¹A. Nathues et al. (>10)*
¹Institute for Solar System Research, Goettingen, Germany
*Find the extensive, full author and affiliation list on the publishers Website

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Reference
Nathues A et al. (2015) Sublimation in bright spots on (1) Ceres. Nature 528, 237–240
Link to Article [doi:10.1038/nature15754]

^1,2Pierre Bonnand, ^1,3Ian J. Parkinson, ^4,5Mahesh Anand
¹Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
²Department of Earth Sciences, University of Oxford, South Parks Roads, Oxford, OX1 3AN, United Kingdom
³School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Clifton BS8 1RJ, Bristol, United Kingdom
⁴Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
⁵Department of Earth Sciences, The Natural History Museum, London SW7 5DB, United Kingdom

We present the first stable chromium isotopic data from mare basalts in order to investigate the similarity between the Moon and the Earth’s mantle. A double spike technique coupled with MC-ICP-MS measurements was used to analyse 19 mare basalts, comprising high-Ti, low-Ti and KREEP-rich varieties. Chromium isotope ratios (δ53Cr) for mare basalts are positively correlated with indices of magmatic differentiation such as Mg# and Cr concentration which suggests that Cr isotopes were fractionated during magmatic differentiation. Modelling of the results provides evidence that spinel and pyroxene are the main phases controlling the Cr isotopic composition during fractional crystallisation. The most evolved samples have the lightest isotopic compositions, complemented by cumulates that are isotopically heavy. Two hypotheses are proposed to explain this fractionation: (i) equilibrium fractionation where heavy isotopes are preferentially incorporated into the spinel lattice and (ii) a difference in isotopic composition between Cr2+ and Cr3+ in the melt. However, both processes require magmatic temperatures below 1200˚C for appreciable Cr3+ to be present at the low oxygen fugacities found in the Moon (IW -1 to -2 log units). There is no isotopic difference between the most primitive high-Ti, low-Ti and KREEP basalts, which suggest that the sources of these basalts were homogeneous in terms of stable Cr isotopes. The least differentiated sample in our sample set is the low-Ti basalt 12016, characterised by a Cr isotopic composition of -0.222 ± 0.025 ‰, which is within error of the current BSE value (-0.124 ± 0.101 ‰). The similarity between the mantles of the Moon and Earth is consistent with a terrestrial origin for a major fraction of the lunar Cr. This similarity also suggests that Cr isotopes were not fractionated by core formation on the Moon.

Reference
Bonnand P, Parkinson IJ, Anand M (2015) Mass dependent fractionation of stable chromium isotopes in mare basalts: implications for the formation and the differentiation of the Moon. Geochimica et Cosmochicmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.041]
Copyright Elsevier

¹S. Yamamoto et al. (>10)*
¹Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, Tsukuba, Japan
*Find the extensive, full author and affiliation list on the publishers website

We report the global distribution of areas exhibiting no absorption features (featureless or FL) on the lunar surface, based on the reflectance spectral data set obtained by the Spectral Profiler onboard Kaguya/SELENE. We found that FL sites are located in impact basins and large impact craters in the Feldspathic Highlands Terrane (FHT), while there are no FL sites in the Procellarum regions nor the South Pole–Aitken basin. FL sites in each impact basin/crater are mainly found at the peak rings or rims, where the purest anorthosite (PAN) sites are also found. At the local scale, most of the FL and PAN points are associated with impact craters and peaks. Most of the FL spectra show a steeper (redder) continuum than the PAN spectra, suggesting the occurrence of space weathering effects. We propose that most of the material exhibiting a FL spectrum originate from space weathered PAN. Taking into account all the occurrence trends of FL sites on the Moon, we propose that both the FL and PAN materials were excavated from the primordial lunar crust during ancient basin formations below the megaregolith in the highlands. Since the FL and PAN sites are widely distributed over the lunar surface, our new data may support the existence of a massive PAN layer below the lunar surface.

Reference
Yamamoto S et al.(2015) Featureless spectra on the Moon as evidence of residual lunar primordial crust. Journal of Geophysical Research, Planets (in Press)
Link to Article [doi: 10.1002/2015JE004935]
Published by arrangement with John Wiley&Sons

^1,2J. Labidi, ¹A. Shahar, ¹C. Le Losq, ¹V.J. Hillgren, ¹B.O. Mysen, ²J. Farquhar
¹Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, USA.
²Department of Geology, University of Maryland, College Park MD, 20740, USA

The Earth’s mantle displays a subchondritic 34S/32S ratio. Sulfur is a moderately siderophile element (i.e. iron-loving), and its partitioning into the Earth’s core may have left such a distinctive isotope composition on the terrestrial mantle. In order to constrain the sulfur isotope fractionation occurring during core-mantle differentiation, high-pressure and temperature experiments were conducted with synthetic mixtures of metal and silicate melts. With the purpose to identify the mechanism(s) responsible for the S isotope fractionations, we performed our experiments in different capsules – namely, graphite and boron nitride capsules – and thus at different fO2, with varying major element chemistry of the silicate and metal fractions.

The S isotope fractionations Δ34Smetal-silicate of equilibrated metal alloys versus silicate melts is +0.2±0.1‰ in a boron-free and aluminum-poor system quenched at 1-1.5 GPa and 1650 ˚C. The isotope fractionation increases linearly with increasing boron and aluminum content, up to +1.4±0.2‰, and is observed to be independent of the silicon abundance as well as of the fO2 over ∼ 3.5 log units of variations explored here. The isotope fractionations are also independent of the graphite or nitride saturation of the metal. Only the melt structural changes associated with aluminum and boron concentration in silicate melts have been observed to affect the strength of sulfur bonding. These results establish that the structure of silicate melts has a direct influence on the S2- average bonding strengths.

These results can be interpreted in the context of planetary differentiation. Indeed, the structural environments of silicate evolve strongly with pressure. For example, the aluminum, iron or silicon coordination numbers increase under the effect of pressure. Consequently, based on our observations, the sulfur-bonding environment is likely to be affected. In this scheme, we tentatively hypothesize that S isotope fractionations between the silicate mantle and metallic core of terrestrial planetary bodies would depend on the average pressure at which their core-mantle differentiation occurred.

Reference
Labidi J, Shahara A, Le Losq C, Hillgren VJ, Mysen BO, Farquhar J (2015) Experimentally determined sulfur isotope fractionation between metal and silicate and implications for planetary Differentiation. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.12.001]
Copyright Elsevier

^1,2J. de León et al. (>10)*
¹Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205, La Laguna, Spain
²Department of Astrophysics, University of La Laguna, 38205, Tenerife, Spain
*Find the extensive, full author and affiliation list on the publishers website

The Polana-Eulalia family complex is located in the inner part of the asteroid belt, bounded by the ν6ν6 and the 3:1 resonances, where we can find another three collisional families of primitive asteroids (Erigone, Clarissa, and Sulamitis), and a low-albedo population of background objects. This region of the belt is believed to be the most likely origin of the two primitive near-Earth asteroids that are the current targets of two sample return missions: NASA’s OSIRIS-REx and JAXA’s Hayabusa 2 to asteroids (101955) Bennu and (162173) Ryugu (also known as 1999 JU3), respectively. Therefore, understanding these families will enhance the scientific return of these missions.

We present the results of a spectroscopic survey of asteroids in the region of the Polana-Eulalia family complex, and also asteroids from the background population of low-albedo, low-inclination objects. We obtained visible spectra of a total of 65 asteroids, using the 10.4m Gran Telescopio Canarias (GTC) and the 3.6m Telescopio Nazionale Galileo (TNG), both located at the El Roque de Los Muchachos Observatory, in the island of La Palma (Spain), and the 3.6m New Technology Telescope (NTT), located at the European Southern Observatory of La Silla, in Chile. From the spectral analysis of our sample we found that, in spite of the presence of distinct dynamical groups, the asteroids in this region present spectral homogeneity at visible wavelengths, showing a continuum of spectral slopes, from blue to moderately red, typical of primitive asteroids classified as B- and C-types. We conclude that visible spectra can not be used to distinguish between members of the Polana and the Eulalia families, or members of the background population.

The visible spectra of the two targets of sample return missions, asteroids Bennu and Ryugu, are compatible with the spectra of the asteroids in this region, supporting previous studies that suggested either the Polana family or the background population as the most likely origins of these NEAs.

Reference
de León J et al. (2015) Visible Spectroscopy of the Polana-Eulalia Family Complex: Spectral Homogeneity. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.11.014]

Copyright Elsevier

¹Caroline Fitoussi, ¹Bernard Bourdon,¹Xueying Wang
¹Laboratoire de Géologie de Lyon (Ecole Normale Supérieure de Lyon, CNRS and Université Claude Bernard de Lyon), ENS Lyon, 46 allée d’Italie, 69364 Lyon Cedex 07, France

The Earth formed in a swarm of Moon- to Mars-sized objects that collided together to build our planet. A large body of work has been dedicated to understanding the Earth’s composition as being made of single groups or mixtures of chondrites, however, these models cannot account for the isotopic and elemental characteristics of the Earth. Here, we test mixtures of meteorites, including achondrites, analyzed for seven isotope systems (O, Cr, Ni, Ti, Mo, Ca and Sr), to reproduce the isotope compositions of the Earth and Mars. Our Monte Carlo inversion (a numerical method based on generation of random numbers used to invert multiparameter models) yields a new compositional model where Earth and Mars come almost entirely from the same source material. This finding is in striking agreement with recent planetary formation models in which Earth and Mars formed in a common narrow zone of the protoplanetary disk with Mars being ejected to its current position which prevented further accretion. An important outcome of the model is that a significant mass fraction of the Earth could have been made of volatile depleted and refractory enriched planetary bodies such as angrites (among the oldest known achondrites). This conclusion is also in agreement with new Si isotope data in angrites which suggest that a component of angrites would help explain the difference in δ30Siδ30Si between the bulk silicate Earth and its building blocks. Our model matches all isotope compositions for both planets, reproduces the volatile element budget of Mars, and accounts for the enrichment in refractory elements of the Earth and Mars compared to chondrites.

Reference
Fitoussi C, Bourdon B, Wang X (2015) The building blocks of Earth and Mars: A close genetic link. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.11.036]

Copyright Elsevier

^1,2,3Vincenzo Stagno, ⁴Luca Bindi, ⁵Changyong Park, ⁶Sergey Tkachev, ⁶Vitali B. Prakapenka, ^1,7H.-K. Mao, ¹Russell J. Hemley, ⁸Paul J. Steinhardt,¹Yingwei Fei
¹Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, U.S.A.
²Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan
³Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan
⁴Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Florence, Italy
⁵HPCAT, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, U.S.A.
⁶Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.
⁷Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, P.R. China
⁸Department of Physics and Princeton Center for Theoretical Science, Princeton University, Princeton, New Jersey 08544, U.S.A.

Icosahedrite, the first natural quasicrystal with composition Al63Cu24Fe13, was discovered in several grains of the Khatyrka meteorite, a CV3 carbonaceous chondrite. The presence of icosahedrite associated with high-pressure phases like ahrensite and stishovite indicates formation at high pressures and temperatures due to an impact-induced shock. Previous experimental studies on the stability of synthetic icosahedral AlCuFe have either been limited to ambient pressure, for which they indicate incongruent melting at ~1123 K, or limited to room-temperature, for which they indicate structural stability up to about 35 GPa. These data are insufficient to experimentally constrain the formation and stability of icosahedrite under the conditions of high pressure and temperature that formed the Khatyrka meteorite. Here we present the results of room-temperature, high-pressure diamond-anvil cells measurements of the compressional behavior of synthetic icosahedrite up to ~50 GPa. High P-T experiments were also carried out using both laser-heated diamond-anvil cells combined with in situ synchrotron X-ray diffraction (at ~42 GPa) and multi-anvil apparatus (at 21 GPa) to investigate the structural evolution and crystallization of possible coexisting phases. The results demonstrate that the quasiperiodic order of icosahedrite is retained over the P-T range explored. We find that pressure acts to stabilize the icosahedral symmetry at temperatures much higher than previously reported. Direct solidification of AlCuFe quasicrystals from an unusual Al-Cu-rich melt is possible but it is limited to a narrow temperature range. Alternatively, quasicrystals may form after crystallization through solid-solid reactions of Al-rich phases. In either case, our results show that quasicrystals can preserve their structure even after hypervelocity impacts spanning a broad range of pressures and temperatures.

Reference
Stagno V, Bindi L, Park C, Tkachev S, Prakapenka VB, Mao H-K, Hemley RJ, Steinhardt PJ,Fei Y (2015) Quasicrystals at extreme conditions: The role of pressure in stabilizing icosahedral Al63Cu24Fe13 at high temperature. American Mineralogist 100, 2412-2418
Link to Article [doi: 10.2138/am-2015-5412]

Copyright: The Mineralogical Society of America

¹Vivian Z. Sun, ¹Ralph E. Milliken
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA

Clay minerals on Mars have commonly been interpreted as the remnants of pervasive water-rock interaction during the Noachian period (>3.7 Ga). This history has been partly inferred by observations of clays in central peaks of impact craters, which often are presumed uplifted from depth. However, combined mineralogical and morphological analyses of individual craters have shown that some central peak clays may represent post-impact, possibly authigenic processes. Here we present a global survey of 633 central peaks to assess their hydrous minerals and the prevalence of uplifted, detrital, and authigenic clays. Central peak regions are examined using high-resolution CRISM and HiRISE data to identify hydrous minerals and place their detections in a stratigraphic and geologic context. We find that many occurrences of Fe/Mg clays and hydrated silica are associated with potential impact melt deposits. Over 35% of central peak clays are not associated with uplifted rocks, thus caution must be used when inferring deeper crustal compositions from surface mineralogy of central peaks. Uplifted clay-bearing rocks suggest the martian crust hosts clays to depths of at least 7 km. We also observe evidence for increasing chloritization with depth, implying the presence of fluids in the upper portions of the crust. Our observations are consistent with widespread Noachian/Early Hesperian clay formation, but a number of central peak clays are also suggestive of clay formation during the Amazonian. These results broadly support current paradigms of Mars’ aqueous history while adding insight to global crustal and diagenetic processes associated with clay mineral formation and stability.

Reference
Sun VZ, Milliken RE (2015) Ancient and recent clay formation on Mars as revealed from a global survey of hydrous minerals in crater central Peaks. Journal of Geophysical Research, Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004918]

Published by arrangement with John Wiley&Sons

¹Hendrix, A. R., ¹Vilas, F., ¹Li, J.-Y.
¹Planetary Science Institute, Tucson, Arizona, USA

Carbon compounds are ubiquitous in the solar system but are challenging to study using remote sensing due to the mostly bland spectral nature of these species in the traditional visible-near-infrared regime. In contrast, carbonaceous species are spectrally active in the ultraviolet (UV) but have largely not been considered for studies of solar system surfaces. We compile existing UV data of carbon compounds—well-studied in contemplation of the ISM extinction “bump”—to review trends in UV spectral behavior. Thermal and/or irradiation processing of carbon species results in the loss of H and ultimately graphitization. Graphitization is shown to produce distinct spectral features in the UV, which are predicted to be more readily detected in the inner solar system, whereas outer solar system bodies are expected to be more dominated by less-processed carbon compounds. Throughout the solar system, we can thus consider a “carbon continuum” where the more evolved carbons in the inner solar system exhibit a stronger UV absorption feature and associated far-UV rise. We compare carbon spectral models with spacecraft data of two bodies from different points in the carbon continuum, Ceres and Iapetus. We find that the apparent strong far-UV upturn in Ceres’ spectrum (in the 150–200 nm range) can be explained by an anthracite-like species while Iapetus’ spectrum features a reflectance peak consistent with polycyclic aromatic hydrocarbons. We make generalized predictions for UV spectral characteristics in other regions of the solar system.

Reference
Hendrix AR, Vilas F, Li J-Y (2015) The UV signature of carbon in the solar system. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12575]

Published by Arrangement with John Wiley&Sons

¹K.K. Larsen,¹M. Schiller,¹M. Bizzarro
¹Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark

The decay of radioactive 26Al to 26Mg (half-life of 730,000 years) is postulated to have been the main energy source promoting asteroidal melting and differentiation in the nascent Solar System. High-resolution chronological information provided by the 26Al-26Mg decay system is, therefore, intrinsically linked to the thermal evolution of early-formed planetesimals. In this paper, we explore the timing and style of asteroidal differentiation by combining high-precision Mg isotope measurements of meteorites with thermal evolution models for planetesimals. In detail, we report Mg isotope data for a suite of olivine-rich [Al/Mg ∼ 0] achondritic meteorites, as well as a few chondrites. Main Group, pyroxene and the Zinder pallasites as well as the lodranite all record deficits in the mass-independent component of μ26Mg (μ26Mg∗) relative to chondrites and Earth. This isotope signal is expected for the retarded ingrowth of radiogenic 26Mg∗ in olivine-rich residues produced through partial silicate melting during 26Al decay and consistent with their marginally heavy Mg isotope composition relative to ordinary chondrites, which may reflect the early extraction of isotopically light partial melts from the source rock. We propose that their parent planetesimals started forming within ∼250,000 years of Solar System formation from a hot (>∼500 K) inner protoplanetary disk region characterized by a reduced initial (26Al/27Al)0 abundance (∼1-2 × 10-5) relative to the (26Al/27Al)0 value in CAIs of 5.25 × 10-5. This effectively reduced the total heat production and allowed for the preservation of solid residues produced through progressive silicate melting with depth within the planetesimals. These ‘non-carbonaceous’ planetesimals acquired their mass throughout an extended period (>3 Myr) of continuous accretion, thereby generating onion-shell structures of incompletely differentiated zones, consisting of olivine-rich residues, overlaid by metachondrites and undifferentiated chondritic crusts. In contrast, individual olivine crystals from Eagle Station pallasites record variable μ26Mg∗ excesses, suggesting that these crystals captured the 26Mg∗ evolution of a magmatic reservoir controlled by fractional crystallization processes during the lifespan of 26Al. Similar to previous suggestions based on isotopic evidence, we suggest that Eagle Station pallasites formed from precursor material similar in composition to carbonaceous chondrites from a cool outer protoplanetary disk region characterized by (26Al/27Al)0 ⩾2.7 × 10-5. Protracted planetesimal accretion timescales at large orbital distances, with onset of accretion 0.3-1 Myr post-CAIs, may have resulted in significant radiative heat loss and thus efficient early interior cooling of slowly accreting ‘carbonaceous’ planetesimals.

Reference
Larsen KK, Schiller M, Bizzarro M (2015) Accretion timescales and style of asteroidal differentiation in an 26Al-poor protoplanetary disk. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.10.036]
Copyright Elsevier