Planetary regolith surface analogs:: optimized determination of Hapke parameters using multi-angular spectro-imaging laboratory data

This work presents a method for a determination of the global set of parameters involved in Hapkeʹs model for planetary surface analogs when dealing with a set of angular conditions representative of the usual range of observation in planetary exploration for spaceborne optical instruments. The present approach is founded on a genetic algorithm: the whole set of Hapke parameters is treated simultaneously without any a priori assumptions. It limits the risk of meeting a local extreme, and the stability and repeatability of the determination are improved. Moreover, it requires less computational time than a Monte Carlo routine. As a demonstration, using multi-angular measurements acquired by a new laboratory wide-field multispectral imaging facility, parameter values for various grain sizes and material compositions of planetary regolith surface analogs are derived and their potential wavelength dependence is studied. The results, coherent with the literature, validate all the steps in our methodology. We focus our interest in the understanding of the physical meaning of Hapkeʹs parameters, in particular of the parameter θ of the shadowing function for macroscopic roughness. It appears that the features smaller than the centimeter scale contribute predominantly to the photometric effect related to the rocky aspect of planetary soil surfaces. The parameter θ may thus be mainly considered as an integral of the roughness properties in the submillimetric-centimetric range. Indeed, the most representative case for describing natural regolith surface lies in texture classes in the millimeter range, not involving necessarily a high macroscopic roughness with large surface slopes at hundreds of meters scale. These findings strengthen the case that θ depends on both the grain size and the material geological properties.