Photometric modeling of the regolith in the Reiner Gamma and Mare Ingenii lunar swirls

Lunar swirls are bright-albedo areas on the surface of the Moon that appear to twist and turn across the surface. Multiple swirl formation processes have been hypothesized, including shielding from space weathering, i.e., small meteoroid impacts and charged particles from the Sun (e.g., Hood & Schubert, Sci 208, 49, 1980), dust accumulation and lofting (e.g., Garrick-Bethell et al., Icar 212, 480, 2011), and comet impacts (e.g., Bruck Syal & Schultz, Icar 257, 194, 2015). One approach to distinguish between the processes is to examine the structure of the regolith, the top layer of the surface, using photometric data, i.e., measuring the brightness of the surface at different viewing angles.

The current study focuses on two swirls: Reiner Gamma, centered at the lunar coordinates (7.5◦ N, 59.0◦ W), and Mare Ingenii, centered at (35.8◦ S, 161.8◦ E). Reiner Gamma is perhaps the best known of lunar swirls, whereas Mare Ingenii represents a multitude of swirls because it is located within both the darker maria and the brighter highlands. Studying Reiner Gamma and Mare Ingenii is useful not only for learning more about swirls and swirl formation processes but also for increasing understanding of the Moon as an atmosphereless object.

In the study presented here, we applied the fractional-Brownian-motion particulate-medium (fBm- PM) model (Parviainen & Muinonen, JQSRT 110, 1418, 2009; Wilkman et al. P&SS 118, 250, 2015; Björn et al., PSJ 5:260, 2024) to photometric data of the swirls to infer the physical properties of their regolith. The fBm-PM model describes a regolith with an fBm surface (Peitgen & Saupe, 1988), which accurately characterizes the surface roughness of an atmosphereless Solar System body. The model has three geometry parameters: the packing density, v, with values between 0.15 and 0.55; the fractal Hurst exponent, H, with values between 0.20 and 0.80; and the amplitude of height variation, sigma, with values between 0.00 and 0.10. The fBm-PM model was compared to photometric observations—the same data as in Weirich et al. (PSJ 4:212, 2023) for Reiner Gamma, and the same data as in Domingue et al. (GeoRL 49, e95285, 2022) for Mare Ingenii. By varying the geometry parameters, we were able to determine the parameter values that agreed best with the observations.

The results suggest that the regolith within the Reiner Gamma swirl is moderately densely packed (v ≈ 0.41) and has moderate horizontal surface roughness (H ≈ 0.60) and large vertical surface roughness (σ ≈ 0.10). A recent study using the same methods derived a similar surface roughness but a higher packing density (v ≈ 0.55) for the average regolith of Mercury (Björn et al., PSJ 5:260, 2024). Our ongoing analysis of Mare Ingenii allows to thoroughly examine two different swirls to infer which swirl formation processes are likely, and to compare lunar swirl regolith to the regolith of Mercury. Preliminary results suggest a regolith structure different from Reiner Gamma, including variations within the swirl itself.