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 equatora 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.
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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