¹Ivy Ettenborough, ¹Anna Szynkiewicz
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116384]
¹Department of Earth and Planetary Sciences, University of Tennessee, 1621 Cumberland Ave., Knoxville, TN 37996, USA
Copyright Elsevier

Secondary sulfate minerals are common throughout the sedimentary deposits of Mount Sharp, located within Gale crater on Mars. However, the source of sulfate (SO42−) and past climatic conditions during their formation are not well understood. Therefore, we investigated the δ34S, δ18O, and δ2H of gypsum veins and other Mg- and Ca- sulfates forming as salt crusts and cement within the shallow sediments of the Rio Puerco watershed in central New Mexico. The δ34S values of vein gypsum and acid-soluble SO42− (cement) varied over the same range (₋33.3 to ₋12.9 ‰ and ₋34.6 to ₋12.1 ‰, respectively), which was similar to the δ34S of bedrock sulfide minerals (₋37.4 to ₋5.9 ‰). This implies that sulfide oxidation is the main source of SO42− in the Rio Puerco aqueous system. The measured δ18O values of SO42− (₋8.9 to +3.1 ‰) as well as δ18O and δ2H values of gypsum hydration water (₋8.9 to +0.6 ‰, and ₋112 to ₋82 ‰, respectively) overlapped with the isotope composition of local meteoric precipitation, suggesting that sulfide oxidation to SO42− and gypsum formation have occurred under semi-arid climate conditions. The isotope results suggest the top-down infiltration of meteoric water leads to leaching of SO42−, Mg+, and Ca2+ from bedrock sulfide weathering followed by abundant formation of Mg- and Ca-sulfates in surface deposits and gypsum veins with depth. Because of spatial and mineralogical similarities in the secondary Mg- and Ca-sulfate mineral occurrences, we hypothesize that chemical weathering of sulfide minerals could have been the main source of SO42− in the aqueous system of Gale crater.

^1,2A. Morbidelli, ³T. Kleine, ⁴F. Nimmo
Earth and Planetary Science Letters 650, 119120 Open Access Link to Article [https://doi.org/10.1016/j.epsl.2024.119120]
¹Collège de France, CNRS, PSL Univ., Sorbonne Univ., Paris, 75014, France
²Observatoire de la Côte d’Azur, Université Cote d’Azur, CNRS, Laboratoire Lagrange, Boulevard de l’Observatoire, 06304 Cedex 4 Nice, France
³Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁴Dept. Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz CA 95060, USA
Copyright Elsevier

The dominant accretion process leading to the formation of the terrestrial planets of the Solar System is a subject of intense scientific debate. Two radically different scenarios have been proposed. The classic scenario starts from a disk of planetesimals which, by mutual collisions, produce a set of Moon to Mars-mass planetary embryos. After the removal of gas from the disk, the embryos experience mutual giant impacts which, together with the accretion of additional planetesimals, lead to the formation of the terrestrial planets on a timescale of tens of millions of years. In the alternative, pebble accretion scenario, the terrestrial planets grow by accreting sunward-drifting mm-cm sized particles from the outer disk. The planets all form within the lifetime of the disk, with the sole exception of Earth, which undergoes a single post-disk giant impact with Theia (a fifth protoplanet formed by pebble accretion itself) to form the Moon. To distinguish between these two scenarios, we revisit all available constraints: compositional (in terms of nucleosynthetic isotope anomalies and chemical composition), dynamical and chronological. We find that the pebble accretion scenario is unable to match these constraints in a self-consistent manner, unlike the classic scenario.

¹Susmita Garai, ¹Peter Olson, ¹Zachary Sharp
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.11.021]
¹Earth and Planetary Sciences, University of New Mexico, United States
Copyright Elsevier

Pebble accretion provides new insights into Earth’s building blocks and early protoplanetary disk conditions. Here, we show that mixtures of chondritic components: metal grains, chondrules, calcium-aluminum-rich inclusions (CAIs), and amoeboid olivine aggregates (AOAs) match Earth’s major element composition (Fe, Ni, Si, Mg, Ca, Al, O) within uncertainties, whereas no combination of chondrites and iron meteorites does. Our best fits also match the ε54Cr and ε50Ti values of Earth precisely, whereas the best fits for chondrites, or components with a high proportion of E chondrules, fails to match Earth. In contrast to some previous studies, our best-fitting component mixture is predominantly carbonaceous, rather than enstatite chondrules. It also includes 16 wt% of early-formed refractory inclusions (CAIs + AOAs), which is similar to that found in some C chondrites (CO, CV, CK), but notably higher than NC chondrites. High abundances of refractory materials is lacking in NC chondrites, because they formed after the majority of refractory grains were either drawn into the Sun or incorporated into terrestrial protoplanets via pebble accretion. We show that combinations of Stokes numbers of chondritic components build 0.35–0.7 Earth masses in 2 My in the Hill regime accretion, for a typical pebble column density of 1.2 kg/m2 at 1 au. However, a larger or smaller column density leads to super-Earth or moon-mass bodies, respectively. Our calculations also demonstrate that a few My of pebble accretion with these components yields a total protoplanet mass inside 1 au exceeding the combined masses of Earth, Moon, Venus, and Mercury. Accordingly, we conclude that pebble accretion is a viable mechanism to build Earth and its major element composition from primitive chondritic components within the solar nebula lifetime.

¹Maria Schönbächler et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14279]
¹Department of Earth Sciences, Institute for Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland
Published by arrangement with John Wiley & Sons

Nucleosynthetic isotope variations are powerful tracers to determine genetic relationships between meteorites and planetary bodies. They can help to link material collected by space missions to known meteorite groups. The Hayabusa 2 mission returned samples from the Cb-type asteroid (162173) Ryugu. The mineralogical, chemical, and isotopic characteristics of these samples show strong similarities to carbonaceous chondrites and in particular CI chondrites. The nucleosynthetic isotope compositions of Ryugu overlap with CI chondrites for several elements (e.g., Cr, Ti, Fe, and Zn). In contrast to these isotopes, which are of predominately supernovae origin, s-process variations in Mo isotope data are similar to those of carbonaceous chondrites, but even more s-process depleted. To further constrain the origin of this depletion and test whether this signature is also present for other s-process elements, we report Zr isotope compositions for three bulk Ryugu samples (A0106, A0106-A0107, C0108) collected from the Hayabusa 2 mission. The data are complemented with that of terrestrial rock reference materials, eucrites, and carbonaceous chondrites. The Ryugu samples are characterized by distinct 96Zr enrichment relative to Earth, indicative of a s-process depletion. Such depletion is also observed for carbonaceous chondrites and eucrites, in line with previous Zr isotope work, but it is more extreme in Ryugu, as observed for Mo isotopes. Since s-process Zr and Mo are coupled in mainstream SiC grains, these distinct s-process variations might be due to SiC grain depletion in the analyzed materials, potentially caused by incomplete sample digestion, because the Ryugu samples were dissolved on a hotplate only to avoid high blank levels for other elements (e.g., Cr). However, local depletion of SiC grains cannot be excluded. An alternative, equally possible scenario is that aqueous alteration redistributed anomalous, s-process-depleted, Zr on a local scale, for example, into Ca-phosphates or phyllosilicates.

¹David T. King Jr,^2,3Michael R. Rampino,¹Lucille W. Petruny
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14290]
¹Department of Geosciences, Auburn University, Auburn, Alabama, USA
²Department of Biology, New York University, New York, New York, USA
³Department of Environmental Studies, New York University, New York, New York, USA
Published by arrangement with John Wiley & Sons

The central uplift area of Araguainha impact structure (Brazil) includes a quartzite pebble- and cobble-bearing stratigraphic facies that have been profoundly affected by impact processes. These quartzite clasts have been studied previously for their planar deformation features (PDFs), but not with regard to their noteworthy transverse fractures. Petrographic study of transverse fractures within seven selected impact-affected cobbles from the conglomeratic fluvial facies of the target Devonian Furnas Formation (near the central uplift of Araguainha impact structure) has revealed that there is a micro-breccia within these transverse fractures, and this micro-breccia originated by comminution of the host cobble. Further, the transverse fractures in these cobbles have different styles (well-defined, poorly defined, complex, and diffuse) and are evidently post-shock, brittle deformation features. We suggest that a late compression-stage process, perhaps collapse of the central peak, may be responsible for the development of these transverse fractures.

¹Kevin Righter
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14289]
¹Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York, USA
Published by arrangement with John Wiley & Sons

ALH 84001 is an orthopyroxenite that is the oldest known Martian meteorite. Given this rock type and age, and the possible source locations, ALH 84001 represents an opportunity to learn more about basic geologic relations in the Martian highlands in the southern hemisphere. Its orthopyroxene-rich mineralogy is unique and also includes C-, S-, P-bearing minerals. ALH 84001 can provide constraints on chronology, geology and surface features, crust formation, paleomagnetism, weathering, climate, magmatism, and interior structure. When it was recognized to be of Martian origin (~1994), there were ~12 known Martian meteorite samples. That number is now >150, with only one other meteorite (NWA 7034) having clasts that are similar in age to ALH 84001. Thus, it remains a unique sample and continues to provide opportunities to understand this early period of Martian history.

^1,2A.P. Broz et al. (>10)
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2024JE008520]
¹Purdue University, West Lafayette, IN, USA
²University of Oregon, Eugene, OR, USA
Published by arrangement with Jiohn Wiley & Sons

The Mars 2020 Perseverance rover discovered fine-grained clastic sedimentary rocks in the “Hogwallow Flats” member of the “Shenandoah” formation at Jezero crater, Mars. The Hogwallow Flats member shows evidence of multiple phases of diagenesis including Fe/Mg-sulfate-rich (20–30 wt. %) outcrop transitioning downward into red-purple-gray mottled outcrop, Fe/Mg clay minerals and oxides, putative concretions, occasional Ca sulfate-filled fractures, and variable redox state over small (cm) spatial scales. This work uses Mastcam-Z and SuperCam instrument data to characterize and interpret the sedimentary facies, mineralogy and diagenetic features of the Hogwallow Flats member. The lateral continuity of bedrock similar in tone and morphology to Hogwallow Flats that occurs over several km within the western Jezero sedimentary fan suggests widespread deposition in a lacustrine or alluvial floodplain setting. Following deposition, sediments interacted with multiple fluids of variable redox state and salinity under habitable conditions. Three drilled sample cores were collected from this interval of the Shenandoah formation as part of the Mars Sample Return campaign. These samples have very high potential to preserve organic compounds and biosignatures. Drill cores may partially include dark-toned mottled outcrop that lies directly below light-toned, sulfate-cemented outcrop. This facies may represent some of the least oxidized material observed at this interval of the Shenandoah formation. This work reconstructs the diagenetic history of the Hogwallow Flats member and discusses implications for biosignature preservation in rock samples for possible return to Earth.

¹J. A. McFadden,¹M. S. Thompson,² L. P. Keller,³R. Christoffersen,²R. V. Morris,⁴C. Shearer, The ANGSA Science Team
Journal of Geophysical Research (Planets)(in Press) Open access Link to Article [https://doi.org/10.1029/2024JE008528]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
²ARES, NASA/JSC, Houston, TX, USA
³Jacobs, NASA Johnson Space Center, Houston, TX, USA
⁴Institute of Meteoritics, University of New Mexico, Albuquerque, NM, USA
Published by arrangement with John Wiley & Sons

Apollo 17 core sample 73001/2 was recently made available to researchers for analysis using state-of-the-art techniques in the framework of a modern understanding of lunar surface processes. In this work, we employ transmission electron microscopic analysis to observe the mineralogy, microstructural, and chemical characteristics of space weathering and solar energetic particle (SEP) track distribution in soil grains in the <20 μm size fraction in core sample 73002. The modal mineralogy and stratigraphic space weathered grain abundance suggests that a geologically recent mixing event affected the top 3 cm of 73002. Surface exposure age distributions derived from SEP tracks demonstrate that individual regolith grains rarely reside on the surface for longer than ∼4 million years. The abundance of surface exposed monomineralic fragments with respect to depth correlates well with bulk measurements of space weathered soils using other techniques, such as ferromagnetic resonance. Exposure age distributions suggest the presence of two unique in situ reworking zones spanning the top 8 cm of the core and median exposure ages decrease with increasing depth for both reworking zones, albeit at different rates. These rates were compared to reworking models and suggest a relationship between median exposure age and reworking rate with respect to depth. Applications of modern transmission electron microscopy to core sample 73001/2 have proven useful in understanding lunar regolith evolution both within the context of the Apollo 17 field site and more broadly via in situ reworking.

¹Juan David Hernández-Montenegro,¹Paul D. Asimow,²Claude T. Herzberg
Journal of Geophysical Research (Planets)(In Press) Link to Article [https://doi.org/10.1029/2024JE008508]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Earth and Planetary Sciences, Rutgers University, Piscataway, NJ, USA
Published by arrangement with John Wiley & Sons

Primary magmas form by partial melting in the mantle of a terrestrial planet and represent the starting material for building its crust. The compositions of primary magmas are critical for understanding the thermal history of planetary interiors, as they can be used to estimate mantle potential temperatures (T_P) and track changes in the conditions of mantle partial melting over time. Here, we introduce PRIMARSMELT, a new member of the PRIMELT software family, calibrated to estimate the composition of Martian primary magmas and their formation conditions. We applied PRIMARSMELT to a comprehensive database of basaltic compositions from Mars. Our results are consistent with their petrology, requiring olivine addition to restore fractionated compositions to their primary parents and olivine subtraction from cumulate rocks. Individual primary magma solutions provide insights into the petrogenesis of specific Martian meteorites, with implications for the near-primary nature of some primitive meteorites and the relationship between lithologies A and B in meteorite EETA 79001. Taken together, our results suggest nearly constant or potentially increasing mantle potential temperatures throughout the geological history of Mars. The average T_P for young shergottite meteorites is ∼1,442 ± 40°C, similar to ambient mantle temperatures inferred from geophysical models. In contrast, older basaltic rocks record potential temperatures as low as ∼1,320 ± 48°C for igneous clasts in meteorites NWA 7034/7533. We suggest that, rather than plume-related magmatism, shergottite meteorites record ambient mantle temperatures, with the thermal evolution trend possibly resulting from inefficient heat loss, as expected for a planet in stagnant-lid mode.

¹Paul G. Lucey et al. (>10)
Journal of Geophysical Research (Planets) (in Press) Open Access Link to Article [https://doi.org/10.1029/2024JE008389]
¹Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI, USA
Published by arrangement with John Wiley & Sons

The lunar surface exhibits an absorption band near 3 μm due to hydration, either water or hydroxyl. In most analyses, the band is variable at least in latitude and temperature. Hypotheses for the variability include infilling of the band by thermal emission, migration of molecular water along temperature gradients, and formation and destruction of metastable hydroxyl as solar wind hydrogen diffuses through lunar surface grains. The degree to which lunar soil exhibits an inherent hydration feature in the absence of environmental influences is an open question. The recent opening of Apollo core sample 73001 that was sealed in vacuum on the lunar surface and curated in dry nitrogen since its return from the Moon affords an opportunity to determine if lunar soil exhibits a spectral feature due to hydration isolated from the lunar environment. To that end, near the close of dissection of the core into samples for allocation to the lunar science community, we introduced an infrared spectrometer into the nitrogen purged curation cabinet and collected reflectance spectra of portions of the core between 2 and 4 μm. We found no evidence of absorption due to hydration to 1.1% band depth uncertainty. The measurements were relative to a diffuse aluminum standard, which itself could possibly absorb light at 3 μm due to a thin film of water; we estimate a possible negative bias of about 50 μg/g equivalent water absorption, leading to a final estimate of core water abundance of 50 μg/g ± 50 μg/g. This finding does not contradict prior estimates of lunar surface hydration as core sample 73001 is immature and may not have had sufficient opportunity to gather enough hydrogen from the solar wind or water from micrometeorites to form detectable hydration. After exposure of the core to laboratory atmosphere, a strong 3 μm absorption developed, equivalent to over 1,000 μg/g at a rate of about 5 μg/g per minute, illustrating the sensitivity of lunar materials to water contamination, and the effectiveness of curation of the sample.