First look by the Yutu-2 rover at the deep subsurface structure at the lunar farside

Abstract

The unequal distribution of volcanic products between the Earth-facing lunar side and the farside is the result of a complex thermal history. To help unravel the dichotomy, for the first time a lunar landing mission (Chang’e-4, CE-4) has targeted the Moon’s farside landing on the floor of Von Kármán crater (VK) inside the South Pole-Aitken (SPA). We present the first deep subsurface stratigraphic structure based on data collected by the ground-penetrating radar (GPR) onboard the Yutu-2 rover during the initial nine months exploration phase. The radargram reveals several strata interfaces beneath the surveying path: buried ejecta is overlaid by at least four layers of distinct lava flows that probably occurred during the Imbrium Epoch, with thicknesses ranging from 12 m up to about 100 m, providing direct evidence of multiple lava-infilling events that occurred within the VK crater. The average loss tangent of mare basalts is estimated at 0.0040-0.0061.

Ejecta From the Orientale Basin at the Chang'E‐4 Landing Site

“…The strength and spatial distribution of echoes are referred for the subjective assignment of the boundaries. estimation is consistent with the depth distribution of echo patterns in LPR radargrams, as the first sharp transition from weak to strong echoes, which was interpreted as the boundary between the younger debris cover and deeper competent mare basalts, is located at ∼45 m (Lai et al, 2020;Zhang et al, 2020).…”

“…It has been a consensus that ejecta from Finsen are the major source of the surface regolith (Li et al, 2019;Gou et al, 2020;Huang et al, 2020;Lin et al, 2019). However, ballistic sedimentation models predicted a dominance of mare basalts in the debris cover that was excavated by distal ejecta (Lai et al, 2020). Essentially, lunar regolith is a weathering product (McKay et al, 1991).…”

“…For comparison, the predicted thickness of Finsen's ejecta and their local excavation was <8 m at the landing site (Huang et al, 2018;Lai et al, 2020), which is contradicted with the top ∼10-13 m thick materials being entirely developed in Finsen's ejecta (Lai et al, 2019). One hydrocode simulation argued that Finsen's ejecta were >30 m thick at the landing site (Di et al, 2019), which was not supported by observations based on reflectance spectra (Li et al, 2020;Qiao et al, 2019) or predictions based on ballistic sedimentation models (Huang et al, 2018;Lai et al, 2020). On the other hand, discontinuous layering of weak and strong echoes are observed in the LPR radargrams ( Figure 1e), with the consensus being emplaced as distal ejecta (Lai et al, 2020(Lai et al, , 2019Li et al, 2020;Zhang et al, 2020).…”

“…The estimated total thickness of primary ejecta from all known source craters is less than 10 m (Huang et al., 2018). Even assuming Alder had delivered impact ejecta to the mare surface (∼8 m thick; Lai et al., 2020), the estimated total thickness of primary ejecta plus local excavation is much less than the observed thickness of post‐mare deposits. Nevertheless, Alder has been frequently cited as a post‐mare crater to explain the thick debris cover (e.g., Zhang et al., 2020).…”

An updated constraint on the local stratigraphy at the Chang’E-4 landing site

“…A scientific target is to understand the formation of regolith at the lunar farside. In the radargram obtained, materials deeper than ~40–45 m are believed to be mare basalts that filled in the floor of Von Kármán; materials shallower than ~10–13 m (Lai JL et al, 2019, 2020; Li CL et al, 2020; Ding CY et al, 2020; Zhang L et al, 2020) have dielectric properties similar to typical lunar regolith.…”