Tikalon Blog by Dev Gualtieri

New Moon

October 25, 2010

Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink.[1]

Aside from minor cinematic excursions,[2,3] the Moon has always been considered to be dry and lifeless. Well, the lifeless part still holds, but the dry description doesn't, at least in selected areas. About a year ago, NASA orchestrated a controlled 5,500 mile/hour impact of a space probe, called the Lunar Crater Observation and Sensing Satellite (LCROSS), and its associated Centaur rocket into a crater on the Moon. This crater, named Cabeus , is located in a southernmost region of the Moon that never gets direct sunlight, and the walls of the crater ensure that at least some portions of this lunar terrain haven't been warmed by the Sun for billions of years. The temperature in these regions is about 40 K . The impact of the Centaur rocket ejected a plume of lunar material nearly ten miles high, and instruments on LCROSS and the Lunar Reconnaissance Orbiter (LRO) detected water ice in the plume. The likely source of this water is comets that have impacted the Moon over its history, although some scientists think this water may have been part of the Moon originally. Most of the water vapor from these comets escaped into space, but some coalesced in cool regions, such as Cabeus.[4-7]

Water is not the end of the story, since a sextet of articles in the current issue of Science show evidence for other volatile materials that may be useful to future lunar explorers.[8-13] The impact revealed methane , ammonia , hydrogen , carbon dioxide and carbon monoxide , as well as relatively large amounts of sodium , mercury , and possibly silver . These volatiles could have formed through reaction of water with the lunar regolith in a process termed, "cold grain chemistry." Since chemical reaction rates are usually very slow at low temperatures, such reactions with water could have taken hundreds of thousands of years. A refinement of the water data indicates that Cabeus as a whole may contain a billion gallons of water. The plume itself contained about 41 gallons of water, and when the data is extrapolated to the crater as a whole, the density of water is about twice that of the Sahara desert.

How, exactly, do you remotely detect chemical elements this way? LCROSS had five cameras (one visible , two near infrared and two mid-infrared ), three spectrometers (one visible and two near infrared) and a photometer . The LRO has several other instruments, including the Lunar Exploration Neutron Detector for measuring hydrogen, the Lyman Alpha Mapping Project (LAMP) that detects elements via their ultraviolet spectra, and the Diviner Lunar Radiometer for measuring temperature. The Diviner registered a maximum impact temperature of 675 degrees Celsius . Plugging in heat capacities of lunar material and the other variables, this would have been sufficient to evaporate about 300 kilograms of water ice. Aside from molecular hydrogen and carbon monoxide, LAMP detected calcium , magnesium , and mercury . It's conjectured that mercury has been boiled off from other parts of the Moon and transported to cooler regions like Cabeus. Mercury exists in about the same concentration as water, so lunar explorers will need to bring some water filters.

Temperatures at the Cabeus crash site about 90 seconds after impact, as measured by the Lunar Reconnaissance Orbiter. The impact generated temperatures of about 675 degrees C, which appear as a tiny glowing dot near the center of the color swath. (NASA/UCLA)

The scant twenty-three pages of articles in Science belie the technical effort required to carry out these experiments. The impact needed to be precise, and the Moon, of course, is a moving target. Spacecraft typically use guide stars to fix their navigation through space. LCROSS needed to use the different approach of utilizing "guide craters." The plan originally called for an impact at a smaller crater called Cabeus A, but hydrogen concentration data from the LRO showed that Cabeus was a better candidate, and the course had to be corrected at a late stage.

The Cabeus impact reminds me of a science fiction short story I read many years ago (title and author unknown). Earth did an experiment to detect the composition of Mars by sending an atomic bomb to explode on its surface. Unfortunately, the bomb destroyed the only city on Mars and the last of the Martians . Hopefully, no Selenites were harmed during the collection of these data.

Still from Méliès' Le Voyage dans la lune. [2]

New Moon

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