Thermal modeling for asteroid surface characterization: Apophis as a case study

The characterization of asteroidal surfaces is of great importance for understanding asteroid formation and evolution as well as for spacecraft landing site selection. Global thermal inertia estimates are available for many asteroids, which can be used to determine material parameters of the surface structure. Specific thermal inertia values can be explained by different surface structures, such as regolith consisting of individual grains or porous rocks. We analyze these cases for the Sq-type asteroid Apophis using the published thermal inertia measurement to derive grain sizes and rock porosities. We also assess their effect on the diurnal temperature evolution. First, we derived these quantities for different surface structure models by comparing the modeled thermal conductivity with the published global thermal inertia estimate. Second, we applied a one-dimensional numerical thermophysical model to compute the corresponding diurnal temperature curves of Apophis' surface. For the best-fit thermal inertia, we derived mean regolith grain radii of 13-28,mm and up to 25-57,mm when assuming an additional grain volume filling factor of 50,%. For a porous rock surface, we find volume filling factors of 0.722 and 0.777. When also considering a hierarchical pore structure and radiative heat transport within the macro pores, we obtain macro filling factors of 0.728-0.879. The macro filling factors decrease when assuming larger pore space diameters. We observe that a regolith and a porous rock surface can be distinguished by their nighttime cooling curves, with the largest temperature differences occurring at sunrise. Measurements of the nighttime temperature evolution allow for the discrimination of the surface structure by remote sensing techniques. A quantitative interpretation of specific surface locations will, however, require accounting for topography, surface roughness, and the exact local rotation history.