The discovery of potentially vast deposits of water ice close to the surface on Mars could breath new life into the notion of sending humans to explore the red planet.
This week, scientists poring over data from NASA's Mars Odyssey orbiter released evidence indicating that large regions near the planet's poles collectively hold enough water to fill Lake Michigan twice.
That amount could be just "the tip of the iceberg," says James Bell III, a planetary geologist at Cornell University in Ithaca, N.Y. Other evidence suggests that much of the planet's surface is a porous layer of rocky rubble more than a half a mile thick. Such a layer, particularly in the polar regions, could trap still more water ice.
The strongest indications of water come from the southern hemisphere, where it's estimated that the icy layer starts at a depth of from one to three feet below the surface.
"This is more like dirty ice rather than dirt containing ice," says William Boynton, a geochemist at the University of Arizona's Lunar and Planetary Laboratory in Tucson, Ariz., and the lead investigator for the instrument suite designed to analyze the chemical makeup of the Martian surface. He estimates that the water content may be as high as 60 percent by volume, meaning that a bucketful of material from this layer would yield more than half a bucket of water when melted.
"If this water is readily accessible, this would change everything about the way we would design the first human missions to Mars and colonize Mars," says James Longuski, professor of aeronautics and astronautics at Purdue University.
Simple solar-powered techniques, he continues, can break water down into hydrogen and oxygen. Hydrogen can be used for rocket propellant, as can methane, which could be produced by combining the hydrogen with carbon from Mars' atmosphere. The water also could be used for drinking, while its oxygen could be extracted for breathing.
As a result, he explains, large quantities of accessible water on Mars could drastically cut the cost and time required to mount a mission to the red planet by reducing the amount of material, including hydrogen, that would have to be launched from Earth. "Now scientists are telling us that the hydrogen is already there waiting for us!" Mr. Longuski says.
For its part, NASA is focusing on the scientific payoff from Mars Odyssey's first few months of mapping the chemical composition of the planet's surface. The results, scheduled for publication on Friday in the journal Science, are of keen interest to researchers trying to piece together the planet's geophysical and climatic history.
"The big questions we are trying to answer are: Where did all that water go? And what are the implications for life?" says Jim Garvin, Mars program scientist at NASA headquarters in Washington.
One of the big questions emerging from the Mars Odyssey observations, Dr. Bell notes, is: How much water has the planet retained?
"There are two extremes. The ice could be a relatively thin deposit that may have condensed out of the atmosphere, slowly accumulated," and migrated below the surface, he says. Or ice deposits "could be enormous, coming from a distant past" when the planet is thought to have had large amounts of water whose effects are seen today scoured into the Martian surface. They would be stored in a relatively loose layer of rubble reaching down more than half a mile, the result of constant pounding from asteroids or comets early in the solar system's history.
"The big unknown is the thickness of the deposits; that's the next piece of the puzzle we have to solve," Bell says.
The evidence for the water ice is indirect, but compelling, researchers say.
The instruments aboard Mars Odyssey are designed to read emissions of neutrons and gamma rays from the planet's surface for the spectral "fingerprints" of key elements. The off-the-charts readings for hydrogen fall neatly into regions of the planet that past studies and computer modeling suggest should be able to hold ice in a stable form.
"Coming in with such a huge home run is really gratifying," Dr. Bennett says, recalling that it's taken him 18 years and one failed orbiter, Mars Observer, to reach this point.
Launched on April 7, 2001, the Mars Odyssey orbiter reached the red planet last October. It began its mapping mission in February and will complete its survey work in August 2004. The orbiter also is slated to act as a radio-relay station for Mars missions planned for next year and 2004.