Ring-moat dome structures (RMDSs) are small circular mounds of diameter typically about 200 m and ∼3–4 m in height, surrounded by narrow, shallow moats. They occur in clusters, are widespread in ancient Imbrian-aged mare basalt host units and show mineralogies comparable to those of their host units. Based on these close associations and similarities, a model has been proposed for the formation of RMDS as the result of late-stage flow inflation, with second boiling releasing quantities of magmatic volatiles that migrate to the top of the flow as magmatic foams and extrude through cracks in the cooled upper part of the flow to produce the small RMDS domes and surrounding moats. In contrast to this model advocating a contemporaneous emplacement of RMDSs and their host lava flows, a range of observations suggests that the RMDS formed significantly after the emplacement and cooling of their host lava flows, perhaps as recently as in the Copernican Period (∼1.1 Ga to the present). These observations include: (a) stratigraphic embayment of domes into post-lava flow emplacement impact craters; (b) young crater degradation age estimates for the underlying embayed craters; (c) regolith development models that predict thicknesses in excess of the observed topography of domes and moats; (d) landform diffusional degradation models that predict very young ages for mounds and moats; (e) suggestions of fewer superposed craters on the mounds than on the adjacent host lava flows, and (f) observations of superposed craters that suggest that the mound substrate does not have the properties predicted by the magmatic foam model. Together, these observations are consistent with the RMDS formation occurring during the period after the extrusion and solidification of the host lava flows, up to and including the geologically recent Late Copernican, that is, the last few hundreds of millions of years of lunar history. We present and discuss each of these contradictory data and interpretations and summarize the requirements for magma ascent and eruption models that might account for young RMDS ages. We conclude with a discussion of the tests and future research and exploration that might help resolve the RMDS age and mode of emplacement conundrum.

中文翻译：

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月球环护城河穹顶结构 (RMDS) 时代难题：与雨海时代的寄主熔岩流同时期还是位于哥白尼？

环形护城河圆顶结构 (RMDS) 是直径通常约为 200 m、高约 3-4 m 的小型圆形土丘，周围环绕着狭窄而浅的护城河。它们成群出现，广泛存在于古老的雨海纪玄武岩寄主单位中，并显示出与其寄主单位相当的矿物学。基于这些密切的联系和相似性，提出了一个模型，用于后期流动膨胀导致 RMDS 的形成，第二次沸腾释放大量岩浆挥发物，它们以岩浆泡沫的形式迁移到流动的顶部并通过气流冷却的上部出现裂缝，产生小的 RMDS 圆顶和周围的护城河。与这种提倡同时安置 RMDS 及其宿主熔岩流的模型相比，一系列观察结果表明，RMDS 是在其宿主熔岩流就位和冷却之后形成的，可能就在哥白尼时期（~1.1 Ga 到现在）。这些观察结果包括： (a) 穹顶在熔岩流就位后撞击坑中的地层分布；(b) 潜在凹坑的年轻陨石坑退化年龄估计；(c) 预测厚度超过观察到的穹顶和护城河地形的风化层发展模型；(d) 预测土丘和护城河非常年轻的年龄的地形扩散退化模型；(e) 建议土丘上的叠加陨石坑比相邻的宿主熔岩流上的少，以及 (f) 对叠加陨石坑的观察表明土墩基底不具有岩浆泡沫模型预测的特性。一起，这些观察结果与在宿主熔岩流挤压和凝固之后的时期发生的 RMDS 形成是一致的，直到并包括地质上最近的哥白尼晚期，即最近几亿年的月球历史。我们呈现并讨论了这些相互矛盾的数据和解释中的每一个，并总结了可能解释年轻 RMDS 年龄的岩浆上升和喷发模型的要求。我们最后讨论了可能有助于解决 RMDS 年龄和就位难题的测试以及未来的研究和探索。最近几亿年的月球历史。我们呈现并讨论了这些相互矛盾的数据和解释中的每一个，并总结了可能解释年轻 RMDS 年龄的岩浆上升和喷发模型的要求。我们最后讨论了可能有助于解决 RMDS 年龄和就位难题的测试以及未来的研究和探索。最近几亿年的月球历史。我们呈现并讨论了这些相互矛盾的数据和解释中的每一个，并总结了可能解释年轻 RMDS 年龄的岩浆上升和喷发模型的要求。我们最后讨论了可能有助于解决 RMDS 年龄和就位难题的测试以及未来的研究和探索。