Paper detail

Mars surface phase function constrained by orbital observations

The photometric properties of the surface of Mars describe how remote measurements of surface reflectance can be linked to hemispherical albedo used for energy balance calculations. A simple Lambert model is frequently assumed for global data processing, even through several local studies have revealed the complexity of Mars surface phase functions. In this paper, we derive a mean Bidirectional Reflectance Distribution Function (BRDF) of Mars as averaged over km-sized remote sensor pixels. OMEGA and CRISM data are used to provide observational constraints at solar wavelengths over a wide range of viewing conditions all over the planet. Atmospheric contribution is quantified and removed using a radiative transfer model. A common phase behavior composed of a 5 to 10% backscattering peak associated with an increase of scattered reflectance with emission angle is observed. Consequently, nadir measurements of surface reflectance typically underestimate hemispherical reflectance, or albedo, by 10%. We provide a parameterization of the mean Mars surface phase function based on Hapke formalism (ω = 0.85, θ = 17, c = 0.6, b = 0.12, B0 = 1 and h = 0.05), and quantify the impact of the diffuse illumination conditions reigning on Mars. Our average phase function can be used as a refinement compared to the Lambert model in global data processing and climate modeling.