The Maskelyne irregular mare patch as interpreted with lunar optical data, including Danuri polarimetric measurements

Irregular Mare Patches (IMP) are peculiar morphological features of debated origin on the Moon. Optical measurements of the elongated (~2.5 × 1.5 km) Maskelyne IMP show that the composition (FeO and TiO 2 content) and degree of optical maturity ( OMAT ) are approximately same for the mounds and the surrounding mare regions and are noticeably distinct from the Maskelyne rough floor areas. The polarimetric data retrieved from the Danuri mission support a similar interpretation; in particular, the rough floor areas display a higher abundance of TiO 2 and larger mean particle size than the other regions. We found that the roof-collapse scenario is consistent with these observations. This implies that in the Maskelyne pit a lava lake was created, which then cooled and solidified from the upper surface downward, forming a solid roof. Meanwhile the underlying lava lake subsequently drained into the cracks, forming a void, with preferentially cooled vertical pillars supporting the roof. Over the subsequent few billion years, regolith-formation processes occurred. Recently, the ancient-roofed void collapsed due to an impact and associated tectonic seismic activity. Many of the superposed craters on the mound surfaces were lost. The rough floor terrain was formed simultaneously from the blocks and debris of the fallen roof and local materials, the composition of which depends on lava-flow stratigraphy. The mounds formed from collapses around the pillars and shedding of regolith. In the northeastern part of Maskelyne there is a large mound that can be interpreted as a ring-moat dome that may have volcanic origin. New craters were then formed after the collapse, but their distribution density is much lower than on the surrounding areas, indicating the recent age of the collapse event that may be related, e.g., with the Tycho impact. This hypothesis explains the similarity of the composition and maturity of the regolith of the mounds and areas surrounding Maskelyne and the difference between the composition of the mounds and the rough floor terrain. The young age of the terrain is consistent with the photopolarimetric data that predict undeveloped fairy-castle microstructure and the presence of large particles in the regolith.