Assessing the spectrophotometric properties of Mercury's hollows through multiangular MESSENGER/MDIS observations

Abstract

The origin and formation mechanism of Mercury's hollows, which are bright, often haloed, small, shallow, irregular, rimless and flat floored depressions, represent one of the major open science questions regarding the Hermean surface and the processes shaping it morphology. In this work, we perform a photometric modelling of multiangular and multiband images of Tyagaraja and Canova craters' hollows to investigate the physical properties of their reflecting material. Thanks to such observations, we demonstrate that we can derive a better topographic correction when compared to the one obtained from the global photometric models of Mercury. Indeed, our parameters, which result from the inversion of the Hapke and Kaasalainen-Shkuratov models, can be useful for both future spectrophotometric analyses of Mercury and laboratory experiments aiming to identify hollows analogue materials. The analysis of our estimated model parameters imply that the Tyagaraja and Canova hollow walls are more backscattering and smoother than the crater floors, in agreement with independent phase ratio analyses. Our results suggest that the hollow forming material is made of roundish particles or particles with a high density of scattering centers, such as holes, vesicles or fractures, consistent with the release of volatiles as part of the hollows' formation mechanism.