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 Here, we report previously unpublished data of the last pre-perihelion passage of this comet, observed between May and September 2008.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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A. Gould¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA.

Using gravitational microlensing, we detected a cold terrestrial planet orbiting one member of a binary star system. The planet has low mass (twice Earth’s) and lies projected at ~0.8 astronomical units (AU) from its host star, about the distance between Earth and the Sun. However, the planet’s temperature is much lower, <60 Kelvin, because the host star is only 0.10 to 0.15 solar masses and therefore more than 400 times less luminous than the Sun. The host itself orbits a slightly more massive companion with projected separation of 10 to 15 AU. This detection is consistent with such systems being very common. Straightforward modification of current microlensing search strategies could increase sensitivity to planets in binary systems. With more detections, such binary-star planetary systems could constrain models of planet formation and evolution.

Reference
Gould et al. (2014) A terrestrial planet in a ~1-AU orbit around one member of a ~15-AU binary. Science 345:46.
[doi:10.1126/science.1251527]
Reprinted with permission from AAAS

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Xuchao Zhao¹, Yangting Lin¹, Qing-Zhu Yin², Jianchao Zhang¹, Jialong Hao¹, Michael Zolensky³ and Peter Jenniskens^4,5

¹Key Laboratory of the Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
²Department of Earth and Planetary Sciences, University of California at Davis, Davis, California, USA
³ARES, NASA Johnson Space Center, Houston, Texas, USA
⁴SETI Institute, Mountain View, California, USA
⁵NASA Ames Research Center, Moffett Field, California, USA

The Sutter’s Mill (SM) carbonaceous chondrite is a regolith breccia, composed predominantly of CM2 clasts with varying degrees of aqueous alteration and thermal metamorphism. An investigation of presolar grains in four Sutter’s Mill sections, SM43, SM51, SM2-4, and SM18, was carried out using NanoSIMS ion mapping technique. A total of 37 C-anomalous grains and one O-anomalous grain have been identified, indicating an abundance of 63 ppm for presolar C-anomalous grains and 2 ppm for presolar oxides. Thirty-one silicon carbide (SiC), five carbonaceous grains, and one Al-oxide (Al₂O₃) were confirmed based on their elemental compositions determined by C-N-Si and O-Si-Mg-Al isotopic measurements. The overall abundance of SiC grains in Sutter’s Mill (55 ppm) is consistent with those in other CM chondrites. The absence of presolar silicates in Sutter’s Mill suggests that they were destroyed by aqueous alteration on the parent asteroid. Furthermore, SM2-4 shows heterogeneous distributions of presolar SiC grains (12–54 ppm) in different matrix areas, indicating that the fine-grained matrix clasts come from different sources, with various thermal histories, in the solar nebula.

Reference
Zhao X, Lin Y, Yin Q-Z, Zhang J, Hao J, Zolensky M and Jenniskens P (in press) Presolar grains in the CM2 chondrite Sutter’s Mill. Meteoritics & Planetary Science
[doi:10.1111/maps.12289]
Published by arrangement with John Wiley & Sons

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Vincent R. Eke^a, Sarah A. Bartram^a, David A. Lane^a, David Smith^a, Luis F.A. Teodoro^b

^aInstitute for Computational Cosmology, Department of Physics, Durham University, Science Laboratories, South Road, Durham DH1 3LE, U.K
^bBAER, Planetary Systems Branch, Space Science and Astrobiology Division, MS: 245-3, NASA Ames Research Center, Moffett Field, CA 94035-1000, U.S.A

Circular Polarisation Ratio (CPR) mosaics from Mini-SAR on Chandrayaan-1 and Mini-RF on LRO are used to study craters near to the lunar north pole. The look direction of the detectors strongly affects the appearance of the crater CPR maps. Rectifying the mosaics to account for parallax also significantly changes the CPR maps of the crater interiors. It is shown that the CPRs of crater interiors in unrectified maps are biased to larger values than crater exteriors, because of a combination of the effects of parallax and incidence angle. Using the LOLA Digital Elevation Map (DEM), the variation of CPR with angle of incidence has been studied. For fresh craters, CPR ∼0.7 with only a weak dependence on angle of incidence or position interior or just exterior to the crater, consistent with dihedral scattering from blocky surface roughness. For anomalous craters, the CPR interior to the crater increases with both incidence angle and distance from the crater centre. Central crater CPRs are similar to those in the crater exteriors. CPR does not appear to correlate with temperature within craters. Furthermore, the anomalous polar craters have diameter-to-depth ratios that are lower than those of typical polar craters. These results strongly suggest that the high CPR values in anomalous polar craters are not providing evidence of significant volumes of water ice. Rather, anomalous craters are of intermediate age, and maintain sufficiently steep sides that sufficient regolith does not cover all rough surfaces.

Reference
Eke VR, Bartram SA, Lane DA, Smith D and Teodoro LFA (in press) Lunar polar craters – icy, rough or just sloping? Icarus
[doi:10.1016/j.icarus.2014.06.021]
Copyright Elsevier

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Patricia M. Doyle^1,2, Paul F. Schofield², Andrew J. Berry^1,2, Andrew M. Walker³ and Kevin S. Knight^4,2

¹Department of Earth Science and Engineering, Imperial College London, South Kensington SW7 2AZ, U.K.
²Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, U.K.
³School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, U.K.
⁴ISIS Science Division, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, U.K.

The structures of eight synthetic samples of hibonite, with variable Ti oxidation state and Ti concentration (2.4–15.9 wt% TiO₂) that span the range reported for natural hibonite found in meteorites, were determined by Rietveld refinements of neutron powder diffraction data. Ti³⁺ was found to exclusively occupy the octahedral face-sharing M4 site irrespective of the presence or absence of Ti⁴⁺. Ti⁴⁺ partitions between the trigonal bipyramidal M2 site and the M4 site. The ratio (Ti⁴⁺ on M2):(Ti⁴⁺ on M4) appears to be constant for all the samples, with an average of 0.18(2) irrespective of the concentrations of Ti³⁺ and Ti⁴⁺. These substitutional sites were shown to be the most stable configurations for Ti in hibonite from calculations using density functional theory, although the predicted preference of Ti⁴⁺ for M4 over M2 is not as strong as is observed. This is attributed to the different Ti contents of the experimental and calculated structures and suggests that the Ti site occupancies might change between these concentrations. Furthermore, it is shown that Ti has a preference to occupy neighboring M4 sites such that Ti-Ti interactions occur with stabilization energies of 83 kJ/mol for Ti³⁺-Ti³⁺ and at least 15 kJ/mol for Ti⁴⁺-Ti⁴⁺. Features in optical spectroscopy and electron spin resonance data from meteoritic and synthetic hibonites that have been used to infer Ti³⁺/Ti⁴⁺ are shown to actually derive from these Ti-Ti interactions. The amount of Ti⁴⁺ in hibonite can be determined from the unit-cell parameters if ∑Ti is determined independently. Ti³⁺/Ti⁴⁺ in hibonite may record the oxygen fugacity (f_O2) of the early solar nebula, however, the existence of Ti³⁺-Ti³⁺ and Ti⁴⁺-Ti⁴⁺ interactions and the potential for Ti⁴⁺-Ti³⁺ interactions need to be considered when interpreting spectroscopic data in terms of Ti valence state and f_O2. Hibonite as a single-mineral oxybarometer must be used with caution due to the potential role of crystal chemistry (including Ti-Ti interactions) to stabilize Ti oxidation states independently of f_O2.

Reference
Doyle PM, Schofield PF, Berry AJ, Walker AM and Knight KS (2014) Substitution of Ti3+ and Ti4+ in hibonite (CaAl12O19). American Mineralogist 99:1369.
[doi:10.2138/am.2014.4532]
Copyright: The Mineralogical Society of America

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Christopher T. Adcock¹, Elisabeth M. Hausrath¹, Paul M. Forster^2,3, Oliver Tschauner^1,3 and Kirellos J. Sefein¹

¹Department of Geoscience, University of Nevada Las Vegas, 4505 South Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.
²Department of Chemistry, University of Nevada Las Vegas, 4505 South Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.
³HiPSEC, University of Nevada Las Vegas, 4505 South Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.

Merrillite [Ca₉NaMg(PO₄)₇] occurs as a dominant primary Ca-phosphate mineral in martian meteorites and therefore presumably also on Mars. The mineral is an important phase in exploring differences in geologic processes between Earth and Mars, and also has astrobiological implications due to its potential role as a significant source of the bio-essential nutrient phosphate. Merrillite does not occur terrestrially as a discrete mineral phase, making it difficult to obtain for Mars-relevant studies. It can, however, be synthesized from a similar terrestrial mineral, whitlockite (natural or synthetic), through dehydrogenation. Here we present methods for synthesizing relatively large quantities (0.5 g or greater per batch) of coarse crystalline (75 μm+) Mg-whitlockite, Fe-whitlockite, mixed Fe/Mg-whitlockites, and from these synthesized minerals produce Mg-merrillite, ferrous and ferric Fe-merrillite, and ferrous and ferric mixed Fe/Mg-merrillite. Chemistry and atomic structures of synthesized Fe- and mixed Fe/Mg-whitlockite and ferrous and ferric Fe- and mixed Fe/Mg- merrillite resulting from single-crystal X-ray diffraction, infrared spectroscopy, and electron microprobe analyses are presented. We also present a mechanism for maintaining charge balance during the formation of merrillite from whitlockite. Our results shed light on these mineral structures for future martian studies, and provide methods for creating coarse crystalline merrillite for use in Mars-relevant thermodynamic, kinetic, soil/dust simulant, crystallographic, astrobiological, and other studies.

Reference
Adcock CT, Hausrath EM, Forster PM, Tschauner O and Sefein KJ (2014) Synthesis and characterization of the Mars-relevant phosphate minerals Fe- and Mg-whitlockite and merrillite and a possible mechanism that maintains charge balance during whitlockite to merrillite transformation. American Mineralogist 99:1221.
[doi:10.2138/am.2014.4688]
Copyright: The Mineralogical Society of America

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Ema Bobocioiu and Razvan Caracas

Laboratoire de Géologie de Lyon (LGLTPE) CNRS UMR 5276, Université Claude Bernard Lyon 1, Ecole Normale Supérieure de Lyon 46, allée d’Italie, 69364 Lyon, France

We study hydrated magnesium sulfate minerals from first-principles calculations based on density-functional theory. We determine the heat of hydration for MgSO₄·nH₂O, compute the Raman and infrared spectra for several phases and calculate the S isotope partitioning as a function of hydration. We find that epsomite and meridianiite with, respectively, n = 7 andn = 11 water molecules per MgSO₄ unit are particularly stable with respect to other individual or combinations of hydration states. The Raman spectra of all phases present clear SO₄ features that are easily identifiable. We use this to show one can use the vibrational spectroscopic information as an identification tool in a remote environment, like the martian surface. We discuss the character and atomic displacement pattern of all vibration modes and compute the ³⁴S/³²S partitioning; this work shows that hydration favors enrichment in the lighter S isotope ³²S with respect to the heavier ³⁴S, which is accumulated in the less hydrous structures. We show for the first time that the signature of ³⁴S/³²S partitioning could be observed by in situ spectroscopy on the surface of Mars. Moreover this can be related to the diurnal cycle of hydration and dehydration and hence it can improve the modeling of the water circulation on Mars.

Reference
Bobocioiu E and Caracas R (2014) Stability and spectroscopy of Mg sulfate minerals: Role of hydration on sulfur isotope partitioning. American Mineralogist 99:1216.
[doi:10.2138/am.2014.4632]
Copyright: The Mineralogical Society of America

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Valerie M. Tu¹, Elisabeth M. Hausrath¹, Oliver Tschauner^1,2, Valentin Iota² and Gerald W. Egeland³

¹Department of Geoscience, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.
²HiPSEC, University of Nevada Las Vegas, 4505 S. Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.
³Department of Mechanical Engineering, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.

Phosphate is an essential nutrient for life on Earth, and therefore if life exists or ever existed on Mars it may have required phosphate. Amorphous Al- and Fe-phosphates rapidly precipitate from acidic solutions and amorphous Al-phosphates likely control phosphate concentrations in some natural waters on Earth. The amorphous fraction of martian soils has also been shown to be enriched in P, and amorphous phosphates are therefore also likely important in the phosphate cycle on Mars. Despite this importance, however, few dissolution rates exist for amorphous Al- and Fe-phosphates. In this study, dissolution rates of amorphous Al- and Fe-phosphates were measured in flow-through reactors from steady state concentrations of Al, Fe, and P. A pH-dependent rate law, log R = log k – npH was determined from the dissolution rates, where R is the dissolution rate, k is the intrinsic rate constant, and n is the reaction order with respect to H+. For amorphous Al-phosphate, log k = −6.539 ± 1.529 and n = 2.391 ± 0.493. For amorphous Fe-phosphate, log k= −13.031 ± 0.558 and n = 1.376 ± 0.221. The amorphous Al-phosphate dissolves stoichiometrically under all experimental conditions measured, and the amorphous Fe-phosphate dissolves non-stoichiometrically, approaching stoichiometric dissolution as pH decreases, due potentially to Fe oxyhydroxides precipitating and armoring grain surfaces. Perhaps due to these effects, amorphous Al-phosphate dissolution rates are approximately three orders of magnitude faster than the amorphous Fe-phosphate dissolution rates measured under these experimental conditions. Amorphous Al-phosphate dissolution rates measured in this study are also faster than published dissolution rates for the crystalline Al-phosphate variscite. The rapid dissolution rates measured in this study therefore suggest that, if these phases are present on Mars, phosphate would be rapidly released into acidic environments.

Reference
Tu VM, Hausrath EM, Tschauner O, Iota V and Egeland GW (2014) Dissolution rates of amorphous Al- and Fe-phosphates and their relevance to phosphate mobility on Mars. American Mineralogist 99:1206.
[doi:10.2138/am.2014.4613]
Copyright: The Mineralogical Society of America

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