Spectrophotometric and Topographic Correlations within the Mare Ingenii Swirl Region: Evidence for a Highly Mobile Lunar Regolith

Abstract

The spectrophotometric properties of two study areas in the Ingenii swirl region show that the combined effects of multiple processes are required to explain the regolith’s mineralogical and physical properties. Production of the swirl regions requires mobilization of the regolith in addition to preferential radiation shielding of subareas. The discovery of topographic correlations between on- and off-swirl (dark lanes) clearly shows that the on-swirl regions are statistically lower than the off-swirl dark lanes by 2–3 m. Photometric analyses show no microscale roughness differences between on- and off-swirl, suggesting no differences in the regolith’s fairy-castle structure between the two swirl regions. The photometric properties of one of the study areas suggest that off-swirl dark lanes may have a more complicated grain structure than on-swirl areas. Enhanced abundances of plagioclase are observed on-swirl in both study areas. Enhanced abundances of FeO and orthopyroxene are observed off-swirl in both areas. No variations in olivine or clinopyroxene abundance were observed for either study area. The discovery of topographic correlations coupled with the similarities in structural properties provides new constraints on the types of processes acting on lunar swirl surfaces.

Near-infrared Photometry of the Moon’s Surface with Passive Radiometry from the Lunar Orbiter Laser Altimeter (LOLA)

“…This interpretation agrees with Kinczyk et al (2022), who used LROC Narrow Angle Camera (NAC) imagery and the Hapke (2012a) model to find that differences in photometric behavior between on-and off-swirl regolith could be explained by the higher single-scattering albedo of the swirl material, without any variations in porosity or roughness. Domingue et al (2023), also using LROC-NAC imagery and the Hapke (2012a) model, likewise found no differences in microscale roughness between on-and off-swirl areas in the Mare Ingenii swirl region. While it remains possible that there are also textural differences between the on-and off-swirl surfaces, the data presented in Figure 22 suggest that the phase curves depend primarily on composition and optical maturity.…”

Photometric Properties within the Reiner Gamma Swirl: Constraining Formation Mechanisms

“…This is suggestive of more complex grain structures (such as agglutinates and more weathered grains) on-swirl, assuming similar composition and grain sizes across all units. However, as has been demonstrated, the compositions are not similar and the topographic differences may contribute to size-sorting of dust grains between on-swirl and off-swirl (Domingue et al 2022(Domingue et al , 2023. The small comparative increase in backward scattering seen on-swirl could be attributed to coarser regolith grains dominated by surface scattering (due to either compositional or structural differences) than the regolith grains in the diffuse-or off-swirl units.…”

“…Correlations between the swirl unit and photometrically derived surface roughness have been observed in some studies (Pinet et al 2000;Kreslavsky & Shkuratov 2003;Kaydash et al 2009;Hess et al 2020Hess et al , 2023 but not in others (Domingue et al 2023;Hess et al 2023). Those that have observed correlations often times are based on empirical models (Pinet et al 2000;Kreslavsky & Shkuratov 2003;Kaydash et al 2009).…”

“…However, these studies either made broad assumptions (like uniform surface roughness between on-swirl and background) or did not directly model the photometric behavior of these regions (Hess et al 2020). For the regions that showed topographic correlations in Mare Ingenii, photometric analysis showed no microscale roughness differences between on-swirl and the darker albedo lanes between them (Domingue et al 2023). This suggests no variations in the "fairy-castle" structure of the regolith.…”

“…The pro and con evidence for each of these processes is many, and has been reviewed in the literature (e.g., Domingue et al 2023). In particular, Domingue et al (2023) reviewed the general spectral properties associated with swirls, and no one process appears to account for the spectral properties while explaining the correlation with magnetic field presence. In addition, recent studies have shown topographic correlations with different albedo units within swirls.…”