A possible interpretation of bright features on the surface of Phobos

Abstract

Rims of many grooves and some craters on Phobos are much brighter than surrounding surfaces when illuminated at low phase angles, but their brightness contrast is much less evident at higher phase angles. These surfaces therefore have both a greater near-normal reflectance and a greater phase coefficient than does most of the satellite's regolith. Materials proposed as compositional analogs to Phobos' regolith consist of mixtures of higher- and lower-albedo components. These materials were simulated in the laboratory as intimate mixtures in various proportions of high-albedo MgO, low-albedo carbon black, and pigment. For low-albedo mixtures, as the fraction of the brighter component increases so do the reflectance and the phase coefficient. These measurements are difficult to explain in terms of the classical shadowing mechanism of the opposition surge, but can be understood as the consequence of a previously-described interference mechanism. These results imply that the groove and crater rims' distinctive optical properties can be explained by a locally greater abundance of a higher-albedo regolith component. Greater abundance of a higher-albedo component in the rim material of bright craters is consistent with the lesser degradation and therefore lesser age of these craters, resulting in a regolith less darkened by interaction with the space environment.