The Ginger-shaped Asteroid 4179 Toutatis: New Observations from a Successful Flyby of Chang'e-2

On 13 December 2012, Chang'e-2 conducted a successful flyby of the near-Earth asteroid 4179 Toutatis at a closest distance of 770 $\pm$ 120 meters from the asteroid's surface. The highest-resolution image, with a resolution of better than 3 meters, reveals new discoveries on the asteroid, e.g., a giant basin at the big end, a sharply perpendicular silhouette near the neck region, and direct evidence of boulders and regolith, which suggests that Toutatis may bear a rubble-pile structure. Toutatis' maximum physical length and width are (4.75 $\times$ 1.95 km) $\pm$10$\%$, respectively, and the direction of the +$z$ axis is estimated to be (250$\pm$5$^\circ$, 63$\pm$5$^\circ$) with respect to the J2000 ecliptic coordinate system. The bifurcated configuration is indicative of a contact binary origin for Toutatis, which is composed of two lobes (head and body). Chang'e-2 observations have significantly improved our understanding of the characteristics, formation, and evolution of asteroids in general.Comment: 21 pages, 3 figures, 1 tabl

Quantitative assessment of water content and mineral abundances at Gale crater on Mars with orbital observations

Context. The information of water content can help to improve atmospheric and climate models, and thus provide a better understanding of the past and present role of water and aqueous alteration on Mars. Mineral abundances can provide unique constraints on their formation environments and thus also on the geological and climate evolution of Mars. Aims. In this study, we used a state-of-the-art approach to derive the hydration state and mineral abundances over Gale crater on Mars, analysing hyperspectral visible/near-infrared data from the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) instrument onboard Mars Express and from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument onboard the Mars Reconnaissance Orbiter. Methods. The Discrete Ordinate Transfer model was used to perform atmospheric and thermal correction of the OMEGA and CRISM hyperspectral data in order to derive the surface single scattering albedos (SSAs) at Gale crater, Mars. Water content was estimated using a linear relationship between the derived effective single-particle absorption thickness at 2.9 μm from SSAs and the water weight percentage. Mineral abundances were retrieved by performing the linear spectral unmixing of SSAs from CRISM data. The results were compared with the ground-truth results returned from Curiosity rover. Results. The water content for most areas at Gale crater derived using the OMEGA data is around 2–3 water weight percent (water wt % hereafter), which is in agreement with that derived from the in situ measurements by Curiosity’s Sample Analysis at Mars instrument. However, the sensitivity tests show that uncertainties exist due to the combination of several factors including modelling bias, instrumental issue, and different sensing techniques. The derived mineral abundances using the orbital data are not fully consistent with that derived by Curiosity, and the discrepancy may be due to a combination of dust cover, texture, and particle size effects, as well as the effectiveness of the quantitative model. Conclusions. The ground-truth data from Curiosity provide a critical calibration point for the quantitative method used in the orbital remote-sensing observations. Our analysis indicates that the method presented here has great potential for mapping the water content and mineral abundances on Mars, but caution must be taken when using these abundance results for geological interpretations

Estimation of surface water content at the Tianwen-1 Zhurong landing site

China’s first Mars rover, Zhurong, successfully landed in the south of Utopia Planitia. The surface water content at the landing area can provide constraints on mineral formation conditions and help us better understand the evolution of the Martian aqueous and geological environment. In this work, the surface kinetic temperature of the Zhurong landing area was derived by analyzing data from the Mars Express Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) spectrometer. Using the Discrete Ordinate Radiative Transfer (DISORT) model, we performed atmospheric correction and thermal correction for the OMEGA data to obtain the surface effective single-particle absorption thickness (ESPAT) parameter to evaluate the surface water content. The surface water content distribution at the landing area was relatively uniform at a lateral scale of ~10 km. At the Zhurong landing site, the surface water content in the topmost layer (a few hundred micrometers) of the regolith was 5−8 weight percent water (wt% H2O), assuming surface particle sizes of <45 μm, or 1.6−2.5 wt% H2O, assuming surface particle sizes in the range of 125−250 μm. The Mars Surface Composition Detector (MarSCoDe) onboard Zhurong also observed significant H2O/OH signals in the landing area. Our results provide an important regional context for the hydration state of the area and can be further verified by the H content derived from the Laser-Induced Breakdown Spectrometer (LIBS) data of MarSCoDe

Spectroscopic and Petrographic Investigations of Lunar Mg-Suite Meteorite Northwest Africa 8687

Magnesian suite (Mg-suite) rocks represent plutonic materials from the lunar crust, and their global distribution can provide critical information for the early magmatic differentiation and crustal asymmetries of the Moon. Visible and near-infrared (VNIR) spectrometers mounted on orbiters and rovers have been proven to be powerful approaches for planetary mineral mapping, which are instrumental in diagnosing Mg-suite rocks. However, due to the scarcity and diversity of Mg-suite samples, laboratory measurements with variable proportions of minerals are imperative for spectral characterization. In this study, spectroscopic investigation and petrographic study were conducted on lunar Mg-suite meteorite Northwest Africa 8687. We classify the sample as a pink spinel-bearing anorthositic norite through spectral and petrographic characteristics. The ground-truth information of the Mg-suite rock is provided for future exploration. Meanwhile, the results imply that the VNIR technique has the potential to identify highland rock types by mineral modal abundance and could further be applied in extraterrestrial samples for primary examination due to its advantage of being fast and non-destructive

Inverted channel belts and floodplain clays to the East of Tempe Terra, Mars:Implications for persistent fluvial activity on early Mars

The climate on early Mars is one of the major unsolved problems. It is unclear whether early Mars was warm and wet or cold and icy. Morphological features on Mars such as sinuous ridges could provide critical constraints to address this issue. Here we investigate several sinuous ridges to the east of Tempe Terra, Mars and find they may have recorded persistent fluvial activity on early Mars. Our analysis indicates that these ridges may represent exhumation of the channel belts and overbank deposits formed from meander rivers over significant geologic time. Layered smectite-bearing minerals, distributed along the ridge flanks, could be detrital or authigenic floodplain clays. Our interpretation of the stratigraphic relationships indicates that the layered smectite-bearing materials lie between channel belt deposits, supporting the floodplain interpretation. Our results suggest that a persistent warming event has persisted for a geologically significant interval (>1500 yr) during the Noachian period of Mars

Phase-angle dependence of colour ratios and potential implications for lunar remote sensing

Context. The colour-ratio technique has been widely used in mapping planetary surfaces, but its solar phase-angle dependence is not well understood. Understanding the phase-angle dependence of the colour ratio would enhance our abilities in interpreting planetary remote sensing data. Aims. We aim to investigate the dependence of the colour ratio indices on mineralogy, phase angle, particle size, and the degree of simulated space weathering. Methods. We measured the multi-band (i.e. 458, 633, 750, and 905 nm) and multi-angle reflectance spectra of four typical lunar-type minerals with different particle sizes using a custom multi-angular imaging device. Results. The colour ratio does have a phase-angle dependence that is more sensitive to the mineralogy and wavelength and less sensitive to particle size distribution. Conclusions. The combined analysis of the colour ratio and its phase dependence can improve efficiency in mapping the lunar surface. With a prior knowledge of the phase behaviours of colour ratios of specific lunar-type minerals, an optimised colour ratio at certain phase angles can be found to efficiently distinguish the composition of a target surface

Geotectonic evolution of lunar LQ-4 region based on multisource data

The Sinus Iridum region, the first choice for China's “Lunar Exploration Project” is located at the center of the lunar LQ-4 area and is the site of Chang'e-3 (CE-3)'s soft landing. To make the scientific exploration of Chang'e-3 more targeted and scientific, and to obtain a better macro-level understanding of the geotectonic environment of the Sinus Iridum region, the tectonic elements in LQ-4 region have been studied and the typical structures were analyzed statistically using data from CE-1, Clementine, LRO and Lunar Prospector missions. Also, the mineral components and periods of mare basalt activities in the study area have been ascertained. The present study divides the tectonic units and establishes the major tectonic events and sequence of evolution in the study area based on morphology, mineral constituents, and tectonic element distribution

Volcanism and Deep Structures of the Moon

Volcanism is the most important endogenic geological process of the Moon, which is closely related to its internal structure and thermal history. Lunar volcanism is one of the most important topics for lunar science and explorations. Recent lunar orbital observations, in situ explorations, and sample return missions have returned a new generation of high-resolution datasets, which greatly enriched our knowledge of lunar volcanism. Here, we summarize recent advances in the duration and spatial distribution of lunar mare volcanism, as well as origin of volcanic landforms such as lava flows, sinuous rilles, lava tubes, domes, and cones. We proposed several outstanding problems in the study of temporal and spatial span of lunar volcanism, the formation mechanism of the varied volcanic landforms, and their relation with deep structures. To solve these problems, more in situ explorations and sample return missions from various volcanic units are needed

Photometric Normalization of Chang’e-4 Visible and Near-Infrared Imaging Spectrometer Datasets: A Combined Study of In-Situ and Laboratory Spectral Measurements

Until 29 May 2020, the Visible and Near-Infrared Imaging Spectrometer (VNIS) onboard the Yutu-2 Rover of the Chang&rsquo;e-4 (CE-4) has acquired 96 high-resolution surface in-situ imaging spectra. These spectra were acquired under different illumination conditions, thus photometric normalization should be conducted to correct the introduced albedo differences before deriving the quantitative mineralogy for accurate geologic interpretations. In this study, a Lommel&ndash;Seeliger (LS) model and Hapke radiative transfer (Hapke) model were used and empirical phase functions of the LS model were derived. The values of these derived phase functions exhibit declining trends with the increase in phase angles and the opposition effect and phase reddening effect were observed. Then, we discovered from in-situ and laboratory measurements that the shadows caused by surface roughness have significant impacts on reflectance spectra and proper corrections were introduced. The validations of different phase functions showed that the maximum discrepancy at 1500 nm of spectra corrected by the LS model was less (~3.7%) than that by the Hapke model (~7.4%). This is the first time that empirical phase functions have been derived for a wavelength from 450 to 2395 nm using in-situ visible and near-infrared spectral datasets. Generally, photometrically normalized spectra exhibit smaller spectral slopes, lower FeO contents and larger optical maturity parameter (OMAT) than spectra without correction. In addition, the band centers of the 1 and 2 &mu;m absorption features of spectra after photometric normalization exhibit a more concentrated distribution, indicating the compositional homogeneity of soils at the CE-4 landing site