Newly Discovered Ring‐Moat Dome Structures in the Lunar Maria: Possible Origins and Implications

Abstract

We report on a newly discovered morphological feature on the lunar surface, here named Ring‐Moat Dome Structure (RMDS). These low domes (a few meters to ~20 m height with slopes <5°) are typically surrounded by narrow annular depressions or moats. We mapped about 2,600 RMDSs in the lunar maria with diameters ranging from tens to hundreds of meters. Four candidate hypotheses for their origin involving volcanism are considered. We currently favor a mechanism for the formation of the RMDS related to modification of the initial lava flows through inflated flow squeeze‐ups and/or extrusion of magmatic foams below a cooling lava flow surface. These newly discovered features provide new insights into the nature of emplacement of lunar lava flows, suggesting that in the waning stages of a dike emplacement event, magmatic foams can be produced, extrude to the surface as the dike closes, and break through the upper lava flow thermal boundary layer (crust) to form foam mounds and surrounding moats.

Ring‐Moat Dome Structures (RMDSs) in the Lunar Maria: Statistical, Compositional, and Morphological Characterization and Assessment of Theories of Origin

“…Figure 10b exhibits three RMDSs showing a complex spatial relationship with each other. The largest RMDS is about 550 m in diameter and tangent to a smaller one (~220 m) to its south, which appears as a satellite of the larger RMDS, and this arrangement was previously reported as dumbbell and short chain patterns by Zhang et al (2017). A small RMDS or lava dome (yellow arrow) is observed on the western flank of the largest RMDS, which partially superposes an ~500 m diameter impact crater to its northeast.…”

“…Some RMDSs are found adjacent to what appear to be morphologically mature and generally circular depressions (white arrows, Figure 8a) or even to partially superpose them (Basilevsky et al, 2019; Zhang et al, 2017). In the examples shown here, the mature craters partially superposed by RMDSs (Figures11a and 11b) usually exhibit a general symmetric profile of the cavity along the line cutting through superposed RMDSs.…”

Geological Characterization of the Ina Shield Volcano Summit Pit Crater on the Moon: Evidence for Extrusion of Waning‐Stage Lava Lake Magmatic Foams and Anomalously Young Crater Retention Ages

“…In addition, bright boulders are usually present within the marginal ring of topographic lows (Figure c). These moat‐mound structures share many morphological similarities with the so‐called ring‐moat dome structures (Zhang et al, ). We interpret these unusual central uplifted structures as possible miniature analogs of Ina mound units, and the marginal ring of lows analogous to the moats frequently occurring along the edges of Ina mounds, where fresh boulders are also often observed (Figure c).…”

Controls on Lunar Basaltic Volcanic Eruption Structure and Morphology: Gas Release Patterns in Sequential Eruption Phases

“…On a timescale of weeks, the resulting compound flows would cool, and the concentration of volatiles into the residual liquid as crystallization occurred would lead to second boiling, with the resulting new population of gas bubbles causing a further, possibly extensive, inflation episode. Extrusion of the resulting magmatic foam through cracks in the lava crust may be an explanation of the ring moat dome structures (RMDSs; Zhang et al, ) found in large numbers on many mare flows.…”

“…In other cases the foam intrusion raises the crust of the crater lake and overflows onto the upper flanks, where partial collapse of the foam produces another type of IMP, such as those on the flanks of Cauchy 5 (Qiao et al, ). Similar features occur on the floor of the Hyginus collapse caldera (Braden et al, ). RMDSs (Zhang et al, ) are interpreted to be the consequence of Phase 4b activity from a large‐volume dike. Earlier phases of the eruption emplace long volatile‐free lava flows.…”