Albedo used in astronomy are the (V-band) geometric albedo (measuring brightness when illumination comes from directly behind the observer) and the Bond albedo (measuring total proportion of electromagnetic energy reflected). Their values can differ significantly, which is a common source of confusion. In detailed studies, the directional reflectance properties of astronomical bodies are often expressed in terms of the five Hapke parameters which semi-empirically describe the variation of albedo with phase angle, including a characterization of the opposition effect of regolith surfaces. The correlation between astronomical (geometric) albedo, absolute magnitude and diameter is: formula_7, where formula_8 is the astronomical albedo, formula_9
Martian surface classical albedo feature Mare Acidalium (Acidalia Planitia) is another prominent dark area that lies north of the main belt. Bright areas, excluding the polar caps and transient clouds, include Hellas, Tharsis, and Arabia Terra. The bright areas are now known to be locations where fine dust covers the surface. The dark markings represent areas that the wind has swept clean of dust, leaving behind a lag of dark, rocky material. The dark color is consistent with the presence of mafic rocks, such as basalt. The albedo of a surface usually varies with the wavelength of light hitting it. Mars reflects