Experimental investigations of the photometric properties of Phobos simulant 

Deriving quantitative regolith properties from photometric remote sensing data remains a challenge. Many photometric models are empirical, where the parameters lack direct physical interpretation. Although Hapke models have parameters that correspond to physical properties of the regolith, they suffer from other issues such as parameter degeneracy (where different physical properties can explain the same behavior) or the fitting algorithm getting stuck in local minima. To reliably interpret remote sensing data, one can validate the findings by reproducing the observed photometric behavior in laboratory. One such study was recently performed by Wargnier et. al. (2024), where they found the single-scattering albedo (SSA) of Phobos simulants to be 3-6 times higher than the observation made by Fornasier et. al. (2024) and Simonelli et. al. (1998) based on Mars Express/HRSC and Viking clear filter images, respectively. However, a closer match with remote sensing observations may be achieved by using wider grain size distributions or higher porosity. In preparation for the upcoming Martian Moons eXploration (MMX) mission to Phobos, multiple new Phobos regolith simulants have been developed for testing purposes [4,5]. A primary objective in preparing these simulants has been achieving spectroscopic similarity to the observations made during previous space missions. However, due to limited knowledge of some key physical properties like particle size distribution, packing density, and surface roughness, they are not tightly constrained in the simulants. Miyamoto et al. [4] prepared three different grain size models for the University of Tokyo Phobos Simulant (UTPS) - a) resembling lunar surface regolith, b) resembling possible regolith of the smooth area of Itokawa, and c) an intermediate distribution between models (a) and (b). While Wargnier et al. [1,4] used different grain size ranges for sample preparation and subsequent analysis, their focus was mainly on the detectability of hydrated minerals and organic bands. Despite these efforts, the effect of different grain size distributions, packing densities, and surface roughness on the photometric behavior of Phobos simulants has yet to be systematically investigated. We conduct an extensive photometric analysis of the UTPS simulant using the PHIRE-2 (PHysikalisches Institut Radiometric Experiment - 2) radio-goniometer at the University of Bern. This instrument is particularly well suited for studying dark samples (like UTPS) and allows observations at phase angles as low as 0.5 degrees [6]. Our study compares the photometric behavior of samples with varying grain size distributions and packing densities against observations from the Mars Express/HRSC. We also plan to investigate wavelength dependency on the photometric properties to assess if the observed red/blue spectral dichotomy on Phobos can be attributed to differences in physical characteristics. These experiments will help us better interpret the observations from Mars Express/HRSC and should give us a more comprehensive understanding of the regolith properties of Phobos.   References: 1. Wargnier, A. et al. (2024), "Spectro-photometry of Phobos simulants: I. Detectability of hydrated minerals and organic bands", Icarus v241, 116216, doi: 10.1016/j.icarus.2024.116216  2. Fornasier, S. et al. (2024), "Phobos photometric properties from Mars Express HRSC observations", A&A, 686, A203, doi: 10.1051/0004-6361/202449220  3. Simonelli, D. et al. (1998) "Photometric Properties of Phobos Surface Materials from Viking Images", Icarus 131(1), p. 52-77, doi:10.1006/icar.1997.5800  4. Miyamoto, H. et. al. (2021), "Surface environment of Phobos and Phobos simulant UTPS", Earth Planets Space 73, 214, doi: 10.1186/s40623-021-01406-3  5. Wargnier, A. et.al. (2023), "Development of a new Phobos spectral simulant: spectral properties from visible to the mid-infrared range", MNRAS v524-3, p3809–3820, doi: 10.1093/mnras/stad2132   6. Jost et. Al. (2016), "Experimental characterization of the opposition surge in fine-grained water–ice and high albedo ice analogs", Icarus v264, 109-131, doi: 10.1016/j.icarus.2015.09.020