From the Galileo
images of Gaspra, photometric properties as a
function of incidence
and emittance angle are derived for the asteroid. The
application of Hapke's [1986] photometric model
results in derivation of five parameters. Average particle single
scattering albedo, 
, amplitude of the rise in
brightness at opposition, B
, and the asymmetry factor of the
phase function, g, are strongly affected
by composition and particle microstructure. The other two,
, the average slope of macroscopic roughness, and h,
angular width over which the opposition surge occurs, represent
macroscopic and microscopic mechanical texture.
Helfenstein et al., [1994] report that Gaspra's
macroscopic surface texture and particle phase-function are
similar to the average values for the Moon. This is not surprising as
their dominant mineral composition consists of mafic silicates, and
both surfaces have been subjected to bombardment over the age of
the solar system. I consider it more significant to understand the
nature of the differences and the processes responsible for those
differences. Gaspra has a slightly larger single particle
albedo, 
=0.36, compared to 0.21 for the Moon, and
the macroscopic surface roughness parameter, 
= 29
,
is greater for Gaspra
than for the Moon, 
= 20
. The larger
value translates to a larger geometric
albedo, p
= 0.22, for Gaspra compared to p
= 0.14 for
the Moon. The visual geometric albedo p
= 0.22 
0.06
calculated from Galileo data is identical to that determined from
ground-based observations of 0.22 
0.03.
Subtle albedo contrasts of 10
exist over Gaspra's northern
hemisphere. Photometrically corrected images showed these to be
significant variations that were classified as brighter, the
same, or darker than the mean albedo. A study of the distribution
of these albedos showed that the brighter areas coincide with blue
regions along ridges, the darkest areas correspond with the reddest
materials and those with the weakest 
m bands. The complete
picture combining morphology, mineralogy and photometric properties
alluded to by these photometrically calibrated images awaits input
from the Near-Infrared Mapping Spectrometer (NIMS) instrument.