In the news this month: latest results from LCROSS
Artists impression of the impact of the LCROSS spacecraft on the Moon back on October 9th CREDIT: NASA
Small lumps of rock hit the Moon quite regularly, but in 2009 two artificial projectiles impacted on the lunar surface in an experiment designed to search for water in the permanent shadows of a crater near the lunar south pole. Hints of subsurface water on the Moon had already been found in 1999 when NASA's Lunar Prospector spacecraft detected signatures of concentrated hydrogen, the "H" in "H2O", near the lunar poles. The Lunar CRater Observation and Sensing Satellite, or LCROSS, was a low-cost mission launched together with the Lunar Reconnaissance Orbiter in June 2009. The mission consisted of the Centaur upper stage of the Atlas-V launch vehicle, and a shepherding spacecraft equipped with various cameras and sensors. Moving at a speed of 1.5 miles per second, the Centaur stage impacted the lunar surface on October 9th 2009, sending up a plume of material from the permanently shadowed floor of the crater Cabeus. The LCROSS spacecraft observed the impact before flying through the plume to impact the surface some four minutes later. In the October 22nd issue of Science magazine, several teams working on data from the impact publish their findings.
Cabeus crater was chosen for the experiment as it contains an area which is permanently in shadow, due to its location close to the lunar south pole. The low temperatures, combined with the movement of soil (regolith) by micrometeorite impacts (known as "impact gardening") which buries accumulated material, makes such craters ideal places to search for volatiles - chemicals which are solid only at very low temperatures.
Previous results from a neutron spectrometer aboard the Lunar Prospector spacecraft suggested that ice could make up between half and one percent of the soil near the lunar poles, and further results from a neutron detector on the Lunar Reconnaissance Orbiter showed a strong hydrogen signal, originally thought to be from water ice. But observations of the LCROSS plume, made with another instrument (LAMP, an ultraviolet spectrograph) on-board LRO, showed that as much of the hydrogen signal comes from molecular hydrogen as it does from water. Water is thought to have accumulated from cometary impacts, distributing water across the lunar surface in the ejecta, but it is far from certain where the molecular hydrogen originated.
The results from the nine instruments on-board the LCROSS shepherding spacecraft, reported in Science on October 22nd, show signatures of numerous different chemicals, including water vapour, water ice and hydroxyl radicals, a common result of the breaking up of water molecules. Using the spectra obtained, the LCROSS team calculated that the maximum amount of water vapour and ice visible in the field of view of the instruments was 155 kilograms. By estimating the amount of material that was excavated by the Centaur impact and became observable by reaching sunlight, they calculated that the concentration of water in the lunar regolith at the impact site was 5.6 percent. They also found that the observed abundances of other volatile compounds, such as ammonia, sulphur dioxide and carbon monoxide, were far higher than the abundances found in comets, suggesting that molecule formation may be going on in these shadowed regions on the surfaces of cold dust grains.
This blog post is a news story from the Jodcast, aired in the November 2010 edition.
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