Chang’E-5 samples reveal high water content in lunar minerals

The formation and distribution of lunar surficial water remains ambiguous. Here, we show the prominence of water (OH/H₂O) attributed to solar wind implantation on the uppermost surface of olivine, plagioclase, and pyroxene grains from Chang’E-5 samples. The results of spectral and microstructural analyses indicate that solar wind-derived water is affected by exposure time, crystal structure, and mineral composition. Our estimate of a minimum of 170 ppm water content in lunar soils in the Chang’E-5 region is consistent with that reported by the Moon Minerology Mapper and Chang’E-5 lander. By comparing with remote sensing data and through lunar soil maturity analysis, the amount of water in Chang’E-5 provides a reference for the distribution of surficial water in middle latitude of the Moon. We conclude that minerals in lunar soils are important reservoirs of water, and formation and retention of water originating from solar wind occurs on airless bodies.

Lunar soils returned by China’s Chang’E−5 (CE5) mission record the unique information of solar wind essential to understanding the preservation and distribution of lunar surficial water. Here the authors report abundant water formed by solar wind implantation in minerals of CE5 lunar soils; the water content in CE5 lunar soils is estimated to be ~ 170 ppm.

Simulated Lunar Surface Hydration Measurements Using Multispectral Lidar at 3 µm

Accurately measuring the variability of spectroscopic signatures of hydration (H₂O + OH) on the illuminated lunar surface at 3 μm as a function of latitude, lunar time of day, and composition is crucial to determining the generation and destruction mechanisms of OH species and understanding the global water cycle. A prime complication in analysis of the spectroscopic feature is the accurate removal of thermal emission, which can modify or even eliminate the hydration feature depending on the data processing methods used and assumptions made. An orbital multispectral lidar, with laser illumination at key diagnostic wavelengths, would provide uniform, zero-phase geometry, complete latitude and time of day coverage from a circular polar orbit, and is agnostic to the thermal state of the surface. We have performed measurement simulations of a four-wavelength multispectral lidar using spectral mixtures of hydrated mid-ocean-ridge basalt (MORB) glasses and lunar regolith endmembers to assess the lidar performance in measuring hydration signatures on the lunar surface. Our results show a feasible system with wavelengths at 1.5 μm, 2.65 μm, 2.8 μm, and 3.1 μm can measure lunar hydration with a precision of 52 ppm (1σ) or better. These results, combined with the uniform measurement capabilities of multispectral lidar make it a valuable spectroscopic technique for elucidating mechanisms of OH/H₂O generation, migration, and destruction.

Evidence of water on the lunar surface from Chang'E-5 in-situ spectra and returned samples

The distribution range, time-varying characteristics, and sources of lunar water are still controversial. Here we show the Chang’E-5 in-situ spectral observations of lunar water under Earth’s magnetosphere shielding and relatively high temperatures. Our results show the hydroxyl contents of lunar soils in Chang’E-5 landing site are with a mean value of 28.5 ppm, which is on the weak end of lunar hydration features. This is consistent with the predictions from remote sensing and ground-based telescopic data. Laboratory analysis of the Chang’E-5 returned samples also provide critical clues to the possible sources of these hydroxyl contents. Much less agglutinate glass contents suggest a weak contribution of solar wind implantation. Besides, the apatite present in the samples can provide hydroxyl contents in the range of 0 to 179 ± 13 ppm, which shows compelling evidence that, the hydroxyl-containing apatite may be an important source for the excess hydroxyl observed at this young mare region.

Laboratory analysis of returned Chang’E-5 samples from the lunar surface show their hydroxyl contents to be on the weak end of lunar hydration features.

Depth to Diameter Analysis on Small Simple Craters at the Lunar South Pole—Possible Implications for Ice Harboring

In this paper, we present a study comparing the depth to diameter (d/D) ratio of small simple craters (200–1000 m) of an area between −88.5° to −90° latitude at the lunar south pole containing Permanent Shadowed Regions (PSRs) versus craters without PSRs. As PSRs can reach temperatures of 110 K and are capable of harboring volatiles, especially water ice, we analyzed the relationship of depth versus diameter ratios and its possible implications for harboring water ice. Variations in the d/D ratios can also be caused by other processes such as degradation, isostatic adjustment, or differences in surface properties. The conducted d/D ratio analysis suggests that a differentiation between craters containing PSRs versus craters without PSRs occurs. Thus, a possible direct relation between d/D ratio, PSRs, and water ice harboring might exist. Our results suggest that differences in the target’s surface properties may explain the obtained results. The resulting d/D ratios of craters with PSRs can help to select target areas for future In-Situ Resource Utilization (ISRU) missions.

Strategies for Deploying a Sensor Network to Explore Planetary Lava Tubes

Recently discovered pits on the surface of the Moon and Mars are theorized to be remnants of lava tubes, and their interior may be in pristine condition. Current landers and rovers are unable to access these areas of high interest. However, multiple small, low-cost robots that can utilize unconventional mobility through ballistic hopping can work as a team to explore these environments. In this work, we propose strategies for exploring these newly discovered Lunar and Martian pits with the help of a mother-daughter architecture for exploration. In this architecture, a highly capable rover or lander would tactically deploy several spherical robots (SphereX) that would hop into the rugged pit environments without risking the rover or lander. The SphereX robots would operate autonomously and perform science tasks, such as getting inside the pit entrance, obtaining high-resolution images, and generating 3D maps of the environment. The SphereX robot utilizes the rover or lander’s resources, including the power to recharge and a long-distance communication link to Earth. Multiple SphereX robots would be placed along the theorized caves/lava tube to maintain a direct line-of-sight connection link from the rover/lander to the team of robots inside. This direct line-of-sight connection link can be used for multi-hop communication and wireless power transfer to sustain the exploration mission for longer durations and even lay a foundation for future high-risk missions.

In situ chemical analysis of geology samples by a rapid simultaneous ultraviolet/visible/near-infrared (UVN) + longwave-infrared laser induced breakdown spectroscopy detection system at standoff distance

The standoff detection range of the simultaneous ultraviolet/visible/near-infrared (UVN) + longwave-Infrared (LWIR) Laser Induced Breakdown Spectroscopy (LIBS) detection system has been successfully extended from merely 10 cm to ≥ 1 meter by adopting a reflecting telescope collection scheme and UVN + LWIR LIBS emission signatures were acquired in various atmospheres from soil and mineral samples. This system simultaneously captured emission signatures from atomic, and simple and complex molecular target species existing in or near the same laser-induce plasma plume within micro-seconds. These pioneer standoff measurements of UVN + LWIR LIBS signatures have revealed an abundance of plasma-generated sample molecular emitting species in their vapor state along with atomic ones which gave intense and distinct signature emissions in both UVN (conventional LIBS) and LWIR (LWIR LIBS) spectral regions. A HITRAN simulation estimates the temperatures of those vapor molecular species to be around 2500 K. Laser-induced plasma emissions in the LWIR region provided direct information on the molecular components of the sample substances. The demonstrable capability of the LWIR LIBS on in situ characterization of carbon- and oxygen-rich materials is expected to find important applications in water discovery and organic materials signatures detection and identification. As a result laser ablation spectroscopy will be greatly augmented in both fundamental knowledge of and capability for chemical analysis.

The delivery of water by impacts from planetary accretion to present

Dynamical models and observational evidence indicate that water-rich asteroids and comets deliver water to objects throughout the solar system, but the mechanisms by which this water is captured have been unclear. New experiments reveal that impact melts and breccias capture up to 30% of the water carried by carbonaceous chondrite-like projectiles under impact conditions typical of the main asteroid belt impact and the early phases of planet formation. This impactor-derived water resides in two distinct reservoirs: in impact melts and projectile survivors. Impact melt hosts the bulk of the delivered water. Entrapment of water within impact glasses and melt-bearing breccias is therefore a plausible source of hydration features associated with craters on the Moon and elsewhere in the solar system and likely contributed to the early accretion of water during planet formation.

Widespread distribution of OH/H2O on the lunar surface inferred from spectral data

Remote sensing data from lunar orbiters have revealed spectral features consistent with the presence of OH or H₂O on the lunar surface. Analyses of data from the Moon Mineralogy Mapper spectrometer onboard the Chandryaan-1 spacecraft have suggested that OH/H₂O is recycled on diurnal timescales and persists only at high latitudes. However, the spatial distribution and temporal variability of the OH/H₂O, as well as its source, remain uncertain. Here we incorporate a physics-based thermal correction into analysis of reflectance spectra from the Moon Mineralogy Mapper and find that prominent absorption features consistent with OH/H₂O can be present at all latitudes, local times, and surface types examined. This suggests the widespread presence of OH/H₂O on the lunar surface without significant diurnal migration. We suggest that the spectra are consistent with the production of OH in space weathered materials by the solar wind implantation of H⁺ and formation of OH at crystal defect sites, as opposed to H₂O sourced from the lunar interior. Regardless of the specific composition or formation mechanism, we conclude that OH/H₂O can be present on the Moon under thermal conditions more wide-ranging than previously recognized.

Remotely distinguishing and mapping endogenic water on the Moon

Water and/or hydroxyl detected remotely on the lunar surface originates from several sources: (i) comets and other exogenous debris; (ii) solar-wind implantation; (iii) the lunar interior. While each of these sources is interesting in its own right, distinguishing among them is critical for testing hypotheses for the origin and evolution of the Moon and our Solar System. Existing spacecraft observations are not of high enough spectral resolution to uniquely characterize the bonding energies of the hydroxyl molecules that have been detected. Nevertheless, the spatial distribution and associations of H, OH- or H2O with specific lunar lithologies provide some insight into the origin of lunar hydrous materials. The global distribution of OH-/H2O as detected using infrared spectroscopic measurements from orbit is here examined, with particular focus on regional geological features that exhibit OH-/H2O absorption band strengths that differ from their immediate surroundings.This article is part of the themed issue 'The origin, history and role of water in the evolution of the inner Solar System'.

ARTEMIS observations of the solar wind proton scattering function from lunar crustal magnetic anomalies

Despite their small scales, lunar crustal magnetic fields are routinely associated with observations of reflected and/or backstreaming populations of solar wind protons. Solar wind proton reflection locally reduces the rate of space weathering of the lunar regolith, depresses local sputtering rates of neutrals into the lunar exosphere, and can trigger electromagnetic waves and small-scale collisionless shocks in the near-lunar space plasma environment. Thus, knowledge of both the magnitude and scattering function of solar wind protons from magnetic anomalies is crucial in understanding a wide variety of planetary phenomena at the Moon. We have compiled 5.5 years of ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) observations of reflected protons at the Moon and used a Liouville tracing method to ascertain each proton's reflection location and scattering angles. We find that solar wind proton reflection is largely correlated with crustal magnetic field strength, with anomalies such as South Pole/Aitken Basin (SPA), Mare Marginis, and Gerasimovich reflecting on average 5-12% of the solar wind flux while the unmagnetized surface reflects between 0.1 and 1% in charged form. We present the scattering function of solar wind protons off of the SPA anomaly, showing that the scattering transitions from isotropic at low solar zenith angles to strongly forward scattering at solar zenith angles near 90°. Such scattering is consistent with simulations that have suggested electrostatic fields as the primary mechanism for solar wind proton reflection from crustal magnetic anomalies.

Space Weathering on Airless Bodies

Space weathering refers to alteration that occurs in the space environment with time. Lunar samples, and to some extent meteorites, have provided a benchmark for understanding the processes and products of space weathering. Lunar soils are derived principally from local materials but have accumulated a range of optically active opaque particles (OAOpq) that include nanophase metallic iron on/in rims formed on individual grains (imparting a red slope to visible and near-infrared reflectance) and larger iron particles (which darken across all wavelengths) such as are often found within the interior of recycled grains. Space weathering of other anhydrous silicate bodies, such as Mercury and some asteroids, produce different forms and relative abundance of OAOpq particles depending on the particular environment. If the development of OAOpq particles is minimized (such as at Vesta), contamination by exogenic material and regolith mixing become the dominant space weathering processes. Volatile-rich bodies and those composed of abundant hydrous minerals (dwarf planet Ceres, many dark asteroids, outer solar system satellites) are affected by space weathering processes differently than the silicate bodies of the inner solar system. However, the space weathering products of these bodies are currently poorly understood and the physics and chemistry of space weathering processes in different environments are areas of active research.

Detection of local H2O exposed at the surface of Ceres

The surface of dwarf planet Ceres contains hydroxyl-rich materials. Theories predict a water ice-rich mantle, and water vapor emissions have been observed, yet no water (H₂O) has been previously identified. The Visible and InfraRed (VIR) mapping spectrometer onboard the Dawn spacecraft has now detected water absorption features within a low-illumination, highly reflective zone in Oxo, a 10-kilometer, geologically fresh crater, on five occasions over a period of 1 month. Candidate materials are H₂O ice and mineral hydrates. Exposed H₂O ice would become optically undetectable within tens of years under current Ceres temperatures; consequently, only a relatively recent exposure or formation of H₂O would explain Dawn's findings. Some mineral hydrates are stable on geological time scales, but their formation would imply extended contact with ice or liquid H₂O.

The negligible chondritic contribution in the lunar soils water

Recent data from Apollo samples demonstrate the presence of water in the lunar interior and at the surface, challenging previous assumption that the Moon was free of water. However, the source(s) of this water remains enigmatic. The external flux of particles and solid materials that reach the surface of the airless Moon constitute a hydrogen (H) surface reservoir that can be converted to water (or OH) during proton implantation in rocks or remobilization during magmatic events. Our original goal was thus to quantify the relative contributions to this H surface reservoir. To this end, we report NanoSIMS measurements of D/H and (7)Li/(6)Li ratios on agglutinates, volcanic glasses, and plagioclase grains from the Apollo sample collection. Clear correlations emerge between cosmogenic D and (6)Li revealing that almost all D is produced by spallation reactions both on the surface and in the interior of the grains. In grain interiors, no evidence of chondritic water has been found. This observation allows us to constrain the H isotopic ratio of hypothetical juvenile lunar water to δD ≤ -550‰. On the grain surface, the hydroxyl concentrations are significant and the D/H ratios indicate that they originate from solar wind implantation. The scattering distribution of the data around the theoretical D vs. (6)Li spallation correlation is compatible with a chondritic contribution <15%. In conclusion, (i) solar wind implantation is the major mechanism responsible for hydroxyls on the lunar surface, and (ii) the postulated chondritic lunar water is not retained in the regolith.