* Astronomy

Title: Mars surface phase function constrained by orbital observations
Authors: Mathieu Vincendon

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 behaviour 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 parameterisation of the mean Mars surface phase function based on Hapke formalism ({\omega} = 0.85, {\theta} = 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 modelling.

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