Photometric Properties of Phobos Surface Materials From Viking Images

Clear-filter Viking images, and an accurate numerical model of the shape of Phobos, have been used to determine this satellite's photometric properties. A global-average Hapke function derived from disk-resolved data confirms previous indications that Phobos has a strong opposition surge. Photometrically corrected images were mosaicked into an albedo map; most of the resulting normal reflectances are in the range 0.06–0.10, and the brightest region on Phobos is the northeast rim of the crater Stickney, the portion of that rim with the highest concentration of grooves. Globally, there are three albedo classes, reasonably separated geographically: (1) Bright material is to the east and south of Stickney, corresponding approximately to the locations on Phobos having the highest, “bluest” visible/near-IR ratio (Murchieet al. 1991). (2) The darkest material is to Stickney's west, correlating with material having an intermediate visible/NIR ratio (Murchieet al.'s “bluish gray” unit). (3) Intermediate-albedo material dominates the anti-Stickney hemisphere, corresponding to material with a lower visible/NIR ratio (Murchieet al.'s “reddish gray” unit). A search for variations in phase behavior across Phobos' surface shows few such effects overall, limited to isolated areas: (1) Stickney's floor darkens with increasing phase faster than average Phobos. This crater floor is both slightly 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 angles in use = 10°–50°). However, these bright rims, overall, display a diversity of photometric behavior and are the most heterogeneous areas on Phobos in terms of regolith properties. (3) We confirm that dark deposits in the floors of smaller craters darken faster with increasing phase than their surroundings (cf., Goguenet al. 1978) and find that these deposits are more backscattering than average Phobos. Isolated regions with unusual phase behavior occur on Phobos but not on Deimos (Thomaset al. 1996). The variable photometric properties of Phobos' isolated craters and grooves, and the association of global albedo features with Stickney, reinforce interpretations that Phobos' regolith is emplaced and modified by discrete cratering events and is not mixed horizontally by extensive downslope creep as is apparently the case on Deimos.