Galileo images of Gaspra make it possible for the first time to determ
ine a main-belt asteroid's photometric properties accurately by provid
ing surface-resolved coverage over a wide range of incidence and emiss
ion angles and by extending the phase angle coverage to phases not obs
ervable from Earth. We combine Earth-based telescopic photometry over
phase angles 2-degrees less-than-or-equal-to alpha less-than-or-equal-
to 25-degrees with Galileo whole-disk and disk-resolved data at 33-deg
rees less-than-or-equal-to alpha less-than-or-equal-to 51-degrees to d
erive average global photometric properties in terms of Hapke's photom
etric model. The microscopic texture and particle phase-function behav
ior of Gaspra's surface are remarkably like those of other airless roc
ky bodies such as the Moon. The macroscopic surface roughness paramete
r, thetaBAR = 29-degrees, is slightly larger than that reported for ty
pical lunar materials. The particle single scattering albedo, omega0BA
R = 0.36 +/- 0. 07, is significantly larger than for lunar materials,
and the opposition surge amplitude, B0 = 1.63 +/- 0.07, is correspondi
ngly smaller. We determine a visual geometric albedo p(v) = 0.22 +/- 0
.06 for Gaspra, in close agreement with p(v) = 0.22 +/- 0.03 estimated
from Earth-based observations. Gaspra's phase integral is 0.47, and t
he bolometric Bond albedo is estimated to be 0.12 +/- 0.03. An albedo
map derived by correcting Galileo images with our average global photo
metric function reveals subdued albedo contrasts of +/- 10% or less ov
er Gaspra's northern hemisphere. Several independent classification al
gorithms confirm the subtle spectral heterogeneity reported earlier (S
. Mottola, M. DiMartino, M. Gonano-Beurer, H. Hoffman, and G. Neukum,
1993, Asteroids, Comets, Meteors, pp. 421-424; M. J. S. Belton et al.,
1992, Science 257, 1647-1652). Whole-disk colors (0.41 less-than-or-e
qual-to lambda less-than-or-equal-to 0.99 mum) vary systematically wit
h longitude by about +/- 5%, but color differences as large as 30% occ
ur locally. Colors vary continuously between end-member materials whos
e areal distribution correlates with regional topography. Infrared: vi
olet (0.99:0.41-mum) color ratios on Gaspra are strongly correlated wi
th local elevation, being largest at lower elevations and smaller at h
igher elevations. No correlation was detected between elevation and th
e green: violet (0.56:0.41-mum) color ratio. Bright materials with a s
trong 1-mum absorption occur primarily in association with craters alo
ng ridges, while darker materials with 30% weaker 1-mum signatures occ
ur downslope. The variations of color and albedo cannot be easily expl
ained by grain-size effects alone or by differences in photometric geo
metry. The trends observed are consistent with those revealed by labor
atory studies of the effects of comminution, glass formation, and segr
egation of metal from silicate components in chondritic meteorites and
also in some silicate mixtures. The relative importance of these vari
ous processes on Gaspra remains to be determined. (C) 1994 Academic Pr
ess, Inc.