¹M.S.Phillips,¹J.E.Moersch,²C.E.Viviano,³J.P.Emery
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114306]
¹Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, United States of America
²Planetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, United States of America
³Department of Astronomy and Planetary Sciences, Northern Arizona University, United States of America
Copyright Elsevier

On Mercury, high-reflectance, flat-floored depressions called hollows are observed nearly globally within low-reflectance material, one of Mercury’s major color units. Hollows are thought to be young, or even currently active, features that form via sublimation, or a “sublimation-like” process. The apparent abundance of sulfides within LRM combined with spectral detections of sulfides associated with hollows suggests that sulfides may be the phase responsible for hollow formation. Despite the association of sulfides with hollows, it is still not clear whether sulfides are the hollow-forming phase. To better understand which phase(s) might be responsible for hollow formation, we calculated sublimation rates for 57 candidate hollow-forming volatile phases from the surface of Mercury and as a function of depth beneath regolith lag deposits of various thicknesses. We found that stearic acid (C18H36O2), fullerenes (C60, C70), and elemental sulfur (S) have the appropriate thermophysical properties to explain hollow formation. Stearic acid and fullerenes are implausible hollow-forming phases because they are unlikely to have been delivered to or generated on Mercury in high enough volume to account for hollows. We suggest that S is most likely the phase responsible for hollow formation based on its abundance on Mercury and its thermophysical properties. We discuss the possibility that S is the phase responsible for hollow formation within the hollow-formation model framework proposed by Blewett et al. (2013). However, several potential limitations with that model lead us to suggest an alternative hollow-formation model: a subsurface heat source (most often impact-induced) generates thermal systems that drive sulfur-rich fumaroles in which S and other phases accumulate on and within the surface at night and sublimate during the day to create hollows. We call this hollow-formation model “Sublimation Cycling Around Fumarole Systems” (SCArFS). We suggest that thermal decomposition of sulfides within LRM is a main contributor to S and S-bearing gases within the proposed fumarole systems and that (re-)precipitation of sulfides may occur at the surface along hollow floors and rims.

¹Shaofan Che,¹Adrian J.Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.027]
¹Department of Earth and Planetary Sciences, MSC03-2040, 1 University of New Mexico, Albuquerque, NM 87131-0001, USA
Copyright Elsevier

Enstatite is a ubiquitous phase in chondritic meteorites, interplanetary dust particles, and cometary samples. In equilibrium condensation models, enstatite is predicted to condense via a reaction between pre-condensed forsterite and gaseous SiO. However, previous studies have shown that some enstatites in chondrite matrices and AOAs do not have a genetic relationship with forsterite, arguing against formation by the predicted forsterite-gas reaction. Here we report the occurrence of enstatite in a unique, fine-grained, spinel-rich inclusion (FGI) Ef1014-01 in the Efremovka CV3 chondrite. Enstatite in this FGI is present as an outer layer on spinel-anorthite-diopside nodules and separates the FGI from an amoeboid olivine aggregate (AOA) -like material. Enstatite shows elevated CaO and Al2O3 contents (up to a few weight percent). Four FIB sections were extracted from this FGI to investigate the microstructures of enstatite and its relationship with other phases using TEM techniques. The TEM observations show that the enstatite is dominantly low-temperature clinoenstatite (LCLEN), which displays abundant twinning, and is sometimes associated with thin orthoenstatite (OREN) lamellae. Clinoenstatite grains commonly have a crystallographic orientation relationship with adjacent diopside, but do not exhibit any replacement relationship with forsterite in the AOA-like material surrounding the FGI. Investigations of several other fine-grained CAIs from the Efremovka and Leoville CV3 chondrites show that enstatite is more common in these inclusions than previously thought and typically forms discontinuous layers or islands on the diopside layers.

Based on SEM and TEM observations, we suggest that the LCLEN-OREN intergrowths in Ef1014-01 formed by transformation from a protoenstatite (PEN) precursor, which may be a product of direct condensation or reheating in the solar nebula. The crystallographic orientation relationship between enstatite and diopside suggests that epitaxial growth of enstatite occurred, lowering the activation energy for nucleation and facilitating direction condensation of enstatite from the gas phase, rather than by reaction of the gas with forsteritic olivine. The microstructures of enstatite are indicative of an extremely rapid cooling rate (∼104 K/h) that is within the range of chondrule cooling rates. Such a rapid cooling rate may imply that the cooling rates of FGIs are indeed much higher than other types of refractory inclusions. Alternatively, the rapid cooling rate may not reflect the primary cooling of the FGIs, but is the result of rapid cooling after a short-lived secondary reheating event in the solar nebula.

A fractionated gas with a lower Mg/Si ratio than the solar value is required to condense enstatite. Such a gas could be produced by isolation of pre-condensed forsterite or repeated evaporation-recondensation processes. The presence of both enstatite-bearing and enstatite-free CAIs in CV3 chondrites suggests that at least two gaseous reservoirs with different Mg/Si ratios were present in the CAI-forming regions.

^1,2,3A.Stephant,¹M.Wadhwa,¹R.Hervig,¹M.Bose,²X.Zhao,²T.J.Barrett,²M.Anand,²I.A.Franchi
Geochimica et cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.01.004]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA
²School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
³Istituto di Astrofisica e Planetologia Spaziali – INAF, 00111 Roma, Italy
Copyright Elsevier

Recent investigations of meteorites thought to originate from the asteroid 4 Vesta have suggested an early accretion of water on rocky bodies in the inner Solar System from a carbonaceous chondrite-like source. However, these studies have been based on the hydrogen isotope compositions (δD) of late-crystallizing apatite grains in eucrites that likely do not record the primary magmatic composition. We have determined the δD and H2O concentrations in some of the earliest-formed silicates (clinopyroxenes) from several eucrites with the goal of constraining the hydrogen isotope composition of their source reservoir on their parent body. The H2O concentrations in clinopyroxenes from eucrites Juvinas, Stannern and Tirhert range from 5 to 18 μg/g, with a weighted average δD of –263 ± 70 ‰. Their apatites and whitlockites exhibit a higher weighted average δD of –165 ± 73 ‰, possibly as a result of H2 degassing during or after phosphate crystallization. Thermal metamorphism of these eucrites has most probably resulted in the loss of H, and an increase in their original δD values. While the weighted average δD value for the eucrite clinopyroxenes reported here is inferred to reflect an upper limit for the isotopic composition of the silicate mantle reservoir on their parent asteroid 4 Vesta, the average δD value of Stannern clinopyroxenes is considered to be closest to the initial δD of the source mantle (i.e., –373 ±127 ‰), which is lighter than that of Earth’s depleted upper mantle and most carbonaceous chondrites. We suggest that at least some of the water in 4 Vesta (and possibly other rocky bodies in the inner Solar System) was derived from a relatively deuterium-poor reservoir in the protosolar nebula, which was incorporated into planetesimals formed early in Solar System history.

¹Emmanuel Jacquet
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.025]
¹Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Muséum national d’Histoire naturelle, Sorbonne Université, CNRS; CP52, 57 rue Cuvier, 75005 Paris, France
Copyright Elsevier

Compound chondrules, i.e. chondrules fused together, make a powerful probe of the density and compositional diversity in chondrule-forming environments, but their abundance among the dominating porphyritic textures may have been drastically underestimated. I report herein microscopic observations and LA-ICP-MS analyses of lobate chondrules in the CO3 chondrites Miller Range 07193 and 07342. Lobes in a given chondrule show correlated volatile and moderately volatile element abundances but refractory element concentrations are essentially independent. This indicates that they formed by the collision of preexisting droplets whose refractory elements behaved in closed system, while their more volatile elements were buffered by the same gaseous medium. The presence of lobes would otherwise be difficult to explain, as surface tension should have rapidly imposed a spherical shape at the temperature peak. In fact, since most chondrules across chondrite groups are nonspherical, a majority are probably compounds variously relaxed toward sphericity. The lack of correlation of refractory elements between conjoined compound chondrule components is inconsistent with derivation of chondrules from the disruption of homogenized melt bodies as in impact scenarios and evokes rather the melting of independent mm-size nebular aggregates. Yet a “nebular” setting for chondrule formation would need to involve not only increased solid concentration, e.g. by settling to the midplane, but also a boost in relative velocities between droplets during chondrule-forming events to account for observed compound chondrule frequencies .

¹B. C. Kaiser,¹J. C. Clemens,²S. Blouin,^3,4P. Dufour,¹R. J. Hegedus,¹J. S. Reding,³A. Bédard
Science 371, 168-172 Link to Article [DOI: 10.1126/science.abd1714]
¹Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC, USA.
²Los Alamos National Laboratory, Los Alamos, NM, USA.
³Département de Physique, Université de Montréal, Montreal, QC, Canada.
⁴Institut de Recherche sur les Exoplanètes, Université de Montréal, Montreal, QC, Canada.
Reprinted with Permission from AAAS

Tidal disruption and subsequent accretion of planetesimals by white dwarfs can reveal the elemental abundances of rocky bodies in exoplanetary systems. Those abundances provide information on the composition of the nebula from which the systems formed, which is analogous to how meteorite abundances inform our understanding of the early Solar System. We report the detection of lithium, sodium, potassium, and calcium in the atmosphere of the white dwarf Gaia DR2 4353607450860305024, which we ascribe to the accretion of a planetesimal. Using model atmospheres, we determine abundance ratios of these elements, and, with the exception of lithium, they are consistent with meteoritic values in the Solar System. We compare the measured lithium abundance with measurements in old stars and with expectations from Big Bang nucleosynthesis.

¹Lucy McGee,^2,3Munir Humayun,¹John Creech,^4,5Brigitte Zanda
Science 371, 164-167 Link to Article [DOI: 10.1126/science.abc8116]
¹Department of Earth and Environmental Sciences, Macquarie University, Sydney, NSW 2109, Australia.
²Department of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, FL 32310, USA.
³National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA.
⁴Institute de minéralogie, de physique des matériaux et de cosmochemie, Muséum National d’Histoire Naturelle, 75005 Paris, France.
⁵Institute of celestial mechanics and ephemeris calculations, Observatoire de Paris, 75014 Paris, France.
Reprinted with Permission from AAAS

Carbonaceous chondritic meteorites are primordial Solar System materials and a source of water delivery to Earth. Fluid flow on the parent bodies of these meteorites is known to have occurred very early in Solar System history (first <4 million years). We analyze short-lived uranium isotopes in carbonaceous chondrites, finding excesses of 234-uranium over 238-uranium and 238-uranium over 230-thorium. These indicate that the fluid-mobile uranium ion U6+ moved within the past few 100,000 years. In some meteorites, this time scale is less than the cosmic-ray exposure age, which measures when they were ejected from their parent body into space. Fluid flow occurred after melting of ice, potentially by impact heating, solar heating, or atmospheric ablation. We favor the impact heating hypothesis, which implies that the parent bodies still contain ice.

¹Gordon M.Gartrelle,²Paul S.Hardersen,³Matthew R.M.Izawa,⁴Matthew C.Nowinski
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114295]
¹University of North Dakota, Grand Forks, ND, USA
²Trouvaille LLC, Tucson, AZ, USA
³Institute for Planetary Materials, Okayama University, Misasa, Japan
⁴George Mason University, Fairfax, VA, USA
Copyright Elsevier

D-type asteroids represent a complex mystery related to the accretional history, composition, and dynamical migration of Outer Solar System objects. These spectrally featureless bodies have revealed few clues while raising many questions over four decades. D-types are dark, difficult to observe and perhaps contain unaltered primordial material. D-type asteroids are abundant in the outer belt, dominant in the Jupiter Trojans, and rare in the inner belt as well as near Earth space. Material spectrally similar to D-types is pervasive on other outer solar system bodies as well. The appearance of dark, spectrally red material in multiple classes of small bodies suggests some unknown geochemical and/or evolutionary connection(s) may exist between them. Our investigation focused on the visible near-infrared (VNIR) (0.7–2.45 μm) spectral distinctions of D-types based on heliocentric location. Twenty-five newly acquired spectra from NASA’s Infrared Telescope Facility (IRTF) plus sixty-one IRTF VNIR spectra from the literature were combined into a single database and extensively analyzed with multiple orbital, observational, and spectral variables included in the examination. Twelve of the newly acquired spectra had not been imaged previously at IRTF.

Pearson’s correlation, simple and multiple regression, slope analysis, Monte Carlo modeling, as well as Principal Component Analysis (PCA) determined D-types show increased reddening with decreasing distance, with the segment from 1.5–2.45 μm, driving the overall trend for the full slope. Principal components show strong connection to the 0.7–1.35 μm slope and inclination of D-type Jupiter Trojans. Principal component combinations, magnitudes, and positive/negative direction relate strongly to both observed and derived differences in the D-type L4 and L5 Trojan population. The L5 population is less evolved spectrally and dynamically than L4 counterparts perhaps due to lower dynamical instabilities inside the L4 cloud.

¹Angela M.Dapremont,¹James J.Wray
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114299]
¹School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA, USA
Copyright Elsevier

Mud volcanism (MV) has been a proposed formation mechanism for positive-relief landforms in the lowland, equatorial, and highland regions of Mars. While visible and near-infrared (VNIR) spectroscopy has been used in a few cases to argue for the presence of MV on the surface of Mars, data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) remain underutilized. We conducted a global examination of proposed Mars MV features using CRISM VNIR data. We observe variable hydration states and place constraints on the composition of these features from orbit. We do not confidently identify phyllosilicates, carbonates, or sulfates associated with suggested Martian mud volcanoes. However, specific structures in Valles Marineris exhibit VNIR signatures consistent with unaltered hydrated glass of a volcanic origin and high-Ca pyroxene. CRISM visible data from MV features reveal consistent nanophase ferric oxide signatures on a global scale, although these signatures are not unique to Mars MV materials. Limitations in specific mineral detection are likely due to the fine grain size and/or textural characteristics of putative MV features. While we do not argue in favor of a specific proposed MV site in the context of future robotic or human missions, the insights of this study could be used as a guide for Mars surface exploration.

¹Marina A.Ivanova,^2,3Ruslan A.Mendybaev,¹Sergei I.Shornikov,¹Cyril A.Lorenz,⁴Glenn J.MacPherson
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.023]
¹Vernadsky Institute of Geochemistry of the Russian Academy of Sciences, Moscow, Russia
²Department of the Geophysical Sciences, University of Chicago, Chicago, IL, United States
³Chicago Center for Cosmochemistry, University of Chicago, Chicago, IL, United States
⁴National Museum of Natural History, Smithsonian Institution, Washington DC, USA
Copyright Elsevier

To address the bulk compositions of CAIs from CH-CB chondrites we have used a new thermodynamic method to model the evaporation of CAI-like melts. The model calculations agree closely with the results of evaporation experiments on individual bulk compositions, and thus could provide a general means of predicting the evaporation trajectory of any CAI bulk composition melt. The model calculations and evaporation experiments show that the initial stages of CAI melt evaporation are controlled by the relative evaporation rates of MgO and SiO2, whereas the late stages are dominated by the initial CaO/Al2O3 ratio of the melt. Application of the model to the puzzling bulk compositions of very refractory CAIs from CH-CB chondrites, many of which are grossite-, hibonite-, and spinel-rich, shows that such compositions can be derived via evaporation of precursors unusually enriched in Al2O3 with CaO/Al2O3 ratios (weight %) < 0.3. This rules out most silicate-rich CAI varieties. Only spinel- and spinel-hibonite-rich fine-grained inclusions with group II REE patterns (common in CV3 chondrites), which may have been present in the region where CH CAIs formed, could be a precursor for the grossite- and hibonite-rich igneous CAIs.

¹Sin-iti Sirono,¹Takuma Shibata²Nagayoshi Katsuta,³Hidekazu Yoshida
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.017]
¹Graduate School of Earth and Environmental Sciences, Nagoya University, Furo-tyo, Tikusa, Nagoya 464-8601, Japan
²Fuculty of Education, Gifu University, Yanagito 1-1, Gifu 501-1193 Japan
³Nagoya University Museum, Nagoya University, Furo-tyo, Tikusa, Nagoya 464-8601, Japan
Copyright Elsevier

Iron oxide concretions are found in sedimentary rocks on both Earth and Mars. On Earth, concretions are common in eolian formations, such as the Jurassic Navajo Sandstone in Utah, USA and those found in the Cretaceous Djadokhta Formation, Gobi Desert, Mongolia. Although it is known that the formation conditions of the iron oxide concretions were affected by the paleoclimate of these regions, quantitative modeling of such formations still requires development, especially concerning initial and diagenetic conditions. A 1-D diffusion-reaction simulation was conducted by assuming that a calcite concretion was initially located in a homogeneous layer of sandgrains. Favorable conditions for the formation of iron oxide concretions have been found to be 4.5⩽pH⩽6, and 10-7⩽[Fe2+]fO2⩽10-5, where [Fe2+] and fO2 are the concentration of ferrous Fe2+ ions and dissolved oxygen relative to the atmospheric value, respectively. An iron-rinded concretion from ferric Fe3+ ions is not possible. For the case of Fe2+ ions, the flow speed of the groundwater should be faster than 2×10-5mms-1. The formation timescale is determined by the diffusion flux of the hydrogen ion, and varies between 2.7×102 and 1.5×104 years for a calcite concretion with an initial radius of 15 mm. Formation conditions of iron-rinded concretions on Earth and Mars are discussed.