Scientists studying Mars say they have new evidence to back a theory that suggests an enormous ocean once covered much of the Red Planet's surface.

Few debate that Mars once held more water than can be observed today, but the theory the planet was once home to a vast ocean has been hotly debated since images taken from the Viking spacecraft in the 1980s revealed two possible ancient shorelines near the planet's north pole.

Subsequent observations from the Mars Global Surveyor in the 1990s appeared to be a blow for the ocean theory. It recorded the elevation of features on both shorelines and found they rose and fell with altitudes varying by several thousand metres. In contrast,Earth's shorelinesare typically constant when measured relative to sea level.

Variation in shoreline elevations could be explained by the movement of the planet's poles in response to the redistribution of mass on the planet,Harvard University scientist Taylor Perron said in a statement released Wednesday.

He and co-authors Michael Manga and Mark Richards from University of California, Berkeley, will publish their findings in this week's issue of Nature.

Planets tend to spin with a bulge at their equator, like a spinning top, so that the Earth and Mars are not actually spheres but have a shape called an oblate spheroid.

When a major shift of mass occurs ona planet, the axis it spins on will tend to move so that the areas with the larger mass are at the equator.

On Earth, for example, scientists have suggested that Ecuador's Mount Chimborazo rests on such a bulge and is actually closer to the moon than Mount Everest, though it has a lower listed elevation than Mount Everest because we measure elevation on this planet relative to sea levels —which would also be affected by the bulge.

Likewise, the most massive bulge on Mars is the Tharsis rise, a range that formed four billion years ago just north of the equator that contains the planet's largest volcano, the Olympus Mons.

Perron suggest two subsequent changes in mass distribution following the formation of Tharsis could have moved the poles and would have had a profound impact on the surface of Mars —enough to have disrupted the landscape of normally flat ocean shorelines, said Perron in a statement.

"On planets like Mars and Earth that have an outer shell, or lithosphere, that behaves elastically, the solid surface will deform differently than the sea surface, creating a non-uniform change in the topography," he said.

According to the authors, the older Arabia shoreline on Mars varied in elevation by up to 2.5 kilometres, while the younger Deuteronilus shoreline varies by about 0.7 km.

Perron and his colleagues suggest a shift of 50 degrees from today's pole would account for the disruptions noted on the Arabia shoreline, while a subsequent shift of 20 degrees would account for the variation of Deuteronilus's shoreline.

Today's north pole and the two suggested ancient poles are all equidistant from the Tharsis rise, the authors said, suggesting the changes in mass distribution were smaller than the Tharsis rise, since the planet would orient to keep it near the equator.

"This alignment is unlikely to occur by coincidence," the team wrote.