L. J. Hallis^a,b,c, M. Anand^b,c, S. Strekopytov^c

^aHawai‘i Institute of Geophysics and Planetology, Pacific Ocean Science & Technology (POST) Building, University of Hawai‘i, 1680 East-West Road, Honolulu, HI 96822, US
^bDepartment of Physical Sciences, CEPSAR, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
^cDepartment of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom

The heterogeneous-source model of mare basalt formation indicates that Lunar Magma Ocean (LMO) overturn produced an uneven mixture of early-formed olivine and pyroxene, and late-formed, ilmenite-rich cumulates, which subsequently partially melted to give rise to mare magmas. These heterogeneous cumulate source regions would not only have been characterised by different mineral modal abundances, but also by different trace element compositions. The aim of this work was to investigate the petrology and geochemistry of a diverse suite of Apollo mare basalts, and utilise trace-element modelling in order to understand their petrogenetic history. Chemical modelling confirms that the mare basalts were produced by relatively small degrees of partial melting (< 10%) of the LMO cumulates, and that the dominant melting type (batch vs. fractional) varies among different basalt groups. Similarly, single-source mineralogy cannot be applied to all mare basalt types, confirming that the lunar mantle was heterogeneous at the time of generation of mare magmas. Plagioclase is not required in the source of most mare basalts, with the notable exception of the Apollo 14 high-Al basalts. Addition of more than 1% plagioclase to the source of other basalts produces weaker negative Eu anomalies than those observed in the samples. AFC calculations demonstrate the compositional differences between materials assimilated into the Apollo 14 high-Al and Apollo 11 high-K mare basalt partial melts, highlighting the complexities of mare basalt petrogenesis.

Reference
Hallis LJ, Anand M and Strekopytov S (in press) Trace-element modelling of mare basalt parental melts: Implications for a heterogeneous lunar mantle. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.012]
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Isabel Egea-González^a and Javier Ruiz^b

^aInstituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
^bDepartamento de Geodinámica, Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, 28040 Madrid, Spain

Mercury is covered by a megaregolith layer, which constitutes a poor thermally conducting layer that must have an influence on the thermal state and evolution of the planet, although most thermal modeling or heat flow studies have overlooked it. In this work we have calculated surface heat flows and subsurface temperatures from the depth of thrust faults associated with several prominent lobate scarps on Mercury, valid for the time of the formation of these scarps, by solving the heat equation and taking into account the insulating effects of a megaregolith layer. We conclude that megaregolith insulation could have been an important factor limiting heat loss and therefore interior cooling and contraction of Mercury. As Mercurian megaregolith properties are not very well known, we also analyze the influence of these properties on the results, and discuss the consequences of imposing the condition that the total radioactive heat production must be lower than the total surface heat loss (this is, the Urey ratio, Ur, must be lower than 1) in a cooling and thermally contracting planet such as Mercury at the time of scarp emplacement. Our results show that satisfying the condition of Ur < 1 implies that the average abundances of heat-producing elements silicate layer is 0.4 times or less the average surface value, placing an upper bound on the bulk content of heat producing elements in Mercury’s interior.

Reference
Egea-González I and Ruiz J (in press) Influence of an insulating megaregolith on heat flow and crustal temperature structure of Mercury. Icarus
[doi:10.1016/j.icarus.2014.01.024]
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Samuel P. Kounaves^a, Nikos A. Chaniotakis^b, Vincent F. Chevrier^c, Brandi L. Carrier^a, Kaitlyn E. Folds^a, Victoria M. Hansen^a, Kyle M. McElhoney^a, Glen D. O’Neil^a, Andrew W. Weber^a

^aDepartment of Chemistry, Tufts University, Medford, MA, 02155, USA
^bDepartment of Chemistry, University of Crete, 71003 Iraklion, Crete, Greece
^cArkansas Center for Space and Planetary Science, University of Arkansas, Fayetteville, AK 72701, USA

In 2008 the Phoenix Mars lander Wet Chemistry Laboratory (WCL) measured 0.6 wt% of perchlorate (ClO₄^–) in the martian soil. A crucial question remaining unanswered is the identity of the parent salt phase(s) of the ClO₄^–. Due to the ClO₄^– ion’s high solubility and stability, its distribution, chemical forms, and interactions with water, could reveal much about the aqueous history of the planet. Until now, the presence of Ca(ClO₄)₂ as a parent salt has been considered unlikely because based on its eutectic point and model calculations, highly insoluble calcium carbonates and sulfates would serve as sinks for Ca²⁺ rather than Ca(ClO₄)_2. Thus, the dominant ClO₄^– parent salt has been assumed to be a hydrated form of Mg(ClO₄)₂. Here we report on the results of a new refined analysis of the Phoenix WCL sensor data, post-flight experiments run on a flight-spare WCL unit, and numerous laboratory analyses. The results show that the response of the Ca²⁺ sensor is extremely sensitive to the counter ion of the ClO₄^– salt, and that addition of martian soil to the Phoenix WCL that contained only Mg(ClO₄)₂ or Ca(ClO₄)₂ would have produced a very different response than what was observed on Mars. A series of analyses were run with Ca²⁺ sensors and calibration solutions identical to those used on Phoenix, and with a Mars simulant formulation known to give the same results as on Mars. The response of the Ca²⁺ sensor at various ratios of added Mg(ClO₄)₂ to Ca(ClO₄)₂ gave the best fit to the Phoenix data with a sample containing ∼ 60% Ca(ClO₄)₂ and 40% Mg(ClO₄)₂. These results suggest that the Ca(ClO₄)₂ in the Phoenix soil has not been in contact with liquid water and thus did not form by evaporation or sublimation processes. Had the highly soluble Ca(ClO₄)₂ come into contact with liquid water, then the presence of soluble sulfates would have, on evaporation formed only CaSO₄. The presence of Ca(ClO₄)₂ and Mg(ClO₄)₂ phases at roughly the CaCO₃ to MgCO₃ ratio suggests that since their production, the ClO₄^–phases have remained in a severely arid environment, with minimal or no liquid water interaction. The formation of the Ca(ClO₄)₂ and Mg(ClO₄)₂ is also consistent with the interaction of atmospherically deposited HClO₄ with Ca- and Mg-carbonates and may also contribute to CO₂ enrichment of ¹⁸O and may contribute in explaining why carbonates on the surface are at much lower levels than expected from an earlier global wet and warm period.

Reference
Kounaves SP, Chaniotakis NA, Chevrier VF, Carrier BL, Folds KE, Hansen VM, McElhoney KM, O’Neil GD and Weber AW (in press) Identification of the perchlorate parent salts at the Phoenix Mars landing site and possible implications. Icarus
[doi:10.1016/j.icarus.2014.01.016]

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Paul Douglas Archer Jr.¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Jacobs, NASA Johnson Space Center, Houston, Texas, USA

The Sample Analysis at Mars (SAM) instrument on the Mars Science Laboratory (MSL) rover Curiosity detected evolved gases during thermal analysis of soil samples from the Rocknest aeolian deposit in Gale Crater. Major species detected (in order of decreasing molar abundance) were H₂O, SO₂, CO₂, and O₂, all at the µmol level, with HCl, H₂S, NH₃, NO, and HCN present at the tens to hundreds of nmol level. We compute weight % numbers for the major gases evolved by assuming a likely source and calculate abundances between 0.5 and 3 wt.%. The evolution of these gases implies the presence of both oxidized (perchlorates) and reduced (sulfides or H-bearing) species as well as minerals formed under alkaline (carbonates) and possibly acidic (sulfates) conditions. Possible source phases in the Rocknest material are hydrated amorphous material, minor clay minerals, and hydrated perchlorate salts (all potential H₂O sources), carbonates (CO₂), perchlorates (O₂ and HCl), and potential N-bearing materials (e.g., Martian nitrates, terrestrial or Martian nitrogenated organics, ammonium salts) that evolve NH₃, NO, and/or HCN. We conclude that Rocknest materials are a physical mixture in chemical disequilibrium, consistent with aeolian mixing, and that although weathering is not extensive, it may be ongoing even under current Martian surface conditions.

Reference
Archer PD Jr.(in press) Abundances and implications of volatile-bearing species from evolved gas analysis of the Rocknest aeolian deposit, Gale Crater, Mars. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004493]
Published by arrangement with John Wiley & Sons

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F. E. DeMeo^1,2 and B. Carry^3,4

¹Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-16, Cambridge, Massachusetts 02138, USA
²Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology (MIT), 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
³Institut de Mécanique Céleste et de Calcul des Éphémérides, Observatoire de Paris, UMR8028 CNRS, 77 avenue Denfert-Rochereau, 75014 Paris, France
⁴European Space Astronomy (ESA) Centre, PO Box 78, Villanueva de la Cañada 28691, Madrid, Spain

We are still seeking a copyright agreement with Nature to display their abstracts.

Reference
DeMeo FE and Carry B (2014) Solar System evolution from compositional mapping of the asteroid belt. Nature 505:629–634.
[doi:10.1038/nature12908]

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Emmanuel Jacquet

Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St Georges Street, Toronto, ON M5S 3H8, Canada

Primitive meteorites are dominated by millimeter-size silicate spherules called chondrules. The nature of the high-temperature events that produced them in the early solar system remains enigmatic. Beside their thermal history, one important clue is provided by their size which shows remarkably little variation (less than a factor of 6 for the mean chondrule radius of most chondrites) despite the extensive range of ages and heliocentric distances sampled. It is however unclear whether chondrule size is due to the chondrule melting process itself, or has been simply inherited from the precursor material, or yet results from some sorting process. I examine these different possibilities in terms of their analytical size predictions. Unless the chondrule-forming “window” was very narrow, radial sorting can be excluded as size-determining processes because of the large variations it would predict. Molten planetesimal collision or impact melting models, which derive chondrules from the fragmentation of larger melt bodies, would likewise predict too much size variability by themselves; more generally any size modification during chondrule formation is limited in extent by evidence from compound chondrules and the considerable compositional variability of chondrules. Turbulent concentration would predict a low size variability but lack of evidence of any accretion bias in carbonaceous chondrites may be difficult to reconcile with any form of local sorting upon agglomeration. Growth by sticking (especially if bouncing-limited) of aggregates as chondrule precursors would yield limited variations of their final radius in space and time, and would be consistent with the relatively similar size of other chondrite components such as refractory inclusions. This suggests that the chondrule-melting process(es) simply melted such nebular aggregates with little modification of mass.

Reference
Jacquet E (in press) The quasi-universality of chondrule size as a constraint for chondrule formation models. Icarus
[doi:10.1016/j.icarus.2014.01.012]
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Colin M. Dundas¹, Shane Byrne², Alfred S. McEwen², Michael T. Mellon³, Megan R. Kennedy⁴, Ingrid J. Daubar², Lee Saper⁴

¹Astrogeology Science Center, U. S. Geological Survey, Flagstaff, Arizona, USA
²Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
³Southwest Research Institute, Boulder, Colorado, USA
⁴Malin Space Science Systems, San Diego, California, USA

Twenty small new impact craters or clusters have been observed to excavate bright material inferred to be ice at mid-latitudes and high latitudes on Mars. In the northern hemisphere, the craters are widely distributed geographically and occur at latitudes as low as 39°N. Stability modeling suggests that this ice distribution requires a long-term average atmospheric water vapor content around 25 precipitable micrometers, more than double the present value, which is consistent with the expected effect of recent orbital variations. Alternatively, near-surface humidity could be higher than expected for current column abundances if water vapor is not well mixed with atmospheric CO₂, or the vapor pressure at the ice table could be lower due to salts. Ice in and around the craters remains visibly bright for months to years, indicating that it is clean ice rather than ice-cemented regolith. Although some clean ice may be produced by the impact process, it is likely that the original ground ice was excess ice (exceeding dry soil pore space) in many cases. Observations of the craters suggest small-scale heterogeneities in this excess ice. The origin of such ice is uncertain. Ice lens formation by migration of thin films of liquid is most consistent with local heterogeneity in ice content and common surface boulders, but in some cases, nearby thermokarst landforms suggest large amounts of excess ice that may be best explained by a degraded ice sheet.

Reference
Dundas CM, Byrne S, McEwen AS, Mellon MT, Kennedy MR, Daubar IJ and Saper L (in press) HiRISE observations of new impact craters exposing Martian ground ice. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004482]
Published by arrangement with John Wiley & Sons

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Kirsten L. Siebach and John P. Grotzinger

Geological and Planetary Sciences Division, California Institute of Technology, Pasadena, California, USA

While the presence of water on the surface of early Mars is now well known, the volume, distribution, duration, and timing of the liquid water have proven difficult to determine. This study makes use of a distinctive boxwork-rich sedimentary layer on Mount Sharp to map fluid-based cementation from orbital imagery and estimate the minimum volume of water present when this sedimentary interval was formed. The boxwork structures on Mount Sharp are decameter-scale light-toned polygonal ridges that are unique compared to previous observations of Martian fractured terrain because they are parallel-sided ridges with dark central linear depressions. This texture and the sedimentary setting strongly imply that the ridges are early diagenetic features formed in the subsurface phreatic groundwater zone. High-resolution orbital imagery was used to map the volume of light-toned cemented ridges. Based on the cemented volume, a minimum of 5.25 × 105 m³ of cement was deposited within the fractures. Using a brine composition based on observations of other Martian cements and modeling the degree of evaporation, each volume of cement requires 800–6700 pore volumes of water, so the mapped boxwork ridge cements require a minimum of 0.43 km³ of water. This is a significant amount of groundwater that must have been present at the −3620 m level, 1050 m above the current floor of Gale Crater, providing both a new constraint on the possible origins of Mount Sharp and a possible future science target for the Curiosity rover where large volumes of water were present, and early mineralization could have preserved a once-habitable environment.

Reference
Siebach KL and Grotzinger JP (in press) Volumetric estimates of ancient water on Mount Sharp based on boxwork deposits, Gale Crater, Mars. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004508]
Published by arrangement with John Wiley & Sons

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Miriam Sharp^a, Iva Gerasimenko^a, Lorne C. Loudin^b, Jingao Liu^a,c, Odette B. James^d, Igor S. Puchtel^a and Richard J. Walker^a

^aDepartment of Geology, University of Maryland, College Park, MD 20742, USA
^bDepartment of Earth Sciences, University of New Hampshire, Durham, NH 03824, USA
^cDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G2E3, Canada
^dEmeritus U.S.Geological Survey, Reston, VA 20192, USA

Concentrations of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, and Pd and ¹⁸⁷Os/¹⁸⁸Os isotopic compositions are reported for seven Apollo 17 impact melt rocks. These data are used to examine the dominant chemical signature of the impactor that formed the melts. Six of the samples (72355, 72435, 72535, 76035, 76055, and 76135) have poikilitic textures; one sample (73235) has an aphanitic texture. Data for the samples define linear correlations when Ir is plotted versus other HSE concentrations, with y-intercepts indistinguishable from zero for most HSE in most rocks. Scatter about some of the trends, and occasional trends with positive y-intercepts, indicate either mixing of additional components that are heterogeneously distributed within several rocks, or modest fractionation of some HSE by volatilization, crystal fractionation, or other processes, during formation and evolution of the melt sheet. There is no statistical difference between the aphanitic and poikilitic samples in terms of HSE ratios after visible granulitic clasts were removed from aphanite 73235. Hence, earlier speculations that the two types of impact melt rocks at this site may have been generated by different impactors are not supported by our data.
Most Apollo 17 samples examined here and in prior studies are characterized by very similar HSE signatures, consistent with a common impactor. These samples are characterized by elevated Ru/Ir, Pd/Ir, and Re/Os, relative to most chondrites. Collectively, the data indicate that the impactor was characterized by the following HSE ratios (2σ): Re/Ir 0.093±0.020, Os/Ir 1.03±0.28, Ru/Ir 1.87±0.30, Pt/Ir 2.36±0.31, Pd/Ir 1.85±0.41, and present-day ¹⁸⁷Os/¹⁸⁸Os of 0.1322±0.0013. The results most likely mean that the impactor was a body with a bulk composition that was just outside the range of meteoritic compositions currently sampled on Earth.

Reference
Sharp M, Gerasimenko I, Loudin LC, Liu J, James OB, Puchtel IS and Walker RJ (in press) Characterization of the Dominant Impactor Signature for Apollo 17 Impact Melt Rocks. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.014]
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David Polishook^a, Nicholas Moskovitz^a, Richard P. Binzel^a, Francesca E. DeMeo^a,b, David Vokrouhlický^c, Jindřich Žižka^c, Dagmara Oszkiewicz^d

^aDepartment of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
^bHarvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
^cInstitute of Astronomy, Charles University, Prague, V Holešovičkách 8, CZ – 18000 Prague 8, Czech Republic
^dAstronomical Observatory Institute, Faculty of Physics, A. Mickiewicz University, Słoneczna 36, 60-286, Poznań, Poland

The rotational-fission of a “rubble-pile” structured asteroid can result in an “asteroid pair” – two un-bound asteroids sharing nearly identical heliocentric orbits. Models suggest that this mechanism exposes material from below the progenitor surface that previously had never have been exposed to the weathering conditions of space. Therefore, the surfaces of asteroid pairs offer the opportunity to observe non-weathered “fresh” spectra.
Here we report near-infrared spectroscopic observations of 31 asteroids in pairs. In order to search for spectral indications of fresh surfaces we analyze their spectral slopes, parameters of their 1μm absorption band and taxonomic classification. Additionally, through backward dynamical integration we estimate the time elapsed since the disintegration of the pairs’ progenitors.
Analyzing the 19 ordinary chondrite-like (S-complex) objects in our sample, we find two Q-type asteroids (19289 and 54827) that are the first of their kind to be observed in the main-belt of asteroids over the full visible and near-infrared range. This solidly demonstrates that the Q-type taxonomy is not limited to the NEA population.
The pairs in our sample present a range of fresh and weathered surfaces with no clear evidence for a correlation with the ages of the pairs. However, our sample includes “old” pairs (2×10⁶ ⩾ age ⩾ 1×10⁶ years) that present relatively low, meteoritic-like spectral slopes (<0.2% per μm). This illustrates a timescale of at least ∼2 million years before an object develops high spectral slope that is typical for S-type asteroids.
We discuss three mechanisms that explain the existence of weathered pairs with young dynamical ages and find that the “secondary fission” model (Jacobson and Scheeres 2011) is the most robust with our observations. In this mechanism an additional and subsequent fission of the secondary component contributes the lion share of fresh material that re-settles on the primary’s surface and recoats it with fresh material. If the secondary breaks loose from the vicinity of the primary before its “secondary fission”, this main source of fresh dust is avoided. We prefer this secondary fission model since i) the secondary members in our sample present “fresh” parameters that tend to be “fresher” than their weathered primaries; ii) most of the fresh pairs in our sample have low size ratios between the secondary and the primary; iii) 33% of the primaries in our sample are fresh, similar to the prediction set by the secondary fission model (Jacobson and Scheeres 2011); iv) known satellites orbit two of the pairs in our sample with low size ratio (D₂/D₁) and fresh surface; v) there is no correlation between the weathering state and the primary shape as predicted by other models.

Reference
Polishook D, Moskovitz N, Binzel RP, DeMeo FE, Vokrouhlický D, Žižka J and Oszkiewicz D (in press) Observations of “Fresh” and Weathered Surfaces on Asteroid Pairs and Their Implications on the Rotational-Fission Mechanism. Icarus
[doi:10.1016/j.icarus.2014.01.014]
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