Rebecca J. Thomas^a, David A. Rothery^a, Susan J. Conway^a, Mahesh Anand^a,b

^aDepartment of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K
^bDepartment of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, U.K

Recent images of the surface of Mercury have revealed an unusual and intriguing landform: sub-kilometre scale, shallow, flat-floored, steep-sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters. These ‘hollows’ appear to form by the loss of a moderately-volatile substance from the planet’s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently (and may be on-going). Hypotheses to explain the volatile-loss have included sublimation and space weathering, and it has been suggested that hollow-forming volatiles are endogenic and are exposed at the surface during impact cratering. However, detailed verification of these hypotheses has hitherto been lacking. In this study, we have conducted a comprehensive survey of all MESSENGER images obtained up to the end of its fourth solar day in orbit in order to identify hollowed areas. We have studied how their location relates to both exogenic processes (insolation, impact cratering, and solar wind) and endogenic processes (explosive volcanism and flood lavas) on local and regional scales. We find that there is a weak correlation between hollow formation and insolation intensity, suggesting formation may occur by an insolation-related process such as sublimation. The vast majority of hollow formation is in localised or regional low-reflectance material within impact craters, suggesting that this low-reflectance material is a volatile-bearing unit present below the surface that becomes exposed as a result of impacts. In many cases hollow occurrence is consistent with formation in volatile-bearing material exhumed and exposed during crater formation, while in other cases volatiles may have accessed the surface later through re-exposure and possibly in association with explosive volcanism. Hollows occur at the surface of thick flood lavas only where a lower-reflectance substrate has been exhumed from beneath them, indicating that this form of flood volcanism on Mercury lacks significant concentrations of hollow-forming volatiles.

Reference
Thomas RJ, Rothery DA, Conway SJ and Anand M (in press) Hollows on Mercury: materials and mechanisms involved in their formation. Icarus
[doi:10.1016/j.icarus.2013.11.018]
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Thomas Mueller^a,b,*, E. Bruce Watson^a, Dustin Trail^a,c, Michael Wiedenbeck^d, James Van Orman^e and Erik H. Hauri^f

^aNew York Center for Astrobiology, Rensselaer Polytechnic Institute, 110 8[th] Street, Troy, NY 12180, USA
^bInstitut für Geologie, Mineralogie & Geophysik, Ruhr-Universität Bochum, D-44801 Bochum, Germany
^cDepartment of Earth and Environmental Sciences, University of Rochester, Rochester NY, 14627.
^dHelmholtz Zentrum Potsdam, GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany
^eDept. of Earth, Environmental and Planetary Sciences, Case Western University, Cleveland, USA
^fCarnegie Institution of Washington, Washington, DC 20015, USA

Carbon is an abundant element of planets and meteorites whose isotopes provide unique insights into both organic and inorganic geochemical processes. The identities of carbonaceous phases and their textural and isotopic characters shed light on dynamical processes in modern Earth systems and the evolution of the early solar system. In meteorites and their parent bodies, reduced carbon is often associated with Fe-Ni alloys, so knowledge of the mechanisms that fractionate C isotopes in such phases is crucial for deciphering the isotopic record of planetary materials. Here we present the results of a diffusion-couple experiment in which cylinders of polycrystalline Fe containing 11,500 and 150 μg/g of C were juxtaposed at 1273K and 1.5 GPa for a duration of 36 min. Diffusion profiles of total C concentration and ¹³C/¹²C were measured by secondary ion mass spectrometry (SIMS). The elemental diffusivity extracted from the data is ∼3.0 × 10^-11 m²s^-1, where ¹³C/¹²C was observed to change significantly along the diffusion profile, reflecting a higher diffusivity of ¹²C relative to ¹³C. The maximum isotopic fractionation along the diffusion profile is ∼30-40‰. The relative diffusivities (D) of the carbon isotopes can be related to their masses (M) by D_13C/D_12C = (M_12C/M_13C)^β; the exponent β calculated from our data has a value of 0.225±0.025. Similarly high β values for diffusion of other elements in metals have been taken as an indication of interstitial diffusion, so our results are consistent with C diffusion in Fe by an interstitial mechanism. The high β-value reported here means that significant fractionation of carbon isotopes in nature may arise via diffusion in Fe(-Ni) metal, which is an abundant component of planetary interiors and meteorites.

Reference
Mueller T, Watson EB, Trail D, Wiedenbeck M, Orman JV and Hauri EH (in press) Diffusive fractionation of carbon isotopes in γ-Fe: experiment, models and implications for early solar system processes. Geochimica et Cosmochimica Acta
[doi:10.1002/2013JE004426]
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M. H. Jones^1,*, D. Bewsher², D. S. Brown²

¹Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, Buckinghamshire MK7 6AA, UK.
²Jeremiah Horrocks Institute, University of Central Lancashire, Preston, Lancashire PR1 2HE, UK.

The gravitational interaction of dust in the zodiacal cloud with individual planets is expected to give rise to ringlike features: Such a circumsolar ring has been observed associated with Earth, but such resonance rings have not been confirmed to exist for other planets. Here, we report on sensitive photometric observations, based on imaging from the STEREO mission, that confirm the existence of a dust ring at the orbit of Venus. The maximum overdensity of dust in this ring, compared to the zodiacal cloud, is ~10%. The radial density profile of this ring differs from the model used to describe Earth’s ring in that it has two distinct steplike components, with one step being interior and the other exterior to the orbit of Venus.

Reference
Jones MH, Bewsher D and Brown DS (2013) Imaging of a Circumsolar Dust Ring Near the Orbit of Venus. Science 342:960-963.
[doi:10.1126/science.1243194]
Reprinted with permission from AAAS

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Samuel P. Kounaves^a, Brandi L. Carrier^a, Glen D. O’Neil^a, Shannon T. Stroble^b, Mark W. Claire^c,d

^aDepartment of Chemistry, Tufts University, Medford, MA, 02155, USA.
^bFranklin Pierce University, Department of Chemistry, Rindge, NH 03461, USA.
^cUniversity of St. Andrews, Department of Earth & Environmental Sciences, St. Andrews, UK.
^dBlue Marble Space Institute of Science, P.O. Box 85561, Seattle, WA 98145, USA.

The results from the Viking mission in the mid 1970’s provided evidence that the martian surface contained oxidants responsible for destroying organic compounds. In 2008 the Phoenix Wet Chemistry Lab (WCL) found perchlorate (ClO₄^–) in three soil samples at concentrations from 0.5 to 0.7 wt%. The detection of chloromethane (CH₃Cl) and dichloromethane (CH₂Cl₂) by the Viking pyrolysis gas chromatograph-mass spectrometer (GC-MS) may have been a result of ClO₄^– at that site oxidizing either terrestrial organic contaminates or, if present, indigenous organics. Recently, the Sample Analysis at Mars (SAM) instrument on the Mars Science Laboratory (MSL) Curiosity directly measured the presence of CH₃Cl, CH₂Cl₂ and, along with measurements of HCl and oxygen, indirectly indicate the presence of ClO₄^–. However, except for Phoenix, no other direct measurement of the ClO₄^– anion in martian soil or rock has been made. We report here ion chromatographic (IC) and isotopic analyses of a unique sawdust portion of the martian meteorite EETA79001 that show the presence by mass of 0.6 ± 0.1 ppm ClO₄^– , 1.4 ± 0.1ppm ClO₃^–, and 16 ± 0.2 ppm NO₃^– at a quantity and location within the meteorite that is difficult to reconcile with terrestrial contamination. The sawdust sample consists of basaltic material with a minor salt-rich inclusion in a mass ratio of ∼300:1, thus the salts may be 300 times more concentrated within the inclusion than the whole sample. The molar ratios of NO₃^– : ClO₄^– and Cl- : ClO₄^–, are very different for EETA79001 at ∼ 40:1 and 15:1, respectively, than the Antarctic soils and ice near where the meteorite was recovered at ∼ 10,000:1 and 5000:1, respectively. In addition, the isotope ratios for EETA79001 with δ¹⁵N = -10.48 ± 0.32 ‰ and δ¹⁸O = +51.61 ± 0.74 ‰ are significantly different from that of the nearby Miller Range blue ice with δ¹⁵N = +102.80 ± 0.14 ‰ and δ¹⁸O = +43.11 ± 0.64 ‰. This difference is notable, because if the meteorite had been contaminated with nitrate from the blue ice, the δ¹⁵N values should be the same. More importantly, the δ¹⁵N is similar to the uncontaminated Tissint Mars meteorite with δ¹⁵N = -4.5 ‰. These findings suggest a martian origin of the ClO₄^–, ClO₃^– and NO₃^– in EETA79001, and in conjunction with previous discoveries, support the hypothesis that they are present and ubiquitous on Mars. The presence of ClO₃^– in EETA79001 suggests the accompanying presence of other highly oxidizing oxychlorines such as ClO₂^– or ClO^–, produced both by UV oxidation of Cl^– and γ- and x-ray radiolysis of ClO₄^–. Since such intermediary species may contribute to oxidization of organic compounds, only highly refractory and/or well-protected organics are likely to survive. The global presence of ClO₄^–, ClO₃^–, and NO₃^–, has broad implications for the planet-wide water cycle, formation of brines, human habitability, organics, and life.

Reference
Kounaves SP, Carrier BL, O’Neil GD, Stroble ST and Claire MW (in press) Evidence of martian perchlorate, chlorate, and nitrate in Mars meteorite EETA79001: implications for oxidants and organics. Icarus
[doi:10.1016/j.icarus.2013.11.012]
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Jian-Yang Li¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Planetary Science Institute, 1700 E. Ft. Lowell Road, Suite 106, Tucson, AZ 85719, USA

We report results from broadband visible images of comet C/2012 S1 (ISON) obtained with the Hubble Space Telescope Wide Field Camera 3 on 2013 April 10. C/ISON’s coma brightness follows a 1/ρ (where ρ is the projected distance from the nucleus) profile out to 5000 km, consistent with a constant speed dust outflow model. The turnaround distance in the sunward direction suggests that the dust coma is composed of sub-micron-sized particles emitted at speeds of tens of m s^-1. A(θ)fρ, which is commonly used to characterize the dust production rate, was 1340 and 1240 cm in the F606W and F438W filters, respectively, in apertures <1.”6 in radius. The dust colors are slightly redder than solar, with a slope of 5.0% ± 0.2% per 100 nm, increasing to >10% per 100 nm 10,000 km down the tail. The colors are similar to those of comet C/1995 O1 (Hale-Bopp) and other long-period comets, but somewhat bluer than typical values for short-period comets. The spatial color variations are also reminiscent of C/Hale-Bopp. A sunward jet is visible in enhanced images, curving to the north and then tailward in the outer coma. The 1.”6 long jet is centered at a position angle of 291°, with an opening angle of ~45°. The jet morphology remains unchanged over 19 hr of our observations, suggesting that it is near the rotational pole of the nucleus, and implying that the pole points to within 30° of (R.A., decl.) = (330°, 0°). This pole orientation indicates a high obliquity of 50°-80°.

Reference
Li J-Y et al. (2013) Characterizing the Dust Coma of Comet C/2012 S1 (ISON) at 4.15 AU from the Sun. The Astrophysical Journal – Letters 342:960-963.
[doi:10.1088/2041-8205/779/1/L3]

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Minitti^a et al. (>>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aApplied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA

During Martian solar days 57–100, the Mars Science Laboratory Curiosity rover acquired and processed a solid (sediment) sample and analyzed its mineralogy and geochemistry with the Chemistry and Mineralogy and Sample Analysis at Mars instruments. An aeolian deposit—herein referred to as the Rocknest sand shadow—was inferred to represent a global average soil composition and selected for study to facilitate integration of analytical results with observations from earlier missions. During first-time activities, the Mars Hand Lens Imager (MAHLI) was used to support both science and engineering activities related to sample assessment, collection, and delivery. Here we report on MAHLI activities that directly supported sample analysis and provide MAHLI observations regarding the grain-scale characteristics of the Rocknest sand shadow. MAHLI imaging confirms that the Rocknest sand shadow is one of a family of bimodal aeolian accumulations on Mars—similar to the coarse-grained ripples interrogated by the Mars Exploration Rovers Spirit and Opportunity—in which a surface veneer of coarse-grained sediment stabilizes predominantly fine-grained sediment of the deposit interior. The similarity in grain size distribution of these geographically disparate deposits support the widespread occurrence of bimodal aeolian transport on Mars. We suggest that preservation of bimodal aeolian deposits may be characteristic of regions of active deflation, where winnowing of the fine-sediment fraction results in a relatively low sediment load and a preferential increase in the coarse-grained fraction of the sediment load. The compositional similarity of Martian aeolian deposits supports the potential for global redistribution of fine-grained components, combined with potential local contributions.

Reference
Minitti ME et al. (in press) MAHLI at the Rocknest sand shadow: Science and science-enabling activities. Journal of Geophysical Research – Planets
[doi:10.1002/2013JE004426]
Published by arrangement with John Wiley & Sons

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Howard^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aInstitute for Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA

We currently seek a copyright agreement with Nature to display abstracts of their cosmochemistry related publications.

Reference
Howard AW et al. (2013) A rocky composition for an Earth-sized exoplanet. Nature 503:381–384.
[doi:10.1038/nature12767]

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Pepe^a et al. (>>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aObservatoire Astronomique de l’Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland

We currently seek a copyright agreement with Nature to display abstracts of their cosmochemistry related publications.

Reference
Pepe F et al. (2013) An Earth-sized planet with an Earth-like density. Nature 503:377–380.
[doi:10.1038/nature12768]

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D. Bodewits^a, J.-B. Vincent^b and M.S.P. Kelley^a

^aDepartment of Astronomy, University of Maryland, College Park MD 20742-2421, USA
^bMax-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany

Asteroid (596) Scheila was the first object for which the immediate aftermath of an inter-asteroidal collision was observed. In Dec. 2010, the 113 km-sized asteroid was impacted by a smaller asteroid of less than 100 m in diameter. The scale of the impactor was established by observations of fading ejecta plumes. Comparison of the light curves obtained before and after the impact allowed us to assess how much of Scheila’s surface was altered. Cratering physics based on the impactor size suggests that the size of the affected area is larger than expected, (effective radii of 3.5 – 10 km depending on the change in the albedo of the surface). Similar but more localized albedo changes have been observed on Vesta and the Martian moons, but are not understood. Empirical laws describing ejecta blankets however indicate that at distances between 3.5 – 10 km from the crater, Scheila’s surface would be covered by a thin layer 2 mm to 2 cm thick. This dusting, possibly mixed with bright impactor material may be enough to explain to observed brightness increase. Our results show that sub-critical impacts may play a significant role in processing the surfaces of asteroids. The large effect of small impacts on asteroidal light curves complicate shape modeling.

Reference
Bodewits D, Vincent J-B and Kelley MSP (in press) Scheila’s Scar: Direct Evidence of Impact Surface Alteration on a Primitive Asteroid. Icarus
[doi:10.1016/j.icarus.2013.11.003]
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S. Mazrouei^a, M.G. Daly^a, O.S. Barnouin^b, C.M. Ernst^b, I. DeSouza^a

^aThe Centre for Research in Earth and Space Science, York University, Toronto, Ont., Canada, M3J 1P3
^bThe Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099, USA

Asteroid 25143 Itokawa is a small elongated asteroid with two distinct parts. The evolution of this two-part body has been the source of speculation. The scenarios for the formation of this asteroid include: two-body capture, catastrophic disruption and rapid reaccretion, YORP spin-up and mass shedding, and disruption (or partial disruption) with two-body reaccretion. In this paper we use the global and regional analyses of block populations and size-frequency distributions as evidence of the probable evolutionary history of Itokawa. The block sample used in this study is believed to be complete for blocks of size >6m and consists of a sample more than twice as large as previous known studies.
Although block size frequency distributions hint at different evolutionary paths for the head and the body, their differences are not statistically significant. The distribution of blocks across each body provides clues as to the histories of each body. The head is populated in a spherically symmetric fashion while the body has a distinct equatorial peak. When considering that the head and the body may have been separate entities for a period of time and estimating a rotational axis using minimum rotational energy considerations, the preferential equatorial distribution becomes even more pronounced. We interpret this as excellent evidence for the partial disruption of a proto-Itokawa, subsequent planarization of a debris field and reaccretion of the head and the body into its present configuration.

Reference
Mazrouei S, Daly MG, Barnouin OS, Ernst CM and DeSouza I (in press) Block Distributions on Itokawa. Icarus
[doi:10.1016/j.icarus.2013.11.010]
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