High-resolution images from Nasa's Mars Global Surveyor spacecraft have concluded there is no evidence of shorelines that would have surrounded former oceans on Mars.

The result will trigger debate about the possibility of water having existed on Mars in the past. At the moment the atmospheric pressure is too low to allow liquid water to exist on the surface. But if Mars had a thicker atmosphere in the past , water could have survived.

New 30m resolution reveals no ancient coastal features

Images sent back by the 1970s Viking missions showed many surface features that may have been made by running water. But was water on Mars a long-term feature in the form of oceans or was it just temporary flash-flooding?

The images from Mars Global Surveyor, taken in 1998, have a resolution five to 10 times better than those that Viking provided. With this closer inspection, scientists now say that they cannot find any evidence of ancient shorelines.

Water for life

"The ocean hypothesis is very important because the existence of large bodies of liquid water in the Martian past would have had a tremendous impact on ancient Martian climate and implications for the search for evidence of past life on the planet," said Dr Kenneth Edgett of Malin Space Science Systems (MSSS) in California.

"The newer images do not show any coastal landforms in areas where previous researchers, working with the lower resolution Viking images, proposed there were shorelines."

"Even on Earth, looking for ancient shorelines from the air or space is a challenge," said Dr Michael Malin, principal investigator for the Mars camera at MSSS.

"Despite these difficulties, we believe these Mars Orbiter Camera images of the proposed shorelines are of a high-enough resolution that they would have shown features indicative of a coastal environment had there been an ancient ocean on Mars."

The vanishing cliffs

One area that might have been a coastline is located northwest of the great extinct volcano Olympus Mons. Looking at Viking images researchers have suggested that there might be a cliff separating the western margin of the Lycus Sulci uplands from the lower-elevation, smoother Amazonis plains.

The proposed cliff looked like the kind that forms on Earth from erosion as waves break against a shoreline.

Three high-resolution images were taken of this proposed coastline. The uplands are roughly textured, while the flat plains appear smoother. The image shows that the contact between the two regions is clearly not a wave-cut cliff, nor are there any features that can be unambiguously identified as coastal landforms, according to Dr Malin.

"While the suggestion that Mars at one time had oceans cannot be ruled out, the foundation for the 'ocean hypothesis' developed in the 1980s on the basis of suspected shorelines appears now to have been incorrect," Dr Malin concluded.

However, Dr Malin said it still seems highly likely that liquid water once stood on Mars, given all the evidence.