A ROUGH-SURFACE THERMOPHYSICAL MODEL FOR AIRLESS PLANETS

This paper describes the disk-integrated thermal emission from a rough planetary surface with nonzero thermal inertia. I use a model that determines diurnal temperatures in spherical-section depressions and includes the effects of subsurface heat flow and direct and scattered sunlight. The beaming parameter η used in radiometric diameter determinations, which describes the zero-phase thermal emission compared to a smooth nonrotating body, is almost independent of albedo for a given surface roughness. The variation of η with roughness and thermal inertia is described. The difference in η between a rough body and an otherwise identical smooth body is remarkably constant with varying geometry and thermal parameters, and depends largely on roughness alone. Both the Moon and asteroids require RMS slopes ≈40° to match the observed zero-phase thermal emission. The thermal phase curve of Ceres is well matched by a model that has prograde rotation, an RMS surface slope of 44°, and a thermal inertia 30% of the lunar value. © 1990.