Infrared Spectroscopy in Outer Space!

Last month NASA announced the presence of a thin layer of water and hydroxyl ions on the surface of the moon. Not one, not two, but three different satellites recorded data to support this conclusion. And guess what technology the three satellites used to detect the water? Infrared Spectroscopy of course! As many of you know from looking at the spectra of liquid water or water vapor, this molecule absorbs strongly around 3500 cm-1, or since astronomers like to use wavelength, around 3 microns. The first evidence that there was water on the moon came from the analysis of the moon rocks brought back by the Apollo missions back in the 1970s (it was watching these missions as a kid that inspired me to become a scientist). Water was found in the rocks, but it was assumed that they had been contaminated with terrestrial moisture.

In 1999 NASA’s Cassini satellite flew past the moon on its way to Saturn and detected water as well. However, at the time the data were thought to be erroneous and were not published until this year. More recently, the Moon Mineralogy Mapper aboard the Indian Space Agencies’ Chandrayaan-1 spacecraft found evidence for water on the moon. In June of this year NASA’s Epoxi spacecraft, flying past the moon on its way to a comet encounter in 2010, also detected water on the moon. For all three of these spacecraft the spectrum of the moon was measured in infrared reflectance. A dispersive grating spectrometer was used rather than an FTIR, perhaps because FTIRs have moving parts and are hard to fix billions of miles from home. The spectra are measured in single beam mode since there is not an identical twin moon available devoid of water to provide a background spectrum. However, the spectra from equatorial regions, where it is known to be drier, can be subtracted from the spectra of polar regions to hopefully remove any contribution from the moon’s surface other than OH and water. Examples of the types of spectra measured are seen above.

Now, it’s not time to pack your swim trunks and head to the moon for a dip. The water and hydroxyl are estimated to be present in concentrations of about 1000 ppm. You would have to wring dry a ton of lunar soil to obtain a quart of lunar water. The water is thought to form when energetic protons in the solar wind collide with oxygen bearing minerals on the moon’s surface. More details at these NASA websites.
(cut and paste these URLs into your browser...blogger won't let me post links!).

http://science.nasa.gov/headlines/y2009/24sep_moonwater.htm

http://www.nasa.gov/topics/moonmars/features/moonm3-images.html