Abstract Nakhlites, clinopyroxene‐rich rocks, are the largest single‐origin suite of samples from Mars. Despite extensive study to discern their petrogenetic histories, nakhlite emplacement mechanisms and environments are not well‐constrained, and it is unknown whether they represent intrusive or extrusive igneous rocks, or a combination. Here, we use X‐ray computed microtomography (XCT) and three‐dimensional (3D) quantitative textural analyses (e.g., 2D–3D modal abundances, crystal size distributions [CSDs], and petrofabrics) to place additional constraints on nakhlite formation and emplacement. Modal abundances between and within the nakhlites are variable on both a 2D and 3D basis, highlighting the significance of XCT and 3D analyses when studying these samples. All nakhlites in our study have similar crystallization conditions and histories based on 3D CSDs. Cumulus phases (=olivine and pyroxene) crystallized from magma(s) with high nucleation densities, likely related to effective undercooling, and subsequently underwent a period of magma storage. The CSD profiles record evidence for magma recharge events. Pyroxene long‐axis orientations in the nakhlites studied here exhibit a magmatic foliation, which likely developed during crystal settling and accumulation in low‐to‐no flow settings, such as magma chambers, shallow intrusions (e.g., sills and dikes), lava lake or pond infills, or thick lava flows. We also show that the pyroxenitic layer of Theo's Flow (Canada) may not be an appropriate terrestrial analog for the nakhlites due to differences in emplacement mechanisms and conditions. Our findings suggest that lava flows may be less prevalent in the martian meteorite collection, while intrusive bodies and rocks may be more common than initially thought.

Abstract Cooling fractures found within impact melt deposits have been manually mapped within several craters on the Moon and Mercury, as their distribution can indicate which heat‐loss processes were most significant in the periods after impact. However, due to the discovery of melt deposits in Lunar impact craters with sub‐km diameters, it is unlikely that the complete mapping of these impact melt fractures (IMFs) on the Moon will be achievable without automation. As such, we have trained a DeepLabV3 semantic segmentation deep convolutional neural network, called IMFMapper, to detect IMFs within Lunar Reconnaissance Orbiter Narrow Angle Camera (LROC NAC) satellite imagery. As a means of maximizing the size of the training data set, “weak” pixel‐level labels were generated by buffering line annotations. In testing upon the IMFs found within Ohm crater, IMFMapper achieved an average ‐score of 69.3%. IMFMapper has also been deployed to map IMFs within the previously surveyed Crookes crater, where we have found new candidate melt deposits within the crater's western and southern walls. In addition, IMFMapper has produced the first map of IMFs within Schomberger A crater, in which IMFs may act as permanently shadowed regions due to the crater's proximity to the Lunar South Pole. The successful mapping of IMFs in Schomberger A also signifies IMFMapper's robustness to extreme solar incidence angles. We also demonstrate that IMFMapper could be implemented for automated mapping of IMFs on Mercury upon the commencement of BepiColombo's science operations.

Abstract Earth wind, namely the particles from the Earth's magnetotail, is an important source of lunar water. Besides tailward flow incident on the lunar nearside when the Moon is in the magnetotail, there exists earthward flow bombarding the farside, affecting the distribution and preservation of lunar water. In this study, we determine the incident fluxes of both the tailward and earthward flows on the lunar surface with the ARTEMIS observations and examine their influences on the concentration and migration of lunar surface water and its reservation in the polar regions using Monte Carlo modeling. It is found that water molecules produced by the earthward flow can stay longer on the surface than those produced by the tailward flow. Our results suggest that the evolution of the Earth's magnetosphere can be inferred from Earth wind particles implanted in the soils of both the lunar nearside and farside.

Abstract Seismic determination of the Moon's crustal thickness is crucial for understanding its formation, composition, and evolution. Additionally, crustal thicknesses below seismic stations serve as anchoring points for 3D crustal thickness maps derived from gravity and topography field variations. However, strong seismic scattering inside the Moon can prevent seismic waves reflected on, or converted at, crust and upper mantle discontinuities from reaching the surface without interacting with scatterers. This study estimates Auto‐Correlation Functions (ACF) of all channels of Apollo seismometers during the coda of high‐signal‐to‐noise seismic events. As expected for a strongly scattering medium, Apollo stations S12 and S15 do not detect late arrivals in the ACF indicative of waves reflected on internal discontinuities. However, station S16 and, to a lesser extent, station S14 show stable late arrivals in the ACF. Arrivals at 14.2 and 27.1 s and at 23.9 and 50.6 s in the ACF of vertical and horizontal mid‐period channels of station S16 are interpreted as and waves reflected at mid‐crustal and crust‐mantle interfaces. A seismic crust and upper mantle model beneath station S16 fits the arrival times of and waves with realistic seismic velocities and ratios ranging from 1.65 to 1.88. The crustal thickness is non‐uniquely estimated at 53 km below station S16. Our results are also compatible with significant variations in the properties of the interfaces, or in strength of seismic scattering between Apollo stations.

Abstract A major mission goal for the Mars Science Laboratory’s rover, Curiosity , is to investigate the transition from clay‐bearing to hydrated‐Mg‐sulfate‐bearing sedimentary strata hypothesized to record a transition from a wet to a dry paleoclimate. Alpha Particle X‐ray Spectrometer (APXS) results from this region, named the Clay‐Sulfate Transition (CST), indicate an overall ∼5% increase in Ca‐sulfate, but Mg‐sulfate enrichment is limited to diagenetic nodules. Sulfates in the CST change sharply at the contact with the overlying Mg‐sulfate unit, which has ∼5% Ca‐sulfate and ∼10% Mg‐sulfate in the bedrock matrix. Despite this change in sulfate assemblage, and the transition from fluvial‐lacustrine to drier aeolian sedimentary deposits, the bulk chemical composition of the aeolian sandstone (sulfate‐free basis) effectively has the same altered basalt chemical fingerprint as the underlying fluvial‐lacustrine mudstone. That is, the composition of rocks that record the transition from a wet to a dry paleoclimate is isochemical. It is remarkable that the aeolian sandstone has the same altered bulk chemical characteristics as the fluvial‐lacustrine mudstone, notwithstanding very different inferred geochemical regimes. We propose a simplified model wherein older basaltic sediment was aqueously altered in a fluvial‐lacustrine regime and reworked, likely during cycles of alteration, salt formation, and reworking. This process led to an averaging of the bulk chemical composition of the Mt. Sharp group sediment, resulting in the isochemical characteristics of the paleoenvironment change.

Abstract Earth's particular style of plate‐tectonics—characterized by localized deformation along dynamic plate boundaries and long‐lived stable plate interiors—appears to be unique among rocky objects in the solar system. However, it is entirely unknown how common plate tectonics and related lithospheric phenomena are among the vast population of exoplanets discovered astronomically or assumed to exist throughout the Universe. In this study, we explore the effect of planetary composition on mylonitization—a set of microphysical processes that is commonly associated with shear localization and plate boundary deformation on Earth. A model for planet compositions, based on stellar spectroscopy, is used to define a plausible range of theoretical mineral abundances in the mantles of rocky Earth‐sized exoplanets. These mineral abundances, along with experimental rock rheology, are used to model microphysical evolution with two‐phase mixing. The model is then used to determine the effect of composition on the time‐scales for shear zone formation. We demonstrate that lithospheres composed of sub‐equal proportions of two mineral phases will form shear zones over relatively short time‐scales, a more favorable condition for forming Earth‐like plate boundaries. In contrast, lithospheres that are nearly monomineralic may require unrealistically long time‐scales to form plate boundary shear zones. Using this approach, we identify specific nearby stars with the optimal range of compositions to be targeted by future astronomical missions, including the Habitable Worlds Observatory.

Abstract The Mount Sharp group in Gale crater is typically interpreted to record a progressive change (with increasing elevation) from wetter fluvio‐lacustrine environments to dry, eolian environments. This shift has been linked to orbital evidence for a global change in environmental and depositional conditions leading to an overall drying out of Mars. The Mars Science Laboratory Curiosity rover has documented evidence at Gale for the periodic continuation of aqueous depositional processes within the Mount Sharp group within the Clay Sulfate Transition, suggesting that, in Gale crater, it was not a simple, unidirectional drying out pattern. We present data on a series of interdune lenses at Gale crater, providing a window into locally changing environmental conditions. They represent wet (fluvial to lacustrine) deposits in the otherwise dry eolian Contigo dune field. Alpha Particle X‐Ray Spectrometer geochemical data reveal compositional similarities between the interdune lenses and both the host Contigo member and unaltered basaltic sediments. Modeling indicates that the lenses may contain up to 52% basaltic sands/soils mixed with local sediment. A high degree of homogeneity in composition and sedimentary structures are identified across the lenses. This suggests a common source of influx material and a repeated process (wet episodes in a dry environment over several cycles of seasonal variations). Furthermore, differences in composition between the interdune lenses and the host Contigo member help to elucidate the timing of alteration within the Mount Sharp group.

Abstract The observation of low viscosity lava flows and shield volcanoes on radar images, combined with X‐ray fluorescence analyses performed by Soviet landers, strongly suggests that Venus's crust is primarily basaltic. However, near‐infrared emissivity data from the Galileo and Venus Express missions indicate that crustal plateaus may be compositionally distinct and contain higher proportions of felsic minerals than the surrounding plains. The hypothesis that these highlands are felsic has led to suggestions of past oceans on Venus, as the generation of such rocks on Earth typically requires significant amounts of water. We use thermodynamic modeling to explore the conditions under which silica‐rich melts can form without water, through deep melting of basaltic crust in thickened crustal roots or during crustal recycling in the mantle. Felsic melts containing 60–65 wt.% can form at depths 60 km. In regions of thickened crust, such melts would be produced along a thermal gradient of 10 K/km, whereas for higher gradients (15 K/km), melting occurs at depths where quartz is not yet present (50 km), resulting in silica‐poor melts. The presence of small amounts of water allows similar felsic melts (in composition and volume) to form at moderately shallower depths (40 km). Our results indicate that large volumes of felsic melt could form even under anhydrous conditions on Venus, via crustal thickening or recycling mechanisms such as subduction or lithospheric dripping. This supports the idea that part of Venus's highlands could be the product of remelted basaltic crust, regardless of whether surface water existed in the past.

<em>Journal of Geophysical Research: Planets</em> is an AGU planetary science journal publishing original research articles spanning planetary geology, geophysics, geochemistry, atmospheres, and exoplanets.

Abstract We observed Io with the James Webb Space Telescope (JWST) NIRSpec/Integral Field Unit (1.0–5.3 μm, ) in August 2023 while the satellite was in eclipse. Thermal emission from Kanehekili Fluctus is consistent with the cooling of lava flows after a vigorous eruption in November 2022. At Loki Patera, after a new brightening event was detected in November 2022, the lava lake was in a quiescent state, as expected from previous analyses. We mapped the SO emission band at 1.707 μm, and detected, for the first time, [SI] emission lines at 1.082 and 1.131 μm. The SO emissions are concentrated above Kanehekili Fluctus, and in two regions in the northern hemisphere. The disk‐averaged brightness is 14.5 kR. The emissions are sourced from SO molecules ejected from 1,500 to 1,700 K vents in an excited state, with a typical SO column density above the northern hemisphere of . Sulfur emissions are distributed homogeneously across a band in the northern hemisphere. The disk‐averaged total brightness is 5.6 kR, versus 9.65 kR in the north. The emissions are produced through direct electron impact excitation by 4 eV electrons in the torus (density 2,500 ) penetrating the atmosphere, and require the atmosphere to be hot (1,700 K) to populate the upper levels before excitation. The sulfur column density over the northern hemisphere is . These same parameters can explain recent 0.7725‐m observations, as well as the 147.9‐nm multiplet emissions observed with HST‐STIS 20 years earlier. This suggests a quite stable system over decades‐long timescales.