¹Ryuki Hyodo,²Hidenori Genda,²Ramon Brasser
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114064]
¹ISAS, JAXA, Sagamihara, Japan
²Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan
Copyright Elsevier

Late accretion is a process that strongly modulated surface geomorphic and geochemical features of Mercury. Yet, the fate of the impactors and their effects on Mercury’s surface through the bombardment epoch are not clear. Using Monte-Carlo and analytical approaches of cratering impacts, we investigate the physical and thermodynamical outcomes of late accretion on Mercury. Considering the uncertainties in late accretion, we develop scaling laws for the following parameters as a function of impact velocity and total mass of late accretion: (1) depth of crustal erosion, (2) the degree of resurfacing, and (3) mass accreted from impactor material. Existing dynamical models indicate that Mercury experienced an intense impact bombardment (a total mass of ∼8 × 1018 − 8 × 1020 kg with a typical impact velocity of 30 − 40 km s−1) after 4.5 Ga. For this parameter range, we find that late accretion could remove 50 m to 10 km of the early (post-formation) crust of Mercury, but the change to its core-to-mantle ratio is negligible. Alternatively, the mantles of putative differentiated planetesimals in the early solar system could be more easily removed by impact erosion and their respective core fraction increased, if Mercury ultimately accreted from such objects. Although the cratering is notable for erasing the older geological surface records on Mercury, we show that ∼40 − 50wt. % of the impactor’s exogenic materials, including the volatile-bearing materials, can be heterogeneously implanted on Mercury’s surface as a late veneer (at least 3 × 1018 − 1.6 × 1019 kg in total). About half of the accreted impactor’s materials are vaporized, and the rest is completely melted upon the impact. We expect that the further interplay between our theoretical results and forthcoming surface observations of Mercury, including the BepiColombo mission, will lead us to a better understanding of Mercury’s origin and evolution.

¹C.Xiang,¹A.Carballido,¹L.S.Matthews,¹T.W.Hyde
Icarus (in Press) Linkto Article [https://doi.org/10.1016/j.icarus.2020.114053]
¹Center for Astrophysics, Space Physics, and Engineering Research, Baylor University, Waco, TX 76798-7316, USA
Copyright Elsevier

In order to characterize the early growth of fine-grained dust rims (FGRs) that commonly surround chondrules, we simulate the growth of FGRs through direct accretion of monomers of various sizes onto the chondrule surfaces. Dust becomes charged to varying degrees in the radiative plasma environment of the solar nebula (SN), and the resulting electrostatic force alters the trajectories of colliding dust grains, influencing the structure of the dust rim as well as the time scale of rim formation. We compare the growth of FGRs in protoplanetary disks (PPD) with different turbulence strengths and plasma conditions to previous models which assumed neutral dust grains (Xiang, C., Carballido, A., Hanna, R.D., Matthews, L.S., Hyde, T.W., 2019). We use a combination of a Monte Carlo method and an N-body code to simulate the collision of dust monomers of radii 0.5 – m with chondrules whose radii are between 500 and m: a Monte Carlo algorithm is used to randomly select dust particles that will collide with the chondrule as well as determine the elapsed time interval between collisions; at close approach, the detailed collision process is modeled using an N-body algorithm, Aggregate Builder (AB), to determine the collision outcome, as well as any restructuring of the chondrule rim. For computational expediency, we limit accretion of dust monomers to a small patch of the chondrule surface. The collisions are driven by Brownian motion and coupling to turbulent gas motion in the protoplanetary disk. The charge distribution of the dust rim is modeled, used to calculate the trajectories of dust grains, and then analyze the resulting morphology of the dust rim. In a weakly turbulent region, the decreased relative velocity between charged particles causes small grains to be repelled from the chondrule, causing dust rims to grow more slowly and be composed of larger monomers, which results in a more porous structure. In a highly turbulent region, the presence of charge mainly affects the porosity of the rim by causing dust particles to deviate from the extremities of the rim and reducing the amount of restructuring caused by high-velocity collisions.

¹Libby D. Tunney,¹Christopher D. K. Herd,^1,2Robert W. Hilts
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13551]
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 Canada
²Department of Physical Sciences, MacEwan University, Edmonton, Alberta, T6J 4S2 Canada
Published by arrangement with John Wiley & Sons

The study of organic compounds in astromaterials represents a unique window into organic matter in the interstellar medium, the solar nebula, and asteroid parent bodies, and insights into pathways that may relate organic matter in these diverse environments. Unfortunately, the Earth’s surface is awash in organic material, which forms a significant source of contamination, especially for specimens of meteorite falls. Here, we employ specimens of the Buzzard Coulee H4 ordinary chondrite, the fall of which in western Saskatchewan, Canada, on November 20, 2008 was widely documented, and from which meteorites were recovered starting shortly after the fall and continuing to over 10 years later. The low intrinsic organic matter content of these H4 specimens enables their use as “blanks” for terrestrial contamination. Using dichloromethane rinses of meteorite specimen exteriors, and analysis of organic compounds by gas chromatograph‐mass spectrometry, we document the sources of terrestrial contamination as a function of location, time of collection relative to the fall, and curation and handling since collection. We find numerous terrestrial organic compounds, most of which can be attributed to the terrestrial surface on which the meteorite specimens were collected, and we consider a variety of factors that may influence the degree of contamination. To determine the source of each contaminant more accurately, we advocate for the collection of terrestrial materials (e.g., soil, vegetation) alongside meteorites. Our results have implications for how specimens from meteorite falls—especially for meteorites expected to have high intrinsic organic content—are collected, documented, and curated.

¹Laurette Piani,¹Yves Marrocchi,¹Thomas Rigaudier,^1,2Lionel G. Vacher,¹Dorian Thomassin,¹Bernard Marty
Science 369, 1110-1113 Link to Article [DOI: 10.1126/science.aba1948
Article]
¹Centre de Recherches Pétrographiques et Géochimiques (CRPG), Centre National de Recherche Scientifique (CNRS)–Université de Lorraine, Vandoeuvre-les-Nancy, F-54500, France.
²Department of Physics, Washington University in St. Louis, St. Louis, MO 63130, USA.
Reprinted with permission from AAAS

The origin of Earth’s water remains unknown. Enstatite chondrite (EC) meteorites have similar isotopic composition to terrestrial rocks and thus may be representative of the material that formed Earth. ECs are presumed to be devoid of water because they formed in the inner Solar System. Earth’s water is therefore generally attributed to the late addition of a small fraction of hydrated materials, such as carbonaceous chondrite meteorites, which originated in the outer Solar System where water was more abundant. We show that EC meteorites contain sufficient hydrogen to have delivered to Earth at least three times the mass of water in its oceans. EC hydrogen and nitrogen isotopic compositions match those of Earth’s mantle, so EC-like asteroids might have contributed these volatile elements to Earth’s crust and mantle.

¹G.David et al. (>10)
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2019JE006314]
¹Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, UPS, CNRS, CNES, Toulouse, France
Published by arrangement with John Wiley & Sons

Curiosity investigated a topographic rise named Vera Rubin ridge (VRR) in Gale crater, for which a distinct hematite‐like signature was observed from orbit. However, the ChemCam and APXS instruments on board the rover did not record any significant iron enrichment in the bulk of the ridge compared to previous terrains. For this study, we have re‐verified ChemCam iron calibration at moderate abundances and developed more accurate calibrations at high‐iron abundances using iron oxide mixtures in a basaltic matrix in order to complete the ChemCam calibration database. The high‐iron calibration was first applied to the analysis of dark‐toned diagenetic features encountered at several locations on VRR, which showed that their chemical compositions are close to pure anhydrous iron oxides. Then, we tracked iron abundances in the VRR bedrock and demonstrated that although there is no overall iron enrichment in the bulk of the ridge (21.2±1.8 wt.% FeO_T) compared to underlying terrains, the iron content is more variable in its upper section with areas of enhanced iron abundances in the bedrock (up to 26.6±0.85 wt.% FeO_T). Since the observed variability in iron abundances does not conform to the stratigraphy, the involvement of diagenetic fluid circulation was likely. An in‐depth chemical study of these Fe‐rich rocks reveals that spatial gradients in redox potential (Eh) may have driven iron mobility and reactions that precipitated and accumulated iron oxides. We hypothesize that slightly reducing fluids were probably involved in transporting ferrous iron. Mobile Fe²⁺ could have precipitated as iron oxides in more oxidizing conditions.

¹Rebecca N. Greenberger,^1,2Bethany L. Ehlmann,^3,4Gordon R. Osinski,^3,4Livio L. Tornabene,²Robert O. Green
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2019JE006218]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
³Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada
⁴Institute for Earth and Space Exploration, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

Connecting the surface expression of impact crater‐related lithologies to planetary or regional subsurface compositions requires an understanding of material transport during crater formation. Here, we use imaging spectroscopy of 6 clast‐rich impact melt rock outcrops within the well‐preserved 23.5‐Ma, 23‐km diameter Haughton impact structure, Canada, to determine melt rock composition and spatial heterogeneity. We compare results from outcrop to outcrop, using clasts, groundmass, and integrated clast‐groundmass compositions as tracers of transport during crater‐fill melt rock formation and cooling. Supporting laboratory imaging spectroscopy analyses of 91 melt‐bearing breccia and clast samples and microscopic x‐ray fluorescence elemental mapping of cut samples paired with spectroscopy of identical surfaces validate outcrop‐scale lithological determinations. Results show different clast‐rich impact melt rock compositions at three sites kilometers apart and an inverse correlation between silica‐rich (sandstone, gneiss, and phyllosilicate‐rich shales) and gypsum‐rich rocks that suggests differences in source depth with location. In the target stratigraphy, gypsum is primarily sourced from ~1 km depth, while gneiss is from >1.8 km depth, sandstone from >1.3 km, and shales from ~1.6–1.7 km. Observed heterogeneities likely result from different excavation depths coupled with rapid quenching of the melt due to high content of cool clasts. Results provide quantitative constraints for numerical models of impact structure formation and give new details on melt rock heterogeneity important in interpreting mission data and planning sample return of impactites, particularly for bodies with impacts into sedimentary and volatile‐bearing targets, e.g., Mars and Ceres.

¹E.K.Kolesnikov,²A.G.Zelensky
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2020.105065]
¹St. Petersburg State University, St. Petersburg, 198504 Russia
²Moscow State Technical University of Civil Aviation, Moscow, 125493 Russia

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

¹Yves Marrocchi,²Lydie Bonal,³Jérome Gattacceca,¹Laurette Piani,²Pierre Beck,⁴Richard Greenwood,²Jolantha Eschrig,¹Anne Basque,⁵Pasquale Mario Nuccio,^6,7Franco Foresta Martin
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13552]
¹CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre‐lès‐Nancy, 54501 France
²Institut de Planétologie et d’Astrophysique de Grenoble, Grenoble, France
³Aix‐Marseille Univ, CNRS, IRD, Coll France, INRAE, CEREGE, Aix‐en‐Provence
⁴PSS, Open University, Walton Hall, Milton Keynes, MK7 6AA UK
⁵Università di Palermo, Palermo, Italy
⁶Instituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, 90146 Palermo, Italy
⁷Laboratorio Museo di Scienze della Terra, Ustica, Palermo, Italy
Published by Arrangement with John Wiley & Sons

The Piancaldoli ordinary chondrite fell in northern Italy on August 10, 1968. Preliminary studies led to its classification as an LL3.4 unequilibrated ordinary chondrite. However, recent developments in classification procedures have prompted us to re‐examine its mineralogical, petrographic, spectroscopic, chemical, and isotopic features in a multi‐technique study. Raman spectra and magnetic properties indicate that Piancaldoli experienced minimal thermal metamorphism, consistent with its high bulk hydrogen content and the Cr contents of ferroan olivines in its type II chondrules. In combination with findings of previous studies, our data thus confirm the variability of Cr contents in ferroan olivines in type II chondrules as a proxy of thermal metamorphism. Furthermore, our results reveal that Piancaldoli is less altered than previously reported and should be reclassified as an LL3.10 unequilibrated ordinary chondrite. Our results also imply that the bulk deuterium enrichment, as observed in Piancaldoli (LL3.10), Bishunpur (LL3.15), and Semarkona (LL3.00), is a specific signature of the most primitive unequilibrated ordinary chondrites. Based on our results, we propose that, to date, Piancaldoli is the second least‐altered unequilibrated ordinary chondrite fall after Semarkona. This work reiterates the importance of meteorite collections worldwide as fundamental resources for studying the formation conditions and evolution of our solar system.

¹Paula Lindgren et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.08.021]
¹Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden
Copyright Elsevier

The CM carbonaceous chondrites have all been aqueously altered, and some of them were subsequently heated in a parent body environment. Here we have sought to understand the impact of short duration heating on a highly aqueously altered CM through laboratory experiments on Allan Hills (ALH) 83100. Unheated ALH 83100 contains 83 volume per cent serpentine within the fine-grained matrix and altered chondrules. The matrix also hosts grains of calcite and dolomite, which are often intergrown with tochilinite, Fe(Ni) sulphides (pyrrhotite, pentlandite), magnetite and organic matter. Some of the magnetite formed by replacement of Fe(Ni) sulphides that were accreted from the nebula. Laboratory heating to 400 °C has caused partial dehydroxylation of serpentine and loss of isotopically light oxygen leading to an increase in bulk δ¹⁸O and fall in Δ¹⁷O. Tochilinite has decomposed to magnetite, whereas carbonates have remained unaltered. With regards to infrared spectroscopy (4000–400 cm^-1; 2.5 – 25 µm), heating to 400 °C has resulted in decreased emissivity (increased reflectance), a sharper and more symmetric OH band at 3684 cm^-1 (2.71 µm), a broadening of the Si-O stretching band together with movement of its minimum to longer wavenumbers, and a decreasing depth of the Mg-OH band (625 cm^-1; 16 µm). The Si-O bending band is unmodified by mild heating. With heating to 800 °C the serpentine has fully dehydroxylated and recrystallized to ∼Fo_60/70 olivine. Bulk δ¹⁸O has further increased and Δ¹⁷O decreased. Troilite and pyrrhotite have formed, and recrystallization of pentlandite has produced Fe,Ni metal. Calcite and dolomite were calcined at ∼700 °C and in their place is an un-named Ca-Fe oxysulphide. Heating changes the structural order of organic matter so that Raman spectroscopy of carbon in the 800 °C sample shows an increased (D1 + D4) proportional area parameter. The infrared spectrum of the 800 °C sample confirms the abundance of Fe-bearing olivine and is very similar to the spectrum of naturally heated stage IV CM Pecora Escarpment 02010. The temperature-related mineralogical, chemical, isotopic and spectroscopic signatures defined in ALH 83100 will help to track the post-hydration thermal histories of carbonaceous chondrite meteorites, and samples returned from the primitive asteroids Ryugu and Bennu.

^1,2Z.Ren,²P.M.Vasconcelos
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.08.019]
¹State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, 430074, China
²School of Earth and Environmental Sciences, The University of Queensland, Brisbane, Qld 4072, Australia
Copyright Elsevier

Jarosite [KFe₃(SO₄)₂(OH)₆] occurs both as a hydrothermal mineral or as the product of weathering and chemical sedimentation. It has been used in ⁴⁰Ar/³⁹Ar geochronology to date water-rock interaction and weathering processes on the surfaces of Earth and Mars, but the lack of information about Ar diffusivity parameters relevant to specific types of jarosites makes the interpretation of geochronological results tentative. We have filled this gap by investigating Ar diffusion parameters in representative supergene and hypogene jarosites. Detailed diffusion studies were carried out on a hypogene jarosite sample from Gilbert, Nevada, and two supergene jarosite samples from Baiyin, China. The diffusion studies were accompanied by in-situ heating investigations in a transmission electron microscope to directly determine the thermal stability and the phase transformations that jarosite undergoes under progressive heating under ultra-high vacuum. The TEM results suggest that jarosite is stable under vacuum up to ∼ 400 °C, when it undergoes phase transition to yavapaiite [KFe(SO₄)₂] and hematite (Fe₂O₃). Incremental-heating experiments reveal average diffusion parameters of E_a = 138.6 ± 4.2 kJ/mol and ln(D_o/a²) = 9.9 ± 0.9 ln(s^-1) for hypogene jarosite; E_a = 110.3 ± 3.2 kJ/mol and ln(D_o/a²) = 5.7 ± 0.7 ln(s^-1) for one supergene jarosites; and E_a = 141.2 ± 7.9 kJ/mol and ln(D_o/a²) = 11.3 ± 1.7 ln(s^-1) for the other supergene jarosite sample from the same weathering profile. Jarosite closure temperatures depend strongly on sieve size. For samples between 500-200 µm (grain size usually used for samples in Ar geochronology), at a cooling rate 100 °C·Ma^-1, the closure temperatures are 143 ± 18 °C for hypogene and 105 ± 8 and 113 ± 14 °C, respectively, for the two supergene jarosites. Forward modelling of incremental-heating results predicts that coarse-grained hypogene jarosite is retentive of Ar below 50 °C for 100 Ma and below 25 °C for 4 Ga. Densely packed supergene jarosite grains larger than 200 µm are suitable for ⁴⁰Ar/³⁹Ar geochronology at the timescales suitable for investigating water-rock interaction at the surface of Earth and Mars. Fine-grained, porous jarosites require detailed diffusion analyses prior to geochronology due to possible high Ar losses over Ma timescales. The absence of jarosites older than ∼40 Ma on Earth suggests that jarosite may require continuous exposure to acid oxidizing conditions, and that it does not survive burial and exhumation. Therefore, the occurrence of jarosite on Earth and Mars may identify segments of the planets’ surfaces continuously exposure to acid-oxidizing conditions since jarosite precipitation.