Ruthenium isotope vestige of Earth’s pre-late-veneer mantle preserved in Archaean rocks

1Mario Fischer-Gödde,1Bo-Magnus Elfers,1Carsten Münker,2Kristoffer Szilas,3Wolfgang D. Maier,1Nils Messling,4,5,6Tomoaki Morishita,7Martin Van Kranendonk,8Hugh Smithies
Nature 579, 240-244 Link to Article [DOIhttps://doi.org/10.1038/s41586-020-2069-3]
1Institut für Geologie und Mineralogie, University of Cologne, Cologne, Germany
2Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark
3School of Earth and Ocean Sciences, Cardiff University, Cardiff, UK
4Faculty of Geosciences and Civil Engineering, Institute of Science and Engineering, Kanazawa University, Kanazawa, Japan
5Lamont-Doherty Earth Observatory, Columbia University, New York, NY, USA
6Volcanoes and Earth’s Interior Research Center, Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
7Australian Centre for Astrobiology, University of New South Wales, Sydney, New South Wales, Australia
8Geological Survey of Western Australia, East Perth, Western Australia, Australia

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

Probing Europa’s subsurface ocean composition from surface salt minerals using in-situ techniques

1,2Tuan H.Vu,1,2Mathieu Choukroun,1,2Robert Hodyss,1,2Paul V.Johnson
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113746]
1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
2NASA Astrobiology Institute, USA
Copyright Elsevier

The composition of Europa’s subsurface ocean is of great interest for understanding the internal geochemistry and potential habitability of this icy body. However, current constraints on the ocean composition need to rely largely on its expression on the surface. In this work, we combine chemical divide modeling with cryogenic Raman and X-ray diffraction experiments to examine freezing of a simple putative brine system containing Na+, Mg2+, Cl, and SO42− across a range of ionic concentrations and freezing rates to assess the feasibility of inferring the ocean’s chemical composition via in-situ techniques. The results suggest that multiple hydrated salts not predicted by chemical models are frequently encountered in the final solid phase, making accurate quantification of the subsurface liquid composition via surface observables rather challenging. In addition, flash freezing of diluted brines often produces water ice together with amorphous hydrated Mg salts, which may significantly hinder their detection. These findings can help inform both analytical protocols for a Raman spectrometer onboard a Europa surface lander as well as potential locations for exploration, in order to best provide meaningful constraints on emplacement mechanisms and the composition of frozen salt minerals on the surface.

The role of Bells in the continuous accretion between the CM and CR chondrite reservoirs

1Elishevah van Kooten,2Larissa Cavalcante,3Daniel Wielandt,3Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1111/maps.13459]
1Institut de Physique du Globe de Paris, Université de Paris, CNRS, UMR 7154, 1 rue Jussieu, 75238 Paris, France
2Institute of Chemistry, University of São Paulo, 03178 São Paulo, Brazil
3Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, DK‐1350 Copenhagen, Denmark
Published by arrangement with John Wiley & Sons

CM meteorites are dominant members of carbonaceous chondrites (CCs), which evidently accreted in a region separated from the terrestrial planets. These chondrites are key in determining the accretion regions of solar system materials, since in Mg and Cr isotope space, they intersect between what are identified as inner and outer solar system reservoirs. In this model, the outer reservoir is represented by metal‐rich carbonaceous chondrites (MRCCs), including CR chondrites. An important question remains whether the barrier between MRCCs and CCs was a temporal or spatial one. CM chondrites and chondrules are used here to identify the nature of the barrier as well as the timescale of chondrite parent body accretion. We find based on high precision Mg and Cr isotope data of seven CM chondrites and 12 chondrules, that accretion in the CM chondrite reservoir was continuous lasting <3 Myr and showing late accretion of MRCC‐like material reflected by the anomalous CM chondrite Bells. We further argue that although MRCCs likely accreted later than CM chondrites, CR chondrules must have initially formed from a reservoir spatially separated from CM chondrules. Finally, we hypothesize on the nature of the spatial barrier separating these reservoirs.

Widespread production of silica- and alkali-rich melts at the onset of planetesimal melting

1Max Collinet,1Timothy L.Grove
Geochimcia et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.005]
1Massachusetts Institute of Technology, Earth, Atmospheric and Planetary Sciences department, 77 Massachusetts avenue, 02139, MA, USA
Copyright Elsevier

We present the results of melting experiments on a suite of carbonaceous and ordinary chondritic compositions (CV, CM, CI, H and LL) performed at low pressure (0.1 to 13.1 MPa) and over a range of oxygen fugacity (log fO2 – (log fO2)IW = -2.5 to -1 and +0.8, IW being the iron-wustite buffer). These experiments constrain the composition of partial melts (F = 5-25 wt.%) of chondritic planetesimals. Most experiments (IW -2.5 to -1) were conducted in Molybdenum-Hafnium Carbide pressure vessels, which prevented the loss of alkali elements from the melt. The results show that all planetesimals not significantly depleted in moderately volatile elements relative to the sun’s photosphere (e.g. CI, H and LL compositions) produced low-degree melts (<15 wt.%) rich in SiO2, Al2O3 and alkali elements, regardless of the fO2. Despite their high apparent viscosities (104-5 Pa.s), such low-density partial melts (2400-2500 kg/m3) were mobilized and, upon crystallizing, formed rocks containing up to 80 vol.% of plagioclase An10-30 (i.e. oligoclase) such as the trachyandesite achondrites Graves Nunataks 06128/9, Northwest Africa 6698 and 11575, the Almahata Sitta clast ALM-A, as well as smaller “albitic clasts” in polymict ureilites and “alkali-silica-rich” inclusions in non-magmatic iron meteorites. We suggest that silica- and alkali- rich melts were widespread in small bodies of the early solar system but that much evidence was erased by subsequent stages of melting and planetary accretion and differentiation.

Formation of primitive achondrites by partial melting of alkali-undepleted planetesimals in the inner solar system

1Max Collinet,1Timothy L.Grove
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.004]
1Massachusetts Institute of Technology, Earth, Atmospheric and Planetary Sciences department, 77 Massachusetts avenue, 02139, MA, USA
Copyright Elsevier

Acapulcoites-lodranites, ureilites, brachinites, brachinite-like achondrites and winonaites are the main groups of primitive achondrites. They are variably depleted in incompatible lithophile elements (Al, Na, K and rare earth elements) and siderophile/chalcophile elements relative to chondrites and are interpreted as the residual mantle of planetesimals from which silicate melts and sulfide/metal melts were extracted. We use a series of melting experiments conducted with various chondritic compositions (CV, CM, CI, H and LL) to constrain the oxygen fugacity (fO2), the temperature, extent of melting and the initial bulk composition of the parent bodies of primitive achondrites. They melted at different and variable fO2: ΔIW -0.5/-1.0 for brachinites, ΔIW -1.3/-2.5 for ureilites, ΔIW -1.6/-2.7 for acapulcoites/lodranites and ΔIW -2.5/-3.0 for winonaites (with ΔIW = log fO2 – (log fO2)IW; IW being the iron-wustite buffer). Those main groups of primitive achondrites, which have nucleosynthetic anomalies characteristic of the “non-carbonaceous” reservoir and the inner solar system, were not initially depleted in Na2O and K2O relative to the sun’s photosphere. This suggests, in accordance with the enrichment in the heavy isotopes of Zn, Rb and K in eucrites, that the depletion of moderately volatile elements in planetesimals that melted to a larger extent (e.g. Vesta, the angrite parent body) resulted from evaporative losses during partial melting. The depletion of moderately volatile elements in terrestrial planets is likely inherited from partial melting and differentiation of small planetary bodies rather than from the incomplete complete condensation of the solar nebula.

Characterizing irradiated surfaces using IR spectroscopy

1R.Brunetto,1C.Lantz,2T.Nakamura,1D.Baklouti,1T.Le Pivert-Jolivet,2S.Kobayashi,3F.Borondics
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113722]
1Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
2Division of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Japan
3SOLEIL Synchrotron, Gif-sur-Yvette, France
Copyright Elsevier

Solar wind ion irradiation continuously modifies the optical properties of unprotected surfaces of airless bodies in the Solar System. This alteration induces significant biases in the interpretation of the spectral data obtained through remote sensing, and it impedes a correct estimation of the composition of the sub-surface pristine materials. However, as the alteration of the surface is a function of time, an in-depth understanding of the phenomenon may provide an original way to estimate the weathering age of a surface. Laboratory experiments show that mid- and far-IR bands are very sensitive to space weathering, as they are significantly modified upon irradiation. These bands can thus constitute a reliable proxy of the time-bound effects of irradiation on an object. We show that the detection of irradiation effects is within the reach of IR spectral resolution of the OSIRIS-REx mission and of the future James Webb Space Telescope. Our results provide a possible evidence for space weathering effects in the IR spectrum of asteroid 101955 Bennu measured by OTES/OSIRIS-REx.

Machine learning approaches for classifying lunar soils

1Gayantha R.L. Kodikara,2Lindsay J.McHenry
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113719]
1Department of Geosciences, University of Wisconsin-Milwaukee, 3209 N. Maryland Avenue, Milwaukee, WI 53211, USA
2Department of Geosciences, University of Wisconsin-Milwaukee, 3209 N. Maryland Avenue, Milwaukee, WI 53211, USA
Copyright Elsevier

We examine the ability of machine learning techniques to determine the physical and mineralogical properties of lunar soil using reflectance spectra. We use the Lunar Soil Characterization Consortium (LSCC) dataset to train and asses the predictive power of classification models based on their type (Mare soil and Highland soil), particle size, maturity, and the dominant type of pyroxene (High-Ca and Low-Ca). Nine ML algorithms including linear methods, non-linear methods, and rule-based methods (three from each) were selected, representing a range of characteristics such as simplicity, flexibility, computational complexity, and interpretability along with their ability to handle different types of data. Fifteen spectral parameters were initially introduced to the models as input features and a maximum of four features was selected as the best feature combinations to classify lunar soils based on their types, particle size, maturity, and the type of pyroxene. The Support Vector Machine with radial basis function (svmRadial) and the penalized logistic regression model (glmnet) performed well for all target variables with high accuracies. Band depths and Integrated band depths at 1 μm, 1.25 μm and 2 μm, band position of the 1 μm band, along with four band ratios (band tilt, band strength, band curvature and olivine/pyroxene) are important features for classifying soil type, grain size, maturity, and type of pyroxene from reflectance spectra. This study shows that proper preprocessing and feature engineering techniques are crucial for high performance of the predictive models.