Since humans first touched the moon and brought pieces of it back to Earth, scientists have thought that the lunar surface was bone-dry. But new observations from three different spacecraft have put this notion to rest with what has been called "unambiguous evidence" of water across the surface of the moon.
The new findings, detailed in Friday's issue of the journal Science, come in the wake of further evidence of lunar polar water ice by NASA's Lunar Reconnaissance Orbiter and just weeks before the planned lunar impact of NASA's LCROSS satellite, which will hit one of the permanently shadowed craters at the moon's south pole in hope of churning up evidence of water ice deposits in the debris field.
The moon remains drier than any desert on Earth, but water is said to exist on the moon in very small quantities. Finding water on the moon would be a boon to possible future lunar bases, acting as a potential source of drinking water and fuel.
Apollo turns up dry
When Apollo astronauts returned from the moon 40 years ago, they brought back several samples of lunar rocks.
The moon rocks were analyzed for signs of water bound to minerals present in the rocks. While trace amounts of water were detected, these were assumed to be contamination from Earth, because the containers in which the rocks were carried had leaked.
"The isotopes of oxygen that exist on the moon are the same as those that exist on Earth, so it was difficult if not impossible to tell the difference between water from the moon and water from Earth," said Larry Taylor of the University of Tennessee at Knoxville, who is a member of one of the NASA-built instrument teams for India's Chandrayaan 1 satellite and has studied the moon since the Apollo missions.
While scientists continued to suspect that water ice deposits could be found in the coldest spots of south pole craters that never saw sunlight, the consensus was that the rest of the moon was dry.
But new observations of the lunar surface made with Chandrayaan 1, the international Cassini spacecraft and NASA's Deep Impact probe are calling that consensus into question. Scientists say the probes made multiple detections of the spectral signal of either water or the hydroxyl group (an oxygen and hydrogen chemically bonded).
Three spacecraft
Chandrayaan 1, India's first-ever moon probe, was aimed at mapping the lunar surface and determining its mineral composition. In August, the orbiter's mission ended 14 months prematurely due to an abrupt malfunction. But while the probe was still active, its NASA-built Moon Mineralogy Mapper, also known as M3, detected wavelengths of light reflected off the surface that signaled the chemical bond between hydrogen and oxygen — the telltale sign of either water or hydroxyl.
Because M3 can only penetrate the top few millimeters of lunar regolith, the newly observed water seems to be at or near the lunar surface. M3's observations also showed that the water signal got stronger toward the polar regions.
Archived data from Cassini, which passed by the moon in 1999 on its way to Saturn, provided confirmation of this water/hydroxyl signal. The water would have to be absorbed or trapped in the glass and minerals at the lunar surface, Roger Clark of the U.S. Geological Survey wrote in a study detailing Cassini's findings.
The Cassini readings show a global distribution of the water signal, though it also appears stronger near the poles (and low in the lunar maria).
Finally, the Deep Impact spacecraft, as part of its extended EPOXI mission, made infrared detections of water and hydroxyl as well. Those observations were made at the request of the M3 team, as part of a calibration exercise during several close approaches of the Earth-moon system en route to its planned flyby of Comet 103P/Hartley 2 in November 2010.
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Month in Space: January 2014
Deep Impact detected the signal at all latitudes above 10 degrees N, though once again, the poles showed the strongest signals. With its multiple passes, Deep Impact was able to observe the same regions at different times of the lunar day. At noon, when the sun's rays were strongest, the water feature was lowest. The feature was strongest during the lunar morning.
"The Deep Impact observations of the moon not only unequivocally confirm the presence of [water/hydroxyl] on the lunar surface, but also reveal that the entire lunar surface is hydrated during at least some portion of the lunar day," the authors wrote in their study.
The findings of all three spacecraft "provide unambiguous evidence for the presence of hydroxyl or water," Paul Lacey of the University of Hawaii said in an essay accompanying the three studies. Lacey was not involved in any of the missions.
The new data "prompt a critical reexamination of the notion that the moon is dry. It is not," Lacey wrote.
Where the water comes from
Combined, the findings show that the moon's hydration process is a dynamic one, driven by the daily changes in solar radiation hitting any given spot on the surface.
The sun might also have something to do with how the water got there.
There are potentially two types of water on the moon: icy material brought from outside sources, such as water-bearing comets striking the surface, or water that has its source on the moon. This endogenic water may arise from the interaction of the solar wind with moon rocks and soils.
The rocks and regolith that make up the lunar surface are about 45 percent oxygen (combined with other elements as mostly silicate minerals). The solar wind — the constant stream of charged particles emitted by the sun — are mostly protons, or positively charged hydrogen ions.
If the hydrogen ions hit the lunar surface with enough force, they may break apart oxygen bonds in soil materials, said Taylor, the M3 team member. Where free oxygen and hydrogen exist, there is a high chance that trace amounts of water will form.
The various researchers also suggest that the daily dehydration and rehydration of the trace water across the moon's surface could lead to the migration of hydroxyl and hydrogen towards the poles, where it can accumulate in the cold traps of the permanently shadowed regions.