Application of multiple photometric models to disk-resolved measurements of Mercury’s surface: Insights into Mercury’s regolith characteristics

Abstract

Photometric analyses are used to standardize images obtained at a variety of illumination and viewing conditions to a common geometry for the construction of maps or mosaics and for comparison with spectral measurements acquired in the laboratory. Many models exist that can be used to model photometric behavior. Two of the most commonly use models, those of Hapke and Kaasalainen–Shkuratov, are compared for their ability to standardize MESSENGER images of Mercury. Analysis of the modeling results shows that photometric corrections using the Kaasalainen–Shkuratov model provides significantly less contrast between images acquired at large differences in emission angle. The contrast seen between images acquired at large differences in either incidence and phase angle is smaller with the Hapke model based corrections, but not significantly better than that provided by the Kaasalainen–Shkuratov model. Photometric studies are also used to infer scattering properties of the surface regolith. The quantitative correlation between photometric model parameters and surface properties is questionable, but laboratory studies do indicate general correlations and trends between parameters and sample properties that allow for comparisons between surfaces based on photometric modeling. Based on comparisons with the Moon and several asteroids that have been observed by spacecraft, the photometric analyses presented here are interpreted to indicate that Mercury’s regolith is smoother on micrometer scales and has a narrower particle size distribution with a lower mean particle size than lunar regolith. Grain structures of regolith particles from Mercury are inferred to be different than those of the Moon or those asteroids observed to date. Mercury’s regolith may contain a component compositionally distinct from lunar regolith.