Photometric and Phase Reddening properties of a Phobos simulant

The physical properties of the surfaces of small Solar System bodies, including particle grain size, porosity, and roughness, are essential for understanding their geological processes and formation history. While reflectance measurements taken at varying wavelengths and geometries can be used to derive these properties through photometry, deriving it remains a challenge due to limitations in the existing photometric models [1]. Therefore, characterizing the photometric behavior of representative surfaces through controlled laboratory experiments can provide valuable constraints for interpreting remote sensing observations. In preparation for Martian Moon eXploration (MMX) mission to Phobos, multiple Phobos simulants have been prepared, and different properties have been characterized in laboratory [2,3].  We have systematically investigated the effects of varying particle size and porosity on the photometric and phase reddening properties of the University of Tokyo Phobos Simulant (UTPS) [2].  

The measurements were conducted using the PHIRE-2 (PHysikalisches Institut Radiometric Experiment - 2) radio-goniometer at the University of Bern [4]. We have found systematic variation in the albedo, opposition effect, and the forward scattering behavior of the sample with the particle size and porosity.  Furthermore, we derived the Hapke parameters [5] for a comparative study against meteorite samples, finding that the phase function of our samples resembles the Allende and Tagish Lake meteorites [6].  The phase reddening behavior of the sample was found to depend strongly on both the particle size and porosity, with finer particles and higher porosity showing higher phase reddening coefficient. Comparing it to the observations from TGO/CaSSIS [7], we found the phase reddening coefficient to be consistent with the values associated with Phobos’s red unit. 

While missions such as Mars Express [8] and TGO [7] continue to observe Phobos, the upcoming Martian Moon eXploration (MMX) mission will provide us with even higher resolution data with more complete coverage. Our work can help in better interpreting spectral and photometric remote sensing measurements, enabling a more comprehensive understanding of Phobos. 

References: 
1. Shkuratov, Y. et al. (2012), "A critical assessment of the Hapke photometric model", Journal of Quantitative Spectroscopy and Radiative Transfer, v113, pp. 2431–2456, doi: 10.1016/j.jqsrt.2012.04.010 

2. 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 

3. 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 

4. Pommerol, A. et al. (2011), "Photometry and bulk physical properties of Solar System surfaces icy analogs: The Planetary Ice Laboratory at University of Bern", Planetary and Space Science, v59, pp. 1601–1612, doi: 10.1016/j.pss.2011.07.009 

5. Hapke, B. (2012), Theory of Reflectance and Emittance Spectroscopy, 2nd ed., Cambridge University Press, Cambridge. 

6. Beck, P. et al. (2012), "Photometry of meteorites", Icarus, v218, pp. 364–377, doi: 10.1016/j.icarus.2011.12.006 

7. Munaretto, G. et al. (2025), "Phase reddening of Phobos and Deimos from TGO/CaSSIS observations", Astronomy & Astrophysics, doi: 10.1051/0004-6361/202555720 

8. Fornasier, S. et al. (2024), "Phobos photometric properties from Mars Express HRSC observations", A&A, 686, A203, doi: 10.1051/0004-6361/202449220