Photometric properties of Asteroid 4179 Toutatis from lightcurves and a radar-derived physical model

Optical lightcurves reported by J. R. Spencer et al. (1995, Icarus 117, 71-89) and a radar-derived shape and spin-state model (R. S. Hudson and S. J. Ostro, 1995, Science 270, 84-86) are used to estimate the Hapke parameters of the Earth-crossing Asteroid 4179 Toutatis. We find a particle single-scattering albedo w = 0.261 +/- 0.019, opposition surge width h = 0.036 +/- 0.023 and amplitude B-0 = 1.20 +/- 0.32, particle phase function asymmetry factor g = -0.29 +/- 0.06, and macroscopic roughness parameter <(theta)over bar> = 32 +/- 8 degrees. The synthetic lightcurves generated with this model provide a good fit (rms residual = 0.12 mag) to the optical data which span phase angles from 0.2 degrees to 121.4 degrees. The addition of optical data in the modeling process results in a slight refinement of the radar-derived estimate of the parameters that describe Toutatis's non-principal-axis spin state. Analysis of the opposition surge width in light of recent radar results supports the hypothesis that a significant fraction of Toutatis's surface is covered by a fine particulate regolith. (C) 1998 Academic Press.