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 (~5–9 m) beneath Eratosthenian unit and on the top of Imbrian unit, implying fast regolith formation and possible high meteoric flux during these periods.

LPR data after data processing:

We discovered one strong and continuous reflection (denoted by R1 in Fig. 2) at about 450 ns (i.e., the two-way travel time of reflected radar waves from subsurface). The polarity reversed R2 reflection at about 700 ns can be identified as well. R3 is reversed again. We illustrated a conceptual model of reversed polarity in the right panel.

Further, we conducted a forward modeling of electromagnetic reflection waves using a series of relative permittivity models by finite-difference method. We aim to match computer simulation LPR data (red) to Chang’E 3 LPR data (blue) to optimize the depth-profile permittivity (in below Figure d).

Reference

Zhu T., Zhang J. and Lin Y.T., (2021). Ultra-thick paleoregolith layer detected by lunar penetrating radar: implication for fast regolith formation between 3.6 and 2.35 Ga. Geophysical Research Letters, 48, e2021GL095282. PDF  https://doi.org/10.1029/2021GL095282