Search

Radargrams obtained by the Yutu‐2 rover reveal that the post‐mare deposits at the Chang'E‐4 landing site are ∼45 m thick, consistent with estimations based on orbital observations. Besides obvious ejecta from Finsen, polarized interpretations exist on whether or not the Alder crater is another contributor to the post‐mare deposits, although predicted thicknesses of ejecta from all potential source craters are not adequately large compared to observations. We recognize that the rover has been exploring along a shared crater wall of two secondaries from Finsen. Mechanics of secondary impacts suggests that the discontinuous layers of coarse and fine materials observed in the radargram are older than Finsen. Cross‐cutting relationships and crater density comparisons show that Alder is older than the mare basalts, and the Orientale basin was the major source for the post‐mare materials. Lower‐crust materials excavated by Orientale constituted a substantial portion of the surface regolith detected by the rover. Plain Language Summary: Understanding surface processes at the lunar farside and revealing potential compositions related with SPA are two scientific targets of the Chang'E‐4 mission. Orbital observations and radargrams obtained by the rover show that the mare basalts at the landing area are covered by > 30 m thick deposits. Finsen has delivered bright rays to this area, but whether or not Alder is another source has been debated. Considering all potential sources that even include Alder, predicted thickness of post‐mare materials is substantially less than observations. We recognize that Yutu‐2 is exploring along the shared wall of two secondaries from Finsen, and subsurface materials are older than Finsen. Orientale's secondaries are traceable on top of Alder. The crater density on Orientale's ejecta deposits is comparable with that of the mare surface at the landing site, and morphological comparison of secondaries indicates that substantial ejecta had been deposited in Von Kármán before the volcanism in Leibnitz was ceased. The limited time window suggests that Orientale's ejecta were the only suitable source for the post‐mare materials, forming the discontinuous layers of coarse and fine materials revealed in radargrams. Surface regolith examined by Yutu‐2 should contain lower‐crust materials that were excavated by Orientale. Key Points: The Yutu‐2 rover has been exploring along the rim of Finsen's secondaries, and pre‐Finsen materials dominate the subsurfaceCrater density comparisons reveal that Orientale is a major source of the post‐mare ejecta, explaining the radargram obtainedSurface regolith detected by Yutu‐2 is dominated by lower‐crust materials ejected by Orientale, not related with SPA [ABSTRACT FROM AUTHOR]

The geologic analysis of lunar tectonic and co‐existing mass wasting features provides important insights into seismicity, seismic ground motion, and the seismic risk of the landed missions. Hence, we performed a detailed geologic mapping of tectonic features and co‐seismic boulder avalanches present at the 930‐km‐diameter lunar Orientale basin. We traced 6869 boulder falls at 141 sites along the basin rings, impact crater walls, and volcanic features. The boulder fields also occur on the graben walls and wrinkle ridges. The boulder trail length and width, and slopes of trail and source region of fallen boulders provide insights into formation processes and transport dynamics. Tens of sites have anomalously larger number of boulder falls and trail density. The cumulative size‐frequency distributions of superimposed impact craters present on the boulder trails provided their model formation ages varying between present and 7.5 Ma with at least eight formation episodes, suggesting the timings of seismic activities along the Cordillera and Outer Rook Rings. Many boulder falls are devoid of superimposed impact craters suggesting their recent origins, including some possibly triggered by the 1972 (MW 2.7) shallow moonquake that occurred near the Cordillera Ring. The formation rates of shallow moonquakes and impact craters along with the seismic ground motion simulations suggest that shallow moonquakes are more dominant sources of ground shaking for triggering the boulder falls in the last 30 million years. Therefore, the Orientale basin has been seismically active for long time and it may be seismically hazardous locally for the future landed missions. Plain Language Summary: Seismic hazard assessment of the Moon provides essential information for developing seismic‐resilient lunar bases where long‐term scientific exploration can be carried out. The 930‐km‐diameter Orientale basin has been proposed to be the landing site for the future human and robotic missions. Hence, we assessed whether the basin is seismically active and hazardous to the landed missions. For this, we relied on the local geology, mapped the boulder fall occurrences throughout the basin interior, characterized their morphology and formation ages, estimated the relative abundances of the shallow moonquakes and impact events, and modeled the seismic ground motion scenarios. Our study confirms that the Orientale basin is seismically active. The formation ages of boulder falls point to episodically occurring shallow moonquakes in the last eight million years. Our seismic ground motion simulations indicate that the shallow moonquakes are the major triggering sources for the origins of boulder falls in the basin. Although the Orientale basin may be locally hazardous to the future landed missions, seismically safe regions are also present in the basin. It is also a great place for the observation of local to globally occurring shallow moonquakes and we recommend a seismic observation network for this basin. Key Points: Thousands of boulder falls of varying density and morphometric properties are traced at hundreds of sites on the Orientale basinAt least eight episodes of boulder fall events ranging in ages from present to 7.5 Ma are recognized in the Orientale basinThe Orientale basin is seismically active for the last few million years and shallow moonquakes triggered the majority of boulder avalanches [ABSTRACT FROM AUTHOR]

Abstract: Secondary impact craters, features created by projectiles ejected from a primary impact, contain important information about the primary cratering event and the nature and distribution of its ejecta. The Orientale impact basin (D ~ 930 km) is the youngest and the least degraded large impact basin on the Moon and has the most recognizable secondary impact craters. We identified and mapped 2,728 secondary craters in the investigated area of ~1.66 × 107 km2, covering an area from the rim of Orientale to six radii. Secondary crater diameters range from ~2 to 27 km, and the median diameter decreases as distance increases. Secondary craters are concentrated predominantly in the northwest and southwest. The ejecta deposit pattern inferred from secondary crater distribution suggests that the Orientale basin was formed by an oblique impact in which the downrange direction was 240°–265° in azimuth, and the incidence angle was steeper than 20°. The cumulative size‐frequency distribution of mapped secondary craters steepens as diameter increases and is very well approximated with a Weibull distribution with an exponent 1.32. A widely used crater scaling relationship predicts that the fragments that produced the secondary craters were predominantly in ~0.5–2‐km diameter range over the investigated area; the diameter of the largest fragment, however, decreases with increasing distance from Orientale. On the basis of the diameter of the largest secondary crater of Orientale, and other craters and basins, the largest secondary crater of the South Pole‐Aitken basin is estimated to be ~40 km in diameter. We explore the implications of these findings for the evolution of the megaregolith and future sample return missions. [ABSTRACT FROM AUTHOR]