Photometric modeling of the regolith in the Reiner Gamma lunar swirl

Lunar swirls are bright areas on the otherwise darker mare regions on the Moon. Our current study focuses on the Reiner Gamma swirl, centered slightly north of the equator and on the side facing Earth, at coordinates 7.5°N, 59.0°W. We are using the same data as prior research to the lunar swirl, namely the data in Weirich et al. (Planet. Sci. J. 4, 212, 2023) and Domingue et al. (Planet. Sci. J. 5, 161, 2024). 

Our analysis of photometric data and the use of the particulate medium (PM) model (Muinonen et al., A&A 531, A150, 2011; Wilkman et al., PS&S 118, 255, 2015) follows our approach with the surface of Mercury (Björn et al., Planet. Sci. J. 5, 260, 2024). However, there is a difference in the extent of the area being examined: here, we apply the methods to a localized area on the Moon, as opposed to the average Mercury surface as in Björn et al. (2024). In addition, the amount of data is orders of magnitude larger than the Mercury data, which prevents the use of the same pipeline as before. Therefore, we use a Python-language implementation, healpy (Zonca et al., J. Open Source Softw. 4, 1298, 2019), of the HEALPix discretization (Górski et al., Astrophys. J. 622, 759, 2005) to reduce the number of data points by averaging the reflectance inside a bin in the incidence-emergence-azimuth angles. We utilize the Reiner Gamma swirl mapping results of Chuang et al. (Planet. Sci. J. 3, 231, 2022) to separately examine areas, or ”units”, with different brightnesses: the bright on-swirl unit, the dark off-swirl unit, and the transitionary diffuse-swirl unit. 

To deduce physical properties of the regolith of Reiner Gamma, we apply the PM model to photometric data. The model describes a regolith with a fractional Brownian motion surface, which characterizes well the surface roughness of atmosphereless bodies in the Solar System (Muinonen et al., 2011). The model has three geometry parameters: the packing density v, the fractal Hurst exponent H, and the amplitude of height variation σ. Figure 1 shows the effects that the geometry parameters have on the regolith. 

Our preliminary results for a subset of the data suggest that the diffuse-swirl regolith around Reiner Gamma is similar in surface roughness but less densely packed than Mercury’s regolith: v ≈ 0.44, H ≈ 0.60, σ ≈ 0.10. For comparison, the packing density of the regolith of Mercury was derived to be v = 0.547±0.004 (Björn et al., 2024). In the future, we plan to extend the analysis to photometric data of the Mare Ingenii swirl (33.7°S, 163.5°E) as well.