Investigating the stratigraphy of Mare Imbrium flow emplacement with Earth‐based radar

The lunar maria are the product of extensive basaltic volcanism that flooded widespread portions of the Moon's surface. Constraining mare volcanic history therefore provides a window into the endogenic processes responsible for shaping the Moon. Due to the low magma viscosity and the associated thin nature of lava units, the majority of mare surface structures are masked and subdued by impact regolith. Subtle individual mare flow morphologies, coupled with spatial limitations in the use of crater size distributions to distinguish surface units close in age, restrict our understanding of mare stratigraphy. Earth-based 70 cm wavelength (P band) radar can reveal features beneath the regolith and highlight very subtle changes in the ilmenite content of the flows, providing a unique means to map mare units. Here we map volcanic units in Mare Imbrium using high-resolution (200 m/pixel), Earth-based P band data. Situated within the heat-producing potassium, rare earth element, and phosphorus terrane, Mare Imbrium experienced some of the most long-lived (and recent) lunar volcanism, and its surface exhibits a significant diversity of basaltic chemistry. Our investigation identifies at least four distinct stages of volcanic activity, originating from multiple sources within Imbrium. The most recent of these stages comprises extensive, yet relatively thin volcanic flow units that left remnant kipukas of older mare material distributed across much of the basin. From a future mission perspective, it may be possible to collect samples expressing a wide range in age from small areas of Mare Imbrium. Our map also places important constraints on the interpretation of the Chang'e-3 Lunar Penetrating Radar measurements.