Journal of Geophysical Research: Planets最新文献

Analysis of space weathering features in lunar regolith using transmission electron microscopy (TEM) allows researchers to characterize the surface exposure timescale of individual grains from the Moon. TEM analysis of regolith grains from Apollo 17 scoop samples, collected from five different locations in the Taurus-Littrow Valley (TLV), each with unique surface exposure histories, was conducted. Results show that the extent of space weathering in both mature and immature soil is significant and demonstrate that non-agglutinitic grains in the samples have almost all experienced exposure to interplanetary space on the lunar surface. Solar energetic particle (SEP) track density derived exposure age distributions, when compared to observations from lunar core samples and regolith development models, suggest evidence of varying degrees of regolith mixing and overturn on the surface of the Moon. Exposure age distributions may be indicative of mixing of mature and immature regolith. Median exposure ages for mature soils saturate between 2.0 and 3.0 Ma while mean values will continue to increase with continued exposure on the lunar surface. The saturation of median exposure ages for individual grains may be attributed to high rates of reworking on the top millimeter of regolith. Recent analysis of LROC reflectance images found that the TLV light mantle could form from reoccurring landslide events. Maximum SEP exposure ages for particles in light mantle soils appear to indicate the relative landslide depositional order. Together, these observations can be used to interpret the mixing and depositional histories of geologic units in the TLV.

Siderophile elements tend to transfer from the magma ocean to the metallic core during core formation in Vesta, but kinetic barriers may prevent equilibrium partitioning. To evaluate the kinetic effect requires knowledge of siderophile element diffusivity in silicate melts, which has yet to be experimentally determined in vestan melts. We have performed diffusion experiments for Ga, Mo and W at 0.5–1 GPa and 1,573–1,873 K in synthesized eucritic and diogenitic melts. The diffusivities of Ga, Mo and W in two vestan melts with distinct melt depolymerization are found to be similar and are 2–3 orders of magnitude higher than those in rhyolitic melts. The modeling results reveal that Ga, Mo and W are enriched into the core to the extent that equilibrium partitioning allows, whereas some highly siderophile elements (HSEs) may maintain a disequilibrium state. However, this disequilibrium does not dramatically affect HSE abundances. The significant variability in HSE abundances in Vesta is mostly due to the brecciation effects and contamination from impactors or metals.

Periodic high-resolution imagery of Io is essential for understanding its surface evolution, from volcanic eruptions to tectonic deformation to large-scale mass wasting. Juno flybys in 2023 and 2024 obtained imagery of the surface with the JunoCam imager at 1.8 km/pixel spatial resolution, comparable to global observations from the Galileo spacecraft (1996–2001). Areas of Io's north pole were imaged for the first time, with low emission (46°–61°), high incidence angle conditions revealing several-kilometers-tall mountains. We are unable to identify detailed changes in Io's mountain features due to the limited overlap in the high-resolution coverage between the Galileo and Juno data sets. However, the improved lighting conditions in the JunoCam imagery allow us to refine our understanding of previously mapped features, including an extension of the rifting relationships previously proposed at Shamshu Patera. Cocytus Montes, a newly identified trio of mountains centered at 60°N 330°W, exhibit intermediate erosional stages with morphologies grading from sharp ridges to eroded hummocks along their slopes, providing a new perspective on the pace and processes of erosion on Io. We present a regional geologic map of this region, and examine the relationships between mountain units and the underlying layered plains which connect them on a raised plateau. Unique crustal blocks strewn across the plateau have sparse analogs elsewhere on Io, and prompt questions about the erosional mechanisms that may have emplaced them. We propose several formation mechanisms and conclude that while we cannot definitively determine which is responsible, regolith creep-modified cliff collapse is the most likely.

The stable isotope composition of carbonates records the environmental formation conditions and can indicate potential biosignatures if formed biologically. Martian meteorite carbonates display unusually high δ¹³C values, not explained using known terrestrial processes. Carbonates have been detected across the martian surface, including observations by the Perseverance Rover in Jezero Crater. However, the stable isotope effects of surface irradiation in carbonate minerals remain poorly constrained. We investigated the influence of ultraviolet (UV) and non-UV radiation on δ¹³C and δ¹⁸O values of carbonates under both Low Earth Orbit (LEO) and laboratory conditions. Three natural carbonates, two calcites (Iceland spar, Bolivian stromatolite) and one aragonite (French microbialite), were exposed to LEO UV and non-UV radiation for 29 ± 4 hr. Laboratory experiments employed reagent-grade calcite and natural aragonite exposed to UV under N₂ atmospheres at 6 mbar (4.5 torr) or 1.73 mbar (1.3 torr) for 24–70 hr in the McMaster Planetary Simulator (MPS). Most carbonates showed no significant radiation-induced changes in their δ¹³C and δ¹⁸O values. However, three cases exhibited minor stable isotope shifts, including carbon and oxygen isotope effects in LEO and MPS carbonates, an oxygen isotope effect in the LEO-UV-exposed French microbialite, and carbon isotope effects in two UV-exposed MPS calcites. These results suggest that short-term radiation cannot explain the ¹³C enrichment in martian meteorites but may initiate subtle stable isotope effects. This study establishes a baseline for radiation-induced stable isotope effects in carbonates, informing interpretations of martian carbonate isotopes, biosignature preservation potential, and analyses planned for Mars Sample Return.

Thermal infrared imaging by Longwave Infrared Camera (LIR) aboard JAXA's Venus orbiter Akatsuki has revealed horizontal structures of large-scale topographic gravity waves (mountain waves) and thermal tides in the Venusian atmosphere. For quantitative analysis of these waves, we developed a radiative transfer model for an atmosphere perturbed by a gravity wave, which represents the mountain waves and the thermal tides in the equatorial region. Combining the infrared images with temperature profiles from the Akatsuki radio occultation, the cloud particle scale height, as well as the vertical wavelengths and growth rates of mountain waves and the semidiurnal thermal tide were determined. The cloud particle scale height around the cloud top was estimated to be approximately equal to the atmospheric scale height, indicating a vertically well-mixed layer of the cloud particles near the cloud top in low latitudes. It was demonstrated that the brightness temperature amplitude observed by LIR is approximately half of the atmospheric temperature amplitude at the cloud top. The waves' impact on the mean flow above the cloud top was assessed by estimating their momentum deposition. The results show that mountain waves can induce an intense but localized deceleration of tens of m s⁻¹/day or larger above 80 km, while the semidiurnal tide provides a global and persistent deceleration that increases with height to reach the order of 1 m s⁻¹/day at 80 km. These results reveal two important but distinct mechanisms that contribute to the momentum budget of the Venusian upper atmosphere.

The solidification of a lunar magma ocean (LMO) after a giant impact event formed the Moon's gravitationally unstable juvenile mantle. Hybridization of the lunar mantle during the overturn of late-crystallized Fe- and Ti-rich ilmenite-bearing cumulates (IBC) in the lunar interior is required to explain the variable TiO₂ and Rare Earth Element (REE) abundances of lunar basalts and picritic melts. We model chemical fractionation during LMO solidification, mantle hybridization, and partial melting of hybridized and unhybridized cumulates to evaluate the formation of lunar picritic glasses. Solidification models indicate that >65% of the plagioclase formed during LMO solidification must be removed by flotation to explain negative Eu anomalies exhibited by lunar picritic glasses. Melting models demonstrate that unhybridized cumulates could produce low-Ti picritic glasses but not Ti enriched glasses. Post-hybridization melting models demonstrate that hybridized garnet-free sources can generate elemental ratios of most Ti enriched picritic glasses. The heavy rare earth element depleted compositions of some orange picritic glasses require a ∼0.25–5% garnet component in the downwelling IBC or hybridized sources. If the LMO extends to the core-mantle boundary, cumulate overturn is likely required for garnet to form in the lunar mantle. Isotopic models indicate that the $��E�$_Nd and $��E�$_Hf trends of lunar basalt sources reflect primary lunar differentiation processes coupled with post differentiation hybridization. The combined elemental and isotopic characteristics of Ti enriched picritic glasses and basalts are strong evidence of cumulate overturn during early lunar differentiation.

Over geologic time-scales, large volumes of exogenic sulfur ions from Io's plasma torus have been supplied to the surface of Europa and Ganymede, which, combined with recent interpretations of orbiter images, dynamical modeling, and surface-subsurface exchange, suggests further sulfur transport into the interior of the icy worlds. These observations motivate mixed-phase spectral modeling for interpreting orbiter spectroscopy data and determination of hydration states of candidate surface materials including hydrous sulfates. In this work, we present a combined experimental and theoretical study of the low temperature and high pressure vibrational spectral signature of the iron-sulfate monohydrate endmember, szomolnokite (FeSO₄·H₂O). By employing synchrotron Fourier-transform infrared spectroscopy (FTIR) in the diamond anvil cell up to 23 GPa and down to 20 K, we explore the extreme range of pressure-temperature domains relevant to icy environments throughout our solar system and beyond. Combined with our density-functional theory quantum-mechanics molecular dynamics results, we demonstrate that experimentally observed infrared features in the O-H stretching region commonly associated with nH₂O (n > 1) hydration states can be attributed to a pure monohydrate without the need for pressure-induced exsolved ice, other coexisting hydrous iron sulfates, or strong overtone and combination modes. We further discuss the possibility of lateral variations in density and shear properties on icy worlds associated with temperature variations and the high-pressure phases of kieserite group monohydrated sulfates.

Lunar volcanic glass beads, as the closest approximations for primary lunar magma, have experienced a significant degree of degassing during eruption. To retrieve their initial fluorine contents requires fluorine diffusivity data, which have yet to be experimentally determined for lunar melts. We have performed diffusion experiments at 0.5 GPa and 1,673–1,883 K in a piston cylinder apparatus for four synthesized lunar melts with compositions corresponding to Apollo green glass (GG), yellow glass (YG), orange glass (OG) and red glass (RG). The fluorine diffusion profiles yield fluorine diffusivities increasing mildly from GG to YG to OG to RG melt. The fluorine diffusivities of lunar melts are greater than F diffusivities in terrestrial melts by at least a factor of 2–10. We develop a general model for F diffusivity in terrestrial and lunar melts as a function of X_Si+Al (mole fraction of Si and Al combined among all cations), which can reproduce experimental data within a factor of 3. Using the new fluorine diffusivity data, modeling of the fluorine profile in Apollo GG bead recovers a pre-eruptive fluorine content of 11.4–15.4 μg/g. Assuming minimal volatile loss before magma fragmentation, the fluorine content is 0.6–1.8 μg/g in its mantle source. The fluorine content in the bulk silicate Moon is inferred to be 4.4–7.4 μg/g, which is consistent with previous estimates. This consistency confirms diffusion degassing as the major mechanism of fluorine depletion for glass beads. Our calculation indicates a depletion degree of 70%–82% for fluorine compared to bulk silicate Earth.

While aspects such as the temperature and composition of the Martian atmosphere are relatively well known thanks to observations from numerous space missions, Martian atmospheric dynamics remain poorly constrained due to the scarcity of direct wind measurements. The use of microwave limb sounders to retrieve winds has been proposed in the past, but no such instrument has yet flown to Mars. The precision of the wind measurements by such instruments has been studied before for a handful of cases, but a thorough study taking into account current knowledge on the variability of the Martian atmosphere has not yet been conducted. In this study, we propose a cross-correlation method to retrieve winds from the Doppler shift of various emission lines as measured by a generic microwave limb sounder, and characterize the variability of the accuracy of the retrieved winds during a whole Mars year assuming a given instrumental configuration. With this instrument setup, and using the combination of CO isotopes with differing line intensities, wind measurements are achieved with an accuracy better than 10 m/s at tangent altitudes between 30 and 140 km during most times of the year, local times, and locations in the planet.

Boninites are high-magnesium volcanic rocks proposed as terrestrial analogs for Mercury's surface, based on elemental data from NASA's MErcury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) mission. In this study, we investigated boninite samples from the Troodos Massif (Cyprus) using a multi-methodological approach to characterize their mineralogical, chemical, and spectroscopic properties, including reflectance and emissivity spectra. Geochemical analyses confirm that the bulk composition of the samples closely matches Mercury's geochemical terrains in terms of SiO₂, MgO, and Al₂O₃ content, though FeO concentrations are higher (∼8 wt% vs. 1–2 wt%). Samples from different localities show some mineralogical differences but generally contain less orthopyroxene and albitic plagioclase than expected on Mercury, along with hydrated minerals from aqueous alteration, which are not expected on the planet's surface. Reflectance spectra in the ultraviolet (UV), visible (VIS), and near-infrared (NIR) range show major absorption features around 1 μm, associated with mafic minerals, and minor bands at ∼1.4 μm, ∼1.9 μm, and 2.2–2.3 μm, linked to hydrated phases, with spectral variations reflecting mineralogical differences. In the mid-infrared (MIR) range and emissivity spectra, we observe Christiansen Features (CF) and Reststrahlen Bands (RB) at different positions, mainly influenced by plagioclase content, and shifts in emissivity minima with increasing temperature. Spectral differences between the boninites and Mercury mainly result from the intrinsic mineralogy of the samples. Nonetheless, Troodos boninites represent one of the best Mercury analogs currently available on Earth, and understanding their spectral behavior in relation to their mineralogy could support future investigations with the ESA/JAXA BepiColombo mission.