THE SURFACE OF DEIMOS - CONTRIBUTION OF MATERIALS AND PROCESSES TO ITS UNIQUE APPEARANCE

Among the well-imaged small satellites and asteroids, Deimos displays a unique surface: very smooth with global-scale albedo features. We ha ve examined the disk-resolved photometry of Deimos using Viking Orbite r images for clues to its distinctive appearance. Hapke parameters wer e fit to characterize the phase behavior and to compute normal reflect ance. The opposition surge amplitude (Be) is smaller for Deimos than f or Phobos. Outside the range of the opposition effect the two martian satellites have similarly shaped phase curves, but Deimos is about 20- 30% brighter than Phobos from 10 degrees-80 degrees phase. The calcula ted mean normal reflectance of Deimos (lambda(eff) = 0.54 mu m) is 0.0 68 +/- 0.007. The brighter and darker areas on Deimos exhibit constant contrast between 0.6 degrees and 81 degrees phase; this characteristi c allows a calculation of the range of normal reflectances over most o f its surface, nearly all of which values are between 0.06 and 0.09. T he trailing side of Deimos has a larger relative distribution of the b righter material, and is on average about 10% brighter than the leadin g side. The mean normal reflectance cannot be formally distinguished f rom that of Phobos (0.071 +/- 0.012; Simonelli, D. P., M. Wisz, A. Swi tala, D. Adinolfi, J. Veverka, P. C. Thomas, and P. Helfenstein 1996. Submitted to Icarus). Although the statistical distribution of normal reflectances on the two satellites is similar, the geography of the al bedo variations is very different. Deimos has gradational changes in a lbedo downslope from ridge crests, primarily manifested in long albedo ''streamers.'' On Phobos there is a more patchy global distribution o f albedos, apparently related to ejecta from the large crater Stickney . Because of the similarity of mean density, spectral properties, mean normal reflectance, the range of normal reflectance, and phase functi on outside the opposition effect, Deimos appears to be made of materia ls with compositions very similar to those on Phobos, although the app arent wider distribution of ejecta on Deimos has been cited as indicat ing a greater role for strength scaling in cratering on Deimos (Lee, S . W., P. Thomas, and J. Veverka 1986. Icarus 68, 77-86). Simple modeli ng of the formation of the albedo patterns by gardening, creep, and '' weathering'' of bright material from crater rims suggests that impact gardening contributes very little to the motion of the material downsl ope, and that vertical mixing and/or ''weathering'' must be important in addition to an unspecified creep process. The distinction of Deimos is primarily in the smooth surface that allows a particularly large s cale of downslope movement of regolith on very gentle slopes. This smo othness is most easily explained by the effects from impact formation of a 10-km concavity at high southern latitudes in the latter half of Deimos' surface history. This impact scar is relatively much larger th an is the crater Stickney on Phobos. The effects of this large impact probably include blanketing by an average of nearly 200 m of ejecta, b ut also may include seismic erasing of craters similar to that propose d for Ida by Asphaug et al. (Asphaug, E., J. M. Moore, D. Morrison, W. Benz, and R. A. Sullivan 1996. Icarus 120, 158-184). (C) 1996 Academi c Press, Inc.