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Basalts from Mare Orientale are representative of lunar flood volcanism, which sheds light on the lunar farside's thermal and volcanic past. We use Chandrayaan's Moon Mineralogy Mapper data to examine the spectral and chemical makeup of the volcanic units located in the Orientale basin; the analysis specifically focuses on three formations: Mare Orientale, Lacus Veris, and Lacus Autumni. The main assemblage in these basaltic units consists of calcic augite and ferroaugite. Pyroxenes in the Orientale volcanic units have an average chemical composition of En35.53 Fs34.11 Wo30.35. The trend in the composition of pigeonites and augites suggests that the magma was fractionated as it crystallized. The pyroxene quadrilateral plot's distinct chemical trends indicate that the Orientale Basin underwent a number of volcanic eruptions from heterogeneous magma sources during the Imbrium to Eratosthenian period. [ABSTRACT FROM AUTHOR]

Previous studies have provided evidence for the synchronous rotation induced cratering asymmetry on lunar surface through numerical simulations and statistical analysis of a limited number of fresh craters. In this study, we reevaluated cratering asymmetry in lunar highland from (70°W, 60°N) to (70°E, 60°S) region using a new crater catalogue with diameters (D) ranging from 1 to 20 km. By utilizing a depth-to-diameter (d/D) ratio constraint to exclude the interference of degraded and secondary craters, we observed significant asymmetry in craters with d/D > 0.15. Moreover, leveraging the characteristic that larger diameter craters (D > 7 km) are less susceptible to degradation, we observed a more pronounced asymmetry with increasing diameter. Particularly, impact craters with larger D and d/D ratios (D > 7 km, d/D > 0.15) displayed an asymmetrical longitudinal distribution, aligning with predictions from the theoretical model. In the diameter range of 10 km–20km, for craters with d/D > 0.15, we observed that new crater influx occurring after 4.0 Ga years ago contributed little to this particular crater population. Therefore, we suggest that the cratering asymmetry was already present before 4.0 Ga. Due to the non-uniform ejecta from the Orientale Basin onto the highland regions, a significant number of smaller impact craters (1–5 km) have degraded or disappeared in the leading region, thereby diminishing the manifestation of the cratering asymmetry. The pronounced asymmetry exhibited in our statistical results might suggest the existence of a significant population of low-velocity impactors in early impact period (>4Ga) around the cis-lunar space. • Lunar impact craters with d/D > 0.15 and D > 5 km exhibit noticeable cratering asymmetry. • The cratering asymmetry was already present before 4.0 Ga. • The eject from the Orientale basin diminishes the manifestation of the cratering asymmetry of smaller craters (1–5 km). • The pronounced asymmetry might suggest the existence of a significant population of low-velocity impactors in early period. [ABSTRACT FROM AUTHOR]

Secondary craters (i.e., secondaries) formed by the landing of impact ejecta are an important crater population in planetary geology. Typical secondaries on planetary surfaces occur in chains and clusters, and they have irregular-shaped crater rims and/or floors. Background secondaries are a special type of secondaries that were formed by more-dispersed impact ejecta, and they have similarly circular morphology and dispersed spatial distribution with same-sized primary craters (i.e., primaries) that are formed by asteroids and comets. While individual background secondaries have been recognized at different planetary bodies, there is no reliable method to differentiate populations of background secondaries from those of primaries. In this work, we design a novel method to extract background secondaries based on multiscale and multifacies topography characteristics of impact craters. To characterize the complex topography fluctuations across continuous ejecta deposits of secondaries, we design a multiwavelet method to present their full-wavelength topography variations. Selecting three populations of secondaries formed by the Orientale basin and a population of primaries, we investigate and compare their crater interior slopes and roughness of continuous ejecta deposits, determining the threshold ranges that can differentiate background secondaries from same-sized and coeval primaries. The two sets of threshold values are then fused together to create a standard T-score terrain indicator, which are combined with our modeled theoretical spatial distribution of secondaries, and abundant previously unrecognized Orientale background secondaries were discovered. Our results demonstrate that the production of background secondaries is much more efficient than manifested by the observed spatial density of obvious secondaries in chains and clusters. This study establishes a promising method to detect populations of background secondaries, which can be extended further to smaller primary craters on various airless planetary bodies. [ABSTRACT FROM AUTHOR]

The specific duration between the impact event and subsequent volcanic flows is highly variable based on previous works. The method of crater size-frequency distribution (CSFD) has been previously used to date the basalt in Orientale Basin, which yielded inconsistent resultant Absolute Model Age (AMA) ranges. The inconsistency may be attributed to the choice of counting area and identified superposed craters. In this study, we integrated the Chang'E-2 (CE-2) imaging data (7 m/pix) and the IIM and 20 m CE-2 DTMS data, re-divided Mare Orientale, and re-estimated the age of the basalts there. The ages revealed that (1) the central basalts had multiphase eruptions, beginning at 3.77 Ga (30 My after the impact event) with the longest duration of 1.51 Gy; (2) the edge basalts have a similar features as the central basalts, beginning at 3.75–3.50 Ga (50–300 My after the impact) with the longest duration of 0.67 Gy. Compared with the basalts along the basinal margin, the central basalts have higher Ti but lower Mg# contents, consistent with the basaltic magma fractionation trend. Spatial distribution characteristics indicate that the basalt eruption occurred in the impact direction upstream and in the center, but almost absent in the impact direction downstream. Accordingly, we speculate that the longevity of the lunar mare basaltic volcanism was affected by gravity changes, material balance, and other post-impact processes. [ABSTRACT FROM AUTHOR]

Basalts from Mare Orientale are representative of lunar flood volcanism, which sheds light on the lunar farside’s thermal and volcanic past. We use Chandrayaan’s Moon Mineralogy Mapper data to examine the spectral and chemical makeup of the volcanic units located in the Orientale basin; the analysis specifically focuses on three formations: Mare Orientale, Lacus Veris, and Lacus Autumni. The main assemblage in these basaltic units consists of calcic augite and ferroaugite. Pyroxenes in the Orientale volcanic units have an average chemical composition of En35.53 Fs34.11 Wo30.35. The trend in the composition of pigeonites and augites suggests that the magma was fractionated as it crystallized. The pyroxene quadrilateral plot’s distinct chemical trends indicate that the Orientale Basin underwent a number of volcanic eruptions from heterogeneous magma sources during the Imbrium to Eratosthenian period.

Light plains on the Moon are distinctive smooth deposits formed by large impact basins. Featuring diverging morphology and frequent spatial association with large secondary craters, light plains were interpreted to be formed either by ground-hugging ejecta flows and/or debris flows triggered by landing of impact ejecta. Light plains located in closed depressions, term ponded light plains (PLPs) here, cover ∼3% of the lunar global surface. They lack diagnostic geomorphology that could be ascribed to secondary impacts or primary ejecta, suggesting a possibly different emplacement mechanism, which has not been investigated before. PLPs feature comparable morphology with both lunar mare that were mainly emplaced by effusive volcanism and large impact melt pools that were deposited by melt-rich ejecta in rims and terraces of large impact basins. For such smooth plain materials, topography roughness at small scales might be an indicative information for their different emplacement styles. This study analyses topography roughness for PLPs and similar-sized melt pools that were formed by the Orientale basin and also coeval mare units on the Moon. We find that mare units have smaller roughness than same-aged melt pools, because large entrained blocks are abundant in melt pools that likely yielded larger viscosity. We further notice that PLPs have a medium topography roughness between coeval mare units and large melt pools, and the topography roughness of PLPs is closer to that of mare units. Correlated observations using high-resolution images suggest that the smoother topography of PLPs than cogenetic melt pools are mainly due to the minor existence of entrained blocks in PLPs. This comparison suggests that while PLPs and melt pools are both basin-related smooth ejecta deposits, PLPs may feature a lower viscosity during the emplacement, and they are most consistent with being melt-rich primary ballistic ejecta that were accumulated in local depressions. Considering that the youngest multiple-ring basin on the Moon, the Orientale basin, is capable of forming numerous PLPs across most parts of the entire Moon, this work predicts that both disrupted and sandwiched melt-rich ejecta deposits should be widespread in the lunar highland crust due to earlier-formed impact basins. • Light plains and melt pools formed by the Orientale basin and contemporaneous mare units are compared. • Small-scale roughness of light plains in closed depressions is closer to that of mare units than melt pools. • Light-plains in closed depressions were emplaced by filling of low-viscosity melt-rich primary ejecta. [ABSTRACT FROM AUTHOR]

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 specific duration between the impact event and subsequent volcanic flows is highly variable based on previous works. The method of crater size-frequency distribution (CSFD) has been previously used to date the basalt in Orientale Basin, which yielded inconsistent resultant Absolute Model Age (AMA) ranges. The inconsistency may be attributed to the choice of counting area and identified superposed craters. In this study, we integrated the Chang’E-2 (CE-2) imaging data (7 m/pix) and the IIM and 20 m CE-2 DTMS data, re-divided Mare Orientale, and re-estimated the age of the basalts there. The ages revealed that (1) the central basalts had multiphase eruptions, beginning at 3.77 Ga (30 My after the impact event) with the longest duration of 1.51 Gy; (2) the edge basalts have a similar features as the central basalts, beginning at 3.75–3.50 Ga (50–300 My after the impact) with the longest duration of 0.67 Gy. Compared with the basalts along the basinal margin, the central basalts have higher Ti but lower Mg# contents, consistent with the basaltic magma fractionation trend. Spatial distribution characteristics indicate that the basalt eruption occurred in the impact direction upstream and in the center, but almost absent in the impact direction downstream. Accordingly, we speculate that the longevity of the lunar mare basaltic volcanism was affected by gravity changes, material balance, and other post-impact processes.

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]

Abstract: Topographical, morphological and spectral reflectance studies have been carried out for a distinct resurface event inside Copernican aged Lowell crater (13.00°S 103.40°W), Orientale basin, using high resolution TC, MI-VIS, LROC-NAC, and M3 data from Kaguya, Lunar Reconnaissance Orbiter (LRO) and Chandrayaan-1 missions. The resurfacing is predominantly gabbroic/basaltic in composition and is confined to nearly a linear ~17km long, a 3–6km wide and a ~100m deep channel, possibly a graben. It is characterised with distinct surface features such as small uplift with melt pond, several lava-like flows, cracks going up to decimetre size, 20–80m pits/craters with small central uplifts or depressions and ~100m craters that emanate liquid. A minimum of three generations of flows have been identified within the unit, the oldest one being less viscous and the subsequent younger ones showing well developed lobes due to the high viscosity. There is a conspicuous absence of unambiguously identified primary impact craters on these flows suggesting their fresh nature. On the basis of these integrated observations we hypothesise that at least the younger portions of this amazingly carved resurfaced unit might be composed of volcanic flows erupted from single or multiple sources subsequent to the emplacement of impact melts from a ~9km diameter crater on the edge of Lowell crater. Gabbroic/basaltic signatures have also been identified at several other locations inside Lowell crater indicating that it would have impacted on a pre-existing basaltic surface or on a gabbroic pluton. These findings have implications to lunar magmatism and understanding of the genesis of young flows on the lunar surface. [Copyright &y& Elsevier]

The lunar multiring mare basins have different ages, compositions and are associated with the diverse unit of mare deposits, providing evidence for the origin and evolution of the lunar crust. The multiring basins offer the opportunity to assess many unknown questions that may reveal the lunar geologic standpoint. The Mare Orientale basin is one of the youngest impact multiring basins on the Moon, which covers about 930 km in diameter and coordinates centered at 200S 950W. In the present study, the compositional, topographical, and chronological studies have been done by using Moon Mineralogy Mapper (M3) from Chandrayaan-1 orbiter and Lunar Reconnaissance Orbiter Camera-Wide Angle Camera (LROC-WAC) image from Lunar Reconnaissance Orbiter (LRO). The M3 data used to map the Optical maturity (OMAT), FeO & TiO 2 concentrations of the basin. The Standard Band Ratio (SBR) of the Orientale basin has been prepared to discriminate the different mare and highland lithologies. The 1 μm and 2 μm band depths have been mapped to demarcate the mafic minerals such as olivine and pyroxenes. The age of the Orientale basin mare units was mapped as Orientale event (3.72 Ga - Upper Imbrian), Orientale South West (3.7 Ga), Orientale West unit (3.37 Ga), Lacus Veris North unit- 1, 2, and 3 (3.1 Ga, 2.63 Ga, and 3.3 Ga), Lacus Veris East (2.9 Ga), Lacus Autumni - North (3.1 Ga), Middle (2.01 Ga), South (2.27 Ga) and Kopff crater (mare) (2.92 Ga) by using LROC-WAC data through Crater Size Frequency Distribution (CSFD) technique. Characterization of M3 reflectance spectra profiles was done to map the Orientale basin lithologies like olivine, pyroxene (low calcic and high calcic), plagioclase, ilmenite, Mg-spinel, and olivine – pyroxene mixtures that are present in the anorthositic, basaltic, noritic, and gabbroic rocks. It is revealed that the Orientale basin could have been formed either by global thermal evolution and basin mare volcanic evolution or pressure release melting and associated with secondary convection. • Mare Orientale basin is considered as a youngest mare basin on the moon which could be a window to the mantle derivatives. • Comp. analysis of Orientale basin done to demarcate lithologies i.e., Anorthositic, Basaltic, Gabbroic, Noritic, &Ilmenite • Low Ti produced by olivine – pyroxene culmulates from deeper sources • Intermediate Ti basalts formed by dynamic mixing of ilmenite bearing rocks with poor Ti early cumulate piles in the basin. Different mare regions chronological ages were calculated through CSFD method which is observed as 3.72 – 2Ga or younger. [ABSTRACT FROM AUTHOR]

The Orientale basin is the most well-preserved multi-ring impact basin on the Moon and it has been used to define the end of the Early Imbrian Epoch. Thus a precise determination of the absolute model age (AMA) of Orientale Basin is important for lunar geology study. The method of crater size–frequency distribution (CSFD) has been used to date Orientale Baisn in previous studies, and the resultant AMA ranges from 3.68 Ga to 3.8 Ga. The inconsistence may be attributed to the choice of counting area and identified superposed craters. In this research we have mapped 27,093 craters larger than 0.7 km in diameter on the entire ejecta of the Orientale Basin, and derived that the AMAs of the Orientale Basin and the pre-existing luanr suface are 3.80 −0.0079 +0.0074 Ga and ~4.14 −0.022 +0.019 Ga, respectively. And the knee point of the CSFD of the entire counting area is ~10 km, corresponding to ~ 2 km of the average thickness of the Orientale Basin ejecta. To further analyze the variation of the ejecta thickness of Orientale Basin, the counting area was divided into eight 45-degree subsections. The analysis of the knee points of the CSFD in the subdivided counting areas demonstrates that Orientale Basin was formed by an oblique impact from the south-west, and the impact incidence was between 45° and 20°. • 27,093 craters larger than 0.7 km in diameter were mapped within Orientale Basin ejecta. • Orientale Basin was formed ~3.80 Ga ago on the pre-existing ~4.14 Ga lunar surface. • Orientale Basin was most likely formed by an oblique impact. [ABSTRACT FROM AUTHOR]