By Will Dunham
WASHINGTON (Reuters) - Long, undulating features on the northern plains of Mars probably are remnants of shorelines of an ocean that covered a third of the planet's surface at least 2 billion years ago, scientists said on Wednesday.
The geological features, stretching thousands of miles (kilometers), were first revealed in the 1980s in Viking spacecraft images. But topographical data collected by NASA's Mars Global Surveyor in the 1990s cast doubt on whether the features truly marked a long-gone sea coast.
The Global Surveyor found big, mountain-sized variations in elevation along the suspected shorelines, whereas a shoreline should be a constant elevation matching sea level.
But scientists writing in the journal Nature said the movement of the Martian poles and also the planet's spin axis by roughly 2,000 miles in the past 2 billion to 3 billion years would have triggered deformation of surface features just like that seen in the suspected coastlines.
"The pole moves and it warps the shorelines," planetary scientist Taylor Perron of Harvard University, the study's lead author, said in a telephone interview.
"We have don't have direct confirmation that there were oceans because, of course, the water isn't there any more. But what we've done is to eliminate one of the main reasons to doubt that they were ever there."
Earth's poles also have moved in the past.
At some point, a big shift of mass on Mars caused its north pole to shift 50 degrees toward its present location and the planet's change in orientation changed the topography of the shorelines, said physicist Jerry Mitrovica of the University of Toronto, one of the researchers.
A MARTIAN PACIFIC Continued...
Mars


University of California, Berkeley scientists have published a report in ''Nature'' that says Mars was once covered by massive oceans.

In the 1980's the Viking spacecraft sent back images to NASA that had shown  thousands of kilometers of ancient shorelines, known as Arabia and Deuteronilus, on the north and south poles of Mars, but the Mars Global Surveyor got a closer look of the shorelines and photographed a 300 meter length of the two shorelines in the 1990's. Those images had shown that the alleged shorelines were too warped and rugged to have been created by water or an ocean.

But the new study now shows that due to a tilting in the axis of Mars by nearly 3,000 kilometers over a period of 2 or 3 billion years, the shorelines might actually have to be more rugged as the water settled, creating land formations that would rise and fall during this process.

"When the spin axis moves relative to the surface, the surface deforms, and that is recorded in the shoreline,"said Michael Manga, a UC Professor and a co-author of the study.

"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," said Taylor Perron the primary author of the study and who is now attending classes Harvard University's Department of Earth and Planetary Sciences to receive his postdoctoral degree.

Mars is said to have an elastic crust and the study shows why the shorelines vary so much in elevation with Arabia at 2.5 kilometers and Deuteronilus at 0.7 kilometers.

"This is a beautiful result that Taylor got. The mere fact that you can explain a good fraction of the information about the shorelines with such a simple model is just amazing. It's something I never would have guessed at the outset. This really confirms that there was an ocean on Mars," said Mark Richards, a professor at UC Berkley of earth and planetary science and study co-author.

The study says that as little as a 50 degree shift in the Martian axis, could cause a significant change in the elevation of the shorelines of Arabia, as much as 3,000 k.m.. As little as a 20 degree shift could do the same with Deuteronilus, but with a 700 k.m. change in the shoreline elevation.

It is estimated that the shorelines on Mars were created between 2 and 4 billion years ago.