A.A. Fraeman^a, S.L. Murchie^b, R.E. Arvidson^a, R.N. Clark^c, R.V. Morris^d, A.S. Rivkin^b and F. Vilas^e

^aWashington University in St. Louis, 1 Brookings Dr, Campus Box 1169, St. Louis, MO, 63130, United States
^bThe Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Road, Laurel, MD, 20723, United States
^cUS Geological Survey, Box25046 Denver Federal Center, Denver, CO 80225, United States
^dARES, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, United States
^ePlanetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ, 85719, United States

Absorption features on Phobos and Deimos in the visible / near infrared wavelength region (0.4 – 3.9 μm) are mapped using observations from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Fe²⁺ electronic absorptions diagnostic of olivine and pyroxene are not detected. A broad absorption centered around 0.65 μm within the red spectral units of both moons is detected, and this feature is also evident in telescopic, Pathfinder, and Phobos-2 observations of Phobos. A 2.8 μm metal-OH combination absorption on both moons is also detected in the CRISM data, and this absorption is shallower in the Phobos blue unit than in the Phobos red unit and Deimos. The strength, position, and shape of both of the 0.65 μm and 2.8 μm absorptions are similar to features seen on red-sloped, low-albedo primitive asteroids. Two end-member hypotheses are presented to explain the spectral features on Phobos and Deimos. The first invokes the presence of highly desiccated Fe-phyllosilicate minerals indigenous to the bodies, and the second invokes Rayleigh scattering and absorption of small iron particles formed by exogenic space weathering processing, coupled with implantation of H from solar wind. Both end-member hypotheses may play a role, and in-situ exploration will be needed to ultimately determine the underlying causes for the pair of spectral features observed on Phobos and Deimos.

Reference
Fraeman AA, Murchie SL, Arvidson RE, Clark RN, Morris RV, Rivkin AS and Vilas F (in press) Spectral Absorptions on Phobos and Deimos in the Visible/Near Infrared Wavelengths and Their Compositional Constraints. Icarus
[doi:10.1016/j.icarus.2013.11.021]
Copyright Elsevier

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