¹Christopher R. Glein, ² Yu (余馨婷),²Cindy N. Luu
The Astrophysical Journal 985, 187 Open Access Link to Article [DOI 10.3847/1538-4357/adced4]
¹Space Science Division, Space Sector, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
²Department of Physics and Astronomy, University of Texas at San Antonio, 1 UTSA Circle, San Antonio, TX 78249, USA

The nature of sub-Neptunes is one of the hottest topics in exoplanetary science. Temperate sub-Neptunes are of special interest because some could be habitable. Here, we consider whether these planets might instead be rocky worlds with thick, hot atmospheres. Can recent James Webb Space Telescope observations of TOI-270 d be understood in terms of such a model? We perform thermochemical equilibrium calculations to infer conditions of quenching of C–H–O–N species. Our results indicate apparent CO2–CH4 equilibrium between ∼900 and ∼1100 K. The CO abundance should be quenched higher in the atmosphere where the equilibrium CO/CO2 ratio is lower, potentially explaining a lack of CO. N2 is predicted to dominate the nitrogen budget. We confirm that the atmosphere of TOI-270 d is strongly enriched in both C and Ogas relative to protosolar H, whereas N is likely to be less enriched or even depleted. We attempt to reproduce these enrichments by modeling the atmosphere as nebular gas that extracted heavy elements from accreted solids. This type of model can explain the C/H and Ogas/H ratios, but despite supersolar C/N ratios provided by solids, the NH3 abundance will probably be too high unless there is a nitrogen sink in addition to N2. A magma ocean may be implied, and indeed the oxygen fugacity of the deep atmosphere seems sufficiently low to support the sequestration of reduced N in silicate melt. The evaluation presented here demonstrates that exoplanetary geochemistry now approaches a level of sophistication comparable to that achieved within our own solar system.

^1,2Alan E. Rubin
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14367]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Maine Mineral & Gem Museum, Bethel, Maine, USA
Published by arrangement with John Wiley & Sons

Meteorite collection inventories show that many related meteorite groups have very different numerical abundances (e.g., lunar versus Martian meteorites; Eagle Station pallasites versus main-group pallasites; eucrites versus diogenites; ungrouped Antarctic irons versus ungrouped non-Antarctic irons; carbonaceous chondrite-related (CC) iron meteorites versus noncarbonaceous chondrite-related (NC) iron meteorites). The number of members of individual meteorite groups reflects the entire history of these rocks from excavation on their parent bodies to recovery on Earth. These numbers are functions of six main selection factors: (1) volume of the parent-body source region, (2) depth of this source region, (3) time spent in interplanetary space, (4) friability of meteoroids in space and during transit through the Earth’s atmosphere, (5) susceptibility of meteorite finds to terrestrial weathering, and (6) post-fall biases resulting from geography, demography, and preferences by meteorite collectors and analysts. The numerical ratio of lunar/Martian meteorites (~1.8) results from several factors including the Moon’s proximity, the short transit time of lunar meteoroids through interplanetary space, the lower crustal depth from which lunar meteorites were excavated, the lower energy required to launch samples off the Moon than off Mars, and the lower porosity and higher mechanical strength of lunar meteorites. The dunite shortage among asteroidal meteorites may have resulted from the deeply buried olivine-rich meteoroids being ejected hundreds of millions of years ago at the same time as pallasites and irons; however, the dunitic meteoroids (with their lower mechanical strength) would have eroded in interplanetary space on much shorter time scales than their metal-rich fellow travelers.

¹Emily L. Fischer, ¹Stephen W. Parman
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2025.116664]
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, 324 Brook St, Providence, RI 02912, United States of America
Copyright Elsevier

Enstatite chondrites are often used as models for the bulk composition of Mercury because they have similarly low oxygen fugacities. However, e-chondrites are too Si-rich to explain the observed composition of Mercury’s lavas. Here we explore a model in which an initially enstatite chondrite-like Mercurian silicate magma ocean loses Si to the large Fe core during early differentiation. We define a Mercury Fractionation Line (MFL) based on average basaltic geochemical terrane compositions and assume Mercury’s bulk silicate composition must fall along this line. We estimate that 26.5–36.7 ± 7.5 % (1σ) Si must be lost from an initial mantle to bring the e-chondrite compositions up to the MFL. Assuming that the Si is partitioned into the core, this implies a core Si content of 2.8–3.9 ± 0.8 wt% and an oxygen fugacity of IW–4.5 ± 1.0. We also show that a model where Mercury was initially ~2 times larger is consistent with more reducing oxygen fugacities (IW–5.0 ± 1.0) and a higher core Si content (~15 wt%). This estimated initial Mercury size is also consistent with predictions from dynamical simulations. We consider how Si partitioning into the core affects the δ30Si composition of the mantle. Though uncertainties are large, we show that as the initial radius of Mercury increases, δ30Si decreases, trending towards the δ30Si composition of enstatite chondrites. Our calculations do not constrain the mechanism by which Mercury’s mantle may have been lost. However, if they are correct, they imply that the mantle loss must have happened after core formation.

¹Lonnie D. Dausend,²Audrey C. Martin, ¹Joshua P. Emery
The Planetary Science Journal 6, 54 Open Access Link to Article [DOI 10.3847/PSJ/ada778]
¹Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, Arizona, 86011, USA
²Department of Physics, University of Central Florida, Orlando, Florida, 32816, USA

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

¹Cristina A. Thomas et al. (>10)
The Planetary Science Journal 6, 115 Open Access Link to Article [DOI 10.3847/PSJ/adceba]
¹Northern Arizona University, Department of Astronomy and Planetary Science, P.O. Box 6010, Flagstaff, AZ 86011, USA

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

^1,2,3Fengke Cao,^2,3Roberta L. Flemming,^2,4Matthew R. M. Izawa,⁵Steven J. Jaret,⁶Jeffrey R. Johnson
Journal of Geophysical Research (Planets)(in Press) Link to Article [https://doi.org/10.1029/2024JE008574]
¹Research Center for Planetary Science, College of Earth and Planetary Sciences, Chengdu University of Technology, Chengdu, China
²Department of Earth Sciences, Western University, London, ON, Canada
³Institute for Earth and Space Exploration, Western University, London, ON, Canada
⁴Institute for Planetary Materials, Okayama University, Misasa, Japan
⁵Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, USA
⁶Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
Published by arrangement with John Wiley & Sons

Plagioclase feldspar is a ubiquitous mineral found in planetary bodies such as Earth, Moon, Mars, large igneous asteroids such as Vesta, numerous achondrites, and every class of chondritic meteorite. Because all solid planetary bodies are potentially subject to hypervelocity impacts, understanding the shock response of plagioclase enables a better understanding of the geological histories of planetary bodies. This study investigates the response of andesine and bytownite to high-pressure shock waves using micro-XRD and Raman. Fourteen andesine and 11 bytownite samples, which had been previously shocked to peak pressures of 0–56 GPa, were examined. Micro-XRD revealed characteristic signatures of shock damage, including weakened diffraction intensities and heightened background signal, reflecting structural collapse under high pressures. Andesine-bearing rock showed the onset of amorphization at 28.4–29.6 GPa, progressing to complete amorphization at 47.5–50 GPa. Bytownite-bearing rock displayed a similar trend but with higher pressure thresholds: partial amorphization occurred at 25.5–27.0 GPa, and complete amorphization at 38.2–49 GPa. To quantify the degree of shock experienced by plagioclase minerals, we measured the Full Width at Half Maximum (FWHMχ) of Debye rings (from 2D XRD images) for samples across different shock levels. We established linear regression models between ΣFWHMχ and peak shock pressure for both andesine (0–28.4 GPa) and bytownite (0–25.5 GPa) using data from samples that remained crystalline. The model is particularly effective for low shock levels, while Raman is more effective at higher shock pressures. These quantitative relationships provide a valuable tool for assessing the shock history recorded in plagioclase minerals.

¹Audrey C. Martin,²Joshua P. Emery,^2,3Mark Loeffler,¹Kerri L. Donaldson Hanna
Journal of Geophysical Research (Planets)(in Press) Open Access Link to Article [https://doi.org/10.1029/2024JE008331]
¹Department of Physics, University of Central Florida, Orlando, FL, USA
²Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, AZ, USA
³Center for Material Interfaces in Research and Applications, Northern Arizona University, Flagstaff, AZ, USA
Published by arrangement with John Wiley & Sons

Mid-infrared (MIR; 5–35 μm) spectroscopy is often used for mineralogical identification on planetary surfaces. Laboratory spectra aiding remote sensing observations are typically performed in reflection geometries, while MIR spectra of planetary surfaces are typically obtained via emission. Here we explore the validity of Kirchhoff’s Law in converting reflectance to emissivity spectra, focusing on the high-porosity regoliths found on airless bodies such as the Moon and asteroids. Specifically, we compared ambient reflectance, ambient emissivity, and simulated asteroid environment (SAE) spectra of fine-particulate olivine and pyroxene with varying regolith porosities, focusing on how spectral features, including the Christiansen feature (CF), reststrahlen bands (RBs), and transparency features (TF), changed under these different conditions. Our results indicate that Kirchhoff’s Law can be effectively employed to interpret 19 MIR reflectance spectra of high-porosity samples, provided environmental spectral effects (i.e., spectral changes due to different pressure and temperature conditions) are considered.

¹E. Dobrică, ²V. Megevand, ¹A.N. Krot, ³A.J. Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.05.020]
¹Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, HI, USA
²Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Paris, France
³Department of Earth and Planetary Sciences, University of New Mexico, NM, USA
Copyright Elsevier

Studies of shock metamorphic effects in apatite and merrillite in nine ordinary chondrites (OCs) of petrologic types 3.5–6 and shock metamorphic stages S1–S5 using transmission electron microscopy (TEM) reveal a correlation between the extent of brittle deformation in phosphates and the shock metamorphic stage of six host meteorites. No correlation is observed in thermally annealed and partially melted phosphates in Kyushu (L6), Paragould (L5), and Hamlet (LL3.5 − 3.9). Apatites in several shocked equilibrated (petrologic type 6) OCs show micro- and nano-scale heterogeneities in volatile elements, suggesting they were locally mobilized during shock metamorphism rather than during thermal metamorphism. In Alfianello (L6, S5) and Kyushu (L6, S5), maskelynite associated with apatite shows clear evidence for melting. We suggest that maskelynite formed during melting processes rather than solid-state deformation, which has significant implications for geochronology and reflects the time of impact rather than the crystallization age of phosphates. Our study demonstrates the inadequacy of optical microscopy methods currently applied to determine shock metamorphic stages of chondrites; incorporation of micro and nanostructural observations will improve the accuracy of these determinations. We suggest that integration of detailed observations of shock and thermal metamorphism and fluid alteration is required for a comprehensive understanding of the secondary processes that modified most small Solar System bodies.

^1,2Nandita Kumari,^2,3Laura B. Breitenfeld,⁴Katherine Shirley,⁵Timothy D. Glotch
Jopurnal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2024JE008814]
¹Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA
²Planetary Science Institute, Tucson, AZ, USA
³Department of Astronomy, Mount Holyoke College, South Hadley, MA, USA
⁴Department of Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, Oxfordshire, USA
⁵Department of Geosciences, Stony Brook University, Stony Brook, NY, USA
Published by arrangement with John Wiley & Sons

The ultramafic and silicic rocks on the lunar surface have played an important role in expanding our knowledge regarding its thermal and magmatic evolution. The surface identification and quantification of these rocks on the global scale can significantly improve our understanding of their spatial extents, relationships and formation mechanisms. Christiansen feature positions using Diviner data have aided in global identification and mapping of relatively silica-rich and silica-poor lithologies on the lunar surface. We have used laboratory thermal infrared spectra of silicic and ultramafic rocks to analyze the variation in Christiansen feature in simulated lunar environment. We have characterized the absolute bulk silica content of the rocks and minerals and their Silica, Calcium, Ferrous iron, Magnesium index. We find that they are linearly correlated to the Christiansen feature despite particle size variations. Furthermore, we find that the Christiansen feature shifts toward longer wavelengths with increase in ilmenite content in the ilmenite-basalt mixtures. We have explored the effect of instrument’s spectral band position on the accuracy of the parabolic method that is currently used for the estimation of Christiansen feature position from Diviner data. We find that this method performs poorly for the estimation of the Christiansen feature for ultramafic and silicic rocks and minerals/mineral mixtures. We propose using a machine learning algorithm to estimate the Christiansen feature with higher accuracy for all kinds of silicate compositions on the Moon. This method will lead to increased accuracy in absolute quantification of bulk silicate composition of the lunar surface at varying spatial scales.

¹Kaydra Barbre,¹Andrew Elwood Madden,¹Caitlin Hodges,¹Megan Elwood Madden
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2024JE008650]
¹School of Geosciences, University of Oklahoma, Norman, OK, USA
Published by arrangement with John Wiley & Sons

Ferrihydrite has been observed within the Martian regolith; therefore, ferrihydrite transformation pathways are likely critical to iron cycling and mineral transformation on Mars and other extraterrestrial systems. Data from Mars rovers and orbiters indicate that ferrihydrite is associated with significant salt deposits. Previous studies show these salts likely formed as the planet desiccated and may rehydrate to form modern brines today that strongly influence(d) mineral alteration. We hypothesize that the salts observed on Mars’ surface may help preserve ferrihydrite for longer periods than typically observed on Earth. This study investigates the effects of brine chemistry on ferrihydrite alteration through laboratory experiments. Lab-synthesized ferrihydrite was reacted with near-saturated brines and ultra-pure water at 20°C for 30 days in a series of batch reactor experiments. X-ray diffraction and Raman spectroscopy showed that ferrihydrite was preserved without evidence of dissolution/transformation in near-saturated solutions of MgSO4, Na2SO4, and NaClO4, while additional iron-oxyhydroxide phases formed in other brines. We also compared mineral reaction products formed from freeze-dried ferrihydrite and undried ferrihydrite slurry. The freeze-dried ferrihydrite was more likely to be preserved, whereas ferrihydrite in a slurry resulted in the precipitation of goethite and lepidocrocite, indicating that particle aggregation and/or drying history affect ferrihydrite stability and alteration. Overall, ferrihydrite remained largely unaltered in the presence of concentrated sulfate and perchlorate brines. In the context of soils/regolith observed on Mars, our research demonstrates that ferrihydrite is more likely to be preserved when found in areas where these salts are dominant, and desiccated in a cold/arid environment prior to brine exposure.