Harry Y. McSween Jr.¹, Theodore C. Labotka¹ and Christina E. Viviano-Beck²

¹Department of Earth and Planetary Sciences and Planetary Geoscience Institute, University of Tennessee, Knoxville, Tennessee, USA
²The Johns Hopkins Applied Physics Laboratory, Laurel, Maryland, USA

Compositions of basaltic and ultramafic rocks analyzed by Mars rovers and occurring as Martian meteorites allow predictions of metamorphic mineral assemblages that would form under various thermophysical conditions. Key minerals identified by remote sensing roughly constrain temperatures and pressures in the Martian crust. We use a traditional metamorphic approach (phase diagrams) to assess low-grade/hydrothermal equilibrium assemblages. Basaltic rocks should produce chlorite + actinolite + albite + silica, accompanied by laumontite, pumpellyite, prehnite, or serpentine/talc. Only prehnite-bearing assemblages have been spectrally identified on Mars, although laumontite and pumpellyite have spectra similar to other uncharacterized zeolites and phyllosilicates. Ultramafic rocks are predicted to produce serpentine, talc, and magnesite, all of which have been detected spectrally on Mars. Mineral assemblages in both basaltic and ultramafic rocks constrain fluid compositions to be H₂O-rich and CO₂-poor. We confirm the hypothesis that low-grade/hydrothermal metamorphism affected the Noachian crust on Mars, which has been excavated in large craters. We estimate the geothermal gradient (>20 °C km⁻¹) required to produce the observed assemblages. This gradient is higher than that estimated from radiogenic heat-producing elements in the crust, suggesting extra heating by regional hydrothermal activity.

Reference
McSween HY, Labotka TC and Viviano-Beck CE (in press) Metamorphism in the Martian crust. Meteoritics & Planetary Science
[doi:10.1111/maps.12330]
Published by arrangement with John Wiley & Sons

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A. Sezer¹ and F. Gök²

¹TÜBİTAK Space Technologies Research Institute, ODTU Campus, Ankara, 06531, Turkey
²Akdeniz University, Faculty of Education, Department of Secondary Science and Mathematics Education, Antalya, 07058, Turkey

In this work, we present results from a ~201.6 ks observation of G352.7–0.1 using the X-ray Imaging Spectrometer on board SuzakuX-ray Observatory. The X-ray emission from the remnant is well described by two-temperature thermal models of non-equilibrium ionization with variable abundances with a column density of N_H ~ 3.3 × 10²² cm^-2. The soft component is characterized by an electron temperature of kT_e ~ 0.6 keV, an ionization timescale of τ ~ 3.4 × 10¹¹ cm^-3 s, and enhanced Si, S, Ar, and Ca abundances. The hard component has kT_e ~ 4.3 keV, τ ~ 8.8 × 10⁹ cm^-3 s, and enhanced Fe abundance. The elemental abundances of Si, S, Ar, Ca, and Fe are found to be significantly higher than the solar values that confirm the presence of ejecta. We detected strong Fe K-shell emission and determined its origin to be the ejecta for the first time. The detection of Fe ejecta with a lower ionization timescale favors a Type Ia origin for this remnant.

Reference
Sezer A and Gök F (2014) Fe-rich Ejecta in the Supernova Remnant G352.7–0.1 with Suzaku. The Astrophysical Journal 790:81.
[doi:10.1088/0004-637X/790/1/81]

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Francis M. McCubbin¹, Charles K. Shearer¹, Paul V. Burger¹, Erik H. Hauri², Jianhua Wang², Stephen M. Elardo¹ and James J. Papike¹

¹Institute of Meteoritics, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, D.C. 20015, U.S.A.

Whitlockite and merrillite are two Ca-phosphate minerals found in terrestrial and planetary igneous rocks, sometimes coexisting with apatite. Whitlockite has essential structural hydrogen, and merrillite is devoid of hydrogen. Whitlockite components have yet to be discovered in samples of extraterrestrial merrillite, despite evidence for whitlockite-merrillite solid solution in terrestrial systems. The observation of merrillite in meteoritic and lunar samples has led many to conclude that the magmas from which the merrillite formed were “very dry.” However, the Shergotty martian meteorite has been reported to contain both apatite and merrillite, and recently the apatite has been shown to contain substantial OH abundances, up to the equivalent of 8600 ppm H₂O. In the present study, we determined the abundances of F, Cl, H₂O, and S in merrillite from Shergotty using secondary ion mass spectrometry (SIMS). We determined that the merrillite in Shergotty was properly identified (i.e., no discernible whitlockite component), and it coexists with OH-rich apatite. The absence of a whitlockite component in Shergotty merrillite and other planetary merrillites may be a consequence of the limited thermal stability of H in whitlockite (stable only at T <1050 °C), which would prohibit merrillite-whitlockite solid-solution at high temperatures. Consequently, the presence of merrillite should not be used as evidence of dry magmatism without a corresponding estimate of the T of crystallization. In fact, if a whitlockite component in extraterrestrial merrillite is discovered, it may indicate formation by or equilibration with hydrothermal or aqueous fluids.

Reference
McCubbin FM, Shearer CK, Burger PV, Hauri EH, Wang J, Elardo SM and Papike JJ (2014) Volatile abundances of coexisting merrillite and apatite in the martian meteorite Shergotty: Implications for merrillite in hydrous magmas. American Mineralogist 99:1347.
[doi:10.2138/am.2014.4782]
Copyright: The Mineralogical Society of America

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Gongjie Li and Konstantin Batygin

Harvard-Smithsonian Center for Astrophysics, The Institute for Theory and Computation, 60 Garden Street, Cambridge, MA 02138, USA

The variation of a planet’s obliquity is influenced by the existence of satellites with a high mass ratio. For instance, Earth’s obliquity is stabilized by the Moon and would undergo chaotic variations in the Moon’s absence. In turn, such variations can lead to large-scale changes in the atmospheric circulation, rendering spin-axis dynamics a central issue for understanding climate. The relevant quantity for dynamically forced climate change is the rate of chaotic diffusion. Accordingly, here we re-examine the spin-axis evolution of a Moonless Earth within the context of a simplified perturbative framework. We present analytical estimates of the characteristic Lyapunov coefficient as well as the chaotic diffusion rate and demonstrate that even in absence of the Moon, the stochastic change in Earth’s obliquity is sufficiently slow to not preclude long-term habitability. Our calculations are consistent with published numerical experiments and illustrate the putative system’s underlying dynamical structure in a simple and intuitive manner.

Reference
Li G and Batygin K (2014) On the Spin-axis Dynamics of a Moonless Earth. The Astrophysical Journal 790:69.
[doi:10.1088/0004-637X/790/1/69]

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Aaron S. Bell¹, Paul V. Burger¹, Loan Le², Charles K. Shearer¹, James J. Papike¹, Steve R. Sutton³, Matthew Newville³ and John Jones⁴

¹Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
²Jacobs, NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
³Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.
⁴NASA, Johnson Space Center, Houston, Texas, 77058, U.S.A

In this work we present a series of experiments that examine the relationship between oxygen fugacity and Cr valence ratio in olivine grown from a basaltic liquid. These experiments are specifically targeted for an olivine-rich martian basalt composition that was modeled after the bulk chemistry of the meteorite Yamato 980459 (i.e., Y-98). The chromium valence ratio in the olivine crystals was measured with X-ray absorption near edge spectroscopy (XANES) at the Advanced Photon Source, Argonne National Laboratory. Results from the XANES measurements indicate that the ratio of divalent to trivalent Cr in the olivine is not only systematically correlated with f_O2, but is also reflective of the molar Cr³⁺/Cr²⁺ in the silicate liquid from which it
grew. In this way, measurements of Cr valence in olivine phenocrysts can yield important information about the oxygen fugacity and molar Cr³⁺/Cr²⁺ of its parental liquid in the absence of a quenched melt phase. Although the results from the experiments presented in this work specifically apply to the Y-98 parental melt, the concepts and XANES analytical techniques discussed within the text present a novel, generalized methodology that may be applicable to any olivine-bearing basalt. Furthermore, the XANES-based measurements are made on a micrometer-scale, thus potential changes of the Cr³⁺/Cr²⁺ in the melt during crystallization could be examined with a great deal of spatial detail.

Reference
Bell AS, Burger PV, Le L, Shearer CK, Papike JJ, Sutton SR, Newville M and Jones J (2014) XANES measurements of Cr valence in olivine and their applications to planetary basalts. American Mineralogist 99:1404.
[doi:10.2138/am.2014.4646]
Copyright: The Mineralogical Society of America

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J. Drążkowska, F. Windmark and C. P. Dullemond

Heidelberg University, Center for Astronomy, Institute of Theoretical Astrophysics, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany

Context. Dust coagulation in protoplanetary disks is one of the initial steps toward planet formation. Simple toy models are often not sufficient to cover the complexity of the coagulation process, and a number of numerical approaches are therefore used, among which integration of the Smoluchowski equation and various versions of the Monte Carlo algorithm are the most popular.
Aims. Recent progress in understanding the processes involved in dust coagulation have caused a need for benchmarking and comparison of various physical aspects of the coagulation process. In this paper, we directly compare the Smoluchowski and Monte Carlo approaches to show their advantages and disadvantages.
Methods. We focus on the mechanism of planetesimal formation via sweep-up growth, which is a new and important aspect of the current planet formation theory. We use realistic test cases that implement a distribution in dust collision velocities. This allows a single collision between two grains to have a wide range of possible outcomes but also requires a very high numerical accuracy.
Results. For most coagulation problems, we find a general agreement between the two approaches. However, for the sweep-up growth driven by the “lucky” breakthrough mechanism, the methods exhibit very different resolution dependencies. With too few mass bins, the Smoluchowski algorithm tends to overestimate the growth rate and the probability of breakthrough. The Monte Carlo method is less dependent on the number of particles in the growth timescale aspect but tends to underestimate the breakthrough chance due to its limited dynamic mass range.
Conclusions. We find that the Smoluchowski approach, which is generally better for the breakthrough studies, is sensitive to low mass resolutions in the high-mass, low-number tail that is important in this scenario. To study the low number density features, a new modulation function has to be introduced to the interaction probabilities. As the minimum resolution needed for breakthrough studies depends strongly on setup, verification has to be performed on a case by case basis.

Reference
Drążkowska J, Windmark F and Dullemond CP(2014) Modeling dust growth in protoplanetary disks: The breakthrough case. Astronomy & Astrophysics 567:A38.
[doi:10.1051/0004-6361/201423708]
Reproduced with permission © ESO

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Jean-David Bodénan^a,b, Natalie A. Starkey^a, Sara S. Russell^b, Ian P. Wright^a, Ian A. Franchi^a

^aPlanetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
^aDepartment of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom

Calcium–aluminium-rich Inclusions (CAIs) and the thin Wark–Lovering (WL) rims of minerals surrounding them offer a record of the nature of changing conditions during the earliest stages of Solar System formation. Considerable heterogeneity in the gas composition in the immediate vicinity of the proto-Sun had previously been inferred from oxygen isotopic variations in the WL rim of a CAI from Allende (Simon et al., 2011). However, high precision and high spatial resolution oxygen isotope measurements presented in this study show that WL rim and pristine core minerals of individual CAIs from meteorites that had experienced only low degrees of alteration or low grade metamorphism (one from Léoville (reduced CV3), two in QUE 99177 (CR3.0) and two in ALHA 77307 (CO3.0)) are uniformly ¹⁶O-rich. This indicates that the previously observed variations are the result of secondary processes, most likely on the asteroid parent body, and that there were no temporal or spatial variations in oxygen isotopic composition during CAI and WL rim formation. Such homogeneity across three groups of carbonaceous chondrites lends further support for a common origin for the CAIs in all chondrites. ¹⁶O-poor oxygen reservoirs such as those associated with chondrule formation, were probably generated by UV photo-dissociation involving self-shielding mechanisms and must have occurred elsewhere in outer regions of the solar accretion disk.

Reference
Bodénan J-D, Starkey NA, Russell SS, Wright IP and Franchi IA (2014) An oxygen isotope study of Wark–Lovering rims on type A CAIs in primitive carbonaceous chondrites. Earth and Planetary Science Letters 401:327.
[doi:10.1016/j.epsl.2014.05.035]
Copyright Elsevier

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P. Mahanti^a, M.S. Robinson^a, D.C. Humm^b, J.D. Stopar^a

^aSchool of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
^bSpace Instrument Calibration Consulting, Annapolis, Maryland, USA

Historically, topographic profiles provided a quantitative means to investigate the morphology and formation processes for impact craters, although no generic mathematical framework was developed to reduce profiles to morphology descriptors. Only need-specific polynomial expressions were utilized in previous studies, thus no standardized automated comparison of craters exists. We employ a Chebyshev polynomial function approximation to describe crater forms in a quantitative and repeatable manner. We show that the Chebyshev polynomials return coefficients that are relatable to crater morphologic characteristics, thus providing a standardized mathematical means for describing crater forms.

Reference
Mahanti P, Robinson MS, Humm DC and Stopar JD (2014) A standardized approach for quantitative characterization of impact crater topography. Icarus
[doi:10.1016/j.icarus.2014.06.023]
Copyright Elsevier

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A. Guilbert-Lepoutre¹, R. Schulz¹, A. Rożek², S. C. Lowry², G. P. Tozzi³ and J. A. Stüwe^{4, 5}

¹European Space Agency – ESTEC, 2200 AG Noordwijk, The Netherlands
²Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Canterbury CT2 7NH, UK
³Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁴Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵Wincor Nixdorf Portavis GmbH, Wendenstrasse 21, 20097 Hamburg, Germany

Context. Comets are believed to hold a relatively pristine record of the physical and chemical processes that occurred during the formation and evolution of the solar system. Thorough investigations of these small bodies, such as the one that will be performed by the ESA/Rosetta cornerstone mission, are thus supposed to bring strong and unique constraints on the origins of the solar system.
Aims. Because comet 67P/Churyumov-Gerasimenko was only recently selected as the target for the ESA/Rosetta mission, there has been little opportunity to study its pre-perihelion activity. This phase is, however, very important for the mission, since the global mapping of the nucleus and the choice of landing site for Philae will be performed during this pre-perihelion phase. Here, we report previously unpublished data of the last pre-perihelion passage of this comet, observed between May and September 2008.
Methods. The gas and dust activity of comet 67P/Churyumov-Gerasimenko are studied through visible spectroscopy and broadband imaging, respectively, covering a range of pre-perihelion heliocentric distances between 2.99 and 2.22 AU.
Results. The data we have gathered on the dust activity are consistent with trends observed by other authors and show a strong asymmetry between the pre- and post-perihelion phases of the orbit. The spectra do not show any lines due to the emission of volatiles, and upper limits on their production rates are typically one order of magnitude lower than at the equivalent post-perihelion heliocentric distances. The asymmetry in the pre- and post-perihelion phases of the activity may be due to a dusty crust quenching the activity at the surface of 67P. We estimate that this crust could be about 12 cm thick, although not uniform across the surface. Even if no gas is individually detected, the coma surface brightness profiles might indicate a possible contamination from gaseous species emitted before the comet actually reaches perihelion.

Reference
Guilbert-Lepoutre A, Schulz R, Rożek A, Lowry SC, Tozzi GP and Stüwe JA (2014) Pre-perihelion activity of comet 67P/Churyumov-Gerasimenko. Astronomy & Astrophysics 567:L2.
[doi:10.1051/0004-6361/201424186]
Reproduced with permission © ESO

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Jorge Meléndez¹, Lucas Schirbel¹, TalaWanda R. Monroe¹, David Yong², Iván Ramírez³ and Martin Asplund²

¹Departamento de Astronomia do IAG/USPUniversidade de São Paulo, rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP Brazil
²Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, Weston ACT 2611, Australia
³McDonald Observatory and Department of Astronomy, University of Texas at Austin, USA

Context. The standard solar model fails to predict the very low lithium abundance in the Sun, which is much lower than the proto-solar nebula (as measured in meteorites). This Li problem has been debated for decades, and it has been ascribed either to planet formation or to secular stellar depletion due to additional mixing below the convection zone, either during the pre-main sequence and thus possibly linked to planet formation, or additionally on secular time-scales during the main sequence. In order to test the evolution of Li, it is important to find solar twins in a range of ages, i.e., stars with about one solar mass and metallicity but in different evolutionary stages. Furthermore, the study of stars similar to the Sun is relevant in relation to the signature of terrestrial planet formation around the Sun, and for anchoring photometric and spectroscopic stellar parameter scales.
Aims. We aim to identify and analyse solar twins using high quality spectra, in order to study Li depletion in the Sun and the possible relation between chemical abundance anomalies and planet formation.
Methods. We acquired high-resolution (R ~ 110 000), high S/N (~300) ESO/VLT UVES spectra of several solar twin candidates and the Sun (as reflected from the asteroid Juno). Among the solar twin candidates we identify HIP 114328 as a solar twin and perform a differential line-by-line abundance analysis of this star relative to the Sun.
Results. HIP 114328 has stellar parameters T_eff = 5785 ± 10 K, log g = 4.38 ± 0.03, [ Fe/H] = −0.022 ± 0.009, and a microturbulent velocity 0.05 ± 0.03 km s^-1 higher than solar. The differential analysis shows that this star is chemically very similar to the Sun. The refractory elements seem slightly more depleted than in the Sun, meaning that HIP 114328 may be as likely to form terrestrial planets as the Sun. HIP 114328 is about 2 Gyr older than the Sun, and is thus the second oldest solar twin analysed at high precision. It has a Li abundance of A(Li)_NLTE ≲ 0.46, which is about 4 times lower than in the Sun (A(Li)_NLTE = 1.07 dex), but close to the oldest solar twin known, HIP 102152.
Conclusions. Based on the lower abundances of refractory elements when compared to other solar twins, HIP 114328 seems an excellent candidate to host rocky planets. The low Li abundance of this star is consistent with its old age and fits very well the emerging Li-age relation among solar twins of different ages.

Reference
Melendez J, Schirbel L, Monroe TR, Yong D, Ramírez I and Asplund M(2014) HIP 114328: a new refractory-poor and Li-poor solar twin. Astronomy & Astrophysics 567:L3.
[doi:10.1051/0004-6361/201424172]
Reproduced with permission © ESO

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