A Whiff of Water Found on the Moon

Target moon. Water molecules have been spotted on their way toward lunar polar craters like Cabeus A (here color-coded for elevation), the target of next month's hunt for water ice.

Yes, the moon is a "wetter" place than the Apollo astronauts ever could have imagined, but don't break out the beach gear just yet. Although three independent groups today announced the detection of water on the lunar surface, their find is at most a part per 1000 water in the outermost millimeter or two of still very dry lunar rock.

The discovery has potential, though. Future astronauts might conceivably wring enough water from not-completely-desiccated lunar "soil" to drink or even to fuel their rockets. Equally enticing, the water seems to be on its way to the poles, where it could be pumping up subsurface ice deposits that would be a real water bonanza.

A less dry moon makes its debut courtesy of the Moon Mineralogy Mapper (M3) that has been orbiting the moon onboard India's now-defunct Chandrayaan-1 spacecraft. A spectrometer, M3 detected an infrared absorption at a wavelength of 3.0 micrometers that only water or hydroxyl--a hydrogen and an oxygen bound together--could have created.

But spectroscopists had long distrusted any sign of water in lunar data because Apollo moon rocks were so bone-dry. So M3 team members asked the researchers operating the spectrometer on NASA's EPOXI spacecraft to take a look as it passed the moon last June on its way to comet Hartley 2. EPOXI observations confirmed the M3 detection, as did a reanalysis of Cassini spectrometer data taken in 1999 on its way to Saturn. The three analyses are reported in separate papers in tomorrow's issue of Science.

The discovery "opens up a whole different avenue to a source of water on the moon," says M3 principal investigator Carlé Pieters of Brown University. EPOXI observations show the water/hydroxyl signal coming and going from the surface over days, notes Pieters, which shows that there's water loosely bound to surface rock, not just tightly bound hydroxyl. The best estimate coming out of the reported observations for water's abundance is 0.2 to 1 part per 1000 of water, she says, and that's in the upper millimeter or two that spectroscopy can penetrate. At those levels, an astronaut would have to process the soil from a baseball-diamond-size plot to get a decent drink of water.

More tantalizing, the water becomes more abundant closer to the poles. That and water's abundance varying with time suggests to Pieters that water is being produced on the moon--perhaps through solar wind hydrogen interacting with surface rock--and then hopscotching from place to place through the moon's vanishingly thin atmosphere. Because a water molecule would stick more securely to colder rock, water would tend to migrate toward the colder polar regions. There, it might become trapped for eons as subsurface ice in permanently shadowed craters, which are currently thought to be among the coldest places in the solar system.

Come 9 October, NASA will crash its LCROSS mission into one such crater in hopes of blasting water into view. "It's very exciting," says planetary scientist Dana Hurley of the Applied Physics Laboratory in Laurel, Maryland. The water detection "is one piece of the puzzle; we still have a lot of pieces missing."

By Richard A. Kerr