for National Geographic News
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.
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.
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