¹Yuri Shimaki et al. (>10)
Icarus (in Press) Link to article [https://doi.org/10.1016/j.icarus.2020.113835]
¹Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
Copyright Elsevier

TIR, the thermal infrared imager on Hayabusa2, acquired high-resolution thermal images of the asteroid 162173 Ryugu for one asteroid rotation period on August 1, 2018 to investigate the thermophysical properties of the asteroid. The surface temperatures of Ryugu suggest that the surface has a low thermal inertia, indicating the presence of porous materials. Thermophysical models that neglect or oversimplify surface roughness cannot reproduce the flat diurnal temperature profiles observed during daytime. We performed numerical simulations of a thermophysical model, including the effects of roughness on the diurnal brightness temperature, the predictions of which successfully reproduced the observed diurnal variation of temperature. The global thermal inertia was obtained with a standard deviation of 225 ± 45 J m⁻² s^−0.5 K⁻¹, which is relatively low but still within the range of the value estimated in our previous study (Okada et al., Nature 579, 518–522, 2020), confirming that the boulders on Ryugu are more porous in nature than typical carbonaceous chondrites. The global surface roughness (the ratio of the variance of the height relative to a local horizontal surface length) was determined as 0.41 ± 0.08, corresponding to a RMS surface slope of 47 ± 5°. We identified a slightly lower roughness distributed along the equatorial ridge, implying a mass movement of boulders from the equatorial ridge to the mid-latitudes.

^1,2L. F. White,^1,2,3A. Černok,⁴J. R. Darling,⁵M. J. Whitehouse,⁶K. H. Joy,⁷C. Cayron,⁴J. Dunlop,^1,2 K. T. Tait,^3,8M. Anand
Nature Astronomy (in Print) Link to Article [DOIhttps://doi.org/10.1038/s41550-020-1092-5]
¹Centre of Applied Planetary Mineralogy, Department of Natural History, Royal Ontario Museum, Toronto, Ontario, Canada
²Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada
³School of Physical Sciences, The Open University, Milton Keynes, UK
⁴School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth, UK
⁵Swedish Museum of Natural History, Stockholm, Sweden
⁶Department of Earth and Environmental Science, University of Manchester, Manchester, UK
⁷Laboratory of ThermoMechanical Metallurgy (LMTM), PX Group Chair, École Polytechnique Fédérale de Lausanne (EPFL), Neuchâtel, Switzerland
⁸Department of Earth Sciences, The Natural History Museum, London, UK

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^1,2Michaël Marsset et al. (>10)
Nature Astronomy (in Press) Link to Article [DOIhttps://doi.org/10.1038/s41550-019-1007-5]
¹Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA, USA
²Astrophysics Research Centre, Queen’s University Belfast, Belfast, UK

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¹Miyamoto, H.,¹Niihara, T.
Natural Resources Research (in Press) Link to Article [DOI: 10.1007/s11053-020-09626-2]
¹Department of Systems Innovation, Graduate School of Engineering, University of Tokyo, Tokyo, Japan

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¹Thomas S.Kruijer,¹Lars E.Borg,¹Josh Wimpenny,¹Corliss K.Sio
Earth and Planetary Science Letters 542, 116315 Link to Article [https://doi.org/10.1016/j.epsl.2020.116315]
¹Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue (L-231), Livermore, CA 94550, USA
Copyright Elsevier

The mantle of Mars probably differentiated through the crystallization of a magma ocean during the first tens of million years (Ma) of Solar System evolution. However, the exact timescale of large-scale silicate differentiation of the martian mantle is debated, and in particular, it remains unclear when differentiation commenced. Here we applied the short-lived 53Mn-53Cr system to martian meteorites in order to date the onset of large-scale mantle differentiation on Mars. The new Cr isotope data demonstrate that martian meteorites exhibit no resolvable radiogenic 53Cr variations, and instead have a uniform +20.2±1.2 (95% conf.) parts-per-million excess in 53Cr/52Cr relative to the terrestrial mantle. The investigated groups of martian meteorites are lithologically varied and derive from diverse mantle sources that probably had variable Mn/Cr. Hence, the lack of 53Cr variability among martian meteorites demonstrates that silicate differentiation on Mars occurred after the extinction of 53Mn. Provided that the sources of the martian meteorites have Mn/Cr variations that are typical of the terrestrial planets, this result implies that the onset of large-scale silicate differentiation must have occurred later than 20±5 Ma after Solar System formation. The onset of silicate differentiation on Mars inferred here is significantly later than time estimates for segregation of the martian core which conservatively occurred within <10 Ma after Solar System formation. Thus, the new Mn-Cr data imply that there was a small, but resolvable, time gap of at least 5 Ma between core formation and magma ocean solidification on Mars. If the age of core segregation is taken at face value, our results imply that the martian magma ocean remained mostly molten over several Ma. This inferred longevity of the magma ocean is inconsistent with thermal models predicting rapid (<1 Ma) solidification of the martian magma ocean. Although there is currently no unique solution to this conundrum, our results can potentially be explained by a protracted history of impact bombardment that delayed differentiation in a shallow magma ocean on Mars, or perhaps more readily, by the presence of an early and dense atmosphere that acted as an insulator and prevented the magma ocean from cooling quickly.

¹Nikitha Susan Saji,¹Daniel Wielandt,¹Jesper Christian Holst,¹Martin Bizzarro
Geochimica et Cosmochimic Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.006]
¹Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, DK-1350 Copenhagen, Denmark
Copyright Elsevier

High-precision Nd isotope measurements of a diverse set of solar system materials including bulk chondrites and achondrites reveal that their Nd isotope composition is governed by several distinct nucleosynthetic components. The full spectrum of non-radiogenic, mass-independent Nd isotope compositions of solar system materials is best explained by heterogeneous distribution of at least three nucleosynthetic components – the classical s-process component, pure p-process component and an anomalous, previously unidentified s-/r-process component. The 142Nd/144Nd variations in solar system reservoirs specifically fall into three distinct trends – those that result from variations in the s-process component, those resulting from variations in the pure p-process component, and those resulting from coupled s-process and p-process variations. The μ148Nd value, a proxy for s-process variations , as well as μ142Nd that has been corrected for s-process heterogeneity to reflect p-process variations, broadly show an inverse correlation with ∊∊54Cr. The linearity in μ148Nd – ∊∊54Cr space for inner solar system bodies, CI chondrite and Allende-type CAIs possibly suggests the thermally labile nature of some s-process carrier grains unlike the mainstream refractory s-process SiC grains. The p-process carrier for Nd is inferred to be a refractory phase enriched in inner solar system materials through thermal processing. The bulk meteorite regression lines that specifically correspond to s- and p-process heterogeneity, largely define μ142Nd intercepts indistinguishable from terrestrial composition within analytical uncertainty, ruling out resolvable radiogenic μ142Nd excess on Earth that cannot otherwise be accounted for by nucleosynthetic heterogeneity.

¹François Robert,²Romain Tartèse,³Guillaume Lombardi,⁴Peter Reinhardt,¹Mathieu Roskosz,¹Béatrice Doisneau,⁵Zhengbin Deng,⁵Marc Chaussidon
Nature Astronomy (in Press) Link to Article [DOIhttps://doi.org/10.1038/s41550-020-1043-1]
¹Muséum National d’Histoire Naturelle, Institut de Minéralogie, Physique des Matériaux et Cosmochimie, CNRS UMR 7590, Paris, France
²Department of Earth and Environmental Sciences, The University of Manchester, Manchester, UK
³Laboratoire des Laboratoire des Sciences des Procédés et des Matériaux, CNRS UPR 3407, Université Paris 13, Sorbonne Paris Cité, Villetaneuse, France
⁴Laboratoire de Chimie Théorique, Sorbonne Université, CNRS UMR 7616, Paris, France
⁵Institut de Physique du Globe de Paris, Université de Paris, CNRS UMR 7154, Paris, France

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^1,2,3Stephen M. Elardo,⁴Matthieu Laneuville,⁵Francis M. McCubbin,⁶Charles K. Shearer
Nature Geoscience 13, 339–343 Link to Article [DOIhttps://doi.org/10.1038/s41561-020-0559-4]
¹Department of Geological Sciences, University of Florida, Gainesville, FL, USA
²Geophysical Laboratory, Carnegie Institution for Science, Washington, DC, USA
³Department of Physics, Astronomy, and Geosciences, Towson University, Towson, MD, USA
⁴Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo, Japan
⁵NASA Johnson Space Center, Houston, TX, USA
⁶Institute of Meteoritics, University of New Mexico, Albuquerque, NM, USA

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^1,2Jessica J. Barnes et al. (>10)
Nature Geoscience 13, 260–264 Link to Article [DOIhttps://doi.org/10.1038/s41561-020-0552-y]
¹NASA Johnson Space Center, Houston, TX, USA
²Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

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^1,2Christopher R. J. Charles,^2,3Phil J. A. McCausland,²Donald W. Davis
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13492]
¹TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada
²Department of Earth Sciences, University of Toronto, 22 Russell St., Toronto, Ontario, M5S 3B1 Canada
³Institute for Earth and Space Exploration, Western University, London, Ontario, N6A 5B7 Canada
Published by arrangement with John Wiley & Sons

Lovina, classified as an ungrouped ataxite, is controversial and its identity as a meteorite has been questioned. In this work, we use Pb isotopes on targeted troilite nodules in Lovina as a test of its antiquity and provenance. Although precise ages cannot be obtained, LA‐ICP‐MS offers a rapid, straightforward procedure to establish the source of lead, whether ancient (meteoritic) or modern (terrestrial). For nine pristine, unweathered nodules in Lovina, we find a lead isotopic composition of: ²⁰⁶Pb/²⁰⁸Pb = 0.492 ± 0.003 (2σ, MSWD 0.79; 95%) and ²⁰⁷Pb/²⁰⁶Pb = 0.852 ± 0.003 (2σ, MSWD 1.09; 95%) with no detectable uranium. All lead compositions of the troilite fall in the range expected for modern environmental and mantle lead and are distinctly different from the primordial Canyon Diablo Troilite (CDT) composition of ancient meteoritic troilite. Although the origin of Lovina remains unknown, we conclude that lead in the Lovina troilite is unsupported by U decay and originated from a terrestrial source.