Features on the surface of Mars once interpreted as evidence of an ancient ocean may in fact have a completely different origin, planetary scientists reported this week.

Projection of MOLA data of Martian north pole to equator. Black lines indicate position interpreted shorelines. Credit: MOLA Science Team

In a paper published in Thursday's issue of the journal Nature, planetary scientists at the University of Arizona and MIT said that features in images of the planet previously thought to be remains of ancient shorelines are more likely linked to the planet's volcanoes.

Evidence for those ancient shorelines, also called paleoshorelines, was published in late 1999. Scientists analyzing data from the Mars Orbiter Laser Altimeter (MOLA) instrument on the Mars Global Surveyor spacecraft found a variety of features in the planet's northern hemisphere that they believed could be explained if they were the remnants of shorelines of an ancient Martian ocean. Those features included smooth, flat boundaries between geological units and a series of terraces parallel to the shorelines that could be explained by receding shorelines.

Those findings seemed to confirm models of an early Mars that was warmer and wetter than the present day. Some scientists believe that the northern plains of the planet, whose low elevation and flat topology closely resembles an ocean basin, could have been submerged under an ocean early in the planet's history, when liquid water could exist on the surface.

Paul Withers of the University of Arizona and Gregory Neumann of MIT decided to reexamine the MOLA data for some of the paleoshoreline features identified in the earlier work. They found, though, a closer correlation between the features and tectonic activity than any processes related to the formation of shorelines.

According to Withers, the key piece of evidence was the terraces identified in the earlier study. "There are the flat terraces, but the ridges are on what would be the oceanward side," he said. "That's difficult to explain if you have an ocean coming in, flattening things smooth over the terrace and then receding again."

Instead, those features are more likely tectonic stress ridges created by massive volcanism earlier in the planet's history. Without a network of plates on the planet's crust that move over time, as is the case on Earth, the volcanoes in areas like the Tharsis Ridge grew to great heights, putting tremendous stress on the crust and generating the ridges seen in the MOLA data.

Their results do not rule out the existence of such an ocean, but further study of the ridges may help scientists understand the nature and origin of Mars's northern plains. "In future work," Withers and Neumann wrote, "we hope that studying these ridges will reveal how the huge Martian volcanoes formed, what the Martian crust and lithosphere were like at the time, and what the northern plains of Mars are like today beneath their blanketing surface layer of Martian dust."