¹Daniela Rommel, ¹Arne Grumpe, ¹Marian Patrik Felder, ¹Christian Wöhler, ²Urs Mall,
³Andreas Kronz
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.10.029]
¹Image Analysis Group, TU Dortmund University, Otto-Hahn-Str. 4, D–44227 Dortmund, Germany
²Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, D–37077 Göttingen, Germany
³Geowissenschaftliches Zentrum Göttingen, Goldschmidtstr. 1, D–37077 Göttingen, Germany
Copyright Elsevier

While the interpretation of spectral reflectance data has been widely applied to detect the presence of minerals, determining and quantifying the abundances of minerals contained by planetary surfaces is still an open problem. With this paper we address one of the two main questions arising from the spectral unmixing problem. While the mathematical mixture model has been extensively researched, considerably less work has been committed to the selection of endmembers from a possibly huge database or catalog of potential endmembers. To solve the endmember selection problem we define a new spectral similarity measure that is not purely based on the reconstruction error, i.e. the squared difference between the modeled and the measured reflectance spectrum. To select reasonable endmembers, we extend the similarity measure by adding information extracted from the spectral absorption bands. This will allow for a better separation of spectrally similar minerals. Evaluating all possible subsets of a possibly very large catalog that contain at least one endmember leads to an exponential increase in computational complexity, rendering catalogs of 20–30 endmembers impractical. To overcome this computational limitation, we propose the usage of a genetic algorithm that, while initially starting with random subsets, forms new subsets by combining the best subsets and, to some extent, does a local search around the best subsets by randomly adding a few endmembers. A Monte-Carlo simulation based on synthetic mixtures and a catalog size varying from three to eight endmembers demonstrates that the genetic algorithm is expected to require less combinations to be evaluated than an exhaustive search if the catalog comprises 10 or more endmembers. Since the genetic algorithm evaluates some combinations multiple times, we propose a simple modification and store previously evaluated endmember combinations. The resulting algorithm is shown to never require more function evaluations than a full exhaustive search and the number of required function evaluations appears to grow less than exponentially. It thus requires considerably less time than an exhaustive search because the number of function evaluations is a hardware independent measure of the computational complexity. To evaluate the spectral similarity measure, we created a spectral reflectance catalog of selected lunar analog minerals. Based on precisely prepared mixtures of two to three components, we show that the proposed spectral similarity measure selects less false endmembers from the catalog than a similarity measure that is purely based on the reconstruction error.

¹Olivier Verhoeven, ¹Pierre Vacher
Planetary and Space Science (in Press) Link to Article [http://dx.doi.org/10.1016/j.pss.2016.10.005]
¹Laboratoire de Planétologie et Géodynamique, UMR-CNRS 6112, Université de Nantes, 2 rue de la Houssinière 44322 Nantes, France

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^1,2,3Rebecca A. Fischer, ¹Andrew J. Campbell, ¹Fred J. Ciesla
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.10.025]
¹University of Chicago, Department of the Geophysical Sciences, 5734 S Ellis Ave, Chicago, IL 60637, USA
²National Museum of Natural History, Smithsonian Institution, PO Box 37012, MRC 119, Washington, DC 20013-7012, USA
³University of California Santa Cruz, Department of Earth and Planetary Sciences, 1156 High St, Santa Cruz, CA 95064, USA
Copyright Elsevier

The Earth and other terrestrial planets formed through the accretion of smaller bodies, with their core and mantle compositions primarily set by metal–silicate interactions during accretion. The conditions of these interactions are poorly understood, but could provide insight into the mechanisms of planetary core formation and the composition of Earth’s core. Here we present modeling of Earth’s core formation, combining results of 100 N-body accretion simulations with high pressure–temperature metal–silicate partitioning experiments. We explored how various aspects of accretion and core formation influence the resulting core and mantle chemistry: depth of equilibration, amounts of metal and silicate that equilibrate, initial distribution of oxidation states in the disk, temperature distribution in the planet, and target:impactor ratio of equilibrating silicate. Virtually all sets of model parameters that are able to reproduce the Earth’s mantle composition result in at least several weight percent of both silicon and oxygen in the core, with more silicon than oxygen. This implies that the core’s light element budget may be dominated by these elements, and is consistent with ≤1–2 wt% of other light elements. Reproducing geochemical and geophysical constraints requires that Earth formed from reduced materials that equilibrated at temperatures near or slightly above the mantle liquidus during accretion. The results indicate a strong tradeoff between the compositional effects of the depth of equilibration and the amounts of metal and silicate that equilibrate, so these aspects should be targeted in future studies aiming to better understand core formation conditions. Over the range of allowed parameter space, core and mantle compositions are most sensitive to these factors as well as stochastic variations in what the planet accreted as a function of time, so tighter constraints on these parameters will lead to an improved understanding of Earth’s core composition.

^aNicole X. Nie, ^aNicolas Dauphas, ^bRichard C. Greenwood
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.09.035]

^aOrigins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, United States
^bPlanetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
Copyright Elsevier

Banded iron formations (BIFs) contain appreciable amounts of ferric iron (Fe³⁺). The mechanism by which ferrous iron (Fe²⁺) was oxidized into Fe³⁺ in an atmosphere that was globally anoxic is highly debated. Of the three scenarios that have been proposed to explain BIF formation, photo-oxidation by UV photons is the only one that does not involve life (the other two are oxidation by O₂ produced by photosynthesis, and anoxygenic photosynthesis whereby Fe²⁺ is directly used as electron donor in place of water). We experimentally investigated iron and oxygen isotope fractionation imparted by iron photo-oxidation at a pH of 7.3. The iron isotope fractionation between precipitated Fe³⁺-bearing lepidocrocite and dissolved Fe²⁺ follows a Rayleigh distillation with an instantaneous ⁵⁶Fe/⁵⁴Fe fractionation factor of +1.2‰. Such enrichment in the heavy isotopes of iron is consistent with the values measured in BIFs. We also investigated the nature of the mass-fractionation law that governs iron isotope fractionation in the photo-oxidation experiments (i.e., the slope of the δ⁵⁶Fe–δ⁵⁷Fe relationship). The experimental run products follow a mass-dependent law corresponding to the high-T equilibrium limit. The fact that a ∼3.8 Gyr old BIF sample (IF-G) from Isua (Greenland) falls on the same fractionation line confirms that iron photo-oxidation in the surface layers of the oceans was a viable pathway to BIF formation in the Archean, when the atmosphere was largely transparent to UV photons.

Our experiments allow us to estimate the quantum yield of the photo-oxidation process (∼0.07 iron atom oxidized per photon absorbed). This yield is used to model iron oxidation on early Mars. As the photo-oxidation proceeds, the aqueous medium becomes more acidic, which slows down the reaction by changing the speciation of iron to species that are less efficient at absorbing UV-photons. Iron photo-oxidation in centimeter to meter-deep water ponds would take months to years to complete. Oxidation by O₂ in acidic conditions would be slower. Iron photo-oxidation is thus likely responsible for the formation of jarosite–hematite deposits on Mars, provided that shallow standing water bodies could persist for extended periods of time.

The oxygen isotopic composition of lepidocrocite precipitated from the photo-oxidation experiment was measured and it is related to the composition of water by mass-dependent fractionation. The precipitate-fluid ¹⁸O/¹⁶O isotope fractionation of ∼+6‰ is consistent with previous determinations of oxygen equilibrium fraction factors between iron oxyhydroxides and water.

^aJ.J. Bellucci, ^aM.J. Whitehouse, ^bT. John, ^a,cA.A. Nemchin, ^aJ.F. Snape, ^cP.A. Bland, ^cG.K. Benedix
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.09.028]

^aDepartment of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
^bInstitut für Geologische Wissenschaften, Freie Universität Berlin, Malteser Str. 74-100, 12249 Berlin, Germany
^cDepartment of Applied Geology, Curtin University, Perth, WA 6845, Australia
Copyright Elsevier

The Cl isotopic compositions and halogen (Cl, F, Br, and I) abundances in phosphates from eight Martian meteorites, spanning most rock types and ages currently available, have been measured in situ   by Secondary Ion Mass Spectrometry (SIMS). Likewise, the distribution of halogens has been documented by x-ray mapping. Halogen concentrations range over several orders of magnitude up to some of the largest concentrations yet measured in Martian samples or on the Martian surface, and the inter-element ratios are highly variable. Similarly, Cl isotope compositions exhibit a larger range than all pristine terrestrial igneous rocks. Phosphates in ancient (>4 Ga) meteorites (orthopyroxenite ALH 84001 and breccia NWA 7533) have positive δ³⁷Cl anomalies (+1.1 to +2.5‰). These samples also exhibit explicit whole rock and grain scale evidence for hydrothermal or aqueous activity. In contrast, the phosphates in the younger basaltic Shergottite meteorites (<600 Ma) have negative δ³⁷Cl anomalies (−0.2 to −5.6‰). Phosphates with the largest negative δ³⁷Cl anomalies display zonation in which the rims of the grains are enriched in all halogens and have significantly more negative δ³⁷Cl anomalies suggestive of interaction with the surface of Mars during the latest stages of basalt crystallization. The phosphates with no textural, major element, or halogen enrichment evidence for mixing with this surface reservoir have an average δ³⁷Cl of −0.6‰, supporting a similar initial Cl isotope composition for Mars, the Earth, and the Moon. Oxidation and reduction of chlorine are the only processes known to strongly fractionate Cl isotopes, both positively and negatively, and perchlorate has been detected in weight percent concentrations on the Martian surface. The age range and obvious mixing history of the phosphates studied here suggest perchlorate formation and halogen cycling via brines, which have been documented on the Martian surface, has been active throughout Martian history.

^a,bJacob A. Richardson, ^aJames A. Wilson, ^aCharles B. Connor, ^bJacob E. Bleacher, Koji Kiyosugi^c
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.09.040]

^aSchool of Geosciences, University of South Florida, Tampa, FL, USA
^bPlanetary Geology, Geophysics, and Geochemistry Laboratory, Code 698, NASA Goddard Space Flight Center, Greenbelt, MD, USA
^cOrganization for Advanced and Integrated Research, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
Copyright Elsevier

Magmatism and volcanism have evolved the Martian lithosphere, surface, and climate throughout the history of Mars. Constraining the rates of magma generation and timing of volcanism on the surface clarifies the ways in which magma and volcanic activity have shaped these Martian systems. The ages of lava flows on other planets are often estimated using impact crater counts, assuming that the number and size-distribution of impact craters per unit area reflect the time the lava flow has been on the surface and exposed to potential impacts. Here we show that impact crater age model uncertainty is reduced by adding stratigraphic information observed at locations where neighboring lavas abut each other, and demonstrate the significance of this reduction in age uncertainty for understanding the history of a volcanic field comprising 29 vents in the 110-km-diameter caldera of Arsia Mons, Mars. Each vent within this caldera produced lava flows several to tens of kilometers in length; these vents are likely among the youngest on Mars, since no impact craters in their lava flows are larger than 1 km in diameter. First, we modeled the age of each vent with impact crater counts performed on their corresponding lava flows and found very large age uncertainties for the ages of individual vents, often spanning the estimated age for the entire volcanic field. The age model derived from impact crater counts alone is broad and unimodal, with estimated peak activity in the field around 130 Ma. Next we applied our volcano event age model (VEAM), which uses a directed graph of stratigraphic relationships and random sampling of the impact crater age determinations to create alternative age models. Monte Carlo simulation was used to create 10,000 possible vent age sets. The recurrence rate of volcanism is calculated for each possible age set, and these rates are combined to calculate the median recurrence rate of all simulations. Applying this approach to the 29 volcanic vents, volcanism likely began around 200–300 Ma then first peaked around 150 Ma, with an average production rate of 0.4 vents per Myr. The recurrence rate estimated including stratigraphic data is distinctly bimodal, with a second, lower peak in activity around 100 Ma. Volcanism then waned until the final vents were produced 10–90 Ma. Based on this model, volume flux is also bimodal, reached a peak rate of 1–8 km³ Myr⁻¹by 150 Ma and remained above half this rate until about 90 Ma, after which the volume flux diminished greatly. The onset of effusive volcanism from 200–150 Ma might be due to a transition of volcanic style away from explosive volcanism that emplaced tephra on the western flank of Arsia Mons, while the waning of volcanism after the 150 Ma peak might represent a larger-scale diminishing of volcanic activity at Arsia Mons related to the emplacement of flank apron lavas.

^aQueenie H. S. Chan, ^aMichael E. Zolensky, ^bRobert.J. Bodnar, ^bCharles Farley, ^cJacob C. H. Cheung
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.10.048]

^aNASA Johnson Space Center, Houston, Texas 77058, USA
^bDepartment of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA
^cMet Office, Exeter, UK
Copyright Elsevier

Carbonates record information regarding the timing, nature and conditions of the fluids circulating through asteroid parent bodies during aqueous alteration events. Determining carbonate abundances and their relationships with organic matter improves our understanding of the genesis of major carbonaceous components in chondritic materials. In this study, five CM2 carbonaceous chondrites (CM2.2 Nogoya, CM2.3 Jbilet Winselwan, CM2.5 Murchison, CM2 Santa Cruz, and CM2TII Wisconsin Range 91600) were studied with Raman spectroscopy. Carbonates were identified in these meteorite samples by the distinctive Raman band in the ∼1100 cm^-1 region, representing the symmetric stretching vibration mode (v₁) of the (CO₃)^2- anion. Carbonates identified in the meteorite samples are all calcite, with the exception of a single dolomite grain in Nogoya. The v₁ positions of the CM calcites are 2−3 cm^-1 higher than in pure calcite, which suggests that they contain significant impurity cations. Typical graphitic first-order D and G bands were identified in the meteorite matrix as well as in ∼25% of the analyzed carbonate grains. From the Raman results, we postulate that the carbonates might not have formed under equilibrium conditions from a single fluid. The first generation of carbonate is interpreted to have formed from highly oxidized fluids that led to the oxidation of organic matter (OM) and produced carbonates that are OM-barren. The second generation of carbonate was formed from a more evolved aqueous fluid with the presence of OM. The Raman parameters of the organics in carbonates clearly deviate from the matrix OM which suggests that the carbonate organics contain very different carbonaceous components that are distinct from the typical amorphous OM of the CM matrix. The occurrence of different generations of carbonate in close proximity may be partly responsible for the wide range in estimated ages of carbonates in carbonaceous chondrites reported in previous studies.

¹Karin I. Öberg and ²Edwin A. Bergin
The Astrophysical Journal Letters 831,L19 Link to Article [http://dx.doi.org/10.3847/2041-8205/831/2/L19]
¹Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
²Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Avenue, Ann Arbor, MI 48109, USA

The compositions of nascent planets depend on the compositions of their birth disks. In particular, the elemental compositions of gas giant gaseous envelopes depend on the elemental compositions of the disk gas from which the envelopes are accreted. Previous models have demonstrated that sequential freeze-out of O- and C-bearing volatiles in disks will result in supersolar C/O ratios and subsolar C/H ratios in the gas between water and CO snowlines. However, this result does not take into account the expected grain growth and radial drift of pebbles in disks, and the accompanying redistribution of volatiles from the outer to the inner disk. Using a toy model we demonstrate that when drift is considered, CO is enhanced between the water and CO snowline, resulting in both supersolar C/O and C/H ratios in the disk gas in the gas giant formation zone. This result appears to be robust for the disk model as long as there is substantial pebble drift across the CO snowline, and the efficiency of CO vapor diffusion is limited. Gas giants that accrete their gaseous envelopes exterior to the water snowline and do not experience substantial core-envelope mixing may thus feature both superstellar C/O and C/H ratios in their atmospheres. Pebble drift will also affect the nitrogen and noble gas abundances in the planet-forming zones, which may explain some of Jupiter’s peculiar abundance patterns.

¹L. Gavilan (>10)*
Astronomy & Astrophysics  595, A102  Link to Article [http://dx.doi.org/10.1051/0004-6361/201628764]
¹LESIA, Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, 5 Place J. Janssen, 92195 Meudon Principal Cedex, France
*Find the extensive, full author and affiliation list on the publishers website

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^aEmily H. Mitchell, ^bUjjwal Rautb, ^bBenjamin D. Teolis, ^aRaúl A. Baragiola
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.11.004]

^aLaboratory for Astrophysics and Surface Physics, Thornton Hall B 113, University of Virginia, Charlottesville, VA 22904
^bSouthwest Research Institute, Space Science and Engineering Division, 6220 Culebra Road, San Antonio, TX 78238
Copyright Elsevier

We have investigated the effects of porosity on the crystallization kinetics of amorphous solid water (ASW). Porosity in ASW films, condensed from the vapor phase at varying incidences at 10 K, was characterized using ultraviolet-visible interferometry and quartz crystal microgravimetry. The films were heated to crystallization temperatures between 130 and 141 K, resulting in partial pore compaction. The isothermal phase transformation was characterized using transmission infrared spectroscopy to monitor the time evolution of the 3.1-µm O-H stretch absorption band. We find that ASW crystallization unfolds in two distinct stages. The first stage, responsible for ∼10% transformation, is initiated from nucleation at the external surface. The dominant second stage begins with nucleation at the internal pore surfaces and completes the transformation of the film at a faster rate compared to the first stage. A key finding is that porosity has major influence on crystallization kinetics; a film with five-times-higher porosity was observed to crystallize ∼15 times faster, compared to the less porous counterpart. We extrapolate our results to predict crystallization times for amorphous ices condensed on Europa’s surface from plume sources, as recently observed by the Hubble Space Telescope.