WASHINGTON (AP) _ Researchers say there is virtually no evidence of limestone formation on Mars, a finding that suggests the Red Planet never had oceans or seas. That conclusion, however, does not alter the possibility of life on Mars, experts say.
Philip Christensen of Arizona State University said that an instrument on NASA's Mars Global Surveyor that searched the entire planet for evidence of carbonate found only trace amounts of the limestone-like mineral.
The finding means it is unlikely that Mars ever had oceans of water as some scientists have suggested, he said.
``Maybe instead of calling them oceans, we should call them glaciers,'' said Christensen. ``A frozen ocean will not form carbonate. I believe Mars has a lot of water, but it is cold and frozen most of the time. That is consistent with what we have seen.''
Other Mars experts said the finding makes a significant contribution to the continuing debate among scientists about how much water there was on Mars, where did it go and how did the planet's intricate patterns of river beds, carved canyons and delta fans form without huge volumes of flowing water.
``This is dramatically important,'' Matt Golombek, a geologist with the Jet Propulsion Laboratory, the lead agency in NASA's program of Mars exploration, said of the new study.
He said there is clear evidence that water flowed on Mars in the past, but yet the thin atmosphere and frigid temperatures of the planet now make liquid surface water impossible. This suggests that Mars was once warmer and wetter and with a denser carbon dioxide atmosphere. The new finding by the Arizona State researchers shows that may not have been the case, said Golombek.
``If you had a warmer, wetter, thicker atmosphere, you would expect to find carbonate somewhere and so far we haven't found it,'' he said. ``This geochemical information is in direct contradiction to an early warmer, wetter Mars.''
In the study, Christensen and his co-authors, Joshua L. Bandfield and T. D. Glotch, used a Global Surveyor spacecraft instrument called the thermal emission spectrometer, or TES, that was designed to search for evidence of carbonate minerals on Mars.
Carbonate is formed in the presence of water and carbon dioxide. On Earth, the mineral is found in the immense deposits of limestone that are present on every continent, in soils and layers of stone formed beneath some lakes, seas and oceans.
Mars' atmosphere is mostly carbon dioxide, so it has long been believed that if the planet at one time had large bodies of water then there would have been large deposits of carbonate. But the TES found only trace amounts of the chemical.
Christensen said that even though there may not have been large bodies of liquid water on Mars, some life forms could still have evolved there.
``When people say there are no oceans or lakes, does that mean there was no life? Not at all,'' he said. ``There's the possibility that ice and snow on Mars melted from time to time, forming those gullies and then refreezing again.''
Areas where this happened on Mars, he said, ``are excellent potential abodes for life and certainly worth looking at.''
Golombek agreed, noting that around the edges of large deposits of ice there are small areas of liquid water that could host life.
``On Earth, there are growing communities of microbes that live at the edge of glaciers where you get flashes of water, even though the dominant feature is ice,'' he said.
Ross Irwin, a geologist with the Smithsonian Institution, said the new finding does not eliminate the possibility that conditions on Mars once allowed for large bodies of standing water on the Red Planet.
He said geological features on Mars, such as basins and river beds, were clearly carved by running water and that it is possible any carbonate formed was carried beneath the surface of the planet, beyond the detection range of the TES.
``Lots of basins have been resurfaced on Mars,'' said Irwin. ``Carbonate could be in the subsurface or buried beneath sediment. There could be extensive carbonate deposits that are difficult to locate.''