Kun Wang^a, James M.D. Day^b, Randy L. Korotev^a, Ryan A. Zeigler^c and Frédéric Moynier^a,d

^aDepartment of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
^bGeosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA
^cAstromaterials Research and Explorations Science Directorate, Acquisition and Curation, NASA Johnson Space Center, 2101 NASA Road 1, Houston, TX 77058, USA
^dInstitut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, 1 rue Jussieu, 75238, Paris Cedex 05, France

New Fe isotope data of feldspar-rich meteorites Graves Nunataks 06128 and 06129 (GRA 06128/9) reveal that they are the only known examples of crustal materials with isotopically light Fe isotope compositions (; δ  ⁵⁶Fe is defined as the per mille   deviation of a sample’s ⁵⁶Fe/⁵⁴Fe ratio from the IRMM-014 standard) in the Solar System. In contrast, associated brachinites, as well as brachinite-like achondrites, have Fe isotope compositions () that are isotopically similar to carbonaceous chondrites and the bulk terrestrial mantle. In order to understand the cause of Fe isotope variations in the GRA 06128/9 and brachinite parent body, we also report the Fe isotope compositions of metal, silicate and sulfide fractions from three ordinary chondrites (Semarkona, Kernouve, Saint-Séverin). Metals from ordinary chondrites are enriched in the heavier isotopes of Fe (average ), sulfide fractions are enriched in the lighter isotopes of Fe (average ), and the δ  ⁵⁶Fe values of the silicates are coincident with that of the bulk rock (average ).
The enrichment of light isotopes of Fe isotopes in GRA 06128/9 is consistent with preferential melting of sulfides in precursor chondritic source materials leading to the formation of Fe–S-rich felsic melts. Conceptual models show that melt generation to form a GRA 06128/9 parental melt occurred prior to the onset of higher-temperature basaltic melting (<1200 °C) in a volatile-rich precursor and led to the generation of buoyant felsic melt with a strong Fe–S signature. These models not only reveal the origin of enrichment in light isotopes of Fe for GRA 06128/9, but are also consistent with petrological and geochemical observations, experimental studies for the origin of Fe–S-rich felsic melts, and for the cessation of early melting on some asteroidal parent bodies because of the effective removal of the major radioactive heat-source, ²⁶Al. The mode of origin for GRA 06128/9 contrasts strongly with crust formation on Earth, the Moon, Mars and other asteroids, where mantle differentiation and/or oxygen activity are the major controls on crustal Fe isotope compositions.

Reference
Wang K, Day JMD, Korotev RL, Zeigler RA and Moynier F (2014) Iron isotope fractionation during sulfide-rich felsic partial melting in early planetesimals. Earth and Planetary Science Letters 392:124–132.
[doi:10.1016/j.epsl.2014.02.022]
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Gonzalo Tancredi

Departamento de Astronomí a, Facultad de Ciencias, Iguá 4225, 11400. Montevideo, URUGUAY

The classification criterion between asteroids and comets has evolved in recent decades, but the main distinction remains unchanged. Comets present gas and dust ejection from the surface at some point of their orbits, therefore, these objects are considered to be active. On the other hand, asteroids do not show any kind of large scale gas and dust ejection, they are inert. Nevertheless, this classification scheme is impractical when we have more than 500,000 asteroids already discovered. In addition, comets are not active all along their orbits. In order for a comet to display activity at present or in the recent past in the inner region of the Solar System (heliocentric distance <2AU), the cometary orbit must be unstable in the time scale on the order of ten thousands of years; otherwise, the object should have completely consumed its volatile component. Close encounters with the most massive planets is the only mechanism that could produce ”macroscopic” instabilities on a short time scale. The macroscopic changes in the orbital elements can be detected in a numerical integration of the dynamical evolution of the object over a time scale of several thousand years. This procedure to identify asteroids in cometary-like orbits is also impractical because it would require months of computing time. Therefore, a classification scheme based on the orbital elements to identify the border cases between the asteroid and comet populations is urgently required.
We present a criterion to classify asteroids and comets and to find the border case based on the Tisserand’s parameter, the Minimum Orbital Intersection Distance (MOID), and considering some information regarding the aphelion and perihelion distances. Objects in mean-motion are disregarded. After applying a filter to the sample of over half a million asteroids already discovered to select the precise orbits and to the sample of 487 short-period comets, we apply the proposed classification criterion. The resulting sample consists of ~331 Asteroids in Cometary Orbits (ACOs). The ACOs are further classified in subclasses similar to the cometary classification. There are 436 Jupiter Family Comets and 203 ACOs of the Jupiter Family type. This new criterion is more strict that the criteria used by other authors to identify ACOs; nonetheless, with the new criterion we ensure that the ACOs have a chaotic dynamical evolution similar to the periodic comets. The discovered dormant or extinct comets seems, if they exist at all, to be a small fraction of the active comets.
We also analyse the available photometric data of ACOs to identify possible large brightness variations. Among the sample of ACOs, there is only one object with brightness variations typical of an active comet: 174P/(60558) Echeclus. But this object has already been double classified as asteroid and comet.

Reference
Tancredi G (in press) A criterion to classify asteroids and comets based on the orbital parameters. Icarus
[doi:10.1016/j.icarus.2014.02.013]
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Miloslav Druckmüller¹, Shadia Rifai Habbal², Peter Aniol^3,4, Adalbert Ding⁵ and Huw Morgan⁶

¹Faculty of Mechanical Engineering, Brno University of Technology, 616 69 Brno, Czech Republic
²Institute for Astronomy, University of Hawaii, Honolulu 96822, Hawaii, USA
³ASTELCO Systems GmbH, D-82152 Martinsried, Germany
⁴KACCOLR, King Abdulaziz University, Jeddah 22254, Saudi Arabia
⁵Institute of Optics and Atomic Physics, Technische Universitaet Berlin, and Institute of Technical Physics, Berlin, Germany
⁶Institute of Mathematics, Physics and Computer Science, Aberystwyth University, Ceredigion, Cymru SY23 3BZ, UK

Much anticipation and speculation were building around comet ISON, or C/2012 S1, discovered on 2012 September 21 by the International Scientific Optical Network telescope in Russia, and bound for the Sun on 2013 November 28, with a closest heliocentric approach distance of 2.7 R_☉. Here we present the first white light image of the comet’s trail through the inner corona. The image was taken with a wide field Lyot-type coronagraph from the Mees Observatory on Haleakala at 19:12 UT, past its perihelion passage at 18:45 UT. The perfect match between the comet’s trail captured in the inner corona and the trail that had persisted across the field of view of 2-6 R_☉ of the Solar and Heliospheric Observatory Large Angle and Spectrometric Coronagraph Experiment/C2 coronagraph at 19:12 UT demonstrates that the comet survived its perihelion passage.

Reference
Druckmüller M, Habbal SR, Aniol P, Ding A and Morgan H (2014) Imaging Comet ISON C/2012 S1 in the Inner Corona at Perihelion. The Astrophysical Journal – Letters 784:L22.
[doi:10.1088/2041-8205/784/2/L22]

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Takayuki Tanigawa¹, Akito Maruta², and Masahiro N. Machida²

¹Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan
²Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581, Japan

We investigate the accretion of solid materials onto circumplanetary disks from heliocentric orbits rotating in protoplanetary disks, which is a key process for the formation of regular satellite systems. In the late stage of the gas-capturing phase of giant planet formation, the accreting gas from protoplanetary disks forms circumplanetary disks. Since the accretion flow toward the circumplanetary disks affects the particle motion through gas drag force, we use hydrodynamic simulation data for the gas drag term to calculate the motion of solid materials. We consider a wide range of size for the solid particles (10^-2-10⁶ m), and find that the accretion efficiency of the solid particles peaks around 10 m sized particles because energy dissipation of drag with circum-planetary disk gas in this size regime is most effective. The efficiency for particles larger than 10 m becomes lower because gas drag becomes less effective. For particles smaller than 10 m, the efficiency is lower because the particles are strongly coupled with the background gas flow, which prevents particles from accretion. We also find that the distance from the planet where the particles are captured by the circumplanetary disks is in a narrow range and well described as a function of the particle size.

Reference
Tanigawa T, Maruta A and Machida MN (2014) Accretion of Solid Materials onto Circumplanetary Disks from Protoplanetary Disks. The Astrophysical Journal 784:109.
[doi:10.1088/0004-637X/784/2/109]

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Matthias Ebert^a,b, Lutz Hecht^a,b, Alexander Deutsch^c, Thomas Kenkmann^d, Richard Wirth^e and Jasper Berndt^f

^aMuseum für Naturkunde (MfN), Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, D-10115 Berlin, Germany
^bFreie Universität Berlin (FU Berlin), Institut für Geologische Wissenschaften, Malteserstr. 74-100, D-12249 Berlin, Germany
^cInstitut für Planetologie, Westfälische Wilhelms-Universität Münster (WWU), Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany
^dInstitut für Geo- und Umweltwissenschaften, Albert-Ludwigs-Universität Freiburg (ALU), Albertstr. 23-B, D-79104 Freiburg, Germany
^eHelmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum (GFZ), 3.3, Telegrafenberg, D-14473 Potsdam, Germany
^fInstitut für Mineralogie, Westfälische Wilhelms-Universität Münster (WWU), Correns-Str. 24, D-48149 Münster, Germany

The possibility of fractionation processes between projectile and target matter is critical with regard to the classification of the impactor type from geochemical analysis of impactites from natural craters. Here we present results of five hypervelocity MEMIN impact experiments (Poelchau et al., 2013) using the Cr-V-Co-Mo-W-rich steel D290-1 as projectile and two different silica-rich lithologies (Seeberger sandstone and Taunus quartzite) as target materials. Our study is focused on geochemical target-projectile interaction occurring in highly shocked and projectile-rich ejecta fragments. In all of the investigated impact experiments, whether sandstone or quartzite targets, the ejecta fragments show (i) shock-metamorphic features e.g., planar-deformation features (PDF) and the formation of silica glasses, (ii) partially melting of projectile and target, and (iii) significant mechanical and chemical mixing of the target rock with projectile material. The silica-rich target melts are strongly enriched in the “projectile tracer elements” Cr, V, and Fe, but have just minor enrichments of Co, W, and Mo. Inter-element ratios of these tracer elements within the contaminated target melts differ strongly from the original ratios in the steel. The fractionation results from differences in the reactivity of the respective elements with oxygen during interaction of the metal melt with silicate melt. Our results indicate that the principles of projectile-target interaction and associated fractionation do not depend on impact energies (at least for the selected experimental conditions) and water-saturation of the target. Partitioning of projectile tracer elements into the silicate target melt is much more enhanced in experiments with a non-porous quartzite target compared with the porous sandstone target. This is mainly the result of higher impact pressures, consequently higher temperatures and longer reaction times at high temperatures in the experiments with quartzite as target material.

Reference
Ebert M, Hecht L, Deutsch A, Kenkmann T, Wirth R and Berndt J (in press) Geochemical processes between steel projectiles and silica-rich targets in hypervelocity impact experiments. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.034]
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Steven R. Chesley^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

The target asteroid of the OSIRIS-REx asteroid sample return mission, (101955) Bennu (formerly 1999RQ₃₆), is a half-kilometer near-Earth asteroid with an extraordinarily well constrained orbit. An extensive data set of optical astrometry from 1999 to 2013 and high-quality radar delay measurements to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect, with a mean semimajor axis drift rate  or . The accuracy of this result depends critically on the fidelity of the observational and dynamical model. As an example, neglecting the relativistic perturbations of the Earth during close approaches affects the orbit with 3σ significance in da/dt.

The orbital deviations from purely gravitational dynamics allow us to deduce the acceleration of the Yarkovsky effect, while the known physical characterization of Bennu allows us to independently model the force due to thermal emissions. The combination of these two analyses yields a bulk density of , which indicates a macroporosity in the range 40±10% for the bulk densities of likely analog meteorites, suggesting a rubble-pile internal structure. The associated mass estimate is  and .

Bennu’s Earth close approaches are deterministic over the interval 1654–2135, beyond which the predictions are statistical in nature. In particular, the 2135 close approach is likely within the lunar distance and leads to strong scattering and numerous potential impacts in subsequent years, from 2175 to 2196. The highest individual impact probability is 9.5×10^-5 in 2196, and the cumulative impact probability is 3.7×10^-4, leading to a cumulative Palermo Scale of −1.70.

Reference

Chesley et al. (in press) Orbit and Bulk Density of the OSIRIS-REx Target Asteroid (101955) Bennu. Icarus
[doi:10.1016/j.icarus.2014.02.020]
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Anna Szynkiewicz^a,b, David M. Borrok^c,b and David T. Vaniman^d

^aEarth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, TN 37996, USA
^bGeological Sciences, University of Texas at El Paso, 500 W University Ave., El Paso, TX 79968, USA
^cUniversity of Louisiana at Lafayette, 611 McKinley Street, Lafayette, LA 70504, USA
^dPlanetary Science Institute, 1700 E Fort Lowell, Tucson, AZ 85719, USA

A distinctive sulfur cycle dominates many geological processes on Mars and hydrated sulfate minerals are found in numerous topographic settings with widespread occurrences on the Martian surface. However, many of the key processes controlling the hydrological transport of sulfur, including sulfur sources, climate and the depositional history that led to precipitation of these minerals, remain unclear. In this paper, we use a model for the formation of sulfate efflorescent salts (Mg–Ca–Na sulfates) in the Rio Puerco watershed of New Mexico, a terrestrial analog site from the semiarid Southwest U.S., to assess the origin and environmental conditions that may have controlled deposition of hydrated sulfates in Valles Marineris on Mars. Our terrestrial geochemical results ( of −36.0 to +11.1‰) show that an ephemeral arid hydrological cycle that mobilizes sulfur present in the bedrock as sulfides, sulfate minerals, and dry/wet atmospheric deposition can lead to widespread surface accumulations of hydrated sulfate efflorescences. Repeating cycles of salt dissolution and reprecipitation appear to be major processes that migrate sulfate efflorescences to sites of surface deposition and ultimately increase the aqueous  flux along the watershed (average 41,273 metric tons/yr). We suggest that similar shallow processes may explain the occurrence of hydrated sulfates detected on the scarps and valley floors of Valles Marineris on Mars. Our estimates of salt mass and distribution are in accord with studies that suggest a rather short-lived process of sulfate formation (minimum rough estimate ∼100 to 1000 years) and restriction by prevailing arid conditions on Mars.ed.

Reference
Szynkiewicz A, Borrok DM and Vaniman DT (2014) Efflorescence as a source of hydrated sulfate minerals in valley settings on Mars. Earth and Planetary Science Letters 393:14–25.
[doi:10.1016/j.epsl.2014.02.035]
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Timothy J. Fagan, Daiju Kashima, Yuki Wakabayashi, Akiko Suginohara

Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku, Tokyo 169-8050

Pyroxene and feldspar compositions indicate that most clasts from the Northwest Africa 773 (NWA 773) lunar meteorite breccia crystallized from a common very low-Ti (VLT) mare basalt parental magma on the Moon. An olivine cumulate (OC), with low-Ca and high-Ca pyroxenes and plagioclase feldspar formed during early stages of crystallization, followed by pyroxene gabbro, which is characterized by zoned pyroxene (Fe# = molar Fe/(Fe+Mg) x 100 from ~35 to 90; Ti# = molar Ti/(Ti+Cr) x 100 from ~20 to 99) and feldspar (~An_90-95Ab_05-10 to An_80-85Ab_10-16). Late stage lithologies include alkali-poor symplectite consisting of fayalite, hedenbergitic pyroxene and silica, and alkaline-phase-ferroan clasts characterized by K-rich glass and/or K,Ba-feldspar with fayalite and/or pyroxene. Igneous silica only occurs with the alkaline-phase-ferroan clasts. This sequence of clasts represents stages of magmatic evolution along a ferroan-titanian trend characterized by correlated Fe# and Ti# in pyroxene, and a wide range of increase in Fe# and Ti# prior to crystallization of igneous silica.
Clasts of Apollo 15 quartz monzodiorite (QMD) also have pyroxene co-existing with silica, but the QMD pyroxene has more moderate Fe# (~70). Thus, in AFM components (A = Na₂O+K₂O, M = MgO, F = FeO), the QMD clasts are similar to the terrestrial calc-alkaline trend (silica-enrichment at moderate Fe#), whereas the ferroan-titanian trend is similar to the terrestrial tholeiitic trend (silica-enrichment only after strong increase in Fe#). However, the variations in SiO₂-contents of QMD clasts are due to variable mixing of SiO₂-rich and FeO-rich immiscible liquids (i.e., not a progressive increase in SiO₂). Immiscibility occurred after fractionation of a KREEP-rich parent liquid.
A third trend is based on zoning relations within the NWA 773 OC, where pyroxene Ti# increases at constant Fe# with proximity to intercumulus, incompatible element-rich pockets rich in K,Ba-feldspar and Ca-phosphates. This type of fractionation (increasing refractory trace elements at constant Fe#) in a cumulate parent rock may have been important for generating lunar rocks that combine low Fe# with high incompatible trace element concentrations, such as KREEP basalts and the magnesian suite.
MELTS (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) models of one VLT, one low-Ti and two high-Ti mare basalts and one KREEP basalt all show evolution from low to high Fe# residual liquids during fractional crystallization; however strong enrichments in FeO-concentrations are limited to the VLT and low-Ti liquids. In the high-Ti liquids, crystallization of Fe-Ti-oxides prevents enrichment in FeO, and the increases in Fe# are due to depletion of MgO. Fe-Ti-oxide fractionation results in steady silica-enrichment in the high-Ti mare compositions. Intervals of FeO-enrichment on the VLT and low-Ti mare liquid lines of descent are linked to shifts from olivine to pyroxene crystallization. The onset of plagioclase feldspar crystallization limits the depletion of FeO during crystallization of one high-Ti mare basalt and of the KREEP basalt composition modeled.

Reference
Fagan TJ, Kashima D, Wakabayashi Y and Suginohara A (in press) Case Study of Magmatic Differentiation Trends on the Moon based on Lunar Meteorite Northwest Africa 773 and Comparison with Apollo 15 Quartz Monzodiorite. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.025]
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Wren Montgomery, Jonathan S. Watson, and Mark A. Sephton

Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London SW7 2AZ, UK

There are a number of key structures that can be used to reveal the formation and modification history of organic matter in the cosmos. For instance, the susceptibility of organic matter to heat is well documented and the relative thermal stabilities of different isomers can be used as cosmothermometers. Yet despite being an important variable, no previously recognized organic marker of pressure exists. The absence of a pressure marker is unfortunate considering our ability to effectively recognize extraterrestrial organic structures both remotely and in the laboratory. There are a wide variety of pressures in cosmic settings that could potentially be reflected by organic structures. Therefore, to develop an organic cosmic pressure marker, we have used state-of-the-art diamond anvil cell (DAC) and synchrotron-source Fourier transform infrared (FTIR) spectroscopy to reveal the effects of pressure on the substitution patterns for representatives of the commonly encountered methyl substituted naphthalenes, specifically the dimethylnaphthalenes. Interestingly, although temperature and pressure effects are concordant for many isomers, pressure appears to have the opposite effect to heat on the final molecular architecture of the 1,5-dimethylnaphthalene isomer. Our data suggest the possibility of the first pressure parameter or “cosmo-barometer” (1,5-dimethylnaphthalene/total dimethylnaphthalenes) that can distinguish pressure from thermal effects. Information can be obtained from the new pressure marker either remotely by instrumentation on landers or rovers or directly by laboratory measurement, and its use has relevance for all cases where organic matter, temperature, and pressure interplay in the cosmos.

Reference
Montgomery W, Watson JS and Sephton MA (2014) An Organic Cosmo-barometer: Distinct Pressure and Temperature Effects for Methyl Substituted Polycyclic Aromatic Hydrocarbons. The Astrophysical Journal 784:98.
[doi:10.1088/0004-637X/784/2/98]

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Julien Monteux¹ and Jafar Arkani-Hamed²

¹Laboratoire de Planétologie et de Géodynamique, Université de Nantes, Nantes, France
²Department of Physics, University of Toronto, Toronto, Ontario, Canada

A giant impact is an increasingly popular explanation for the formation of the northern lowland on Mars. It is plausible that at the impact time both Mars and the impactor were differentiated with solid silicate mantles and liquid iron cores. Such a large impact likely resulted in merging of the cores of both bodies, a process which will have implications on the thermal state of the planet. We model the evolution of the Martian mantle following a giant impact and characterize the thermochemical consequences of the sinking of an impactor’s core as a single diapir. The impact heating and the viscous heating induced during the core merging may affect the early thermal state of Mars during several tens of million years. Our results show that large viscosity contrasts between the impactor’s core and the surrounding mantle silicates can reduce the duration of the merging down to 1 kyr but do not modify the merging temperature. When the viscosity contrast between the diapir and the surrounding silicates is larger than a factor of 1000, the descent of the diapir can lead to some entrainment of the relatively shallow silicates to deepest regions close to the core-mantle boundary. Finally, the direct impact heating of Martian core leads to thermal stratification of the core and kills the core dynamo. It takes on the order of 150–200 Myr to reinitiate a strong dynamo anew. The merging of the impactor’s core with the Martian core only delays the reinitiation of the dynamo for a very short time.

Reference
Monteux J and Arkani-Hamed J (in press) Consequences of giant impacts in early Mars: Core merging and Martian dynamo evolution. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004587]
Published by arrangement with John Wiley & Sons

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