Handwerk
for National Geographic News

June 13, 2007

The search for water—and possible life—on Mars got a boost this week as scientists announced evidence of an ancient ocean on the planet's northern plains.

The massive, sediment-filled basin surrounded by "shoreline-like" features was first spotted when scientists were mapping Mars's surface using images from the late-1970s Viking missions.

The features made theories of an ancient Martian ocean quite popular—until NASA's Mars Global Surveyor highlighted some seemingly insurmountable problems.

The Arabia and Deuteronilus "shorelines" are thousands of kilometers long. But they undulate like a long wave, rising and falling in height as much as several kilometers (more than a mile) along their length.

"That doesn't seem to jibe with the idea that they are shorelines, because shorelines form at sea level," said lead author Taylor Perron, a postdoctoral fellow in Harvard University's Department of Earth and Planetary Sciences.

But there really was an ocean there, Perron and colleagues now suggest in a finding that adds to a number of recent studies bolstering evidence of Mars's watery past. (Related: "Mars Rovers Find 'Best Evidence Yet' of Water" [May 23, 2007].)

The researchers believe that Mars's poles, along with the axis the planet spins on, have moved about 1,850 miles (3,000 kilometers) during the past two or three billion years. (Related: "Mars Pole Holds Enough Ice to Flood Planet, Radar Study Shows" [March 15, 2007].)

The process, known as "true polar wander" can cause dramatic topographic changes in a planet's surface—in this case making the once-flat shorelines rise and fall over enormous distances.

"When the spin axis moves relative to the surface, the surface deforms, and that is recorded in the shoreline," said co-author Michael Manga, professor of earth and planetary science at the University of California, Berkeley.

The team's research appears this week in the journal Nature.

Wandering Poles

True polar wander has probably occurred on planets other than Mars. On Earth the phenomenon is believed to have caused more modest deformation, ranging from centimeters over the course of years to hundreds of meters over tens of millions of years.

The phenomenon occurs because planets contain irregular and shifting distributions of mass.

Any change in the distribution of a planet's mass, whether on its surface or in its interior, will make it spin around a different axis as it shifts areas of large mass to its equator—the area farthest from the rotation axis.

On the Martian equator one finds the Tharsis rise—a massive bulge that holds the colossal volcano Olympus Mons. (See a map of Mars.)

"The largest mountain in the solar system is sitting almost right on the equator, and that's right where you should expect such a large surface load to be," Perron said.

"That tells you that if Mars experienced any true polar wander after the creation of Tharsis, it should reorient in such a way that keeps Tharsis at the equator—a circle 90 degrees from Tharsis."

The current locations of the poles, along with calculations of their previous positions based on the deformation of the shorelines, suggest just such an alignment. The authors argue that this is unlikely to have occurred by chance.

Not Obvious

The findings are sure to spark debate among the many professionals in search of a liquid "smoking gun" on the red planet.

Jeff Moore, a planetary geologist at NASA's Ames Research Center who is unaffiliated with the study, finds it plausible that early Mars had large standing bodies of water—either liquid or frozen.

But while the new study presents a possible explanation for why the shorelines don't follow a constant contour, it's not a conclusive one, Moore said.

The problem, in his view, is that so many things have happened on Mars in the billions of years since surface water existed that the clues have been muddled.

"What you don't see on Mars, expressed in the landforms of today, is obvious evidence," he said.

"The Mars that we see represents the last stage of hydrological activity. It's like looking at the desiccated corpse of an 80-year-old rocker and trying to imagine him at 20."

So the search for additional—and better—evidence will go on.

"We're all trying to answer the same questions here really," study leader Perron said. "How much water was there on Mars, what form was it in, and how much is there now?"

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