Moon much wetter than thought

by Carolyn Gramling
Thursday, January 5, 2012

The moon isn’t quite the bone-dry place scientists once thought; instead, its surface is covered in water, according to a landmark finding announced by scientists at NASA today. Teams from three different NASA missions — the Moon Mineralogy Mapper (M3) mission that launched with the Indian Chandrayaan-1 satellite currently in orbit around the moon, the Deep Impact mission and the Cassini mission — pooled their data to confirm a finding that M3 principal investigator Carle Pieters said was “completely conclusive.”

“There is no question whatsoever,” Pieters said at the press conference to announce the finding. “Widespread water has been detected on the surface of the moon.”

The most common theory for the moon’s formation was that a Mars-sized object smashed into Earth about 4.5 billion years ago; the resulting particles coalesced into the moon, while nearly all of the volatile molecules that may have been present — including water — boiled away into space.

But about a decade ago, scientists found evidence that a few small pockets of water may still linger on the moon. In the late 1990s, NASA’s Lunar Prospector mission orbited the moon’s polar regions; the satellite included an instrument that measured neutrons emitted from the moon’s surface as it was bombarded by cosmic rays. As the instrument passed over polar craters that were in permanent shadow — never lit by the sun — the neutron flux dropped significantly, scientists noted. Because water absorbs neutrons, that dip in the neutron flux was one possible indicator of the presence of water in the craters.

But the new findings, which will be published tomorrow in three papers in Science, say that there is water present in soil across the entire surface of the moon — not just lurking in those permanently shadowed craters. Although still much drier than any of Earth’s deserts, the moon holds perhaps a liter of water per ton of lunar material, the scientists noted.

All three datasets from the different missions were key to the discovery, the scientists said — particularly because the finding itself was so difficult to believe that multiple independent datasets were needed to confirm it.

The M3 instrument observes and measures the wavelengths and brightness of light reflecting from the moon’s surface. Water and hydroxyl, or OH-, each have a unique fingerprint based on their reflectances at different wavelengths. When the M3 team studied the data they were receiving, they discovered “a highly diagnostic signature for detecting water on the surface of the moon,” Pieters said: At the surface, infrared light was being absorbed at wavelengths consistent with the presence of both water and hydroxyl.

At first, the team itself didn’t quite believe what they saw, she said. “We thought … it’s not possible, the moon doesn’t do this.”

But the data continued to accumulate, and data from other missions offered independent confirmation. The Cassini spacecraft, which flew by the moon in 1999 on its way to Saturn, had on board the Visual and Infrared Mapping Spectrometer (VIMS). VIMS went through a series of calibrations; the moon data were originally intended to calibrate the instrument, which was again recalibrated from 2004 to 2008. By then, the Cassini scientists were able to distinguish the signature of water on the moon in their data. They found evidence of water and hydroxyl across the moon’s surface from the equator to the poles — even in regions under direct sunlight, where it was not thought possible that water could exist, said Roger Clark, a Cassini team member and co-investigator for the M3 mission.

The VIMS and M3 instruments, although measuring different ranges of wavelengths at different temperatures, gave close to the same answer, he said. A third clue came from the Deep Impact mission, which was originally intended to study comets. But in 2007, the instrument made several observations of the moon — also originally for calibration purposes, said Jessica Sunshine, the deputy principal investigator for Deep Impact and a co-investigator on the M3 team. Deep Impact also “unequivocally” saw water and hydroxyl at both the poles and the equator, she said.

Because it took two images about a week apart, Deep Impact also glimpsed part of a daily water cycle on the moon’s surface. In that time, the moon had rotated 90 degrees relative to the sun — so the scientists were able to see how the spectral absorption of the same face of the moon changed as it rotated in and out of the sunlight, mimicking a change in the time of day. The strongest absorption was at dawn and dusk — indicating more water — while the weakest was at noon, Sunshine said. “We were seeing an entire cycle of loss and recovery of water,” she said.

What was behind that lunar water cycle is uncertain, Sunshine said, but one possible explanation would be the interaction of the solar wind with the moon’s surface. At dawn, the solar wind, bearing abundant hydrogen ions, could interact with oxygen in the lunar soil to produce water, which vaporizes in the heat of the day at noon. Then, as the surface cools down toward dusk, the water could again begin to accumulate. What that would mean, she added, is that regardless of the type of terrain or the latitude, the moon’s entire surface would be hydrated for at least part of the lunar day. Furthermore, that explanation could be extrapolated to other rocky bodies in the solar system, such as Mercury or asteroids, that have abundant oxygen and no atmosphere to protect them from the solar wind.

The source of the moon’s water, and whether it is ice or in some other physical form, also remain a mystery. “It could occur as one monolayer, it could be mixed into the soil, it could be altered minerals on the surface,” Pieters said. That, she added, will be one of the many topics the scientists plan to actively pursue in the next few years.

But one thing is certain, said Jim Green, director of the Science Mission Directorate’s Planetary Science Division at NASA headquarters in Washington, D.C.: These new findings amount to a “new dimension in the water story on the moon.”

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