Airless Bodies Research Articles

No Thick Atmosphere on the Terrestrial Exoplanet Gl 486b

A primary science goal for JWST is to detect and characterize the atmospheres of terrestrial planets orbiting M dwarfs (M-Earths). The existence of atmospheres on M-Earths is highly uncertain because their host stars’ extended history of high X-ray and ultraviolet irradiation may act to completely remove their atmospheres. We present two JWST secondary eclipse observations of the M-Earth Gl 486b (also known as GJ 486b) between 5 and 12 μm. We combined these observations with a precise analysis of the host star parameters to derive a planetary dayside temperature of T p = 865 ± 14 K. We compared this temperature to the maximum expected temperature for a zero albedo, zero heat redistribution bare rock and derived a temperature ratio of R=Tp,daysideTp,max=0.97±0.01 . This value is consistent with an airless body with a slight nonzero albedo or a thin atmosphere with <1% H2O or <1 ppm CO2. However, it is inconsistent with an Earth- or Venus-like atmosphere, and the spectrum shows no clear emission or absorption features. Additionally, our observations are inconsistent with the water-rich atmospheric scenario allowed by previous transit observations and suggest the transmission spectrum was instead shaped by stellar contamination. Given the potential for atmospheric escape throughout the system’s ≥6.6 Gyr lifetime, we conclude that the observations are likely best explained by an airless planet. This result is the most precise measurement yet of terrestrial exoplanet thermal emission with JWST, which places a strong constraint on the position of the “cosmic shoreline” between airless bodies and those with atmospheres.

Depth profiling of implanted D in silicates: Contribution of solar wind protons to water in the Moon and terrestrial planets

The solar wind protons implanted in silicate material and combined with oxygen are considered crucial for forming OH/H O on the Moon and other airless bodies. This process may also have contributed to hydrogen delivery to planetary interiors through the accretion of micrometre-sized dust and planetesimals during early stages of the Solar System. This paper experimentally investigates the depth distribution of solar wind protons in silicate materials and explores the mechanisms that influence this profile. We simulated solar wind irradiation by implanting 3 keV D ions in three typical silicates (olivine, pyroxene, and plagioclase) at a fluence of sim 1.4 × 10 ions/cm . Fourier transform infrared spectroscopy was used to analyse chemical bond changes, while transmission electron microscopy (TEM) characterised microstructural modifications. Nanoscale secondary ion mass spectrometry (NanoSIMS) was employed to measure the D/ O ratio and determine the depth distribution of implanted deuterium. The newly produced OD band (at 2400–2800 cm –1 ) in the infrared spectrum reveals the formation of O–D bonds in the irradiated silicates. The TEM and NanoSIMS results suggest that over 73&lt;!PCT!&gt; of the implanted D accumulated in fully amorphous rims with a depth of 70 nm, while 25&lt;!PCT!&gt; extended inwards to sim 190 nanometres, resulting in partial amorphisation. The distribution of these deuterium particles is governed by the collision processes of the implanted particles, which involve factors such as initial energy loss, cascade collisions, and channelling effects. Furthermore, up to 2&lt;!PCT!&gt; of the total implanted D penetrated the intact lattice via diffusion, reaching depths ranging from hundreds of nanometres to several micrometres. Our results suggest that implanted solar wind protons can be retained in silicate interiors, which may significantly affect the hydrogen isotopic composition in extraterrestrial samples and imply an important source of hydrogen during the formation of terrestrial planets.

Experimental study of the UV irradiation influence on the activation of dust particles of atmosphereless bodies regolith simulators

The activity of dust particles on airless bodies has been recorded since the early automated missions to the Moon. Since then, numerous theoretical and experimental studies of this effect have been conducted, yet at present, there is no clear understanding of the influence of external factors on the dynamics of this phenomenon. Experimental work has been carried out to determine the contribution of hard UV radiation to the activity of dust particles. It has been shown that the impact of UV radiation significantly affects the dynamics of the particles. The results on determining the conditions for particle detachment from the surface are in line with theoretical calculations.

Revisiting lunar dust charging and dynamics

Under the dynamic influence of near-surface plasma, intricate dynamics of lunar dust have been observed during the Surveyors and Apollo missions in the form of Lunar horizontal glow. These dynamics are primarily driven by electrostatic forces generated by the continual bombardment of solar wind and highly energetic UV photons on the lunar surface and dust particles. This paper revisits the phenomenon of dust charging within the lunar photoelectron sheath and subsequent dynamics. The investigation has been carried out using a comprehensive model of the lunar photoelectron sheath characterized by observed solar spectra, latitude-dependent Fermionic photoelectrons, non-Maxwellian solar wind electrons, and cold ions. A test dust particle is introduced into the sheath, and equilibrium charge and static levitation conditions are derived. The result of dynamical evolution suggests the existence of a narrow parametric regime corresponding to the periodic hopping trajectory of the dust particle over the lunar surface. In other cases, the dust particles are found to re-impact the surface after a single ballistic hop. We further identify that the discrete charging of the dust could be crucial in determining the dust dynamics, particularly in the tenuous plasmas. The analysis of the discrete dust charging model reveals significant discrepancies with the continuous dust charging model and suggests a lower likelihood of static dust levitation in the lunar environment. The present study is important for unraveling the fundamental processes governing surface evolution on the Moon and other airless bodies throughout the Solar System.

The thermal impact of the self-heating effect on airless bodies. The case of Mercury’s north polar craters

Thermal models are essential for studying airless planetary surfaces, as the interaction between topography and thermophysical properties plays a crucial role in determining a surface’s response to localized illumination. Accurate temperature distribution calculations require a comprehensive investigation of sunlight scattering, a process that, despite its computational challenges, cannot be overlooked, especially when high resolution is necessary. Furthermore, thermal analysis is fundamental for assessing the stability of volatiles in polar regions. In this study, we introduce a novel approach by discretizing the Sun into 100 individual elements, allowing for a highly precise simulation of solar flux—an innovation crucial for accurately capturing temperature distributions in Mercury’s polar craters, given the planet’s proximity to the Sun. This level of discretization significantly enhances the accuracy of the thermal model, ensuring a more realistic depiction of how sunlight interacts with crater topography. We developed a dual-model approach that simulates both direct solar illumination and its scattering on two craters, Laxness and Fuller, located at Mercury’s north pole. The illumination and thermal model predict temperature distribution and heat transfer based on the material’s thermal properties and topography. The study examines the interaction between direct sunlight, causing localized heating, and scattered light, which influences the thermal response of surface materials. Detailed illumination maps and temperature profiles were generated over two Hermean years, revealing the significant impact of the self-heating effect on temperature distribution. The results show that specific regions experience indirect solar flux due to the craters’ morphology, particularly in permanently shadowed regions (PSRs) that are heated exclusively by scattered radiation. Maximum temperature profiles for the Laxness and Fuller craters show a substantial temperature increase within PSRs compared to areas exposed to direct illumination. However, while self-heating does not affect the stability of water ice in the Laxness crater, in the Fuller crater, a section within the radar-bright material reaches temperatures of up to 210 K, potentially threatening the stability of water ice. Further investigation with the onboard SIMBIO-SYS instrument on the BepiColombo mission will help to better understand the current state of these craters and their volatile deposits.

Simulated Solar-wind-induced Space Weathering of Olivine Powders: Spectral Alterations in the Ultraviolet, Visible, and Near-infrared

Bombardment by solar wind ions is one of the main drivers of space weathering on airless bodies. Here, we simulate the solar-wind-driven spectral alteration of loosely packed olivine powders by irradiation with 1.2 keV helium ions (He+). We measured the reflectance spectra of the olivine powder in the ultraviolet–visible–near-infrared (UV–Vis–NIR) wavelength range (0.2–2 μm) as a function of ion fluence. In the Vis–NIR range, we observed spectral darkening, absorption band shallowing, and spectral reddening, in agreement with lunar-style space weathering and previous laboratory studies. In the UV–Vis, spectral darkening was also observed. However, a spectral bluing took place at wavelengths below 400 nm. As the simulated space weathering progressed, the spectral slopes shifted from steep-UV/shallow-NIR slopes to shallow-UV/steep-NIR slopes. Moreover, the change in the UV slope was almost 10 times larger than in the NIR, supporting the hypothesis that the UV spectral slope could be an earlier indicator of space weathering.

Near-infrared spectral behavior of space-weathered olivine with varying iron content

Space weathering alters the surfaces of airless celestial bodies, thereby modifying their spectra significantly. Olivine plays a crucial role in responding to space weathering on silicate planets. However, the spectral variations that occur in olivine with varying iron content as a result of space weathering conditions remain unclear. We aim to systematically characterize the spectral variability of surface iron-rich olivine in the space weathering environments of Phobos and the Moon. We conducted nanosecond pulsed laser irradiation experiments on a set of synthetic Fe-rich olivine (Fa29, Fa50, Fa71, and Fa100). The energy levels were simulated for Phobos and the Moon. We analyzed the available near-infrared (NIR) spectroscopy. We find that olivine with higher Fe content undergoes stronger weathering under the same irradiation energy, shifting absorption centers around 1.08 mu m and 1.35 mu m to longer wavelengths. When comparing the high energy and low frequency, spectral changes are more pronounced at low energy and high frequency. The olivine with the same iron content exhibits a more noticeable shift around 1.08 mu m under various irradiation levels, while the band center around 1.35 mu m remains stable. When the same amount of radiation energy is received, changes in the spectrum are more noticeable at low energy and high impact frequency than at high energy and low impact frequency. The absorption position at $ $ 1.35 mu m is a good indicator of the Mg$\#$ value of space-weathered olivine.

Experimental Method for Measuring Cohesion of Regolith via Electrostatic Lofting

The hypothesized electrostatic lofting of individual regolith grains on the Moon and asteroids has been investigated extensively in laboratory studies. Cohesion may dominate how regolith behaves on these small, airless bodies, yet the magnitude of this force remains uncertain. We induce the electrostatic detachment of dust as a mechanism to break cohesive bonds between individual zirconia-silica microspheres in order to measure the interparticle cohesive force between them, likely dominated by capillary bridges. A high-speed camera imaged centroid positions of the lofted microspheres over time. Using the centroids from the initial detachment, we numerically calculated initial accelerations to solve for the cohesion that had been restraining the microspheres. Unexpectedly, the electrostatic lofting of clumps of particles was observed and experimental results showed that clumps were a nonnegligible portion of the lofted object population.

Extremely large Cl isotopic fractionation in Chang'e-5 impact glass beads

Lunar materials have recorded a very large δ37Cl variation, and the mechanism causing this variation has yet to be determined. We measured the F and Cl contents and the δ37Cl in Chang'e-5 impact glass beads using a nanoscale secondary ion mass spectrometry. These glass beads exhibit the largest δ37Cl variation observed to date, ranging from –0.7 ‰ to 119 ‰. Furthermore, the δ37Cl values are roughly negatively correlated with the Cl concentration. The correlations between F and Cl concentrations differ for homogeneous and heterogeneous glass beads. Our calculations indicate that NaCl (g) and HCl (g) degassing may have been the pivotal mechanism that elevated the δ37Cl value, with >50 % of Cl in the melt evaporating during glass formation. The glass beads may have incorporated the chlorine species condensed from early evaporation. Our results provide direct evidence to constrain the impact-induced degassing process of Cl on airless celestial bodies.

Space-weathered rims on lunar ilmenite as an indicator for relative exposure ages of regolith

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.

Three‐Dimensional Electrostatic Hybrid Particle‐In‐Cell Simulations of the Plasma Mini‐Wake Near a Lunar Polar Crater

AbstractLunar polar region has become the focus of future explorations due to the possible ice reservoir in the permanently shadowed craters. However, the space environment near the polar crater is quite complicated, and a plasma mini‐wake can be caused by the topographic obstruction. So far, three‐dimensional (3D) numerical simulations of the mini‐wake around a crater far larger than the Debye length are still limited. Here we present a 3D electrostatic hybrid particle‐in‐cell model to study the plasma mini‐wake of a polar crater on scale of about 1 km. It is found that the mini‐wake can begin upstream from the crater with a cone angle of about 8.8°. There is a plasma void with extra electrons near the leeward crater wall, where the electric potential can be as low as −60 V. A part of solar wind ions can be diverted into the crater, and the ratio of the diverted flux is about 4% on the crater bottom and about 18% on the windward crater wall, which provide an important source for the surface sputtering. Further studies show that the mini‐wake can change with the solar wind parameters and the crater shapes. Our results are helpful to assess the space environment and the water loss rate of a polar crater, and have general implications in studying the plasma mini‐wake caused by a crater on the other airless bodies.

Estimate of Water and Hydroxyl Abundance on Asteroid (16) Psyche from JWST Data

Our understanding of solar system evolution is closely tied to interpretations of asteroid composition, particularly the M-class asteroids. These asteroids were initially thought to be the exposed cores of differentiated planetesimals, a hypothesis based on their spectral similarity to iron meteorites. However, recent astronomical observations have revealed hydration on their surface through the detection of 3 μm absorption features associated with OH and potentially H2O. We present evidence of hydration due mainly to OH on asteroid (16) Psyche, the largest M-class asteroid, using data from the James Webb Space Telescope (JWST) spanning 1.1–6.63 μm. Our observations include two detections of the full 3 μm feature associated with OH and H2O resembling those found in CY-, CH-, and CB-type carbonaceous chondrites, and no 6 μm feature uniquely associated with H2O across two observations. We observe 3 μm depths of between 4.3% and 6% across two observations, values consistent with hydrogen abundance estimates on other airless bodies of 250–400 ppm. We place an upper limit of 39 ppm on the water abundance from the standard deviation around the 6 μm feature region. The presence of hydrated minerals suggests a complex history for Psyche. Exogenous sources of OH-bearing minerals could come from hydrated impactors. Endogenous OH-bearing minerals would indicate a composition more similar to E- or P-class asteroids. If the hydration is endogenous, it supports the theory that Psyche originated beyond the snow line and later migrated to the outer main belt.

Formation of nanophase metallic iron through charge disproportionation of ferrous iron during micrometeoroid impact‐induced splash melting

AbstractCharge disproportionation of ferrous iron has been considered as one of the mechanisms for the formation of metallic iron on the lunar surface. However, the detailed mechanism of the disproportionation reaction on the Moon is yet to be elucidated. We provide direct evidence for the ferrous disproportionation reaction that produces nano phase metallic iron (npFe0) during a rapid cooling process after splash melting from a lunar sample returned by China's Chang'e‐5 mission. Space weathering processes have resulted in the formation of three distinct zones at the rim of a pyroxene fragment, as observed through transmission electron microscopy. These zones, made up of splashed melts, newly formed melts from the substrate, and the mineral, are distinguished as I, II, and III. Quantitative analyses of the iron valence state by electron energy loss spectroscopy show that disproportionation reactions occurred in zone II at a low temperature of &lt;570°C during a rapid cooling process. The reaction led to the production of α‐structure npFe0 and Fe3+ reserve in the glass phase. The npFe0 produced by the disproportionation reaction has a larger grain size than those formed from solar wind irradiation, implying that micrometeoroid impacts mainly contribute to the darkening of visible and near‐infrared reflectance. These findings reveal a novel rim structure by repeated space weathering and a universal formation mechanism of npFe0 during micrometeoroid impacts, suggesting that the disproportionation reaction could be widespread on airless bodies with impact‐induced splash processes.

Metal impact and vaporization on the Moon's surface: Nano‐geochemical insights into the source of lunar metals

AbstractMillimeter‐to‐nanometer‐sized iron‐ and nickel‐rich metals are ubiquitous on the lunar surface. The proposed origin of these metals falls into two broad classes which should have distinct geochemical signatures—(1) the reduction of iron‐bearing minerals or (2) the addition of metals from meteoritic sources. The metals measured here from the Apollo 16 regolith possess low Ni (2–6 wt%) and elevated Ge (80–350 ppm) suggesting a meteoritic origin. However, the measured Ni is lower, and the Ge higher than currently known iron meteorites. In comparison to the low Ni iron meteorites, the even lower Ni and higher Ge contents exhibited by these lunar metals are best explained by impact‐driven volatilization and condensation of Ni‐poor meteoritic metal during their impact and addition to the lunar surface. The presence of similar, low Ni‐bearing metals in Apollo return samples from geographically distant sites suggests that this geochemical signature might not be restricted to just the Apollo 16 locality and that volatility‐driven modification of meteoritic metals are a common feature of lunar regolith formation. The possibility of a significant low Ni/high Ge meteoritic component in the lunar regolith, and the observation of chemical fractionation during emplacement, has implications for the interpretation of both lunar remote‐sensing data and bulk geochemical data derived from sample return material. Additionally, our observation of predominantly meteoritic sourced metals has implications for the prevalence of meteoritic addition on airless planetary bodies.

Multiple sources of water preserved in impact glasses from Chang'e-5 lunar soil.

The existence of molecular H2O and evolution of solar wind-derived water on the lunar surface remain controversial. We report that large amounts of OH and molecular H2O related to solar wind and other multiple sources are preserved in impact glasses from Chang'e-5 (CE5) lunar soil based on reflectance infrared spectroscopy and nanoscale secondary ion mass spectrometry analyses. The estimated water content contributed by impact glasses to CE5 lunar soil was ~72 ppm, including molecular H2O of up to 15 to 25 ppm. Our studies revealed that impact glasses are the main carrier of molecular H2O in lunar soils. Moreover, water in CE5 impact glasses provides a record of complex formation processes and multiple water sources, including water derived from solar wind, deposited by water-bearing meteorites/micrometeorites, and inherited from lunar indigenous water. Our study provides a better understanding of the evolution of surficial water on airless bodies and identifies potential source and storage pathways for water in the terrestrial planets.

Laser Irradiation of Carbonaceous Chondrite Simulants: Space-weathering Implications for C-complex Asteroids

Surfaces of carbonaceous asteroids (C-complex) have shown diverse, contrasting spectral variations, which may be related to space weathering. We performed laser irradiation experiments on CI and CM simulant material under vacuum to mimic the spectral alteration induced by micrometeorite impacts. We used in situ ultraviolet-visible and near-infrared reflectance spectroscopy to analyze spectral alterations in response to pulsed laser irradiation, as well as scanning electron microscopy and X-ray photoelectron spectroscopy to search for microstructural and compositional changes. Laser irradiation causes an increase in spectral slope (reddening) and a decrease in the albedo (darkening), and these changes are stronger in the ultraviolet-visible region. These spectral changes are likely driven by the excess iron found in the altered surface region although other factors, such as the observed structural changes, may also contribute. Additionally, while the 0.27 μm band appears relatively stable under laser irradiation, a broad feature at 0.6 μm rapidly disappears with laser irradiation, suggesting that space weathering may inhibit the detection of any feature in this spectral region, including the 0.7 μm band, which has typically been used an indicator of hydration. Comparing our laboratory results with optical spectrophotometry observations of C-complex asteroids, we find that the majority of objects are spectrally red and possess colors that are similar to our irradiated material rather than our fresh samples. Furthermore, we also find that “younger” and “older” C-complex families have similar colors, suggesting that the space-weathering process is near equal or faster than the time it takes to refresh the surfaces of these airless bodies.

Nonmagnetic framboid and associated iron nanoparticles with a space-weathered feature from asteroid Ryugu

Extraterrestrial minerals on the surface of airless Solar System bodies undergo gradual alteration processes known as space weathering over long periods of time. The signatures of space weathering help us understand the phenomena occurring in the Solar System. However, meteorites rarely retain the signatures, making it impossible to study the space weathering processes precisely. Here, we examine samples retrieved from the asteroid Ryugu by the Hayabusa2 spacecraft and discover the presence of nonmagnetic framboids through electron holography measurements that can visualize magnetic flux. Magnetite particles, which normally provide a record of the nebular magnetic field, have lost their magnetic properties by reduction via a high-velocity (>5 km s–1) impact of a micrometeoroid with a diameter ranging from 2 to 20 μm after destruction of the parent body of Ryugu. Around these particles, thousands of metallic-iron nanoparticles with a vortex magnetic domain structure, which could have recorded a magnetic field in the impact event, are found. Through measuring the remanent magnetization of the iron nanoparticles, future studies are expected to elucidate the nature of the nebular/interplanetary magnetic fields after the termination of aqueous alteration in an asteroid.

Helium reservoirs in iron nanoparticles on the lunar surface

The Moon’s surface is directly exposed to the space environment and subject to alteration by space weathering. One agent of space weathering, the solar wind, enriches the lunar surface with helium. Although we understand how helium is delivered to the Moon, certain aspects of helium concentration processes on the surface remain unknown, such as why impact-generated glass aggregates contain more helium than equally sized soil grains of other types. Here we have analyzed the contents of vesicular iron nanoparticles in lunar impact glasses using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy and show that the nanoparticles contain high concentrations of helium (10-24 atoms/nm3). The widespread occurrence of vesicular iron nanoparticles among lunar samples suggests that they may be an important helium reservoir. These results also suggest that space weathering of iron-rich minerals plays a role in helium sequestration on the Moon and potentially on other airless bodies.

Multiband Spectropolarimetry of Lunar Maria, Pyroclastics, Fresh Craters, and Swirl Materials

Imaging polarimetry is a well-known method for examining the small-scale structure of the surface regolith of airless celestial bodies. In this study, we examine (for the first time) the wavelength-dependent polarization behavior of selected lunar areas, including maria, highlands, fresh craters, pyroclastic deposits, and the Reiner Gamma swirl, based on telescopic multiband UBVRI imaging polarimetry at phase angles within the range of the positive polarization branch. The terrain-dependent spectropolarimetric behavior is studied for the first time in this work. For each study area, we conduct a mapping of the relative regolith grain size, an analysis of the exponent of the Umov law, and the wavelength dependence of the degree of linear polarization. Furthermore, we perform area-specific principal component analyses of the degree of linear polarization, followed by unsupervised machine learning (clustering) to segment different terrain types. We find that fresh mare craters and high-titanium pyroclastic deposits have an increased regolith grain size, whereas crater ray material, low-titanium pyroclastic material, and the Reiner Gamma swirl are more finely grained than the average regolith. The degree of linear polarization decreases with increasing wavelength-dependent albedo according to a power law whose exponent is itself positively correlated with the albedo. For a constant albedo and grain size, the degree of linear polarization increases linearly with wavelength. The clustering step yields a library of terrain-dependent prototype spectra of the degree and angle of linear polarization.

Quantitative grain size estimation on airless bodies from the negative polarization branch

This work explores characteristics of the negative polarization branch (NPB), which occurs in scattered light from rough surfaces, with particular focus on the effects of fine particles. Factors such as albedo, compression, roughness, and the refractive index are considered to determine their influence on the NPB. This study compiles experimental data and lunar observations to derive insights from a wide array of literature. Employing our proposed methodology, we estimate the representative grain sizes on the lunar surface to be D ~ 1–2 µm, with D ≲ 2–4 µm, consistent with observed grain size frequency distributions in laboratory settings for lunar fines. Considering Mars, we propose that the finest particles are likely lacking (D ≫ 10 µm), which matches previous estimations. This study highlights the potential of multiwavelength, particularly near-infrared, polarimetry for precisely gauging small particles on airless celestial bodies. The conclusions provided here extend to cross-validation with grain sizes derived from thermal modeling, asteroid taxonomic classification, and regolith evolution studies.