For decades, the moon was thought to be bone dry; drier than Earth's own desolate Atacama Desert.
Not even the most sanguine lunar water advocates expected to find significant quantities of the liquid on or beneath the lunar surface. As a result, this dearth of water has always been a nagging obstacle for permanent lunar habitation, as Miller-McCune noted last month in reviewing progress toward space colonization.
So, the recent detection of surface water by the Indian space agency's Chandrayaan-1 spacecraft completely rattled the lunar science community. Chandrayaan-1 detected evidence for water about a millimeter deep over much of the lunar surface.
Now, NASA is set to follow up on this hunt for water in the form of ice, by crashing two spacecraft into Cabeus, a permanently shaded crater near the lunar South Pole. The lunar South Pole has long been mentioned as a possible site for a permanent lunar base.
NASA's LCROSS, the Lunar Crater Observation and Sensing Satellite mission, will have an auspicious finale, ending with the orchestrated crash of both its Centaur upper rocket stage and the LCROSS shepherding spacecraft around 4:30 a.m. PDT Friday.
The hope is that LCROSS will fly through a plume of debris created by the crash of the upper stage, corroborating the presence of water ice inside the crater. Then, four minutes later, the shepherding spacecraft will crash into the same polar crater.
Both sets of crash debris and ejecta may be observed by four large telescopes on Mauna Kea in Hawaii. If successful, these ground-based observations would confirm the spectral signature of water.
"The big ground based telescopes may see a flash and a spray of ejecta," said Paul Lowman, a geophysicist at NASA Goddard Space Flight Center in Maryland. "But to do compositional mapping under those conditions is basically impossible. I'm very pessimistic about this whole thing. I don't think they will detect any water."
Lowman - whose 1959 proposal to look for water-bearing rocks on the moon helped him get his first job at NASA - calls the Chandrayaan-1 water discovery "a fine piece of work," but he cautions against exaggerating its importance in long-term habitation.
"What Chandrayaan saw was a form of space weathering," said Lowman. "The water was formed by chemical reaction of solar wind protons with [oxygen-rich] silicates in the [lunar] regolith," or dusty surface soil.
It's this hydrogen proton reaction with oxygen in the lunar silicates that creates such a thin and heretofore elusive veneer of water across the lunar surface.
Lowman said that as the solar wind's high-velocity hydrogen protons hit the lunar surface, they chemically react with the lunar regolith to produce water. Whether this water evaporates soon after formation, however, remains an open question.
Mining the regolith for this water would be a big help for long-term settlements, he noted, adding that such water would best be used as life support rather than rocket fuel.
Currently, NASA estimates that it may take a ton of raw lunar material to make up a liter of this bottled water. Lowman said the Apollo astronauts didn't detect water simply because there was so little of it.
"All the rocks and soil we brought back were bone dry," said Lowman. "So this film of water is just like the rust on a car bumper, an outer coating."
But if there is a solid signal for water inside Cabeus, Wendell Mendell, a planetary scientist at NASA's Johnson Space Center in Houston, said that lunar colonists will have a more readily available source of water and will not have to mine for water in craters that are kilometers deep and virtually inaccessible.
Lane Patterson, a University of Arizona biosystems engineering graduate student and current advocate of a lunar greenhouse complex at the lunar South Pole, said that if there really is a significant amount of water in such cold ancient craters, then it would certainly be put to good use in sustaining long-term life support.
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