1Shuai Li, 1Ralph E. Milliken
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005035]
1Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA
Published by arrangement with John Wiley & Sons
Radiance measured by the Moon Mineralogy Mapper (M3) at wavelengths beyond ~2 µm commonly includes both solar reflected and thermally emitted contributions from the lunar surface. Insufficient correction (removal) of the thermal contribution can modify and even mask absorptions at these wavelengths in derived surface reflectance spectra, an effect that precludes accurate identification and analysis of OH and/or H2O absorptions. This study characterized thermal effects in M3 data by evaluating surface temperatures measured independently by the Lunar Reconnaissance Orbiter Diviner radiometer, and results confirm that M3 data (Level 2) currently available in the Planetary Data System (PDS) often contain significant thermal contributions. It is impractical to use independent Diviner measurements to correct all M3 images for the Moon because not every M3 pixel has a corresponding Diviner measurement acquired at the same local time of lunar day. Therefore, a new empirical model, constrained by Diviner data, has been developed based on the correlation of reflectance at 1.55 µm and at 2.54 µm observed in laboratory reflectance spectra of Apollo and Luna soil and glass-rich samples. Reflectance values at these wavelengths follow a clear power law, inline image, for a wide range of lunar sample compositions and maturity. A nearly identical power law is observed in M3 reflectance data that have been independently corrected using Diviner-based temperatures, confirming this is a general reflectance property of materials that typify the lunar surface. These results demonstrate that reflectance at a thermally-affected wavelength (2.54 µm) can be predicted within 2% (absolute) based on reflectance values at shorter wavelengths where thermal contributions are negligible and reflectance is dominant. Radiance at 2.54 µm that is in excess of the expected amount is assumed to be due to thermal emission and is removed during conversion of at-sensor radiance to reflectance or I/F. Removal of this thermal contribution using this empirically-based model provides a more accurate view of surface reflectance properties at wavelengths >2 µm, with the benefit that it does not require independent measurements or modeling of surface temperatures at the same local time as M3 data were acquired. It is demonstrated that this model is appropriate for common lunar surface compositions (e.g., mare and highlands soils, pyroclastic deposits), but surface compositions with reflectance properties that deviate strongly from these cases (e.g., pyroxene-, olivine-, or spinel-rich locations with minimal space weathering) may require the use of more sophisticated thermal correction models or overlapping Diviner temperature estimates.
1Shuai Li, 1Ralph E. Milliken