  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 ({\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
modeling.
