Data from a comet-bound NASA probe, a robotic mission to Saturn, and a U.S. instrument aboard an Indian spacecraft have provided clear evidence that at least trace amounts of water exist on the moon's surface, researchers said Thursday.
While scientists have long suspected that water ice from comet impacts is trapped in cold, permanently shadowed craters near the moon's poles, the new data indicates that water molecules form and dissipate across broader areas, even in lunar daylight.
While the data represent a major surprise and a "really profound discovery," one scientist said, researchers cautioned that the moon remains an extremely dry place, by human standards.
"The observations presented here show a combination of hydroxyl, OH (oxygen hydrogen molecules), and H2O (water) that resides in the upper few millimeters of the lunar surface," said Jim Green, director of NASA's Planetary Science Division. "The average amount of water reported, if we were to extract it, is about a quart of water per ton (of surface soil)."
To put it another way, he said, about 16 ounces of water might be present for every 1,000 pounds of surface soil near the moon's poles. For soil near the equator, only about two tablespoons of water is believed to be present in every 1,000 pounds.
"Even the driest deserts on the Earth have more water than are at the poles and the surfaces of the moon," Green said.
But scientists agreed that the results open a new chapter in humanity's understanding of the moon and the processes at work across the entire solar system that could lead to water formation on other airless asteroids and moons.
"Having any water or hydroxyl in the sunlit areas of the moon is as surprising as it is intriguing," Bruce Betts, director of projects for the Planetary Society, said in a statement. "Will such results turn out to be the tip of the iceberg, or will the moon remain a dry desert with slightly more moisture than we thought?"
On a related front, NASA unveiled new findings from the Mars Reconnaissance Orbiter on Thursday that show clear evidence of huge subsurface ice sheets extending from the poles of the Red Planet halfway to its equator.
The buried ice was spotted in debris thrown up in five recent northern hemisphere impact craters. The ice is surprisingly pure and easy to see in high-resolution pictures from the Mars Reconnaissance Orbiter.
Scientists believe that the ice is a remnant of a more humid period in Mars' recent history, when the planet's polar ice caps extended much farther toward the equator.
"Every indication is that this is forming a broad, continuous sheet beneath the surface," said Ken Edgett, a camera team member with Malin Space Science Systems of San Diego. "We have five separate impact sites, all showing more or less the same thing.
"I'd say the volume of water--and this is a guess--the volume of water is probably comparable to the volume we would have in, say, the Greenland ice sheet on the Earth, in the buried ice deposits (and the North Pole ice cap)."
Shane Byrne, a member of the High Resolution Imaging Science Experiment team at the University of Arizona, said the ice sheet is probably about a yard or so thick.
"These buried ice sheets that extend from the poles all the way down to 45 degrees or so (north and south latitude) don't quite cover half of the planet, but (they) come close," he said. "So we're talking about maybe a (half) million cubic kilometers of ice in total."
Water ice is a critical resource for future space travelers, as well as a requirement for the development of life as it is currently known. The presence of ice on Mars is not a surprise, although the purity and extent of the buried ice sheets is. Water on the moon, however, is another matter.
Three spacecraft--India's Chandrayaan-1 lunar orbiter, NASA's Saturn-bound Cassini probe, and the agency's Deep Impact comet mission--all detected evidence of water molecules on the moon's surface. In a surprise, it appears that water molecules are present, even in the heat of direct sunlight.
"Finding water on the moon in daylight is a huge surprise, even if it is only a small amount of water and only in the form of molecules stuck to soil," Jessica Sunshine, an astronomer at the University of Maryland who helped analyze data from NASA's Deep Impact spacecraft, said in a statement. "In the Deep Impact data, we're essentially watching water molecules form and then dissipate right in front of our eyes."
What causes the water to form is not yet clear, but Sunshine said the mechanism might involve electrically charged hydrogen ions in the solar wind interacting with oxygen-rich minerals in the lunar soil to form water and hydroxyl molecules.
"We aren't certain yet how this happens," she said, "but our findings suggest a solar-driven cycle in which layers of water only a few molecules thick form, dissipate, and reform on the surface each lunar day.
"This water is formed in the morning, substantially lost by lunar midday, and reformed as the lunar surface cools towards evening."
Finding water on the moon has long been one of the holy grails of modern lunar exploration because solar power and ice deposits, assuming they are close enough to the surface, could provide a source of water, air, and rocket fuel for future moon explorers or colonists.
The discoveries announced this week don't necessarily mean that abundant water supplies are available across the moon's surface--the solar-driven cycle implied by Deep Impact would produce only trace amounts--but they show that the moon isn't the totally dry place scientists long thought it was.
Earlier data indicated possible ice deposits in permanently shadowed craters near the moon's poles, where water from comet impacts could have been trapped over the moon's long history.
Orbiting the moon at an altitude of just 31 miles, the LRO spacecraft is designed to map the lunar surface in unprecedented detail to help identify possible landing sites for future manned missions. Lcross is focused specifically on water.
If all goes well, the spent second stage of the rocket that boosted LRO and Lcross to the moon will crash into a permanently shadowed crater on October 9, blasting presumably ice-bearing soil into sunlight for direct analysis by LRO, the Hubble Space Telescope and ground-based observatories. Lcross will fly through the plume, beaming back data before it, too, crashes to the surface.