Mapping Lunar Water from a Groundbased Observatory

Discovery of water on the lunar surface and in lunar samples within the last decade have caused a shift in our understanding of the evolution of the Moon and its volatile budget. Recently we showed that the lunar 3 micron water absorption band, detected using near infrared data from three spacecraft in 2009, can be observed using the groundbased NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawai'i. Our groundbased observations provide access to the entire lunar nearside at spatial resolutions of 1 to 2 km with a wavelength coverage from 1.7 to 4.2 microns. The spectral range of the IRTF provides new spectral constraints to test currently controversial thermal model corrections. Combined, the spectral and spatial parameters provided by the IRTF enables investigations of small geologic target mineralogy beyond the spectral coverage provided by the Chandryaan-1 M³spectrometer.

We have begun to operate the IRTF SpeX infrared spectrograph as an imaging spectrometer by scanning its slit across targets as its two-dimensional array is read out. We have produced maps of Copernicus, Bullialdus and Aristarchus craters, the silicic domes Mairan, Mairan T and Gruithiusen, and have observed moderate to polar latitudes to examine water abundance as a function of latitude. We find strong spectral anomalies in the geologic targets in the 3 to 4 micron region, a spectral range not available in existing spacecraft data at the high resolutions provided by the IRTF. These spectral anomalies may be due to variations in water-bearing mineralogy. Observations of these central peaks and volcanic features provide insights into magmatic, and therefore, interior lunar water.