Albedo formula_5 (also called blue-sky albedo) can then be given as: Directional-hemispherical reflectance is sometimes referred to as black-sky albedo and bi-hemispherical reflectance as white-sky albedo. These terms are important because they allow the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface. The albedos of planets, satellites and minor planets such as asteroids can be used to infer much about their properties. The study of albedos, their dependence on wavelength, lighting angle ("phase angle"), and variation in time comprises a major part of the astronomical field of photometry. For small
Martian surface Rocky materials do not vary much in density and specific heat, so variations in thermal inertia are mainly due to variations in thermal conductivity. Solid rock surfaces, such as outcroppings, have high thermal conductivities and inertias. Dust and small granular material in the regolith have low thermal inertias because the void spaces between grains restrict thermal conductivity to the contact point between grains. Thermal inertia values for most of the Martian surface are inversely related to albedo. Thus, high albedo areas have low thermal inertias indicating surfaces that are covered with dust and other fine granular material. The dark gray,