Shifting ocean: tipsy Mars may explain undulating shoreline.

By proposing that the Red Planet was tipped halfway over on its side several billion years ago, astronomers this week provide a new perspective on--and new support for--the long-standing notion that Mars once held a vast ocean.

Viking-spacecraft images of the northern lowlands of Mars, taken in the 1980s, showed what appeared to be two ancient shorelines, each several thousand kilometers long. The features resembled those found along coasts on Earth. Researchers suggested that the shorelines enclosed a basin that covered one-third of the planet and was filled with water a few billion years ago.

But in the late 1990s, measurements by the Mars Global Surveyor spacecraft showed that the shorelines weren't all at the same altitude. That surprised researchers, who had assumed that shorelines ought to be level. Some scientists began to doubt whether the Red Planet ever had an ocean.

In the June 14 Nature, Taylor Perron of Harvard University and his colleagues suggest a way out of the dilemma. In their model, undulating shorelines are a consequence of a phenomenon that astronomers call true polar wander, in which Mars' axis of rotation slowly drifts in direction. Some 2 to 3 billion years ago, Perron and his team propose, the planet's axis, and therefore its poles, lay 50[degrees] away from their current positions. That's a movement of 3,000 kilometers along the surface.

A spinning object tends to flatten at its poles as mass bulges around its equator. As the spin axis drifts, this equatorial bulge also migrates. Perron and his colleagues invoke that effect to explain why the Martian shorelines today have uneven elevations. As Mars' spin axis wanders, the solid surface of the planet would deform and the water level in the ocean would also vary. The shape of Mars' stiff outer shell wouldn't change nearly as much as the ocean's surface and shoreline. The height of the ocean could change by as much as a few kilometers, Perron explains.

"The results could help to resolve some long-standing, uncertain aspects of the interpretation of a former ocean in the northern lowlands of Mars," says planetary scientist Jim Head of Brown University in Providence, R.I. "This hypothesis makes a number of predictions, such as the formation of an ocean in tropical regions, not high latitudes, that can be further tested during present and future exploration of Mars."

The team proposes that the 2.5-km variation in one of the shorelines arose when the rotation axis of Mars was pointed 50[degrees] away from its current position. Changes in the other shoreline, which has height variations of about 0.7 km, occurred somewhat later, when the axis was tipped by 10[degrees].

That still leaves open the question of why the rotation axis would move in the first place. A large shift in mass, like a volcanic eruption, could cause such a movement. Perron even suggests that the formation of an ocean on Mars could itself have triggered polar wander. Some scientists have proposed that the upwelling of hot material in Earth's mantle caused our planet to tip completely on its side some 800 million years ago.