MARS PATHFINDER IMAGING RESULTS AT PHOBOS AND DEIMOS: CONSISTENCY WITH PREVIOUS DATA. S. Murchie1, N. Thomas2, D. Britt3, K. Herkenhoff4, and J.F. Bell, III5, 1Applied Physics Laboratory, The Johns Hopkins University, Laurel, MD, 2Max Planck Institute for Aeronomy, Kathenburg-Lindau, Germany, 3Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, 4Jet Propulsion Laboratory, Pasadena, CA, 5Cornell University, Ithaca, NY. The Imager for Mars Pathfinder (IMP) assess the presence of an Fe mineral absorption acquired reflectance spectra of both Phobos and feature near 1 mm; and to determine the Deimos, simultaneously with spectra of two relationhip, if any, between albedo and spectral standard stars. The IMP spectrum of Deimos is properties of the two moons. highly consistent with previous ground-based Procedures. Each IMP satellite observation and HST measurements. From the results of the consisted of three multispectral image sets, multispectral imager on Phobos 2, the covered averaged to improve uniformity in CCD region of Phobos is expected to be a mixture of response. The sub-Mars leading hemispheres of spectrally bluer and redder regions. The both satellites were measured, during Martian measured IMP spectrum of Phobos is early morning. The data were calibrated against intermediate to composite Phobos 2 spectra of standard stars in nearby parts of the sky, to the two color units, and it is consistent with minimize affects of atmospheric opacity, and Mariner 9 UVS and HST spectra of an were converted to geometric albedo using a overlapping region. Both Martian moons are photometric model. For comparison with IMP redder than most asteroids and very different data, previous observations were selected for spectrally than C-types. There is no definitive coverage of the same geographic regions. evidence on either moon for a strong absorption Where the data were in units of relative due to Fe minerals. Spectrally, the moons most reflectance, they were converted to estimated closely resemble two very dissimilar analog albedo using Viking clear-filter albedos of the materials, putatively clay- and organic-rich D- observed regions [7,8]. like asteroids and highly space-weathered, Results. Phobos 2 multispectral imaging mafic-rich assemblages such as lunar mare soils. results suggest that the region of Phobos Further measurements at infrared wavelengths measured by IMP is predominantly the may distinguish between these possibilities. spectrally bluer unit, with some fraction of the Background. The C asteroid-like spectra of redder unit. The IMP spectrum is intermediate to Phobos and Deimos assembled from different composite, multi-instrument Phobos 2 spectra of Mariner 9 and Viking Lander observations have the two units, as expected (Fig. 1). It is also been considered important evidence for the consistent with Mariner 9 UVS data and HST composition and origin of the two Martian data, which cover larger regions that overlap moons [1,2]. More recent observations from spatially the area covered by IMP (Fig. 2). For ground-based spectroscopy [3], HST [4], and the leading hemisphere of Deimos, there is Phobos 2 [5,6] have yielded a very different excellent consistency between spatially picture of the two moons. Both are redder overlapping data sets (Fig. 3). These results spectrally than most asteroids, and Phobos validate the recent view of the Martian satellites exhibits two spectrally dissimilar units with as dissimilar to C-type asteroids and spectrally relatively redder and bluer spectral properties. very red. Murchie and Erard [6] showed that the C-like The measured region of Phobos does not spectrum of Phobos can be explained as an exhibit a 1-mm absorption greater than several artifact of mixing of measurements covering percent in depth. This excludes a freshly these dissimilar regions at different wavelength exposed, highly mafic-rich composition, but ranges. Analog materials having as red a excludes neither (a) an organic-rich composition spectrum as Phobos or Deimos include D-like nor (b) a mafic but highly optically altered asteroids and space-weathered mafic silicates. composition. The measured region of Deimos These would would imply drastically different may exhibit a shoulder approaching 1 mm, but satellite compositions. scatter in the data rules out a definitive Each of the two hypotheses for the moons' identification of an absorption. There is enough composition makes predictions about scatter between adjacent IMP filters that occurrence of Fe mineral absorptions and about interpration of narrower, single-filter "spectral relationships between albedo and spectral features" is likely problematic. properties. IMP observations were designed: to Comparison to Analogs. The spectra of both observe independently the two moons' spectral moons are grossly similar, but differ in albedo properties, to validate previous measurements; to and spectral character near 1 mm. Deimos is both MARS PATHFINDER SPECTRA OF PHOBOS AND DEIMOS: S. Murchie et al. higher in albedo and exhibits more of a wavelengths diagnostic of the presence of mafic shoulder near 1 mm. A similar relationship silicates or organics and clays. between albedo and depth of a 1-mm absorption Acknowledgments. We thank P. Thomas for is seen in mafic assemblages space-weathered to Viking albedo maps of both Martian moons. different degrees [9], but the small number of References. [1] Pang, K. et al., Science, 199, IMP spectra, scatter in the data, and lack of 64-66, 1978. [2] Pang, K. et al., J. Geophys. wavelength coverage beond 1 mm preclude Res., 88, 2475-2485, 1983. [3] Grundy, W. and definitive statements about an analgous trend. U. Fink, in Asteroids, Comets, Meteors 1991, pp. The moons are redder than most asteroids, and 215-218, 1991, [4] Zellner, B. and E. Wells, are most similar to red, putatively clay- and Lunar Planet. Sci. XXV, 1541-1542, 1994. [5] organic-rich D-like asteroids including D- and Murchie, S. et al., J. Geophys. Res., 96, 5925- T-types. Alternatively, the satellites compare 5945, 1991. [6] Murchie, S. and S. and Erard, equally well with highly space-weathered mafic Icarus, 123, 63-86, 1996. [7] Thomas, P. et al., mineral rich materials such as lunar mare soils. Icarus, 123, 536-556, 1996. [8] Simonelli, D. et Drastically different compositional implications al., Icarus in press. [9] Pieters, C., in Remote of these analog materials may be resolved by Geochemical Analysis, ed. by C. Pieters and P. further spectral measurements, covering infrared Englert, pp. 309-339, Cambridge Univ., 1993

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