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Bulk density of the lunar regolith is a key factor affecting its geophysical and geotechnical properties. In this study, a new method for estimating the bulk density of the lunar regolith is developed based on the geometric characteristic (i.e., hyperbolic shape) of radar echoes in groundpenetrating radar (GPR) image. As an application, bulk density of the lunar regolith at China's Chang'E‐3 (CE‐3) landing site is estimated using the Lunar Penetrating Radar data. In total, 57 hyperbolas are identified in the Lunar Penetrating Radar image and their eccentricities are used to estimate relative permittivity of the regolith. Then, bulk density of the lunar regolith is estimated using an empirical relation through its dependence on relative permittivity. The results show that bulk density of the regolith at the CE‐3 landing site increases with depth from 0.85 g/cm 3 at the surface to a steady‐state value of 2.25 g/cm 3 at 5 m, with an average gradient much smaller than that based on the Apollo regolith samples. The bulk density corresponds to a regolith porosity of 74.5% at the surface and 32.3% at 5 m depth over the CE‐3 landing region. Given that the landing site is only ∼50 m from the east rim of a 500 m diameter crater, named as Zi Wei, the steady‐state bulk density indicates a 29% volume fraction of subsurface rocks within the continuous ejecta of this crater. Plain Language Summary In groundpenetrating radar observations, the distance between a radar antenna and a discrete subsurface object can produce a hyperbolic curve that is very common in groundpenetrating radar images. The shape of a hyperbolic curve depends on antenna height, depth of the object, and dielectric permittivity of subsurface material. Based on these relationships, relative permittivity (i.e., the ratio of the dielectric permittivity of a material to that of vacuum) of the lunar regolith to a depth of ∼5 m at China's Chang'E‐3 (CE‐3) landing site is first estimated using the Lunar Penetrating Radar observation. Then, bulk density and porosity of regolith and volume fraction of subsurface rocks are estimated through their dependences on relative permittivity, grain density of lunar regolith, and density of lunar rocks. The results show that bulk density increases with depth from 0.85 g/cm 3 at the surface to 2.25 g/cm 3 at 5 m, indicating a regolith porosity of 74.5% at the surface and 32.3% at 5 m depth. The results also indicate a volume fraction of 29% for subsurface rocks at the CE‐3 landing site. All these results can improve our understanding of the subsurface property of the lunar regolith, which was not revealed by previous missions because of the limited penetration depth (e.g., Apollo core tube experiment and Diviner radiometer estimation). Key Points A method for estimating bulk density of lunar regolith from GPR image is developed Bulk density of regolith at CE‐3 landing site increases from 0.85 g/cm 3 at the surface to 2.25 g/cm 3 at 5 m depth Volume fraction of subsurface rocks at the CE‐3 landing site could be as high as 29%

Exploring the subsurface structure and stratification of Mars advances our understanding of Martian geology, hydrological evolution and palaeoclimatic changes, and has been a main task for past and continuing Mars exploration missions 1 – 10 . Utopia Planitia, the smooth plains of volcanic and sedimentary strata that infilled the Utopia impact crater, has been a prime target for such exploration as it is inferred to have hosted an ancient ocean on Mars 11 – 13 . However, 45 years have passed since Viking-2 provided ground-based detection results. Here we report an in situ ground-penetrating radar survey of Martian subsurface structure in a southern marginal area of Utopia Planitia conducted by the Zhurong rover of the Tianwen-1 mission. A detailed subsurface image profile is constructed along the roughly 1,171 m traverse of the rover, showing an approximately 70-m-thick, multi-layered structure below a less than 10-m-thick regolith. Although alternative models deserve further scrutiny, the new radar image suggests the occurrence of episodic hydraulic flooding sedimentation that is interpreted to represent the basin infilling of Utopia Planitia during the Late Hesperian to Amazonian. While no direct evidence for the existence of liquid water was found within the radar detection depth range, we cannot rule out the presence of saline ice in the subsurface of the landing area. A ground-penetrating radar survey of Martian subsurface structure in a southern marginal area of Utopia Planitia constructed a detailed subsurface image profile showing a roughly 70-m-thick, multi-layered structure below regolith.

The bombardment of impactors (leftover planetesimals, asteroids and comets) created numerous impact craters on the Moon. The giant planets in the outer Solar System are believed to have experienced a dynamical instability, in which the migration of giant planets delivers impactors to the inner Solar System bodies1,2. The difference between the population of large (diameter more than ~5 km) impact craters observed on heavily cratered lunar highlands and that on the lunar maria3 was thought to support the lunar Late Heavy Bombardment, which started ~0.6 billion years after planet formation and could have been caused by the late instability of giant planets4–6. However, large craters on various-aged lunar surfaces have similar size–frequency distributions when considering the preferential erasure of small craters7,8. In addition, dynamical and geochemical evidence favour an early instability of giant planets at ~4.5 Gyr ago2,5,9. Here, we report the evolution at geological scales of regolith thickness on the Moon, which is a proxy for the change of the size–frequency distribution slope for Earth–Moon impactors with diameters less than ~50 m (which generate craters with diameters less than ~1 km (ref. 10)). We found an abnormally slow growth of regolith thickness per unit of impact flux before 3.5−0.6+0.3 Gyr ago (3σ uncertainty), which can best be explained by a population of craters of less than ~1 km whose size–frequency distribution had a shallower power-law slope (−2.57−0.16+0.30) than that afterward (−3.24−0.06+0.03). The transition time at ~3.5 Gyr ago supports the early instability of giant planets, in which dominant Earth–Moon impactors changed from leftover planetesimals to asteroids5. The value of −3.24 is consistent with the preferential delivery of small asteroids via Yarkovsky–YORP effects11.The change in growth of the lunar regolith thickness around 3.5 Gyr ago, a consequence of a change in population of the impactor bodies from planetesimals to asteroids, indicates that the instability of giant planets happened early.

The Moon has experienced an intense bombardment history since its formation 1 . Fragments of the impactor can remain on the lunar surface 2 – 4 and can provide evidence of the evolution of the impactor composition and impact population in the Earth–Moon system 3 – 5 . However, the retained impactor fragments previously identified in the Apollo samples have been well mixed into bulk lunar regolith due to the subsequent impact gardening, and their properties cannot be easily isolated 3 , 6 , 7 . Here we report observations of a two-metre-sized crater that formed less than one million years ago obtained by the Yutu-2 rover of Chang’e-4. Hyperspectral images in the visible and near-infrared range (0.45–0.945 μm) with a spatial resolution less than 1 mm per pixel highlight the presence of glassy material with high concentration (47%) of carbonaceous chondrites. We identify this material as remnants of the original impactor that was not entirely vaporized by the impact. Although carbonaceous chondrite fragments have been found in Apollo samples 8 , 9 , no carbonaceous chondrite remnant had been directly observed on the lunar surface by remote sensing exploration. We suggest that carbonaceous chondrite-like bodies may still provide one of the sources of water to the present Moon. The Chang’e-4 rover observed a small crater formed less than one million years ago, finding glassy materials with spectral characteristics similar to those of carbonaceous chondrites that are identified as remnants of the original impactor that have not yet been affected by weathering processes.

Solar wind irradiation, as a crucial space weathering mechanism, alters the microscopic characteristics and reflectance spectrum of the lunar regolith, and its cumulative effect is strongly related to the exposure time. Ilmenite is highly resistant to solar wind irradiation. Here, we combined transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy to systematically illustrate the diverse space-weathered rims on the shoveled and drilled Chang’e-5 ilmenites resulting from varying degrees of solar wind irradiation, revealing that solar wind plays a key role in the early-stage alteration of exposed lunar soil. In addition, the space weathering microstructure observations are consistent with the model-predicted exposure history at the Chang’e-5 landing site and prove that the Chang’e-5 deep-layered drilled (~65 cm) and surface samples (<3 cm) have been exposed for a longer time than that of the intermediate-layered drilled samples. We conclude that the ilmenite rims have the potential to be an effective indicator for comparing the relative exposure ages of regolith on airless bodies.Ilmenite grains collected in lunar regolith sampled by the Chang’e-5 sample return mission reveal that the 1 m deep core contains a mixture of space-weathered material, buried ejecta from the nearby Xu Guangqi crater, and a lunar paleoregolith.

There is ongoing debate about whether lunar magnesian suite (Mg-suite) magmatism was a global, nearly synchronous event with a genetic link to potassium, rare-earth element and phosphorus components (KREEP). Arguin 002, the first whole-rock meteorite classified as a lunar norite, offers a unique opportunity to explore the genesis and timing of Mg-suite rocks. Here we investigated the petrology, mineralogy, geochemistry, and chronology of Arguin 002, revealing it to be an evolved, KREEP-free Mg-suite rock with chemical similarities to atypical Apollo-15 Fe-norites. It likely formed through plutonic magmatism originating from low-degree partial melting of a deep, KREEP-free mantle source and has a 207 Pb/ 206 Pb age of 4341.5  ± 9.3 Ma. The potential source of Arguin 002 is within the South Pole-Aitken basin, near the Chang’e-6 landing site. These findings indicate that Mg-suite magmatism was a global and nearly synchronous event, potentially driven by rapid global mantle overturn. The lunar norite meteorite Arguin 002 likely formed through plutonic magmatism originating from low-degree partial melting of a deep, KREEP-free mantle source, according to mineralogical and geochemical analyses.

The lunar surface temperature (LST) derived from thermal infrared (TIR) measurements can aid in understanding the physical properties of the lunar surface. The Diviner Lunar Radiometer Experiment (herein, Diviner) sensor provides global lunar surface observation in seven TIR channels. However, its retrieval of LST constantly uses a single emissivity value (i.e., 0.95) by ignoring the spatial variation of lunar surface, thereby reducing the accuracy of temperature and day–night temperature difference. To overcome this problem, this study developed a physical method called temperature–emissivity separation (TES) algorithm to retrieve LST and lunar surface emissivity from the daytime observation in three Christiansen Feature (CF) channels (7.55–8.05, 8.10–8.40, and 8.38–8.60 μm) of the Diviner, and then used the emissivity from daytime observation to inverse LST at nighttime observation. Findings showed that the TES algorithm could retrieve LST and emissivity with an error of less than 0.8 K and 0.008, respectively. However, observation noise significantly affected the retrieval accuracy, particularly for the low‐temperature pixels; moreover, high retrieval accuracy requires a surface temperature higher than 240 K. The new algorithm was applied to obtain the daytime and nighttime LST and emissivity from the Diviner images. Results showed that the LST retrieved from the algorithm differed approximately 3.9 K from that calculated from a single emissivity 0.95. Finally, an example of global surface temperature and emissivity were obtained. Consequently, the CF pixels were found to distribute in the latitude range from −60° to 60°; however, they did not have a large distribution in high‐latitude and near‐polar regions. Key Points A physical algorithm was proposed to estimate lunar surface temperature and emissivity from Diviner daytime and nighttime observations Lunar surface temperature retrieved from the new algorithm differed 3.9 K from that calculated from a fixed emissivity 0.95 Global lunar Christiansen Feature pixels were identified and found to mainly distribute in the latitude range from −60° to 60°

Due to its significance in astrobiology, assessing the amount and state of liquid water present on Mars today has become one of the drivers of its exploration. Subglacial water was identified by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard the European Space Agency spacecraft Mars Express through the analysis of echoes, coming from a depth of about 1.5 km, which were stronger than surface echoes. The cause of this anomalous characteristic is the high relative permittivity of water-bearing materials, resulting in a high reflection coefficient. A determining factor in the occurrence of such strong echoes is the low attenuation of the MARSIS radar pulse in cold water ice, the main constituent of the Martian polar caps. The present analysis clarifies that the conditions causing exceptionally strong subsurface echoes occur solely in the Martian polar caps, and that the detection of subsurface water under a predominantly rocky surface layer using radar sounding will require thorough electromagnetic modeling, complicated by the lack of knowledge of many subsurface physical parameters. Higher-frequency radar sounders such as SHARAD cannot penetrate deep enough to detect basal echoes over the thickest part of the polar caps. Alternative methods such as rover-borne Ground Penetrating Radar and time-domain electromagnetic sounding are not capable of providing global coverage. MARSIS observations over the Martian polar caps have been limited by the need to downlink data before on-board processing, but their number will increase in coming years. The Chinese mission to Mars that is to be launched in 2020, Tianwen-1, will carry a subsurface sounding radar operating at frequencies that are close to those of MARSIS, and the expected signal-to-noise ratio of subsurface detection will likely be sufficient for identifying anomalously bright subsurface reflectors. The search for subsurface water through radar sounding is thus far from being concluded.

KREEP materials were thought to be last crystallized at the lunar crust and mantle boundary. Impact cratering and volcanism are mainly responsible for their distributions on the lunar surface. Therefore, observation of global KREEP materials and investigation of distributions in the areas of large basins are of critical importance to understand the geologic history of the Moon. Here we report the new global potassium distribution on the Moon detected by Chang'E-2 Gamma-ray Spectrometer. We found that our new measurements are in general agreement with previous observation. A new finding and an important difference is that relatively higher K abundances in the Mare Crisium and Mare Orientale than their surrounding rims were detected for the first time. In light of our observations in these two areas, we propose that Crisium and Orientale basin-forming impact events may have penetrated to the lower crust and excavate the deeper materials to the lunar surface.

Space-borne high frequency (HF) radar sounder is an effective tool for investigation of lunar subsurface structure in lunar exploration. The primary strategy of radar sounder technology for subsurface structure detection is utilization of the nadir echoes time delay and intensity difference from the lunar surface and subsurface. It is important to fully understand electromagnetic wave propagation, scattering, and attenuation through the lunar media in order to retrieve information of lunar layering structure from weak nadir echoes of the subsurface, which is simultaneously interfered by strong off-nadir surface clutters. Based on the Kirchhoff approximation (KA) of rough surface scattering and the ray tracing of geometric optics, a numerical simulation of radar echoes from lunar layering structures is developed. According to the lunar surface feature, the topography of mare and highland surfaces is numerically generated, and the triangulated network is employed to make digital elevations of the whole lunar surface. Scattering from the lunar surface and subsurface is numerically calculated using KA approach. Radar echoes and its range images are numerically simulated, and their dependence on the parameters of lunar layering interfaces is discussed. The approach of this paper can also be utilized to investigate subsurface structures in Mars and other planetary exploration.