Mars surface phase function constrained by orbital observations

Abstract

The bidirectional 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's law is frequently assumed for global data processing, even though several local studies have revealed the complexity of Mars surface phase functions. In this paper, we derive a mean Bidirectional Reflectance Distribution Function (BRDF) representative of widespread typical Martian terrains. OMEGA and CRISM orbital observations 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. We observe a common phase behavior consisting of a 5%-10% backscattering peak and, outside the backscattering region, a 10%-20% reflectance increase with emergence angles. Consequently, nadir measurements of surface reflectance typically underestimate hemispherical reflectance, or albedo, by 10%. We provide a parameterization of our 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 which change surface albedo as a function of local time and season. Our average phase function can be used as a refinement compared to the Lambertian surface model in global data processing and climate modeling.