NASA’s Deep Impact spacecraft conducted an extended mission in 2008 through 2010, designated EPOXI. The EPOXI mission included observations of Earth as an analog to a habitable terrestrial exoplanet in order to test retrieving properties from spatially unresolved spectrophotometry in the visible and spectroscopy in the near-infrared at 1-5 µm wavelength (Livengood et al. 2011). Observations in May 2008 additionally included measurements to characterize a nominally airless rocky body, Earth’s Moon, as part of a transit of Earth viewed from the spacecraft. A separate campaign of spatially resolved observations by Sunshine et al. (2009) as part of the EPOXI mission revealed the widespread presence of water or hydroxyl hydrating lunar surface minerals, increasing toward the terminator and implying diurnal variability. The May 2008 EPOXI spectra of the Moon support the conclusions drawn by Sunshine et al. (2009), with the additional feature that the hydration signature is clearly detectable in the spatially unresolved lunar spectrum seen in analog to first quarter, with the subspacecraft point near the dawn terminator (Figure). The present work revisits the global near-IR spectroscopy of the Moon to investigate the 3-µm hydration signature as well as other observed spectral features in the global spectrum.
Figure caption: The lunar hydration signature is detected in the Moon’s disc-integrated spectrum. (Left) The 2.8 µm hydration feature appears in the disc-integrated 1-5 µm spectrum observed over the dawn terminator, divided by a model for reflectance plus thermal emission. The displayed spectrum was extracted from measurements when the Moon was further from the transit than the displayed image, to minimize risk of spectral contamination from Earth scattered light. (Right) A map of lunar hydration feature band depth in June 2009, looking down on the north pole, showed greatest hydration at the terminator (inset from Sunshine et al. 2009).
References:Livengood, T. A., L. D. Deming, M. F. A'Hearn, D. Charbonneau, T. Hewagama, C. M. Lisse, L. A. McFadden, V. S. Meadows, T. D. Robinson, S. Seager, and D. D. Wellnitz (2011). Properties of an Earth-Like Planet Orbiting a Sun-Like Star: Earth Observed by the EPOXI Mission. Astrobiology 11, 907-930, doi: 10.1089/ast.2011.0614.
Sunshine, J. M., T. L. Farnham, L. M. Feaga, O. Groussin, F. Merlin, R. E. Milliken, and M. F. A'Hearn (2009). Temporal and Spatial Variability of Lunar Hydration As Observed by the Deep Impact Spacecraft. Science 326, 565-568, doi: 10.1126/science.1179788.