South Pole-Aitken Research Articles

Rock abundance and evolution of the shallow stratum on Chang'e-4 landing site unveiled by lunar penetrating radar data

The Yutu-2 rover, part of the Chang'e-4 (CE-4) mission to the lunar farside, has been exploring the Von Kármán crater (VK) within the South Pole-Aitken (SPA) gathering data on its geology. The Lunar Penetrating Radar's (LPR) 500 MHz antenna reveals the subsurface structure up to 40 m in depth, which helps to construct models on the evolution of the shallow stratum. This work presents the analysis and interpretation of the data collected during the first twenty lunar days, from January 2019 to July 2020. Below an 11±4-meter surface regolith layer, the radar image reveals well-defined reflectors that are interpreted as discrete depositional layers. These marked density boundaries could indicate a geologically brief time interval between each arrival, which prevented the materials to undergo a deeper homogenization through space weathering. To investigate further, we generated a subsurface rock distribution map using the correlation of dual-channel antennas data from the same channel. Clusters of rocks located at the bottom of each ejecta stratum are revealed, probably sorted by size due to the statistically less frequent occurrence of larger impact events admixing the deeper-seated materials. The bowl-shaped outline discovered in both rock and LPR energy distribution maps is interpreted to represent a 150±30 m wide and 10±5 m deep paleo-crater that was excavated within three thin ejecta layers. The CE-4 LPR reveals a complex geological history within the Von Kármán crater with no fewer than four impact events delivering meters thick ejecta at the CE-4 landing area following the end of the infilling phase.

Sample Return from the Moon’s South Pole-Aitken Basin

Whitepaper #290 submitted to the Planetary Science and Astrobiology Decadal Survey 2023-2032. Topics: interior evolution and volcanism; Mercury and/or the Moon; solar system formation, dynamics processes, and chronology

Lunar meteorites from northern Africa

AbstractWe report bulk composition data for 235 stones of ~77 lunar meteorites from northern Africa and 33 lunar meteorites from elsewhere. About 27% of the African meteorites are typical feldspathic lunar rocks, all breccias of similar bulk composition, for example, 3–5% FeO and low concentrations of incompatible elements. Nevertheless, these meteorites have a large range inMg'(whole‐rock mole% Mg/[Mg + Fe]), 57–77 (mean: 65.5), and this parameter does not correlate with either albite content of the plagioclase or concentrations of incompatible elements. From this observation, we conclude that feldspathic lunar meteorites do not support the hypotheses that the anorthositic, precursor plutonic rocks of the meteorite breccias all crystallized from a common magma. Among feldspathic lunar meteorites,Mg'increases with normative olivine abundance but is uncorrelated with normative pyroxene abundance, that is, high‐Mg'feldspathic rocks of the early lunar crust were troctolitic, not noritic. The NWA 5744 (Northwest Arica) clan of lunar meteorites are anorthositic troctolites (59–75% plagioclase, 15–26% olivine, Fo77–80) with very low concentrations of incompatible elements (2.6 × CI) compared to troctolites in the Apollo collection. They may represent the lower feldspathic crust or differentiated impact melt from portions of the highlands distant from the PKT (Procellarum KREEP Terrane). About 31% of the lunar meteorites from northern Africa, all breccias, are, by lunar standards, moderately mafic (5–13% FeO) but more magnesian than mixtures of typical feldspathic meteorite and mare basalt, implying that they represent moderately mafic regions of the highlands or originate from the South Pole–Aitken basin. With ~6 µg g−1Th, two NWA meteorites likely originate from the PKT. Including the unique NWA 773 clan, at least six of the northern Africa meteorites are, or are dominated by, unbrecciated basalts and gabbros of mare affinity. We estimate that the ~341 lunar meteorite stones (Africa and elsewhere) for which there are data represent 131–147 terrestrially unpaired meteorites and 109–134 lunar launch sites.

Investigating the Influences of Crustal Thickness and Temperature on the Uplift of Mantle Materials Beneath Large Impact Craters on the Moon

AbstractIn this work, we examine variations in the mantle uplift associated with large lunar impact craters and basins between major terranes. This study is based on Bouguer gravity anomalies of 100–650‐km diameter impact craters using Gravity Recovery and Interior Laboratory (GRAIL) observations and the Lunar Orbiter Laser Altimeter (LOLA) crater database. The Bouguer gravity anomalies of 324 large impact craters analyzed herein are primarily controlled by the uplifted crust‐mantle (Moho) interface in the central region of these impact craters, while postimpact mare deposits that represent ∼25%–35% of the postimpact deposit column also contribute to the gravity anomalies of mare craters. The central uplift of the Moho interface is primarily controlled by impact energy and increases to ∼ 30 km for a 650‐km diameter crater. Analyses of nonmare craters in the Feldspathic Highlands Terrane (FHT) with varied crustal thickness (Tc) reveal that the onset crater diameter (Dmin) with an uplifted Moho interface is dependent on the local Tc: Dmin ∼ 150 + 1.3Tc (in a unit of km). This equation also provides a quantification of the depth‐dependent attenuation of impact‐induced structural uplift, using the Moho uplift as a proxy for the structural uplift. The Moho uplift of large craters in the South Pole‐Aitken (SPA) Terrane is not statistically different from the FHT craters, consistent with limited overall thermal difference between these two terranes and compensational thermal effects at crater collapse and viscoelastic relaxation stages.

Analysis of Topography and Composition of the Von Kármán Impact Crater

The Vo n Kármán impact crater in the Chang’E-4 landing area is located in in the Northwestern South Pole-Aitken (SPA) basin on the back of the Moon, and the analysis of its topography and compositional characteristics is essential for the landing of the probe and the exploration of the lunar surface, which plays a fundamental role in obtaining information on the geomorphology and geology of the lunar surface. In this paper, the topography of the Vo n Kármán impact crater is analyzed based on DEM, slope and terrain roughness, then the compositional characteristics of the Von Kármán impact crater are analyzed according to TiO2 and FeO content through combining LRO LOLA data and Clementine UV-VIS data. This study is a basis for revealing the surface characteristics and structure of the Moon.

Estimation of the flux density of galactic comets in the Orion–Cygnus branch based on number of shield volcanoes on Venus, craters on Mars and marine basins on the Moon

Based on data on shield volcanoes of Venus and the distribution of craters on Mars and sea basins on the Moon, the flux density of galactic comets in the Orion–Cygnus branch was calculated. The comets flux density is 5 × 10−10 year−1 km−2. This value was used to estimate the number, energy and fallings frequency of galactic comets on terrestrial planets in the period from ≈ 5 to 1 million years ago. The article shows, that the falls of such comets can explain the phenomena of “newest uplifts of Earth’s crust” and “young volcanism” on Earth, the appearance of shield volcanoes on Venus, the asymmetric structure of Mars, as well as origin of large craters, seas and mascons on the Moon, Mars and Mercury. A hypothesis of formation on the Moon of the South Pole–Aitken basin by galactic comets is proposed.

In situ lunar phase curves measured by Chang’E-4 in the Von Kármán Crater, South Pole-Aitken basin

Context.The Yutu-2 rover of the Chang’E-4 (CE-4) mission measured the lunar phase curves in the Von Kármán crater, South Pole-Aitken basin.Aims.We aim to study the photometric properties of the regolith at CE-4’s landing site and compare them with those of Chang’E-3 (CE-3) in order to understand the regolith physical properties of the two landing sites.Methods.We extracted theinsitulunar phase curves measured by CE-4 with a very wide phase angle coverage (1°–144°) and performed photometric model inversions using both the Hapke model and the Lumme-Bowell model.Results.Compared with the CE-3 measurement taken in Mare Imbrium, the CE-4 phase curves show the colorimetric opposition effect and have a steeper and narrower opposition spike. The surface regolith at the CE-4 site is much darker, more porous, more forward scattering, and has a larger slope angle (Hapke model) than that of CE-3.Conclusions.The CE-4 site may have experienced more space weathering alterations than the CE-3 site, which is consistent with their different surface model ages (~3.6 Ga for CE-4 and ~3 Ga for CE-3).

Geochemistry of the Von Kármán crater floor and thickness of the non-mare ejecta over the Chang'e-4 landing area

China's Chang'e-4 mission has carried out the first ever lunar farside landing exploration on the floor of the Von Kármán crater, a geologically complex region located in the most ancient and deepest South Pole-Aitken (SPA) basin. In order to demonstrate the characteristics of materials in the landing area, we investigated the regional geochemistry and thickness of non-mare ejecta overlaying the mare basalts. Comparative analyses of FeO, TiO2 and Th concentrations suggest that the landing site surface is dominated by non-mare ejecta from nearby craters (e.g., Finsen crater) with part of basaltic materials. The ejecta thickness is estimated based on the excavation depth of dark-haloed and non-dark-haloed craters by using support-vector machine, a supervised machine learning method for classification. The results show that the ejecta thickness in the region of 40 km across the landing site varies from near zero to ~80 m with a mean value of ~41 m. The ejecta at the Chang'e-4 landing site is ~40 m thick, which is comparable to the in situ observations by the Lunar Penetrating Radar onboard the Yutu-2 rover. Our results provide valuable information for interpretation of the on-going returned data and geologic analysis of the Chang'e-4 exploration region.

Evidence for a Stratified Upper Mantle Preserved Within the South Pole‐Aitken Basin

AbstractThe evolution and compositional structure of the lunar mantle has been extensively modeled but insufficiently constrained by observations. Here, we identify and characterize mantle materials exposed by the Moon's largest impact basin to better understand the composition, stratigraphy, and evolution of the upper mantle. The vast South Pole‐Aitken Basin (SPA) exhibits a broad, crescent‐shaped thorium and potassium distribution. These incompatible elements are predicted to be concentrated in the dregs of the lunar magma ocean during end‐stage crystallization. Through consideration of basin formation models convolved with subsequent geologic evolution, we demonstrate that the distribution and implied stratigraphy of Th‐ and K‐bearing materials across SPA are consistent with an upper mantle ejecta origin. The most pristine exposures of these materials are confined to northwest SPA and also exhibit elevated Ti and Fe (relative to the farside highlands) in association with a gabbronoritic mineralogy. This is consistent with late‐stage magma ocean assemblages predicted by petrologic models. In contrast, SPA impact melt derived from greater depths is associated with a low‐Ca pyroxene‐dominated assemblage. Together, these compositional patterns are evidence for a stratified ancient upper mantle. Importantly, the incompatible‐element‐enriched, ilmenite‐bearing ferroan gabbronoritic cumulates evidently had not participated in gravitational overturn at the time of SPA formation. Contrary to recent hypotheses invoking nearside sequestration of incompatible elements to explain hemispherical differences in crustal building and volcanic resurfacing, it follows that incompatible elements were globally distributed in the magma ocean at the time of SPA formation.

Thickness of orthopyroxene-rich materials of ejecta deposits from the south pole-Aitken basin

The South Pole-Aitken (SPA) basin is the largest impact structure on the Moon and is believed to have excavated the orthopyroxene (Opx)-rich lower crust and/or upper mantle materials. For the complex craters outside of the SPA transient cavity, the origin of the Opx-rich central peaks is possibly from either the Opx-rich materials of the SPA ejecta deposits or the unexcavated lower crust and/or upper mantle. To estimate the thickness of the Opx-rich materials of the SPA ejecta deposits, this study investigated large complex craters (dozens of kilometers) that have penetrated the Opx-rich materials and exposed deeper mafic-poor crustal material based on spectra extracted from small fresh craters (sub-kilometer scale). The amount of foreign material introduced to these large complex craters by other lunar impact events was estimated to guarantee the least influence on the compositional analysis. The study results suggest that a 56 km-diameter crater at ~640 km northwest of the SPA center is large enough to penetrate the Opx-rich materials of the SPA ejecta deposits, which are thinner than ~4.7 km at this location. This result also indicates that the intense bombardment history of the large craters and basins outside of the SPA transient cavity excavated and redistributed a large amount of crustal material across the basin, possibly resulting in the heterogeneous distribution of mafic-rich and mafic-poor materials on the SPA surface.

Global tectonic patterns of the Moon

Planetary tectonics provide a record of the myriad of processes that shape planetary surfaces and interiors. While there is a long history of mapping and modeling planetary tectonics, stresses from disparate processes are not generally captured by a single model. We present a comprehensive and general stress and tectonics model that can consider multiple stress-generating mechanisms simultaneously. The model is applicable to mass loading with arbitrary geometry, rotational and orbital perturbations, and arbitrary elastic lithosphere thicknesses. This wholistic approach to tectonic modeling has important implications for understanding both lunar evolution and tectonics across the solar system. We apply this model to the Moon, which exhibits a global pattern of thrust faults. The ubiquitous presence of young thrust faults suggests that isotropic contraction plays a dominant role. However, their non-random orientation requires additional stress-generating mechanisms that are not isotropic. Best-fit solutions correspond to models combining isotropic contraction with orbit recession, despinning, and South Pole-Aitken ejecta loading and the corresponding true polar wander. Contraction and despinning assuming an elastic shell with a thinner equatorial region can lead to misfits that are smaller than those assuming a constant thickness elastic shell. The young age of the faults favors recent contraction and recession; however, unrelaxed stresses from older processes combined with recent contractional stresses can also generate young faults. This possibility is supported by the Moon's ability to preserve a fossil figure.

Lunar regolith and substructure at Chang’E-4 landing site in South Pole–Aitken basin

The South Pole–Aitken (SPA) basin is the oldest and largest impact structure on the Moon, and it gives particular insight on the lunar interior composition1–3. However, the surface of the SPA basin has been substantially modified by consequent impacts and basalt flooding. The exploration of the surficial material and the substructure of the SPA basin is one of the main scientific goals of the Chinese spacecraft Chang’E-4 that landed in the Von Karman crater inside the SPA basin4,5. Here we report the lunar penetrating radar profiles along the track of the lunar rover Yutu-2, which show a three-unit substructure at the landing site. The top unit consists of the ~12-m-thick lunar regolith and ~120 m multilayered ejecta that were delivered from several adjacent craters. The middle unit is the mare basalts filling the Von Karman crater. The lowest unit is another ejecta layer with a thickness of ≥200 m, likely from the Leibnitz crater. These discoveries fully support the local stratigraphy and geological explanation presented previously6. Our results reveal that the surface materials at the Chang’E-4 landing site are unambiguously dominated by the ejecta from the Finsen crater with a minor contribution from other neighbouring craters. The regolith measured by Yutu-2 is representative of the initial lunar deep interior materials, rather than the later erupted basalts. Three different layers can be distinguished in the first 500 metres of depth beneath the South Pole–Aitken basin on the Moon: a first layer made up by regolith and ejecta material from different craters, followed by a middle unit of mare basalts and finally a >200-m-thick layer of ejecta from the Leibnitz crater.

Chronological sequence of Chang'E-4 landing zone within Von Kármán crater

Sequence and timing are critical for understanding the geologic evolution. Here we performed detailed crater size-frequency distribution measurements within the Von Kármán crater, where China's Chang'E-4 (CE-4) mission landed, and its surroundings including Alder and Finsen craters, to understand the geologic evolution of the landing zone. The formation time of Von Kármán crater was estimated to be ~4.2-0.03+0.03 Ga, approaching the age of South Pole-Aitken basin. Three major layers under the landing site were dated: The ejecta from Alder crater was considered to be ~3.5-0.2+0.01 Ga old, belonging to Imbrian. The floor of Von Kármán crater was divided into mare basalt units and Finsen ejecta unit based on spectral differences. Two mare basalt units were estimated to be ~3.2-0.1+0.07 and 3.3-0.2+0.08 Ga old (Imbrian), respectively, while Finsen crater and its ejecta, where CE-4 landed, were estimated to be ~3.1-0.5+0.3 Ga (Eratosthenian). The chronological sequence established above indicates that the CE-4 landing zone has experienced a very complex evolutionary history.

EVALUATION OF DEBLOCKING METHODS FOR CHANG’E-4 DESCENT IMAGES

Abstract. Chang’e-4 lunar probe has successfully landed on the far side of the moon in Von Kármán crater inside the South Pole-Aitken (SPA) basin at 10:26 am on January 3, 2019. Due to the reduction of the coding rate, obvious block effects appear at the boundaries of descent images. Unblock, adaptive fast bilateral filtering, structure-texture enhancement and high-order Markov random field methods, are applied to remove the block effect of the descent images. Based on analysing the quality of descent images, quantitative comparison of four methods is performed using simulated compressed 1:64 descent images and real images. Comprehensive analysis was performed using typical measures such as PSNR, SSIM and NIQE. Experimental results show that adaptive fast bilateral filtering is better than other methods. The deblocked 1:64 image sequences have been used to assist localizing the landing point quickly during the mission.

COMPUTER VISION IN THE TELEOPERATION OF THE YUTU-2 ROVER

Abstract. On January 3, 2019, the Chang'e-4 (CE-4) probe successfully landed in the Von Kármán crater inside the South Pole-Aitken (SPA) basin. With the support of a relay communication satellite "Queqiao" launched in 2018 and located at the Earth-Moon L2 liberation point, the lander and the Yutu-2 rover carried out in-situ exploration and patrol surveys, respectively, and were able to make a series of important scientific discoveries. Owing to the complexity and unpredictability of the lunar surface, teleoperation has become the most important control method for the operation of the rover. Computer vision is an important technology to support the teleoperation of the rover. During the powered descent stage and lunar surface exploration, teleoperation based on computer vision can effectively overcome many technical challenges, such as fast positioning of the landing point, high-resolution seamless mapping of the landing site, localization of the rover in the complex environment on the lunar surface, terrain reconstruction, and path planning. All these processes helped achieve the first soft landing, roving, and in-situ exploration on the lunar farside. This paper presents a high-precision positioning technology and positioning results of the landing point based on multi-source data, including orbital images and CE-4 descent images. The method and its results have been successfully applied in an actual engineering mission for the first time in China, providing important support for the topographical analysis of the landing site and mission planning for subsequent teleoperations. After landing, a 0.03 m resolution DOM was generated using the descent images and was used as one of the base maps for the overall rover path planning. Before each movement, the Yutu-2 rover controlled its hazard avoidance cameras (Hazcam), navigation cameras (Navcam), and panoramic cameras (Pancam) to capture stereo images of the lunar surface at different angles. Local digital elevation models (DEMs) with a 0.02 m resolution were routinely produced at each waypoint using the Navcam and Hazcam images. These DEMs were then used to design an obstacle recognition method and establish a model for calculating the slope, aspect, roughness, and visibility. Finally, in combination with the Yutu-2 rover mobility characteristics, a comprehensive cost map for path search was generated.By the end of the first 12 lunar days, the Yutu-2 rover has been working on the lunar farside for more than 300 days, greatly exceeding the projected service life. The rover was able to overcome the complex terrain on the lunar farside, and travelled a total distance of more than 300 m, achieving the "double three hundred" breakthrough. In future manned lunar landing and exploration of Mars by China, computer vision will play an integral role to support science target selection and scientific investigations, and will become an extremely important core technology for various engineering tasks.

Stratigraphy of the Von Kármán Crater Based on Chang'E‐4 Lunar Penetrating Radar Data

AbstractThe Chang'E‐4 (CE‐4) achieved the first successful soft landing on the floor of the Von Kármán crater located in the northeast of the South Pole‐Aitken (SPA) basin on the Moon. Here, through analysis and processing of the dual‐channel lunar penetrating radar (LPR) data, the shallow regolith structure and deep geological strata below the landing site are exposed. The high‐frequency data (CH‐2, 500 MHz) show that the Von Kármán crater underwent remodeling because of ejecta from multiple craters within a thickness of 40 m. Under the surface fine‐grained regolith, superpositions among these crater materials and mare basalt units establish shallow regolith stratigraphic sequences. The low‐frequency data (CH‐1, 60 MHz) show a deep stratigraphic structure to the depth of 360 m that indicates the previous geological evolution history of the Von Kármán crater. The LPR results provide an important scientific basis for understanding the geological history of the far side of the Moon.

Why the lunar South Pole-Aitken Basin is not a mascon

Lunar mascon basins exist for only a range of observed diameters. We modeled the full formation of South-Pole Aitken basin using a sequential two-code (hydrocode and Finite Element Model) approach to understand why this range does not extend to the largest lunar basin. Similar to previous work, we found that the best-fit hydrocode had impact parameters of a 170km in diameter dunite projectile striking at 10km/s, with a pre-impact lunar thermal gradient of 50K/km (until a depth of 28 km at which an adiabat is reached), and pre-impact crustal thickness value of 40 km. Unlike previous work, we matched the crustal distribution of the inner basin by utilizing a weaker melt rheology in our model. The crust in the models with a weaker melt rheology flowed inward after crater collapse to cover the basin center; therefore, not requiring an additional step of melt differentiation. Given the large diameter of South-Pole Aitken and the high geothermal gradient at impact, the steady state configuration after relaxation and thermal cooling was driven by isostatic forces without a significant contribution from lithospheric rigidity. Without lithospheric rigidity, South-Pole Aitken relaxed to a slightly negative free-air gravity signature, not forming a mascon, in agreement with GRAIL observations.

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

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

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

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.

Geologic context and potential EVA targets at the lunar south pole

The lunar south pole is being targeted for exploration, in part, because it contains topographical high points with >50% illumination needed for solar power. Additionally, the south pole is being targeted because it contains permanently shadowed regions (PSRs), which may sequester resources in the form of volatile materials. Geologically, the pole lies on the rim of ~21 km diameter Shackleton crater, which is located on the topographic rim of the ~2,500 km diameter South Pole-Aitken (SPA) basin, the largest and oldest basin on the Moon. To prepare for future missions, we conducted a photogeologic analysis of the walls, rim, and ejecta of Shackleton crater. Two types of underlying (target) terrains were identified. The impact penetrated and exposed (1) purest anorthosite (PAN) representative of primitive crust and (2) a layered terrain that is likely a series of impact ejecta deposits that stratigraphically cover the crystalline crust. Crew performing extravehicular activities (EVAs) near the south pole may be able to sample PAN; impact melt from Shackleton, SPA, and other pre-Nectarian and Nectarian-age impacts; and polar regolith, including material from small PSRs that may contain volatile components. The topography in the south polar region is dramatic, often producing slopes in excess of 15°, creating mobility challenges for astronauts during EVAs.