By Maggie Fox, Health and Science Correspondent
WASHINGTON (Reuters) - NASA scientists have found visual evidence of water on the surface of Mars, a finding that, if verified, would have strong implications for the search for life beyond Earth, an Internet-based space news service said on Wednesday.
The evidence comes from photographs taken by the Mars Global Surveyor, which was launched in November 1996 and which has been orbiting the planet since April of last year.
NASA declined to confirm the report, published on http://www.space.com/, that evidence of water in a liquid form had been found. But it scheduled a news conference for 11 a.m. on Thursday.
``Because some press reports over the past two days on the subject have been incorrect in significant areas, NASA has arranged to hold the press conference earlier than originally planned so that the principal investigators, Drs. Michael Malin and Ken Edgett, can discuss their findings in person,'' NASA said in a statement.
The findings were scheduled to be published in the journal Science later this month, but space.com said it learned of the report from sources at the U.S. space agency.
Editors at Science said there was a paper on Mars in an upcoming issue, but declined to comment on it, and said the actual paper would be released at 10 a.m. (1400 GMT) on Thursday.
Any findings would have to be confirmed by additional study. Reports about evidence of water on Mars have been subjected to rigorous scientific debate.
But if confirmed, the finding would give a huge boost to space exploration and to theories that life exists, or has existed in the past, in places other than Earth.
It would also be a much-needed turnaround for NASA, which lost its Mars Polar Lander mission in September -- a mission aimed at finding physical evidence of water on the surface -- and its Mars Climate Orbiter in September.
NASA is putting together plans for a new mission in 2003 and another one in 2005, both with the primary aim of finding water.
They would build on the enormous success of the 1997 Pathfinder mission, in which a small robot roamed and photographed the surface, sending back up-close views of the red rocks and sand that give Mars its rusty hue.
Scientists know that water exists on Mars, but it is frozen in the polar icecaps, locked away as ice beneath the surface or drifting about the atmosphere in thin clouds.
Published reports suggested that what had now been found were changes on the surface that could have been made by seeping water, although the surface is regularly scoured by powerful winds and dust storms.
Water flowing on the surface would be accessible to astronauts on any future Mars mission, and would make it much more likely that life may have existed on the planet.
The idea that water might flow on Mars first arose in 1877 when Italian astronomer Giovanni Schiaparelli saw what he called ``canali'' on the surface of the planet. The word means ''channels,'' but was mistranslated as ``canals'' and people believed for years that some civilization must have built a network of canals that crisscrossed the planet's surface.
In 1965, the Mariner 4 space probe found no evidence of such canals. Since then, missions to Mars have, however, found what look like dried-up riverbeds and giant lakes or oceans.
In December, scientists at Brown University reported they had found evidence of a former wide ocean with beaches in the northern lowlands of Mars.
Water ice has been detected in the icecap at the north pole of the planet and possibly at the south pole. And there is evidence that water once percolated through rocks believed to be from Mars that have fallen to Earth as meteorites.
But most of the surface of today's Mars is far too harsh for liquid water to exist. With average surface temperatures of -63 degrees F (-53 degrees Celsius), any water would be frozen, and the thin, dry atmosphere provides little protection.
The sun warms the surface of Mars to as high as 80 degrees F (27 degrees Celsius) at the equator at midday and although there is no evidence of current geothermal activity on Mars, in theory water from such a source could bubble to the surface in liquid form.
The best place to look would be inside craters -- and scientists are peering into craters on Earth's moon and even on Mercury to see if water could be preserved on the shady, protected walls of deep craters.
In 1997, the Mars Global Surveyor took pictures of what some scientists said could be the remains of springs or seeping water inside a crater in the planet's Southern Hemisphere. But they said the formations could also have come from volcanic activity.
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