Issue Date: September 28, 2009
Moon's Surface Holds Water
Less than two weeks before a spacecraft is set to slam into the moon’s surface in search of water, a flurry of new reports from other spacecraft offer convincing evidence that the moon’s surface is lightly permeated with either water or its precursor, hydroxyl radicals. The possibility that water exists on the moon improves prospects that living things—including humans arriving on future space flights—might be able to survive there more easily than if the moon were dry.
Although scientists found no evidence of water in lunar rocks brought back to Earth by Apollo astronauts, in the past few decades, the idea that stores of water ice might be cached in permanently shadowed craters at the moon’s poles has gained popularity.
Now, using a variety of instruments, international teams have found key spectral evidence that H+2O or HO• covers the moon’s surface (Science, DOI: 10.1126/science.1178658, 10.1126/science.1179788, and 10.1126/science.1178105). “These instruments make it possible to map the lunar hydrogen content on the surface as never before,” said James Green, director of the Planetary Science Division at NASA headquarters, in Washington, D.C., at a press conference announcing the discovery.
Team scientists estimate the abundance of water at about 1,000 ppm, which is about a quart of water per ton of soil.
“Perhaps the most valuable result of these new observations is that they prompt a critical reexamination of the notion that the moon is dry,” writes astronomy professor Paul G. Lucey of the University of Hawaii in a perspective accompanying the papers. “It is not.”
The teams include a group led by Brown University planetary science professor Carle M. Pieters. She monitored visible and near-infrared wavelengths through NASA’s moon mineralogy mapper on Chandrayaan-1, India’s first mission to the moon.
Another group, led by astronomer Jessica M. Sunshine of the University of Maryland, College Park, confirmed the results from Chandrayaan-1 using spectrometers on board NASA’s Deep Impact spacecraft during that craft’s recent flybys of the moon.
And astronomer Roger N. Clark of the U.S. Geological Survey in Denver examined visible and IR data captured by the Saturn-exploring Cassini spacecraft during its lunar flyby in 1999 and also found spectral evidence of adsorbed H2O and HO•.
Coincidentally, on Oct. 9, NASA’s LCROSS spacecraft is slated to twice bombard the moon in search of water. The search focuses on the moon’s permanently dark crater, Cabeus A, located near the south pole because scientists believe that dark craters may contain relic frozen water from bombarding comets. First, the spacecraft will eject the spent second stage of its launch rocket, which will crash onto the surface, throwing up a large amount of debris. LCROSS and its sister spacecraft, NASA’s Lunar Reconnaissance Orbiter, will look for water in the ejected material. Then LCROSS itself will plunge to the surface, tossing up yet another plume.
The new reports, however, suggest that dark craters are not the only source of lunar water. Sunshine’s team proposes that the solar wind may provide an essential ingredient for surface water: energetic H+. In the team’s scenario, the H+ flux strikes the moon’s surface, releasing oxygen atoms bound to minerals in the soil, forming HO•, which can then easily form H2O. The group posits that as temperatures climb, more water molecules are released. Similarly, when temperatures decrease, water collects, creating a steady state.
Pieters cautioned in a statement that “when we say ‘water on the moon,’ we are not talking about lakes, oceans, or even puddles. Water on the moon means molecules of water and hydroxyl that interact with molecules of rock and dust in the top millimeters of the moon’s surface.”
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