GALILEO PHOTOMETRY OF ASTEROID 951-GASPRA

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.