¹B. E. Strauss,¹J. M. Feinberg,^2,3C. L. Johnson
Journal of Geophysical Research, Planets Link to Article [DOI: 10.1002/2016JE005054]
¹Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota, USA
²Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, VancouverBC, Canada
³Planetary Science Institute, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

Mercury and Earth are the only inner solar system planets with active, internally generated dynamo magnetic fields. The MESSENGER mission recently detected magnetic fields on Mercury that are consistent with lithospheric magnetization. We investigate the physical and chemical environment of Mercury’s lithosphere, past and present, to establish the conditions under which magnetization may have been acquired and modified. Three factors are particularly crucial to the determination of crustal composition and iron mineralogy: redox conditions in the planet’s crust and mantle, the iron content of the lithosphere, and, for any remanent magnetization, the temperature profile of the lithosphere and its evolution over time. We explore potential mechanisms for remanence acquisition and alteration on Mercury, whose surface environment is both hot and highly reducing. The long-term thermal history of Mercury’s crust plays an important role in the longevity of any remanent crustal magnetization, which may be subject to remagnetization through thermal, viscous, and shock mechanisms. This thermal and compositional framework is used both to constrain plausible candidate minerals that could carry magnetic remanence on Mercury and to evaluate their capacity to acquire and retain sufficient magnetization to be detectable from satellite orbit. We propose that iron metal and its alloys are likely to be the dominant contributors to induced and remanent magnetization in Mercury’s lithosphere, with additional contributions from iron silicides, sulfides, and carbides.

¹Jorge A.G. Davalos, ¹Jorge Márcio Carvano, ²Julio Blancob
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.10.022]
¹Observatório Nacional, rua Gal. José Cristino 77, São Cristóvão, 20921-400, Rio de Janeiro, Brazil
²Observatorio Astronómico del Instituto de Profesores Artigas, Avenida del Libertador 2025, Montevideo, Uruguay
Copyright Elsevier

Radiative transfer models in particulate media (Hapke, 1981 and Hapke, 1993; 2012b; Shkuratov et al., 1999) are the most versatile tool that can be used to retrieve both composition and surface physical properties from observation of asteroids and other atmosphereless bodies of the Solar System. One caveat is that these methods require as input a sufficiently comprehensive set of optical constants of suitable template materials. These optical constants are the real and imaginary parts of the refractive indexes of the material as function of wavelength, and have to be derived from laboratory measurements of samples of minerals and meteorites. Optical constants can be calculated from a variety of types of measurements, and each has its problems and limitations. In particular, a problem with the determination of optical constants from measurement of reflectance is that the measurements need to be themselves interpreted using radiative transfer models. This is an issue because the number of parameters used in the most accurate versions of the radiative transfer models is large, and for most of the samples many of these parameters were not measured independently. As a result, attempts in the literature to retrieve optical constants from reflectance measurements tend to assume values for the unknown parameters, which can lead to uncertainties in the retrieved optical constants that can be difficult to quantify. In this work we propose a numerical method that allows the simultaneous inversion of the optical constant and the model parameters. This model is then applied to a set of reflectance spectra of 5 HED meteorites from the RELAB database that were measured with the same setup for samples with several particle size intervals. Our results indicate that our method is able to retrieve optical constants which are able to reproduce the measured reflectance of the samples over a large range (View the MathML source25−500μm) of particle diameters. It is also found that the solutions obtained in this way are non-unique, in the sense that many combination of the model parameters can yield different sets of optical constants that fit equally well the reflectance spectra. Thus, in the absence of the independent determination of at least some of the model parameter the method is unable to decide which solution correspond to the physical optical constants of the materials. Even so, the dispersion of the model parameters (in particular effective particle diameter and porosity) for acceptable solutions (defined as the ones that reproduce the measured reflectance spectra at all size range with residues smaller than 10%) is within a radius of around 30% of the value of the best fit solution for each parameter. Given the ability of the the optical constants derived with this method to reproduce the sample spectra over a large range of particle sizes, they can be used without other restriction to assess if a given meteorite assemblage is contributing to the observed spectra of asteroids. However, quantitative informations that can also be derived using these optical constants, like particle sizes, porosity and volumetric fractions of each end-member in a mixture should be regarded only as rough estimates.

¹Vicki Darlington,^1,2Tom Blenkinsop,¹Paul Dirks,³Jess Salisbury,³Andrew Tomkins
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12734]
¹College of Science and Engineering, James Cook University, Townsville, Queensland, Australia
²School of Earth and Ocean Science, Cardiff University, Cardiff, UK
³School of Geosciences, Monash University, Melbourne, Victoria, Australia
Published by arrangement with John Wiley & Sons

The Lawn Hill Impact Structure (LHIS) is located 250 km N of Mt Isa in NW Queensland, Australia, and is marked by a highly deformed dolomite annulus with an outer diameter of ~18 km, overlying low metamorphic grade siltstone, sandstone, and shale, along the NE margin of the Georgina Basin. This study provides detailed field observations from sections of the Lawn Hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the Lawn Hill impact. 40Ar-39Ar dating of impact-related melt particles provides a time of impact in the Ordovician (472 ± 8 Ma) when the Georgina Basin was an active depocenter. The timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed Thorntonia Limestone at the time of impact. The exposed annulus is remarkably well preserved, with preservation attributed to postimpact sedimentation. The LHIS has an atypical crater morphology with no central uplift. The heterogeneous target materials at Lawn Hill were probably low-strength, porous, and water-saturated, with all three properties affecting the crater morphology. The water-saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus, and restricted melt production. The impact timing raises the possibility that the Lawn Hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid-Ordovician (470 ± 6 Ma).

¹E. Chassefière,^2,3J. Lasue,⁴B. Langlais,⁵Y. Quesnel
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12784]
¹GEOPS, Univ. Paris-Sud, CNRS, Universite Paris-Saclay, Rue du Belvedere, Bat. 504-509, 91405 Orsay, France
²Universite de Toulouse, UPS-OMP, IRAP, Toulouse, France
³CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France
⁴LPG-CNRS & Universite de Nantes, Nantes, France
⁵Aix-Marseille Universite, CNRS, IRD, CEREGE UM34, Aix-en-Provence, France
Published by arrangement with John Wiley & Sons

CH4 has been observed on Mars both by remote sensing and in situ during the past 15 yr. It could have been produced by early Mars serpentinization processes that could also explain the observed Martian remanent magnetic field. Assuming a cold early Mars, a cryosphere could trap such CH4 as clathrates in stable form at depth. The maximum storage capacity of such a clathrate cryosphere has been recently estimated to be 2 × 1019 to 2 × 1020 moles of methane. We estimate how large amounts of serpentinization-derived CH4 stored in the cryosphere have been released into the atmosphere during the Noachian and the early Hesperian. Due to rapid clathrate dissociation and photochemical conversion of CH4 to H2, these episodes of massive CH4 release may have resulted in transient H2-rich atmospheres, at typical levels of 10–20% in a background 1–2 bar CO2 atmosphere. The collision-induced heating effect of H2 present in such an atmosphere has been shown to raise the surface temperature above the water freezing point. We show how local and rapid destabilization of the cryosphere can be induced by large events (such as the Hellas Basin or Tharsis bulge formation) and lead to such releases. Our results show that the early Mars cryosphere had a sufficient CH4 storage capacity to have maintained H2-rich transient atmospheres during a total time period up to several million years or tens of million years, having potentially contributed to the formation of valley networks during the Noachian/early Hesperian.

^1,2Ruslan A. Mendybaev, ³Curtis D. Williams, ⁴Michael J. Spicuzza, ^1,2Frank M. Richter, ⁴John W. Valley, ^1,2Alexei V. Fedkin, ³Meenakshi Wadhwa
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.08.034]
¹Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, United States
²Chicago Center for Cosmochemistry, University of Chicago, Chicago, IL 60637, United States
³School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, United States
⁴Department of Geoscience, University of Wisconsin, Madison, WI 53706, United States
Copyright Elsevier

We present the results of laboratory experiments in which a forsterite-rich melt estimated to be a potential precursor of Allende CMS-1 FUN CAI was evaporated into vacuum for different lengths of time at 1900°C. The evaporation of this melt resulted in residues that define trajectories in chemical as well as magnesium, silicon and oxygen isotopic composition space and come very close to the measured properties of CMS-1. The isotopic composition of the evaporation residues was also used to determine the kinetic isotopic fractionation factors [α2,1 (vapor-melt) defined as the ratio of isotopes 2 and 1 of a given element in the evaporating gas divided by their ratio in the evaporating source] for evaporation of magnesium (α25,24 for 25Mg/24Mg), silicon (α29,28 for 29Si/28Si) and oxygen (α18,16 for 18O/16O) from the forsterite-rich melt at 1900°C. The values of α25,24 = 0.98383±0.00033 and α29,28 = 0.99010±0.00038 are essentially independent of change in the melt composition as evaporation proceeds. In contrast, α18,16 changes from 0.9815±0.0016 to ∼ 0.9911 when the residual melt composition changes from forsteritic to melilitic. Using the determined values of α25,24 and α29,28 and present-day bulk chemical composition of the CMS-1, the composition of the precursor of the inclusion was estimated to be close to the clinopyroxene+spinel+forsterite assemblage condensed from a solar composition gas. The correspondence between the chemical composition and isotopic fractionation of experimental evaporation residues and the present-day bulk chemical and isotopic compositions of CMS-1 is evidence that evaporation played a major role in the chemical evolution of CMS-1.

¹Katherine H. Joy, ²Ian A. Crawford, ¹Natalie M. Curran, ^3,4Michael Zolensky, ⁵Amy F. Fagan, ³David A. Kring
Earth, Moon, and Planets (in Press) Link to Article [DOI: 10.1007/s11038-016-9495-0]
¹School of Earth and Environmental Sciences, University of Manchester, Manchester, UK
²Department of Earth and Planetary Sciences, Birkbeck College, University of London, London, UK
³Center for Lunar Science and Exploration, The Lunar and Planetary Institute – USRA, Houston, USA
⁴ARES, NASA Johnson Space Center, Houston, USA
⁵Geosciences and Natural Resources Department, 331 Stillwell Building, Western Carolina University, Cullowhee, USA

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¹Yuhei Umeda, ¹Atsushi Takase, ¹Nao Fukunaga ^1,2Toshimori Sekine, ³Takamichi Kobayashi, ⁴Yoshihiro Furukawa, ⁴Takeshi Kakegawa
Physics and Chemistry of Minerals (in Press) Link to Article [doi:10.1007/s00269-016-0849-y]
¹Department of Earth and Planetary Systems Science Hiroshima University Higashi-Hiroshima Japan
²Center for High Pressure Science and Technology Advanced Research Pudong People’s Republic of China
³National Institute for Materials Science Tsukuba Japan
⁴Department of Earth and Planetary Materials Science Tohoku University Sendai Japan

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^1,2,3Jangmi Han, ¹Adrian J. Brearley
Geochmica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.10.014]
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
²Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, USA
³NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA
Copyright Elsevier

We have studied four melilite-rich calcium-aluminum-rich inclusions (CAIs) from the Allan Hills A77307 CO3.0 chondrite using transmission electron microscopy with the focused ion beam sample preparation technique. This type of CAI represents one of the dominant types of refractory inclusions in CO3 chondrites. Individual melilite-rich CAIs 04 to 07 record complex formational histories involving high-temperature gas-solid condensation that occurred under both equilibrium and disequilibrium conditions. CAI 04 contains two texturally- and compositionally-distinct occurrences of perovskite: fine-grained perovskite within a melilite-rich core and aggregates of perovskite grains that surround the core. The textural and compositional differences suggest that the perovskite aggregates condensed after core formation under different conditions. CAI 05 consists of a compact melilite-rich core surrounded by a porous mantle, and likely formed by at least two different condensation events under different conditions. In CAI 06, complex intergrowth layers surrounding a melilite-rich core indicate reaction of spinel and melilite with a nebular gas to form Al-Ti-rich diopside following core formation. CAI 07 is dominated by melilite with a narrow compositional range and equilibrated textures, suggesting its formation by condensation over a limited temperature range. Collectively, we infer that the melilite-rich inclusions formed by a generalized sequence of high-temperature gas-solid condensation that involved: (1) formation of CAI cores by aggregation of primary equilibrium condensate grains (i.e., perovskite, spinel, and melilite), (2) back-reactions of the primary core minerals with a nebular gas under disequilibrium conditions, forming diopside that evolves in composition from Al-Ti-rich at the interface with the inclusion core to Al-Ti-poor on the exterior of the inclusions. The change in formation conditions may have been achieved by transport and injection of the core materials into a region of a partially-condensed gas that still contained refractory elements in the gas phase.

¹Patrick N. Peplowski
Planetary and Space Science (in Press) Link to Article [http://dx.doi.org/10.1016/j.pss.2016.10.006]
¹The Johns Hopkins University Applied Physics Laboratory, Laurel MD 20723

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^1,2Timothy Gregory,¹Katherine Helen Joy,³Stanislav Strekopytov,¹Natalie Mary Curran
Meteoritics & Planetary Science (in Press) Link to Article [10.1111/maps.12782]
¹School of Earth and Environmental Science, University of Manchester, Manchester, UK
²School of Earth Sciences, University of Bristol, Bristol, UK
³Imaging and Analysis Centre, The Natural History Museum, London, UK
Published by arrangement with John Wiley & Sons

The howardite-eucrite-diogenite (HED) clan of meteorites, which most likely originate from the asteroid Vesta, provide an opportunity to combine in-depth sample analysis with the comprehensive remote-sensing data set from NASA’s recent Dawn mission. Miller Range (MIL) 11100, an Antarctic howardite, contains diverse rock and mineral fragments from common HED lithologies (diogenites, cumulate eucrites, and basaltic eucrites). It also contains a rare pyroxferroite-bearing lithology—not recognized in HED until recently—and rare Mg-rich (Fo86-91) olivine crystals that possibly represent material excavated from the Vestan mantle. Clast components underwent different histories of thermal and impact metamorphism before being incorporated into this sample, reflecting the diversity in geological histories experienced by different parts of Vesta. The bulk chemical composition and petrography of MIL 11100 suggest that it is akin to the fragmental howardite meteorites. The strong lithological heterogeneity across this sample suggests that at least some parts of the Vestan regolith show heterogeneity on the mm-scale. We combine the outcomes of this study with data from NASA’s Dawn mission and hypothesize on possible source regions for this meteorite on the surface of Vesta.