LAUSR

Ring‐moat dome structures (RMDSs) are positive morphologic features found clustered across many mare regions on the Moon, of which only a few isolated examples have been previously reported. Our continuing survey has expanded the known locations of the RMDSs from ~2,600 to over 8,000, indicating that RMDSs are more common geological features than previously thought. This work presents a detailed geomorphological analysis of 532 RMDSs identified in several mare basins. The combination of detailed elemental mapping, morphological and morphometric analyses, spatial distribution relationships with other geologic structures, and comparison with terrestrial analogs lead us to conclude that (1) RMDSs represent low circular mounds with diameters of a few hundred meters (average about 200 m) and a mean height of 3.5 m. The mounds are surrounded by moats ranging from tens to over 100 m in width and up to several meters in depth; (2) there is a wide variation of titanium abundances, although RMDSs are more commonly found in mare regions of moderate‐to‐high titanium content (>3 wt% TiO2); (3) RMDSs are found to occur on or around fractures, graben, and volcanic edifices (small shields and cones); (4) a spatial association between RMDSs and Irregular Mare Patches (see Braden et al., 2014, https://doi.org/10.1038/ngeo2252) is observed, suggesting that both may form from related lava flows; (5) comparisons between RMDSs and lava inflationary structures on Earth support an inflation‐related extrusive nature and a genetic relationship with host lava flow processes; and (6) RMDS embayment relationships with craters of different degradation ages superposed on the host mare, and regolith development models, produces conflicting age relationships and divide theories of RMDS origin into two categories, (1) synchronous with the emplacement and cooling of the host lava flows ~3–4 Ga and (2) emplaced substantially after the host mare lava unit, in the period ~0–3 Ga. We outline the evidence supporting this age conundrum and implications for the different theories of origin and describe future research avenues to help resolve these outstanding questions. Plain Language Summary The research reported in this paper is focused on the statistical, compositional, and morphological characterization of a newly documented lunar lava flow surface feature characterized by very low, circular‐ or ellipse‐shaped mounds typically several hundred meters across and surrounded by ring depressions. Detailed measurement of 532 mounds shows that their diameters range from 68 to 645 m, and heights from 0.4 to 14 m. The surrounding ring depressions are tens to more than 100 m in width and have depths up to several meters. These mounds have complex relationships with circular and irregular depressions or degraded craters. Compositional mapping results show that this type of mound preferentially forms in lava flow fields of moderate to high titanium content (>3 wt% TiO2). Flow features, fractures, graben, and small shields and cones associated with ring‐moat dome structures support their extrusive nature and a genetic relationship with late stages of host lava flow emplacement and cooling. Some embayment relationships suggest, however, that ring‐moat dome structures could have formed many hundreds of millions of years to billions of years after emplacement of their host lava units. The documentation of these features has important implications for the understanding of mare basalt eruption and lava‐emplacement ©2020. American Geophysical Union. All Rights Reserved. RESEARCH ARTICLE 10.1029/2019JE005967 Key Points: • Ring‐moat dome structures in the lunar maria are characterized quantitatively • Greater than 8,000 RMDSs are mapped, more commonly occur in moderate‐high Ti mare • Candidate origins are derived: (1) contemporaneous with host mare (3–4 Ga) and (2) recent (0–3 Ga) Correspondence to: F. Zhang, [email protected] Citation: Zhang, F., Head, J. W., Wöhler, C., Bugiolacchi, R., Wilson, L., Basilevsky, A. T., et al. (2020). Ring‐moat dome structures (RMDSs) in the lunar Maria: Statistical, compositional, and morphological characterization and assessment of theories of origin. Journal of Geophysical Research: Planets, 125, e2019JE005967. https://doi.org/ 10.1029/2019JE005967 Received 7 MAR 2019 Accepted 8 APR 2020 Accepted article online 30 APR 2020 ZHANG ET AL. 1 of 41 mechanisms, and for the age and duration of mare basalt volcanism, one of the most important indicators of lunar thermal evolution and mantle geodynamics.