"We don't see the paleo (ancient) shorelines that some people see," said Paul Withers of the Lunar and Planetary Laboratory at the University of Arizona. "We see an network of tectonic ridges that helps us explain how the Tharsis volcano formed. I wouldn't make too many sweeping conclusions about whether there was or was not an ocean based on this. The northern plains are still flat and nobody has a clue why."

Unlike plate tectonics, which collide, disrupt and mold huge land forms on Earth, the tectonic forces at Mars result from the stresses required to hold up mountains and volcanoes -- some of which reach as high as 10 miles (16 kilometers).

Those stresses build up over time and cause crustal compression that eventually squeezes large slabs of land and forms the ridges that initially look like shorelines, Withers said. The features likely formed about 3 billion years ago.

Withers and his colleague Gregory Neumann of the Massachusetts Institute of Technology and the NASA Goddard Space Flight Center in Maryland published their brief analysis in this week's issue of the journal Nature.

The tip-off to a non-oceanic origin for the ridges included raised lips adjacent to ridges on what is the most featureless landscape in the solar system, Withers said.

"Specifically what we saw on one group was a nice flat terrace you'd associate with a shorelines, and then a raised lip on what would be the ocean-ward side of this terrace," Withers said. "It's difficult to imagine how you'd form this by an ocean or sea washing in and then receding away to leave this lip."

But the new data show a network of ridges crisscrossing the plains, as well as many smaller ridges there, Withers said -- features you wouldn't expect on an ocean floor.

"All the attempts to find shorelines or not -- they don't seem to be succeeding too well," Withers said, referring to similar work by Mike Malin and colleagues, where they searched for evidence of shorelines and failed to find them.

Withers remained open to the idea that the flatlands once harbored an ocean, but said, thus far, the evidence is not apparent.

"No one has a clear [idea] what a billion-year-old shoreline on Mars is going to look like, but it's unlikely to look like the candidates that we've tested," he said. "This featureless area on Mars is covered with this network of tectonic features will give geophysicists something to think about."

James Head is thought to be working on a longer analysis of the ridge features that could be published later this year. He was out of the country Wednesday and could not be reached for comment.

Victor Baker, a planetary geologist at the University of Arizona who also has studied Mars' northern flatland ridges, was an early advocate of the northern ocean idea.

Competing teams of researchers have been studying somewhat different sets of ridge features in this area on Mars, Baker said.

"The ones that Head and others had interpreted had always been suspiciously possible tectonic forms because of their scale," he said, "so that's not an interpretation that would surprise me."

And Withers' and Neumann's finding actually could still point to the idea of a northern ocean, Baker said.

"It's not inconsistent with this having been a basin which could be a possible receptacle for water because obviously the Earth's ocean basins are tectonic features. Tectonic processes resulted in those depressions. So the fact that these are tectonic features that line up along the margins doesn't say anything about whether there was ponded water there."