Ultra-Thick Paleoregolith Layer Detected by Lunar Penetrating Radar: Implication for Fast Regolith Formation Between 3.6 and 2.35 Ga

Lunar paleoregolith was formed by repeated asteroid impact and space weathering and then buried by later lava flows, serving as important records for early solar system history. However, direct observational evidence for the paleoregolith layer is rather limited. We present the evidence for the existence of the paleoregolith layer by processing 60 MHz lunar penetrating radar data acquired by the Chang'E-3 Yutu rover. We find successive reflections with reversed polarities due to a low permittivity (paleoregolith) layer sandwiched in two high permittivity (lava) layers. From modeling and migration imaging of radar reflections, we determine an ultra-thick paleoregolith layer (similar to 5-9 m) beneath the Eratosthenian unit and on the top of the Imbrian unit, suggesting a high regolith production rate of 5.8-10.5 m/Ga between late Imbrian and early Eratosthenian periods compared to the previous estimation similar to 2 m/Ga, implying fast regolith formation and possible high meteoric flux during these periods. Plain Language Summary The surface of the Moon experienced multiple volcano eruptions since its birth and the large basin was filled with dark lava flows. Within this period due to repeated asteroid impact and space weathering between lava flows, the paleoregolith layers will be likely formed and buried by younger lava flows. The paleoregolith layers have been undisturbed since their formation thus are critical for determining early impacting and volcano history of the Moon and are regarded as important records for uncovering early Moon's history and early solar system history. We present the evidence for the existence of the paleoregolith layer in the northern Mare Imbrium basin by processing 60 MHz lunar penetrating radar data acquired by the Chang'E-3 Yutu rover. From modeling and migration imaging of radar reflections, we determine an ultra-thick paleoregolith layer (similar to 5-9 m) beneath the Eratosthenian unit and on the top of the Imbrian unit, implying fast regolith formation and possible high meteoric flux during these periods. Key Points Successive reflections with reversed polarities due to a low permittivity layer sandwiched in two high permittivity layers An ultra-thick paleoregolith layer (similar to 5-9 m) beneath Eratosthenian unit and on the top of Imbrian unit A high regolith production rate of 5.8-10.5 m/Ga between late Imbrian and early Eratosthenian periods compared to previous estimate similar to 2 m/Ga