Characterization of the surface of the Ina irregular Mare patch: A synthesis of lunar optical data, including Danuri polarimetric measurements

Optical measurements of the enigmatic Ina lunar Irregular Mare Patch (IMP) summit pit crater show that the composition (FeO and TiO₂ content) and degree of optical maturity (OMAT) are almost identical for the flat-topped mounds and the surrounding pit crater flank areas of the Ina formation. In contrast, the Ina rough floor unit and the flank regions are very different. The same relationships are observed when using PolCam polarimetric data of the Danuri mission, as well as Earth-based telescope measurements. In particular, with optical data the rough floor areas display a higher abundance of TiO₂ and larger mean particle size. Chandrayaan 1 M³ spectral data for Ina reveal a locally more intensive absorption band near 3$\mathrm{\mu m}$, indicating an excess of H₂O/OH⁻ compounds. Using these data, we analyze different models for the formation of the Ina pit crater, finding that the roof-collapse scenario appears most consistent with the observations. In this scenario, ancient basalt eruptions from the proto-Ina vent created the shield volcano. Next, the lava lake in the volcanic pit crater cooled and solidified from the upper surface downward, and roofed over, while the underlying lava lake, assisted by magmatic degassing, subsequently drained into the vent below, forming a void, with preferentially cooled vertical pillars supporting the roof. Over the next several billion years, detailed traces of the roofing event were obscured by regolith-formation processes. Recently, in the last tens of millions of years, the ancient, roofed void collapsed due to an impact and associate impact/tectonic seismic activity. The seismic effects, which accompany the formation of the crater Tycho, are considered as a feasible cause of the collapse. During this collapse, many of the superposed craters, and a portion of the regolith on the mound surfaces were lost. The rough floor terrain formed simultaneously from the collapse of blocks, debris, and shedding of regolith of the roof materials fallen in voids. The terrain were formed around groups of the pillars. These flat-topped mounds are interpreted as surviving roof remnants. New craters were then formed, but their density distribution is much lower than on the surrounding Ina shield volcano, indicating the recent age of the collapse event. This model explains the similarity of the composition and maturity of the regolith of the mounds and surrounding areas, as well as difference between the composition of the mounds and rough floor terrain. This recent roof-collapse model is also in agreement with the excess of H₂O/OH⁻ compounds in the rough floor terrain that are considered to be young. The young age of the rough floor terrain is consistent with the photometric and polarimetric data that predict undeveloped fairy-castle microstructure and the availability of large particles in the regolith.