Infrared Spectra of Deimos (1-13 μm) and Phobos (3-13 μm)

Abstract

The 3-13 mu m spectra of Deimos and Phobos were obtained on the nights of 2003 August 19, 20, and 21 UT, within a week of opposition ( August 28). Spectra in the 1.5-2.5 mu m region were taken a few weeks earlier on July 26. Observations were made near greatest elongation for each satellite in order to minimize scattered light from Mars. The L, M, and narrowband N ( 10.2 mu m) magnitudes for Deimos were 7.7 +/- 0.1, 4.9 +/- 0.1, and 0.79 +/- 0.09, respectively. The L, M, and narrowband N ( 10.2 mu m) magnitudes for Phobos were 6.4, 3.8, and -0.52, respectively, all +/-0.07. The 5-12 mu m color temperatures of Deimos and Phobos were 344 +/- 5 and 357 +/- 5 K, respectively, significantly higher than the blackbody thermal equilibrium temperature for either a fast rotator at Mars's heliocentric distance ( 237 K) or a slow rotator ( 282 K). The high brightness temperatures of Deimos and Phobos in the thermal infrared of 310-330 and 320-340 K, respectively, are not understood, but they are somewhat consistent with the high color temperatures. Within the errors, there was little evidence for spectral features indicative of surface composition, although there were some 1-2 sigma variations that warrant further observations.