García-Izquierdo et al., 2026

This study is part of an ongoing project in which we experimentally examine the effect of composition (refractive index) and surface roughness on the scattering matrix elements of a set of well-characterized rough regolith simulants. A set of four cylindrical Mars regolith simulants with controlled degrees of porosity and surface roughness is studied. Photopolarimetric measurements, spanning scattering angles from 94° to 177°, were conducted at a wavelength of 488 nm at the Instituto de Astrofísica de Andalucía Cosmic Dust Laboratory. These results are compared with those previously obtained at 640 nm. The higher absorption of the Mars simulants at 488 nm compared to 640 nm produces a significant effect on all elements of the scattering matrix. The trends previously observed in the diagonal elements of the scattering matrix and in the albedo as a function of surface roughness remain unaffected by the differences in refractive indices at both wavelengths. The comparison with the previous study enables the investigation of sample reddening across the scattering-angle range, with a trend also observed between surface roughness and reddening. Moreover, within our measured angular range, the effect of absorption is consistent with Umov’s law, whereas the effects of surface roughness do not appear to support its validity. Nearly all samples exhibit a shallow negative polarization branch.