Francisco J. García-Izquierdo

The early evolution of dust in protoplanetary disks remains uncertain, as the maximum particle size inferred from (sub)millimeter polarization can differ by up to an order of magnitude from that inferred from spectral energy distribution modeling. To test whether porosity and morphology can reduce this tension, we perform light-scattering numerical simulations for two dust populations: (i) consolidated porous particles computed with the discrete dipole approximation ( ADDA ) and (ii) highly porous aggregate models, including fractal and hierarchical aggregates, computed with the multiple-sphere T -matrix method ( MSTM ). Using DSHARP optical constants, we compute scattering matrices, cross sections, and effective albedo ω ¯ eff for a size distribution n ( a ) ∝ a ^q with q = −3.5, a min = 0.1 μ m , and 10 wavelengths from 0.87 to 10 mm. We find that increasing porosity strengthens forward scattering and enhances polarization near θ ≈ 90 ^∘ . For compact spheres, P ( 9 0 ∘ ) ω ¯ eff peaks near a max ∼ λ / 2 π and then declines, whereas porous particles show a broader peak extending to larger sizes, keeping polarization-based constraints compatible with a max ∼ 1 mm . Porosity also lowers κ _abs at fixed dust mass relative to compact spheres, implying larger inferred dust masses for a given continuum flux.

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.

Aims. We experimentally examined the impact of the surface roughness of regolith simulants on the elements of the light-scattering Mueller matrix. Methods. We processed a Mojave Mars Simulant (MMS2) powder sample to produce a set of aggregates with a controlled degree of porosity. The final samples present a cylindrical shape of 0.2 cm radius by 0.4 cm height. The measurements, spanning scattering angles from 94° to 177°, were conducted at a wavelength of 640 nm at the IAA Cosmic Dust Laboratory (CODULAB), which was specifically adapted for surface studies. This marks the first scattering experiment on surfaces performed at CODULAB. Results. Our measurements reveal the influence of surface roughness on the scattering matrix elements, with trends directly correlated with the degree of roughness. Additionally, we observe an inverse relationship between surface roughness and albedo. Across all samples, a shallow negative polarization branch near the backward direction is detected, a characteristic attributed to the single-particle and coherent backscattering mechanisms commonly observed in comets and asteroids. The results also highlight the significant role of large-scale surface structures in determining the scattering behavior, particularly through enhanced multiple scattering. Future work will explore the wavelength dependence of the scattering properties of these rough surfaces.