Yutu-2 Rover Research Articles

Analysis of a large buried impact crater and vertical mineral composition at the Chang'E-4 landing site by multi-source remote sensing data

Exploring the concealed subsurface structures and materials beneath the lunar surface can reveal significant insights into geological history. This study offers a comprehensive analysis of the stratigraphic interpretation and subsurface material composition at the Chang'E-4 landing site, integrating both in-situ and orbital radar with multispectral datasets. We report the identification of a subsurface structure, which resembles a buried impact crater (∼420 m in diameter) under the Yutu-2 rover's path. This crater could degrade over a period of 0.42 to 0.53 Ga, with an initial diameter of 293 to 323 m and an initial depth of 45.9 to 51.4 m. Surface material above the buried crater, evaluated by the in-situ visible and near-infrared imaging spectrometer (VNIS) detector, shows a higher abundance of clinopyroxene compared to surrounding areas, where a near-equal mix of clinopyroxene and orthopyroxene is observed. Assessment of crater diameters in proximity to the Chang'E-4 landing site, along with the mineral compositions at their epicenters, reveals a decrease in the abundance of clinopyroxene and plagioclase with depth. Conversely, the quantities of orthopyroxene and olivine increase, implying that orthopyroxene-rich Finsen ejecta significantly influenced the Chang'E-4 landing site's geological composition. Two potential stratigraphic boundary depths are identified at 13.5 and 22 m, based on pronounced variations in mineral abundance, offering fresh insights into subsurface delineation beyond radar data. Considering the VNIS and vertical mineral composition, we propose the buried crater's formation resulted from Finsen crater's ejecta. Also, we identify eight potential historical impacts by comparing subsurface relief variations with mineral composition ratios between clinopyroxene and orthopyroxene. The integration of subsurface structure, along with surface and subsurface mineral composition, enables a more robust stratigraphic interpretation, facilitates shallow material source analysis, and allows for historical impact tracing.

Study on the Degradation Pattern of Impact Crater Populations in Yutu-2′s Rovering Area

A detailed analysis of the panoramic camera data from the 27th to 33rd lunar days was conducted on the high-resolution scenes captured by the Yutu-2 rover stations. This analysis aimed to determine the detailed morphological parameters of the 2015 impact craters within the inspection area. The levels of degradation observed in the impact craters were determined alongside the surface features. Subsequently, the degradation patterns of the impact craters located within the Yutu-2’s roving area and the distribution patterns of the morphological parameters were analysed and investigated. The results of the analysis indicate that 94% of the impact craters exhibited severe degradation, 80% had depth-to-diameter ratios (DDRs) ranging from 0.07 to 0.17, and the remaining craters were moderately degraded. The DDRs of the impact craters exhibited a declining trend with an increase in the dimensions of the impact craters. Additionally, the degree of degradation of impact crater populations demonstrated a decreasing trend. In general, the impact craters along the rover’s route exhibited severe degradation, with the population of degradation degrees gradually decreasing with increasing diameter.

Energy spectra of energetic neutral hydrogen backscattered and sputtered from the lunar regolith by the solar wind

Context. The solar wind impinging on the lunar surface results in the emission of energetic neutral atoms. This particle population is one of the sources of the lunar exosphere. Aims. We present a semi-empirical model to describe the energy spectra of the neutral emitted atoms. Methods. We used data from the Advanced Small Analyzer for Neutrals (ASAN) on board the Yutu-2 rover of the Chang’E-4 mission to calculate high-resolution average energy spectra of the energetic neutral hydrogen flux from the surface. We then constructed a semi-empirical model to describe these spectra. Results. Excellent agreement between the model and the observed energetic neutral hydrogen data was achieved. The model is also suitable for describing heavier neutral species emitted from the surface. Conclusions. A semi-analytical model describing the energy spectrum of energetic neutral atoms emitted from the lunar surface has been developed and validated by data obtained from the lunar surface.

Lunar rock investigation and tri-aspect characterization of lunar farside regolith by a digital twin

Yutu-2 rover conducted an exciting expedition on the 41st lunar day to investigate a fin-shaped rock at Longji site (45.44°S, 177.56°E) by extending its locomotion margin on perilous peaks. The varied locomotion encountered, especially multi-form wheel slippage, during the journey to the target rock, established unique conditions for a fin-grained lunar regolith analysis regarding bearing, shear and lateral properties based on terramechanics. Here, we show a tri-aspect characterization of lunar regolith and infer the rock’s origin using a digital twin. We estimate internal friction angle within 21.5°−42.0° and associated cohesion of 520-3154 Pa in the Chang’E-4 operational site. These findings suggest shear characteristics similar to Apollo 12 mission samples but notably higher cohesion compared to regolith investigated on most nearside lunar missions. We estimate external friction angle in lateral properties to be within 8.3°−16.5°, which fills the gaps of the lateral property estimation of the lunar farside regolith and serves as a foundational parameter for subsequent engineering verifications. Our in-situ spectral investigations of the target rock unveil its composition of iron/magnesium-rich low-calcium pyroxene, linking it to the Zhinyu crater (45.34°S, 176.15°E) ejecta. Our results indicate that the combination of in-situ measurements with robotics technology in planetary exploration reveal the possibility of additional source regions contributing to the local materials at the Chang’E-4 site, implying a more complicated geological history in the vicinity.

In-situ mapping of iron and titanium with the visible and near-infrared image spectrometer (VNIS) along the Yutu-2 rover traverse on the farside of the moon

In January 2019, China's Chang'E-4 probe successfully landed in the Von Kármán crater of the Moon's farside's South Pole-Aitken basin. Till August 2021, the Yutu-2 rover had worked for >32 lunar days. The Visible and Near-infrared Image Spectrometer (VNIS) is one of scientific payloads on board the rover that can collect in-situ spectral data. This provides a unique perspective for investigating the composition of the lunar surface materials. In this work, a local-scale FeO and TiO2 abundance mapping approach based on the in-situ VNIS data was proposed to utilize the fine spectral-spatial hyperspectral information. Improved FeO and TiO2 abundance inversion models were performed on each unshaded pixel to generate abundance maps. Experimental results obtained on 115 VNIS observation data along the Yutu-2's routing path confirm the effectiveness of the proposed approach, especially in the complex observation scenarios. Comparing with the in-orbit data products and the in-situ abundance results obtained by the reference methods, the effectiveness of the proposed in-situ FeO and TiO2 abundance mapping approach is validated. Furthermore, the proposed approach allows a detailed analysis of chemical properties and spatial distribution of lunar surface materials, and helps to discover abnormal targets according to the serial spectral observations along the rover's routing path, which reveals the great potential in more precise lunar surface material identification and analysis applications.

Slope Stability Analysis and Soil Mechanical Properties of Impact Craters around the Lunar South Pole

Water ice has been found in the permanently shadowed regions of impact craters around the lunar South Pole, which makes them ideal areas for in situ exploration missions. However, near the rim of impact craters, construction and exploration activities may cause slope instability. As a result, a better understanding of the shear strength of lunar soil under higher stress conditions is required. This paper mainly uses the finite element method to analyze slope stability to determine the position and shape of the slip surface and assess the safety factor. The height and gradient of the slope, the shear strength of lunar soil, and the lunar surface mission all influence the stability of the slope. We also analyze the soil mechanical properties of a soil slope adjacent to the traverse path of the Chang’E-4 Yutu-2 rover. Determining the stability of the slope at the lunar South Pole impact crater under various loading conditions will enhance the implementation of the lunar surface construction program. In this respect, this paper simulates a lunar mission landing at the Shackleton and Shoemaker craters and indicates that areas with higher cohesion lunar soil may be more stable for exploration in the more complex terrain of the South Pole.

Yutu-2 Radar Observations at the Chang’E-4 Landing Site: The Shallow Geological Structure and Its Dielectric Properties

China has successfully carried out five lunar exploration missions since 2007. These missions indicate that China has successfully implemented a three-step lunar exploration program of “orbiting, landing, and returning”. Among them, the Lunar Penetrating Radar (LPR) carried by the Yutu-2 rover in the Chang’E-4 (CE-4) mission is the only one still operating on the far side of the Moon. Up to now, the Yutu-2 radar has measured a large amount of scientific data, and its observations are of great significance to human cognition of the geological evolution of the lunar surface and the exploration of possible lunar in situ resources. This paper reviews the scientific results obtained by previous researchers based on the radar exploration data of Yutu-2, focusing mainly on three aspects, e.g., the geological structure of the shallow surface at the CE-4 landing site, the dielectric properties of the shallow subsurface materials and the special geological features. Finally, the prospects of Yutu-2 radar research priorities and future exploration, and the application trend of Moon-based ground-penetrating radar are given.

Three-Dimensional Reconstruction and Geometric Morphology Analysis of Lunar Small Craters within the Patrol Range of the Yutu-2 Rover

Craters on the lunar surface are the most direct method for the study of geological processes and are of great significance to the study of lunar evolution. In order to fill the research gap on small craters (diameter less than 3 m), we focus on the small craters around the moving path of the Yutu-2 lunar rover and carry out a 3D reconstruction and geometrical morphology analysis on them. First, a self-calibration model with multiple feature constraints is used to calibrate the navigation camera and obtain the internal and external parameters. Then, the sequence images with overlapping regions from neighboring stations are used to obtain the precise position of the rover through the bundle adjustment (BA) method. After that, a cross-scale cost aggregation for a stereo matching network is proposed to obtain a parallax map, which can further obtain 3D point clouds of the lunar surface. Finally, the indexes of the craters are extracted (diameter D, depth d, and depth–diameter ratio dr), and the different indicators are fitted and analyzed. The results suggest that CscaNet has an anomaly percentage value of 1.73% in the KITTI2015 dataset, and an EPE of 0.74 px in the SceneFlow dataset, both of which are superior to GC-Net, DispNet, and PSMnet, and have higher reconstruction accuracy. The correlation between D and d is high and exhibits a positive correlation, while the correlation between D and dr is low. The geometric morphology expressions of small craters fitted by using D and d are significantly different from the expressions proposed by other scholars for large craters. This study provides a priori knowledge for the subsequent Von Karmen crater survey mission in the SPA Basin.

Large Area High-Resolution 3D Mapping of the Von Kármán Crater: Landing Site for the Chang’E-4 Lander and Yutu-2 Rover

We demonstrate the creation of a large area of high-resolution (260 × 209 km2 at 1 m/pixel) DTM mosaic from the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images over the Chang’E-4 landing site at Von Kármán crater using an in-house deep learning-based 3D modelling system developed at University College London, called MADNet, trained with lunar orthorectified images and digital terrain models (DTMs). The resultant 1 m DTM mosaic is co-aligned with the Chang’E-2 (CE-2) and the Lunar Orbiter Laser Altimeter (LOLA)—SELenological and Engineering Explorer (SELENE) blended DTM product (SLDEM), providing high spatial and vertical congruence. In this paper, technical details are briefly discussed, along with visual and quantitative assessments of the resultant DTM mosaic product. The LROC NAC MADNet DTM mosaic was compared with three independent DTM datasets, and the mean differences and standard deviations are as follows: PDS photogrammetric DTM at 5 m grid-spacing had a mean difference of −0.019 ± 1.09 m, CE-2 DTM at 20 m had a mean difference of −0.048 ± 1.791 m, and SLDEM at 69 m had a mean difference of 0.577 ± 94.940 m. The resultant LROC NAC MADNet DTM mosaic, alongside a blended LROC NAC and CE-2 MADNet DTM mosaic and a separate LROC NAC, orthorectified image mosaic, are made publicly available via the ESA planetary science archive’s guest storage facility.

A new compositional, mineralogical and chronological study of the Leibnitz crater within the SPA basin

Leibnitz is an impact crater 237 ​km in diameter located on the lunar farside within the South Pole–Aitken (SPA) basin. It shares much of its southern rim with the Von Kármán crater (186 ​km in diameter), which is currently the focus of in situ exploration by the Yutu-2 rover, part of the Chang'e−4 mission. This complementary study analyzes Leibnitz's surface materials mineralogy and age using remote sensing data. We focused on the spectral information from 95 fresh craters within Leibnitz to minimize the effects of weathering on the exposed materials. Subsequently, we employed the Modified Gaussian Model to derive a distribution map for the high to low Ca pyroxene ratio values. Finally, we derived absolute model age estimations of the mare basalts using the lunar crater chronology method. A new geologic map of the Leibnitz crater summarizes the results of this study. The results show that the interiors of Von Kármán and Leibnitz share a degree of similarity in mineral composition, with strong absorption features around the 1 ​μm and 2 ​μm bandcenters, due to the spectral dominance of mafic minerals. The ejecta from the adjacent Finsen crater have higher Ca/Fe and HCP/LCP ratios than the exposed materials of the Leibnitz crater. This study also finds evidence to support an older age of formation of the crater than age estimates to date from previous studies of the Leibnitz crater.

Moon-Based Ground-Penetrating Radar Observation of the Latest Volcanic Activity at the Chang’E-4 Landing Site

The Moon-based ground penetrating radar (GPR) onboard the Chang’E-4 (CE-4) Yutu-2 rover has deployed on the far side of the Moon, which provides the Moon’s far side, providing an unprecedented opportunity to study the shallow surface geological process and the history of the volcanic eruption of the Moon. The high-frequency radar observed a buried lens structure ~27 m below the lunar surface, which was interpreted as paleo-regolith by previous studies. In this study, we conducted a quantitative analysis of the dielectric properties of the buried lens structure observed during the CE-4 mission. Our results reveal that the relative permittivity and loss tangent are of the structure is ~10.5 and ~0.037, respectively. Comparing our estimated dielectric properties with those of Apollo samples and radar-observed lava flows shows that the buried lens is neither regolith nor ejecta material but possibly basalt flow. Additionally, we speculate that the buried basalt flow may represent the latest volcanic eruption on the Moon, possibly from the Eratosthenian-aged volcano that occurred approximately 2.5-2.2 billion years ago. Finally, we update the interpretation of the regolith stratigraphic structure observed by the Yutu-2 high-frequency radar at the CE-4 landing site.

Simulation of Lunar Comprehensive Substructure With Fracture and Imaging of Later LPR Data From Chang’e-4 Mission

As one of the most important geophysical methods for detecting the lunar underground structure of the Moon, the lunar penetrating radar (LPR) is applied to the Chang’e-4 (CE-4) mission to explore the Von Kármán crater on the far side of the Moon. With the Yutu-2 rover continues to move towards the Zhinyu crater at the west, radar will likely detect the fractures created by the impact that formed the Zhinyu Crater. Therefore, the establishment of a comprehensive lunar subsurface structure model with fracture, random media and fractal terrain is crucial for the LPR data processing and the understanding of the lunar geological impact process. In addition, the method that can image the radar data with fractures well has great significance to the interpretation of the LPR data. In this paper, we establish a comprehensive lunar subsurface structure model firstly, the considering factors including fractures, random media, fractal terrain, and the radar response are calculated through forward modeling. Secondly, according to the high resolution and the sensitive to inhomogeneities of LPR data, we design a set of processing flow including plane-wave destruction, velocity analysis based on the focusing analysis method and migration imaging based on the velocity continuation. Finally, the results including simulation data and Antarctic fracture data are used to verify the effectiveness of imaging method.

Sub-surface stratification and dielectric permittivity distribution at the Chang’E-4 landing site revealed by the lunar penetrating radar

Context.In 2019, China’s Chang’E-4 (CE-4) probe landed on the far side of the Moon: a first in lunar exploration. The Lunar Penetrating Radar (LPR) mounted on the Yutu-2 rover allows the mapping of the near-surface structure and the dielectric permittivity of the landing area. The dielectric properties of the lunar soil affect the propagation of the LPR signals, which can be used to infer the depth of sub-surface boundaries and derive the composition of the component materials.Aims.Our objectives are to estimate the fine-resolution spatial distribution of relative permittivity and to improve the interpretation of the geological processes combined with the radargram of the CE-4 landing area.Methods.We used a modified method that combines the F-K migration and the minimum entropy of the ground penetrating radar (GPR) signals to estimate the velocity and permittivity values; this has the advantage of obtaining the appropriate velocity and permittivity, even with the incomplete or unnoticeable hyperbolic curves in the radar imageResults.The sub-surface stratification of the CE-4 landing area is seen in the first 31 lunar days of the LPR data. A fine-resolution dielectric permittivity profile ranging from ~2.3 to ~6.3 is obtained with our method, and the actual depths of the observed prominent sub-surface interfaces are determined, giving a maximum average depth of ~38 m. The thickness of the regolith layer is in the range of ~5.7–15.6 m, with an average of 11.8 m. The permittivity of the near-surface regolith (<30 cm) is ~2.78 ± 0.01, the bulk density is 1.57 ± 0.01 g cm−3, which is close to the results of ~1.61 g cm−3at the Apollo 15 landing area. The permittivity map is consistent with the radargram; the regolith and the paleo-regolith layer have relatively low permittivity and low echo strengths, while the rock debris has high permittivity and shows strong echos in the radargram. Two buried craters of different diameters beneath the navigation sites 4–11 and 16–31 are revealed in the radar profile. The permittivity distribution map can show detailed variations of material properties both inside and outside craters.

Yutu-2 radar observation of the lunar regolith heterogeneity at the Chang’E-4 landing site

Context.The lunar penetrating radar (LPR) carried by the Yutu-2 rover performed the first in situ measurement of the subsurface structure and physical properties of the subsurface materials on the far side of the Moon. It provides an unprecedented opportunity to study the formation and evolution of the lunar surface.Aims.This paper aims to quantitatively estimate the heterogeneity of the lunar regolith using the high-frequency Yutu-2 radar observation and constrain the modeling parameters (e.g., autocorrelation length) on a radar simulation.Methods.The heterogeneity of the lunar regolith was quantified by comparing the simulation and observation acquired by the high-frequency Yutu-2 radar within the first 17 lunar days after its landing. The radar simulation was determined by the numerical calculation of the stochastic regolith model. The change in the autocorrelation length to the modeling was derived by calculating the coarseness of the model.Results.The disturbance range of the lunar regolith with a thickness of ~12 m at the Chang’E-4 landing site is constrained to be ~0.20 ± 0.06 m, indicating a high self-similarity. The stochastic model’s spatial disturbance is controlled by the autocorrelation length and is also scaled by the model size, and the radar scattering echo strength decreases with the increase in autocorrelation length.Conclusions.We conclude that the heterogeneity of lunar regolith is positively related to the geological age. The application of the disturbance range at the decimeter scale might provide a valuable reference to assist in interpreting the radar observation data of the Moon (e.g., Arecibo radar, Min-SAR and Mini-RF, and in situ LPR).

Near-infrared spectroscopy of boulders with dust or patina coatings on the Moon: A two-layer radiative transfer model

Previous remote sensing studies focus on lunar surface regolith, which contains abundant mixtures of rock fragments and dust, making it hard to track the petrologic origin. Igneous boulders exposed on lunar surface, however, carry pristine mineralogy and chemistry since its formation, therefore are direct evidence of lunar thermal evolution events. High spatial-resolution remote sensing images and rover explorations of the Moon allow us to study the spectroscopy of igneous boulders. We modeled the optical scattering properties of rocks using the Legendre and Double Henyey–Greenstein phase functions, porosity parameter and grain size, and provided a modified radiative transfer model for rocks rather than powdered minerals. Considering that space weathering could generate a layer of dust or patina on the surface of boulders, we introduced a two-layer radiative transfer modeling algorithm to solve the spectroscopy of the substrate rock for dust- or patina-coated boulder. The modeled substrate rock spectra show less reddening, larger reflectance, and stronger absorption band depth compared to dust- or patina-coated rock, consistent with the measurements of Apollo rock samples. We applied this two-layer model on the dust-coated boulder detected by Yutu-2 rover and derived the spectrum of the substrate rock. Using Kaguya Multiband Imager data, we calculated the substrate rock spectra for an anorthosite boulder, and our result shows good consistency with laboratory measured anorthosite rock spectrum. This work is a beginning of understanding lunar boulder spectroscopy for a more precise interpretation of lunar thermal history.

APPLICATIONS OF PHOTOGRAMMETRY AND REMOTE SENSING TECHNOLOGIES IN CHANG’E-4 MISSION

Abstract. This paper presents a brief overview of photogrammetry and remote sensing technologies and applications in the Chang’e-4 mission. The developed technologies, such as image matching, block adjustment, 3D mapping, spectral analysis and mineral retrieval, have been applied in topographic mapping, rover localization, and rover path planning, and directly supported daily surface operations of the Yutu-2 rover. The technologies and products have also been supportive in various scientific investigations, such as topographic evolution of Von Kármán crater, multi-scale crater morphologies, mineral composition of lunar deep materials, regolith maturity and space weathering effects, etc.

Lunar Terrestrial Analog Experiment on the Spectral Interpretations of Rocks Observed by the Yutu-2 Rover

A visible and near-infrared imaging spectrometer (VNIS) loaded by the Chang’e-4 rover is the primary method for detecting the mineral composition of the lunar surface in the landing region. However, different data processing methods yield inconsistent mineral modes in measured lunar soil and rocks. To better constrain the mineral modes of the soil and rocks measured by Chang’e-4 VNIS, a noritic-gabbroic rock with a mineral composition similar to that of the lunar highland rocks is measured by scanning electron microscopy (SEM), the spare flight model of Chang’e-4 VNIS and TerraSpec-4 of ASD. Backscattered electron and energy dispersive spectrometry show that olivine, pyroxene, and plagioclase modal mineral abundances are 12.9, 35.0, and 52.2%, respectively. The estimated results of the spectrum by the Hapke radiative transfer model are 7.5, 39.3, and 53.2% for olivine, pyroxene, and plagioclase, respectively, which is consistent with to those of SEM mapping within error. In contrast, the estimated results of the modified Gaussian model are 29 and 71% for olivine and pyroxene, respectively, indicating the absence of plagioclase. Based on our implemented Hapke model, we decode the data of the two rocks detected by the rover on the 3rd and 26th lunar days of mission operations. The obtained results suggest that both rocks are norite or gabbro with noticeable differences. The first rock, with more olivine and pyroxene, may have been excavated from the Finsen crater. The second rock, with more plagioclase, may have been ejected from the southwestern edge of the Von Kármán crater, indicating the initial lunar crust.

A novel precise pose prediction algorithm for setting the sleeping mode of the Yutu-2 rover based on a multiview block bundle adjustment

AbstractTo set the sleeping mode for the Yutu-2 rover, a visual pose prediction algorithm including terrain reconstruction and pose estimation was first studied. The terrain reconstruction precision is affected by using only the stereo navigation camera (Navcam) images and the rotation angles of the mast. However, the hazard camera (Hazcam) pose is fixed, and an image network was constructed by linking all of the Navcam and Hazcam stereoimages. Then, the Navcam pose was refined based on a multiview block bundle adjustment. The experimental results show that the mean absolute errors of the check points in the proposed algorithm were 10.4 mm over the range of $\boldsymbol{L}$ from 2.0 to 6.1 m, and the proposed algorithm achieved good prediction results for the rover pose (the average differences of the values of the pitch angle and the roll angle were −0.19 degrees and 0.29 degrees, respectively). Under the support of the proposed algorithm, engineers have completed the remote setting of the sleeping mode for Yutu-2 successfully in the Chang’e-4 mission operations.

Yutu-2 rover boulder and nearby soil data

Yutu-2 rover boulder and nearby soil data

A 2-year locomotive exploration and scientific investigation of the lunar farside by the Yutu-2 rover.

The lunar nearside has been investigated by many uncrewed and crewed missions, but the farside of the Moon remains poorly known. Lunar farside exploration is challenging because maneuvering rovers with efficient locomotion in harsh extraterrestrial environment is necessary to explore geological characteristics of scientific interest. Chang’E-4 mission successfully targeted the Moon’s farside and deployed a teleoperated rover (Yutu-2) to explore inside the Von Kármán crater, conveying rich information regarding regolith, craters, and rocks. Here, we report mobile exploration on the lunar farside with Yutu-2 over the initial 2 years. During its journey, Yutu-2 has experienced varying degrees of mild slip and skid, indicating that the terrain is relatively flat at large scales but scattered with local gentle slopes. Cloddy soil sticking on its wheels implies a greater cohesion of the lunar soil than encountered at other lunar landing sites. Further identification results indicate that the regolith resembles dry sand and sandy loam on Earth in bearing properties, demonstrating greater bearing strength than that identified during the Apollo missions. In sharp contrast to the sparsity of rocks along the traverse route, small fresh craters with unilateral moldable ejecta are abundant, and some of them contain high-reflectance materials at the bottom, suggestive of secondary impact events. These findings hint at notable differences in the surface geology between the lunar farside and nearside. Experience gained with Yutu-2 improves the understanding of the farside of the Moon, which, in return, may lead to locomotion with improved efficiency and larger range.