Solar storms, like a big one that affected Earth last year, might have torn away the water that used to cover parts of Mars, NASA scientists said on Thursday.
Astronomers believe Mars once had oceans of surface water, enough to support long-ago life, but they have not determined where that water went some 3.5 billion years ago.
Now researchers monitoring the after-effects of a monster solar storm that hit Earth in October and November 2003 said they think repeated buffeting by this kind of space weather could have ripped away Mars' watery veil.
"These (solar) radiation events can affect the surface of Mars because Mars has so little protection," said Ed Stone of NASA's Jet Propulsion Laboratory in Pasadena, California.
Unlike Earth, which has a protective magnetosphere that guards the planet against bombardment by high-energy particles during a solar storm, Mars has only isolated zones of protection, astronomers said in a telephone-and-Internet briefing.
Observations by the Mars rovers Spirit and Opportunity have bolstered the idea of plentiful Martian water, according to Thomas Zurbuchen of the University of Michigan at Ann Arbor.
"Where did it go?" Zurbuchen asked rhetorically about Mars' water. "One of the key ideas that people are talking about is the connection to these space storms ... Over 3.5 billion years, there's kind of a gradual erosion of this water."
{{/usCountry}}"Where did it go?" Zurbuchen asked rhetorically about Mars' water. "One of the key ideas that people are talking about is the connection to these space storms ... Over 3.5 billion years, there's kind of a gradual erosion of this water."
{{/usCountry}}The astronomers referred to a video simulation of what might have happened on Mars.
The brief video, Movie 4 at the Web site, showed water seemingly blowing away from the planet.
Scientists worked with a small fleet of robotic spacecraft to watch the impact of last year's "Halloween" solar storm, the most powerful ever monitored.
Starting with the SOHO spacecraft which monitors the sun from its vantage point near Earth, the astronomers also tracked the solar blast wave with the Ulysses craft near Jupiter and the Cascraft that just began orbiting Saturn.
They followed the wave all the way out to the fringes of the solar system, where the two relatively ancient Voyager probes, launched in 1977, are aiming for interstellar space.
On Earth, the storm caused the rerouting of aircraft and the disruption of some long-distance radio communications and satellite operations. In space, astronauts aboard the International Space Station had to move periodically into the Russian-supplied Service Module, which offered better shielding from solar storms.
Online:http://www.gsfc.nasa.gov/topstore/2004/0708flare1.htm.