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K. E. Simmons, A. R. Hendrix (LASP/ U. of CO)
Observations of the asteroids Gaspra and Ida were performed by the Galileo Ultraviolet Spectrometer (UVS) during the Galileo spacecraft encounters with these asteroids in 1991 and 1993, respectively. The Galileo UVS is very similar to its precursor, the Mariner 9 UVS, which orbited Mars in 1971-72 and observed the moons Phobos and Deimos, which are presumed to be captured asteroids. It is known from visible and infrared observations that Gaspra and Ida are both S-type asteroids, exhibiting spectral properties due to metal, olivine and pyroxene. In contrast, Phobos and Deimos appear to consist of carbonaceous chondritic material, typical of the dark C-type asteroids. In this study, we investigate how the visible-IR asteroid classifications extend into the ultraviolet. This may help us to identify trends in spectral features due to composition, space weathering, and/or thermal modifications. Our work expands on early studies of Mariner 9 UVS Phobos and Deimos spectra by investigating the entire data sets of the moons. In addition, we look for spatial variations in albedo and absorption features by employing the Galileo Graphics and Geometry Software (GGGS), an observation visualization tool. Using GGGS and a spacecraft orbit file constructed from mission geometry, we calculate the size of the body as it appeared in the UVS field-of-view and visualize the orientation and features at the time of the observation. By using this software, we interpret the Phobos and Deimos spectra in terms of photometric variations, as well as spatial variations due to geologic features. In the ultraviolet, Phobos and Deimos are spectrally very similar; their albedos are lower and flatter than the albedos of Ida and Gaspra, consistent with longer wavelength results. This work is supported by NASA's Galileo Project and the Jet Propulsion Laboratory.