Lunar Sosigenes IMP as analyzed with orbital spectrophotometry and polarimetry

Lunar Irregular Mare Patches (IMP) are small sharp-looking geomorphic features of debated origin. Optical data of the Sosigenes (IMP) obtained by the Kaguya mission show that the composition (FeO and TiO₂ content) and degree of optical maturity (OMAT) are almost identical between the Sosigenes mounds and the surrounding IMP flank areas. In contrast, the hummocky floor areas and the flank regions differ significantly in these properties. The same relationships are observed in PolCam polarimetric data from the Danuri mission. In particular, the hummocky floor areas demonstrate a larger mean particle size of the regolith than that of surrounding areas. Spectral measurements of Sosigenes carried out with the Chandrayaan 1 M³ instrument reveal that the hummocky areas are also characterized by a more pronounced 3μm absorption band, indicating an excess of H₂O/OH⁻ compounds. The young age of the areas derived from crater statistics is consistent with LROС NAC photometric data that predict undeveloped fairy-castle microstructure of the regolith. Among the proposed formation mechanisms, the roof-collapse model best explains our optical observations. In this model, the lava lake of the Sosigenes dike cooled and roofed over. The underlying lava drained, forming a void with pillars supporting the roof. In the last tens of millions of years, the roof collapsed due to impact/tectonic seismic activity. As a result, many craters and portions of the regolith on the mounds were removed, and the floor areas formed by blocks of the collapsed roof and the local material. The collapse did not occur from the full height of the Sosigenes structure, but from a height of a few meters. Preservation of the mounds and the absence of severe damage to their edges support this hypothesis.