In the News this month: discovery of water on the Moon
A very young lunar crater on the side of the moon that faces away from Earth, as viewed by NASAs Moon Mineralogy Mapper on the Indian Space Research Organizations Chandrayaan-1 spacecraft CREDIT: ISRO/NASA
It is thought that the Moon was formed about four and a half billion years ago by the collision of a Mars-sized object with the Earth. The heat from the impact and subsequent accretion of material created a magma ocean which would have caused the loss of most of the volatile materials from the surface, so-called because they have low boiling points and evaporate easily. In a press conference at NASA on Thursday 24th September, results were announced from three separate spacecraft showing evidence of water on the lunar surface in far greater quantities than has previously been seen. Two of these spacecraft, Cassini and Deep Impact, observed the Moon as they passed by on their way to other parts of the solar system while the third, India Space Research Organisation's Chandrayaan-1, observed the lunar surface from orbit around the Moon.
What each of these probes detected was an absorption feature in the infra-red part of the spectrum at a wavelength of about 3 microns, a wavelength characteristic of absorption by hydroxyl - a hydrogen atom joined together with an oxygen atom. Add another hydrogen to hydroxyl and you produce H20, water, which also absorbs infra-red light near 3-microns. It has been known since the observations of the Lunar Prospector spacecraft in the late 1990s that there is an estimated 10 to 300 million metric tones of water ice buried in permanently shadowed craters at the lunar poles. These new results however, show that the hydroxyl and water signature is in fact present over large parts of the lunar surface, not just at the poles.
Launched on October 22nd 2008, India's Chandrayaan-1 carried several scientific instruments on board. One of these was the Moon Mineralogy Mapper, or M3, built by NASA, a spectrograph operating in the infra-red part of the spectrum. Although Chandrayaan-1 sadly ceased operations last month, it had already returned many months of usable data from the instruments on board. When the data from the M3 experiment was analysed, researchers found absorption features consistent with patterns expected for water and hydroxyl over most of the lunar surface. Although M3 only probed the top few millimetres of the lunar regolith, they found a strong hydroxyl signature across the surface, stronger towards the lunar poles at higher latitudes, and varying throughout the lunar cycle, suggesting that the Sun has some effect on the distribution.
According to the scientists, the most likely origin for this water is a reaction between protons in the solar wind and oxygen atoms already present in the lunar dirt. The M3 results were subsequently confirmed by observations by the Deep Impact spacecraft which also has instruments that observe in the infra-red and regularly uses observations of the Moon for calibration purposes, and also in old data from the Cassini spacecraft which observed the Moon during a flyby in 1999. The data show that there may be as much as 0.1 to 1 per cent water by weight in the regolith, in contrast to the rocks brought back by the Apollo missions which were incredibly dry. This is roughly equivalent to a litre of water per cubic metre of regolith but, since it is only present in the top few millimetres of soil, extracting usable amounts of water would require processing a very large surface area.
The results from the three spacecraft were announced together to coincide with the publication of three papers in the journal Science on September 24th, and come just two weeks before another spacecraft, NASA's LCROSS, the Lunar Crater Observation and Sensing Satellite, is due to crash into the Moon's surface near the south pole in an attempt to kick up water ice buried in the regolith in craters which rarely see sunlight.
Sunshine, J., Farnham, T., Feaga, L., Groussin, O., Merlin, F., Milliken, R., & A'Hearn, M. (2009). Temporal and Spatial Variability of Lunar Hydration as Observed by the Deep Impact Spacecraft Science DOI: 10.1126/science.1179788
Pieters, C., Goswami, J., Clark, R., Annadurai, M., Boardman, J., Buratti, B., Combe, J., Dyar, M., Green, R., Head, J., Hibbitts, C., Hicks, M., Isaacson, P., Klima, R., Kramer, G., Kumar, S., Livo, E., Lundeen, S., Malaret, E., McCord, T., Mustard, J., Nettles, J., Petro, N., Runyon, C., Staid, M., Sunshine, J., Taylor, L., Tompkins, S., & Varanasi, P. (2009). Character and Spatial Distribution of OH/H2O on the Surface of the Moon Seen by M3 on Chandrayaan-1 Science DOI: 10.1126/science.1178658
Clark, R. (2009). Detection of Adsorbed Water and Hydroxyl on the Moon Science DOI: 10.1126/science.1178105