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Mars was once a blue planet, a team of Canadian and American scientists have found, with a third of its surface covered by ancient seas.

The University of Toronto's Jerry Mitrovica and his colleagues reassessed and reinterpreted data about the topography of the Red Planet collected over the years by spacecraft visiting Mars.

“This really confirms that there was an ocean on Mars,” said Mark Richards, professor of earth and planetary science at University of California, Berkeley.

Does this mean there was life on the Red Planet?

“Water is inextricably linked to life,” said Dr. Mitrovica, a geophysicist, so one of the conditions for life was in place billions of years ago.

“Whether it was sufficient or not, we don't know,” he said.

He and colleagues, who publish their findings in Thursday's edition of the British journal Nature, have resurrected an old theory about water on Mars.

In the 1980s, Viking spacecraft images revealed what looked like a sediment-filled ocean basin surrounded by shoreline near the planet's north pole.

“Imagine the Pacific Ocean. You drain the Pacific Ocean and you are left with this rim. You would have no problem recognizing the shoreline,” Dr. Mitrovica said.

But a decade later, information from NASA's Mars Global Surveyor killed the theory that there were oceans on Mars two to four billion years ago. It turned out the shorelines climbed several kilometres uphill. The rise was gradual, but significant. Walking on the Martian beach would be like moving from the Prairies to the foothills.

That doesn't sound like a shoreline. The theory was dismissed.

Now Dr. Mitrovica and his colleagues have found that billions of years ago the shorelines were more uniform, and in a different part of the planet, closer to the equator. They moved, and were deformed, over time.

That process is called “true polar wander,” and it sounds a little freaky. Essentially, the planet tips over. Imagine New York moving to the North Pole. It didn't happened all of a sudden, Dr. Mitrovica says, but took millions of years, and was likely prompted by a major shift of mass on a planet – perhaps a massive volcanic eruption.

The team also tried to figure out where all the water that was held in the ancient oceans went. Their calculations showed it couldn't all have evaporated into space, says Taylor Perron, a co-author on the paper who is doing a postdoctoral fellowship at Harvard University. This means it must have drained downward, beneath the surface of the planet, he says. It is probably still there today – as ice.

There is plenty of ice on the surface of Mars. Last year, another team reported that there was enough pure water in the ice that blankets the Martian south to submerge the entire planet in a layer 11 metres deep.

Water doesn't stay liquid for long in the low temperatures and thin air of Mars today. It either freezes or turns into vapour.

But there is plenty of evidence the planet was not always the frozen desert it is today. Previous missions have sent back images of dried-up riverbeds. Last year, scientists reported on deposits that were most likely produced by a network of springs.

This new paper suggests there may be reserves of frozen water beneath the surface of the planet, the remnants of ancient seas that could have been home to some form of life.