Illumination Conditions at Phobos: Implications for Surface Processes, Volatiles and Exploration

Illumination conditions at airless bodies, such as Phobos, control or significantly influence many aspects of their environment, including temperature of the surface and subsurface, volatile sequestration and transport. In this study we simulate the average present-day illumination conditions at Phobos and assess whether locations exist where water ice could be either thermally stable in the near subsurface on billion year timescales, or accumulated by the "thermal ice pump" effect. Previous investigations of Phobos temperature and volatile sequestration have assumed a sphere or triaxial ellipsoid shape for Phobos. Here we use more realistic shape models in order to capture the shadowing caused by both its global-scale irregular shape and smaller scale structures, such as craters, that can have a crucial role in determining illumination conditions. Phobos is frequently eclipsed by Mars, especially around equinoxes, and this important effect is accounted for in the shadowing simulations. Preliminary predictions for the average incident solar flux using the Thomas [1993] shape model reveal many interesting features, such as high fluxes around crater rims (e.g., Stickney), doubly-shadowed craters (e.g., Limtoc) and strong asymmetries between poleward- and equatorward-facing crater walls (e.g., Drunlo). Perhaps most significantly, there are regions (most likely small craters) that are predicted to receive very low average solar flux ( 40-50 Wm-2). These will likely be, on average, the coldest places on Phobos. Should water ice, or other volatiles, be present at relatively shallow depths at certain locations on Phobos, then this could serve as a witness plate to the history of the Mars system, as well as provide valuable resources to future explorers.