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]
Copyright Elsevier

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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]
Copyright Elsevier

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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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