Impact melt breccia and surrounding regolith measured by Chang'e-4 rover

Abstract

Chang'e-4 rover discovered a dark greenish and glistening impact melt breccia in a crater during its traverse on the floor of Von Kármán crater within the South Pole Aitken (SPA) basin on the lunar farside. The discovered breccia, being 52 × 16 cm, resembles the lunar impact melt breccia samples 15466 and 70019 that returned by the Apollo missions. It was formed by impact-generated welding, cementing and agglutinating of lunar regolith and breccia. Clods surrounds the breccia-hosting crater were crushed into regolith powders by the rover's wheels, indicating the regolith may be compacted slightly and becomes blocky and friable. Relative mineral fractions are estimated from the in situ measured spectra by a Hapke model-based unmixing algorithm. Unmixing reveals that plagioclase (PLG, 45 ± 6%) is dominant in the regolith, followed by almost equal fractions of pyroxene (PYX, 7 ± 1%) and olivine (OL, 6 ± 2%), indicating the regolith is likely related to noritic rocks. The regolith measured by Chang'e-4 rover was actually a highly mixture of multiple sources, with ejecta from Finsen crater being primary and possible contributions from Alder crater. Finsen and Alder craters are on the margin of the proposed impact melt pool produced by the SPA basin-forming event. Therefore, the ultimate source of the regolith might originate from a differentiated melt pool or from a suite of igneous rocks.