PHOTOMETRIC PROPERTIES OF PHOBOS SURFACE MATERIALS FROM VIKING IMAGES

Clear-filter Viking images, and an accurate numerical model of the sha pe of Phobos, have been used to determine this satellite's photometric properties, A global-average Hapke function derived from disk-resolve d data confirms previous indications that Phobos has a strong oppositi on surge. Photometrically corrected images were mosaicked into an albe do map; most of the resulting normal reflectances are in the range 0.0 6-0.10, and the brightest region on Phobos is the northeast rim of the crater Stickney, the portion of that rim with the highest concentrati on of grooves. Globally, there are three albedo classes, reasonably se parated geographically: (1) Bright material is to the east and south o f Stickney, corresponding approximately to the locations on Phobos hav ing the highest, ''bluest'' visible/near-IR ratio (Murchie et al, 1991 ). (2) The darkest material is to Stickney's west, correlating with ma terial having an intermediate visible/NIR ratio (Murchie et al.'s ''bl uish gray'' unit). (3) Intermediate-albedo material dominates the anti -Stickney hemisphere, corresponding Po material with a lower visible/N IR ratio (Murchie et al.'s ''reddish gray'' unit).A search for variati ons in phase behavior across Phobos' surface shows few such effects ov erall, limited to isolated areas: (1) Stickney's floor darkens with in creasing phase faster than average Phobos. This crater floor is both s lightly more backscattering and significantly rougher than the global average; the latter effect may be related to the slumping hinted at in low-resolution images of Stickney, (2) We confirm that in many cases, the contrast between the bright rims of small craters and grooves and their surroundings drops noticeably with increasing phase (phase angl es in use = 10 degrees-50 degrees). However, these bright rims, overal l, display a diversity of photometric behavior and are the most hetero geneous areas on Phobos in terms of regolith properties. (3) We confir m that dark deposits ire the floors of smaller craters darken faster w ith increasing phase than their surroundings (cf,, Goguen et al. 1978) and find that these deposits are more backscattering than average Pho bos, Isolated regions with unusual phase behavior occur on Phobos but not on Deimos (Thomas et al, 1996). The variable photometric propertie s of Phobos' isolated craters and grooves, and the association of glob al albedo features with Stickney reinforce interpretations that Phobos ' regolith is emplaced and modified by discrete cratering events and i s not mixed horizontally by extensive downslope creep as is apparently the case on Deimos. (C) 1998 Academic Press.