Rima Marius, the Moon: Formation of lunar sinuous rilles by constructional and erosional processes

Lunar sinuous rilles are large (kilometers long: tens of meters wide) channels commonly found on the lunar maria. Although there is broad consensus that sinuous rilles were generated during lava-flow emplacement, disagreement remains regarding the relative contributions of erosional and constructional processes during rille formation. Careful mapping of portions of Rima Marius (centered at 16.37 degrees N, 49.54 degrees W) using Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera (NAC) images with resolutions as high as 0.5 m/pixel reveal morphologic evidence for the origin of sinuous rilles. Specifically, shallow (<10s of meters), narrow (< 500m wide) rilles (herein called rillettes) are observed to anastamose around, and be cross-cut by, the main rille of Rima Marius. We interpret these rillettes to represent the early phases of emplacement of a sheet-like lava flow with a continuous surface crust (similar to terrestrial compound pahoehoe lava flows), in which flowing lava became concentrated into preferred pathways: the rillettes represent those early, and subsequently abandoned, preferred lava pathways. As the eruption continued, lava flow concentrated into a single lava tube (Rima Marius). Observations of active lava tubes at Kilauea volcano, Hawaii, indicate that basaltic lava flowing through an established lava tube can erode the underlying surface, even while the lava is advancing in a laminar fashion, and we infer similar processes occurred at Rima Marius. Thus, the rillettes represent an early, constructional phase of sinuous rille formation, and Rima Marius formed by subsequent downcutting caused by thermal and mechanical erosion. (C) 2018 Published by Elsevier Inc.