Von Karman crater (diameter = ~186 km), lying in the northwestern South Pole-Aitken (SPA) basin, was formed in the pre-Nectarian. The Von Karman crater floor was subsequently flooded with one or several generations of mare basalts during the Imbrian period. Numerous subsequent impact craters in the surrounding region delivered ejecta to the floor, together forming a rich sample of the SPA basin and farside geologic history. We studied in details the targeted landing region (45.0–46.0°S, 176.4–178.8°E) of the 2018 Chinese lunar mission Chang'E-4, within the Von Karman crater. The topography of the landing region is generally flat at a baseline of ~60 m. Secondary craters and ejecta materials have covered most of the mare unit and can be traced back to at least four source craters (Finsen, Von Karman L, Von Karman L', and Antoniadi) based on preferential spatial orientations and crosscutting relationships. Extensive sinuous ridges and troughs are identified spatially related to Ba Jie crater (diameter = ~4 km). Reflectance spectral variations due to difference in both composition and physical properties are observed among the ejecta from various-sized craters on the mare unit. The composition trends were used together with crater scaling relationships and estimates of regolith thickness to reconstruct the subsurface stratigraphy. The results reveal a complex geological history of the landing region and set the framework for the in situ measurements of the CE-4 mission, which will provide unique insights into the compositions of farside mare basalt, SPA compositional zone including SPA compositional anomaly and Mg-pyroxene annulus, regolith evolution, and the lunar space environment.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018JE005577

Geological Characteristics of Von Kármán Crater, Northwestern South Pole-Aitken Basin: Chang'E-4 Landing Site Region

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019GL084531

Mineral Abundances Inferred From In Situ Reflectance Measurements of Chang'E-4 Landing Site in South Pole-Aitken Basin

Lunar mare regolith is traditionally thought to have formed by impact bombardment of newly emplaced coherent solidified basaltic lava. We use new models for initial emplacement of basalt magma to predict and map out thicknesses, surface topographies and internal structures of the fresh lava flows, and pyroclastic deposits that form the lunar mare regolith parent rock, or protolith. The range of basaltic eruption types produce widely varying initial conditions for regolith protolith, including (1) autoregolith, a fragmental meter-thick surface deposit that forms upon eruption and mimics impact-generated regolith in physical properties, (2) lava flows with significant near-surface vesicularity and macroporosity, (3) magmatic foams, and (4) dense, vesicle-poor flows. Each protolith has important implications for the subsequent growth, maturation, and regional variability of regolith deposits, suggesting wide spatial variations in the properties and thickness of regolith of similar age. Regolith may thus provide key insights into mare basalt protolith and its mode of emplacement.

/pdf/rethinking-lunar-mare-basalt-regolith-formation-new-concepts-1d4whnxqo1.pdf

Rethinking Lunar Mare Basalt Regolith Formation: New Concepts of Lava Flow Protolith and Evolution of Regolith Thickness and Internal Structure


 The South Pole–Aitken (SPA) basin, located between the South Pole and Aitken crater on the far side of the Moon, is the largest confirmed lunar impact structure. The pre-Nectarian SPA basin is a 2400 × 2050 km elliptical structure centered at 53°S, 191°E, which should have exposed lower crust and upper mantle due to the enormous excavation depth. Olivine, the dominant mineral in Earth’s mantle, has only been identified in small and localized exposures in the margins of the SPA basin, and the dominant mafic component is, instead, pyroxene. These mineralogical characteristics could be explained by the recent hypothesis that the lunar upper mantle is dominated by low-calcium pyroxene, not olivine. Here, we present observations from imaging and spectral data from China’s Chang’E-4 (CE-4) lunar mission in the first 4 synodic days, especially the first in situ visible/near-infrared spectrometer observations of an exposed boulder. We identified a variety of rock types, but not the recently reported olivine-rich materials in the landing region. The results are consistent with orbital observations. The obtained mineralogical information provides a better understanding of the nature and origin of SPA materials.

Diverse rock types detected in the lunar South Pole–Aitken Basin by the Chang’E-4 lunar mission

Lunar irregular mare patches (IMPs) comprise dozens of small, distinctive, and enigmatic lunar mare features. Characterized by their irregular shapes, well-preserved state of relief, apparent optical immaturity, and few superposed impact craters, IMPs are interpreted to have been formed or modified geologically very recently (<~ 100 Ma; Braden et al. 2014). However, their apparent relatively recent formation/modification dates and emplacement mechanisms are debated. We focus in detail on one of the major IMPs, Sosigenes, located in western Mare Tranquillitatis, and dated by Braden et al. (2014) at ~ 18 Ma. The Sosigenes IMP occurs on the floor of an elongate pit crater interpreted to represent the surface manifestation of magmatic dike propagation from the lunar mantle during the mare basalt emplacement era billions of years ago. The floor of the pit crater is characterized by three morphologic units typical of several other IMPs, i.e., (1) bulbous mounds 5– 10 m higher than the adjacent floor units, with unusually young crater retention ages, meters thick regolith, and slightly smaller subresolution roughness than typical mature lunar regolith; (2) a lower hummocky unit mantled by a very thin regolith and significantly smaller subresolution roughness; and (3) a lower blocky unit composed of fresh boulder fields with individual meter-scale boulders and rough subresolution surface texture. Using new volcanological interpretations for the ascent and eruption of magma in dikes, and dike degassing and extrusion behavior in the final stages of dike closure, we interpret the three units to be related to the late-stage behavior of an ancient dike emplacement event. Following the initial dike emplacement and collapse of the pit crater, the floor of the pit crater was flooded by the latest-stage magma. The low rise rate of the magma in the terminal stages of the dike emplacement event favored flooding of the pit crater floor to form a lava lake, and CO gas bubble coalescence initiated a strombolian phase disrupting the cooling lava lake surface. This phase produced a very rough and highly porous (with both vesicularity and macroporosity) lava lake surface as the lake surface cooled. In the terminal stage of the eruption, dike closure with no addition of magma from depth caused the last magma reaching shallow levels to produce viscous magmatic foam due to H2 O gas exsolution. This magmatic foam was extruded through cracks in the lava lake crust to produce the bulbous mounds. We interpret all of these activities to have taken place in the terminal stages of the dike emplacement event billions of years ago. We attribute the unusual physical properties of the mounds and floor units (anomalously young ages, unusual morphology, relative immaturity, and blockiness) to be due to the unusual physical properties of the substrate produced during the waning stages of a dike emplacement event in a pit crater. The unique physical properties of the mounds (magmatic foams) and hummocky units (small vesicles and large void space) altered the nature of subsequent impact cratering, regolith development, and landscape evolution, inhibiting the typical formation and evolution of superposed impact craters, and maintaining the morphologic crispness and optical immaturity. Accounting for the effects of the reduced diameter of craters formed in magmatic foams results in a shift of the crater size– frequency distribution age from < 100 Myr to billions of years, contemporaneous with the surrounding ancient mare basalts. We conclude that extremely young mare basalt eruptions, and resulting modification of lunar thermal evolution models to account for the apparent young ages of the IMPs, are not required. We suggest that other IMP occurrences, both those associated with pit craters atop dikes and those linked to fissure eruptions in the lunar maria, may have had similar ancient origins.

/pdf/the-role-of-substrate-characteristics-in-producing-2crvgfth4a.pdf

The role of substrate characteristics in producing anomalously young crater retention ages in volcanic deposits on the Moon: Morphology, topography, subresolution roughness, and mode of emplacement of the Sosigenes lunar irregular mare patch

The visible and near-infrared imaging spectrometer on board the Yutu Rover of Chinese Chang'E-3 mission measured the lunar surface reflectance at a close distance (similar to 1m) and collected four spectra at four different sites. These in situ lunar spectra have revealed less mature features than that measured remotely by spaceborne sensors such as the Moon Mineralogy Mapper instrument on board the Chandrayaan-1 mission and the Spectral Profiler on board the Kaguya over the same region. Mineral composition analysis using a spectral lookup table populated with a radiative transfer mixing model has shown that the regolith at the landing site contains high abundance of olivine. Themineral abundance results are consistent with that inferred from the compound measurement made by the on board alpha-particle X-ray spectrometer.

/pdf/in-situ-optical-measurements-of-chang-e-3-landing-site-in-4hqr6l9nvi.pdf

In situ optical measurements of Chang'E-3 landing site in Mare Imbrium: 1. Mineral abundances inferred from spectral reflectance

Context. Visible and near-infrared spectra of asteroids are known to be susceptible to nanophase irons produced by space weathering processes, thus making mineral identifications difficult. Mid-infrared spectroscopy may retain more mineral features owing to its lattice vibrational nature.Aims. We investigate the structure and reflectance spectral feature changes of olivine grains before and after simulated space weathering. Methods. We irradiate olivine grains by using pulsed laser to simulate varying degrees of micrometeorite bombardments. Reflectance measurements from 0.5 to 25 μ m and radiative transfer calculations were carried out in order to compare them with each other. Results. Both the experimental simulations and modeling results indicate that the mid-infrared spectral features of olivine grains can survive the intense irradiations. Although the Christansen Feature is slightly shifted to longer wavelength, major vibrational bands remain essentially unchanged, because the lattice structure is quite immune to even the strongest irradiations, as revealed by both the X-ray diffraction and Raman scattering measurements.Conclusions. Mid-infrared spectroscopy is much more immune to productions of nanophase irons and amorphous materials and thus may be used more reliably in remote detections of minerals on asteroid surfaces.

/pdf/optical-spectroscopic-characterizations-of-laser-irradiated-1jk9zj14cc.pdf

Optical spectroscopic characterizations of laser irradiated olivine grains

/pdf/optical-spectroscopic-characterizations-of-laser-irradiated-3pcqc6sppi.pdf

Optical spectroscopic characterizations of laser irradiated olivine grains (Corrigendum)

The panorama cameras onboard the Yutu Rover of the Chang'E-3 lunar mission acquired hundreds of high-resolution color images of the lunar surface and captured the first in situ lunar opposition effect (OE) since the Apollo era. We extracted the phase curve and the color ratio in three bands with the phase angle range from 2 degrees to 141 degrees. Photometric inversions using the Hapke model reveal that submicroscopic dusts are present in the landing area and both the coherent backscattering and the shadow hiding are responsible for the strong OE. Compared with spaceborne measurements, the grains in the landing site are brighter, more transparent, and appear to be better crystallized than the average maria basaltic grains. The results show that the phase-reddening effect appears to be present in the in situ phase curves. The current phase curve can be used as the ground-truth validations of any future spaceborne phase curve measurement over the landing site region.

/pdf/in-situ-optical-measurements-of-chang-e-3-landing-site-in-34z7xo5zf2.pdf

In situ optical measurements of Chang'E-3 landing site in Mare Imbrium: 2. Photometric properties of the regolith

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In situ optical measurements of Chang'E-3 landing site in Mare Imbrium: 1. Mineral abundances inferred from spectral reflectance

Optical spectroscopic characterizations of laser irradiated olivine grains

Optical spectroscopic characterizations of laser irradiated olivine grains (Corrigendum)

In situ optical measurements of Chang'E-3 landing site in Mare Imbrium: 2. Photometric properties of the regolith