Paper: The Global Distribution of Water and Hydroxyl on the Moon as Seen by the Moon Mineralogy Mapper (M3)
Authors: Roger N. Clark, Neil C. Pearson, Thomas B. McCord, Deborah L. Domingue, Keith Eric Livo, Joseph W. Boardman, Daniel P. Moriarty, Amanda R. Hendrix, Georgiana Kramer, Maria E. Banks
Figure caption: (a) Continuum-removed FTIR reflectance spectra from USGS spectral library 6 (Clark et al. 2007) showing 3 and 6 μm absorptions. The 6 μm absorptions are due to a H–O–H bend, while the 3 μm absorption is due to the O–H stretch in the water molecule. The reflectance at 6 μm is very low, reducing the mean optical path length in the mineral compared to 3 μm, so the band depth is weaker. In transmission spectra with equal path lengths, the absorptions are similar in strength. Note that the short-wavelength side of the 3 μm absorption has a sharper rise than the long-wavelength side. This characteristic is seen in spectra of the Moon where water is present. (b) Example mineral spectra from the USGS spectral library (Clark et al. 2007) with similar 3 μm absorptions as seen in M3 spectra. The spectra are from the USGS spectral library 6 (Clark et al. 2007). Water is confirmed in each of these samples by spectroscopy in the 6 μm region, showing the H–O–H bend due to water. The lab spectra have been convolved to the sampling and bandpass of the M3 instrument. (c) Example M3 spectra compared to mineral spectra from the USGS spectral library (Clark et al. 2007) with similar 3 μm absorptions. We know that the 3 μm absorption is due to water because the ∼6.25 μm H–O–H bend due to water is present. Note that the anorthite HS201.3B spectral library image shows a small abundance of slightly brown grains, indicative of a small amount of weathering. That weathering may be responsible for the observed 2.8 μm OH absorption. The lab spectra have been convolved to the sampling and bandpass of the M3 instrument. (d) Spectra of the Moon from Cassini VIMS and M3 are compared to show areas with a 3 μm water absorption compared to water-free areas. From Clark (2009). The dashed red lines show the same spectra before thermal emission is removed using the Clark et al. (2011) algorithm. Note that the 2.8 μm feature is seen in spectra with and without thermal removal. Note that the water absorption starts near 2.7 μm. Thermal emission is a smoother additive increase without fine spectral features (dashed red lines) and starts at shorter wavelengths. Overcorrection of thermal emission will cause a decrease rolling off toward longer wavelengths, as noted by Clark et al. (2011). (e) M3 spectra from an I/F image cube before thermal removal. The Messier east spectrum shows a strong upturn from 2.5 to 3 μm, indicative of moderate thermal emission. The other spectra are of cooler areas (at the time the spectra were acquired) and show only weak thermal emission. Copernicus, NE Tycho, and the Plato spectra show clear evidence of a 2.8 μm absorber that is narrow (black arrows) and characteristic of the presence of hydroxyl. The Messier east spectrum also indicates a weak hydroxyl feature superimposed on the thermal emission (black arrow). A dotted line continuum is shown for each hydroxyl absorption for easier visualization.