Polarimetric and photometric modelling of regolith simulants

Atmosphereless Solar System objects (SSOs), such as asteroids, exhibit special features in the way they scatter unpolarized incident sunlight: mainly, the nonlinear increase in brightness at small phase angles (the angle between the Sun and the observer seen from the object, α) and the negative surge in the degree of linear polarization at α ≤ 30°. The phenomena are likely due to the surface regolith consisting of closely packed small particles.

In order to examine the features, first we have modelled the photometric and polarimetric phase curves of lunar observations with a novel version of the radiative-transfer coherent-backscattering (RT-CB) algorithm developed by Muinonen et al. [1] using the empirical scattering matrix of the dark lunar regolith simulant JSC-1A [2], for which the matrix elements have been parametrised [3] (Figure 1).

 

 

Figure 1: The photometric phase curve of the Moon from observations by Rougier [4] and averaged by Bowell et al. [5] (left figure, red dots) and the polarimetric phase curve of the waning Moon by Lyot [6] (right figure, red dots) modelled with the RT-CB algorithm using the parametrised empirical scattering matrix of the JSC-1A regolith simulant. The resulting modelled curves (blue line) were computed with 1 million rays, with a mean-free-path length of 5 μm and a single-scattering albedo of 0.723.

 

Furthermore, we have measured the intensity and the linear polarization of a flat surface of the JSC-1A containing particles smaller than 106 μm at zenith incidence angles 0° to 70° and zenith emergence angles −80° to 80° with the spectrogoniometer at the University of Helsinki Astrophysical scattering laboratory. We will similarly measure a brighter substance for the purpose of mimicking a higher-albedo asteroid, and present RT-CB modelling results for both materials.

[1] K. Muinonen, A. Leppälä, J. Markkanen, JQSRT 330, (2025).
[2] O. Muñoz, E. Frattin, J. Martikainen et al., JQSRT 331, (2025).
[3] K. Muinonen, A. Leppälä, in preparation, (2025).
[4] G. Rougier, Ann. Obs. Strasbourg 2, (1933).
[5] E. Bowell et al., Asteroids II, (1989).
[6] B. Lyot, PhD. Thesis, (1929).