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.