Nancy L. Chabot^a, E. Alex Wollack^b, Rachel L. Klima^a and Michelle E. Minitti^a

^aThe Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
^bPrinceton University, Princeton, NJ 08540, USA

The recent discovery of high S concentrations on the surface of Mercury by spacecraft measurements from the MESSENGER mission provides the potential to place new constraints on the composition of Mercuryʼs large metallic core. In this work, we conducted a set of systematic equilibrium metal–silicate experiments that determined the effect of different metallic compositions in the Fe–S–Si system on the S concentration in the coexisting silicate melt. We find that metallic melts with a range of S and Si combinations can be in equilibrium with silicate melts with S contents consistent with Mercuryʼs surface, but that such silicate melts contain Fe contents lower than measured for Mercuryʼs surface. If Mercuryʼs surface S abundance is representative of the planetʼs bulk silicate composition and if the planet experienced metal–silicate equilibrium during planetary core formation, then these results place boundaries on the range of possible combinations of Si and S that could be present as the light elements in Mercuryʼs core and suggest that Mercuryʼs core likely contains Si. Except for core compositions with extreme abundances of Si, bulk Mercury compositions calculated by using the newly determined range of potential S and Si core compositions do not resemble primitive meteorite compositions.

Reference
Chabot NL, Wollack EA, Klima RL and Minitti ME (2014) Experimental constraints on Mercuryʼs core composition. Earth and Planetary Science Letters 390:199–208.
[doi:10.1016/j.epsl.2014.01.004]
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N.G. Rudraswami^a, M. Shyam Prasad^a, J.M.C. Plane^b, T. Berg^c, W. Feng^b and S. Balgar^a

^aNational Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403 004, India
^bSchool of Chemistry, University of Leeds, Leeds LS29JT, UK
^cInstitut für Physik, Johannes Gutenberg-Universität, Staudingerweg 7, D-55128 Mainz, Germany

Out of the three basic cosmic spherule types collected from the seafloor, RMNs (Refractory Metal Nuggets) have been reported from I-type spherules commonly, rarely from S-type spherules and never from the G-type spherules. Nuggets in the I-type cosmic spherules have formed by melting and complete oxidation during atmospheric entry, whereas no clear understanding emerged so far regarding the formation of the rare nuggets in S-type spherules. We collected cosmic spherules by raking the deep seafloor with magnets, and carried out systematic and sequential grinding, polishing and electron microscopic investigations on 992 cosmic spherules to identify RMNs. Fifty-four nuggets (RMNs) are identified, out of which 23, 26, and 5 nuggets are recovered from 23 I-, 21 S- and 5 G-type cosmic spherules, respectively.
The nuggets in all the three spherule types follow a pattern indicative of their formation by metal segregation during atmospheric entry due to heating and oxidation, however, there are differences in their elemental distribution patterns. The refractory metal elements (RMEs) in the I-type spherules show a sequence of volatilization form a chondritic source, however, the relatively volatile RMEs in these spherules seem to be either depleted or distributed in numerous smaller nuggets. However, RMNs in the G-type spherules show closer conformity to CI chondrites and do not have a large volatile RME depletion. Whereas, the RMEs in the nuggets found in the S-type spherules are enriched in the volatile as well as the refractory elements. Also all the spherules show enrichment patterns and elemental ratios that are close to CI composition for refractory elements suggesting a common mechanism of formation. Pulse heating during atmospheric entry seems to be an efficient mechanism for RME segregation into nuggets. The patterns of RME enrichment and elemental ratios when compared with the nuggets in CAIs, show marked variations, outlining their differences in the process of formation. In addition, we also discovered a fremdling-like object in a cosmic spherule which has a nugget encased in Fe-Ni and sulfide phases, similar to those typically observed in CAIs of CV or CO chondrites. The atmospheric entry for this rare cosmic spherule appears to have taken place at a high zenith angle with a low entry velocity, so that its volatile phases are well preserved.

Reference
Rudraswami NG, Prasad MS, Plane JMC, Berg T, Feng W and Balgar S (in press) Refractory Metal Nuggets In Different Types Of Cosmic Spherules. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.026]
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Matthew R.M. Izawa^a, Edward A. Cloutis^a, Daniel M. Applin^a, Michael A. Craig^b, Paul Mann^a and Matthew Cuddy^a

^aHyperspectral Optical Sensing for Extraterrestrial Reconnaissance Laboratory, Dept. Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba R3B 2E9, Canada
^bDepartment of Earth Sciences/Centre for Planetary Science and Exploration, University of Western Ontario, 1151 Richmond Street, London, Ontario N6A 5B7, Canada

Reflectance spectroscopy of Apollo lunar soil samples curated in an air- and water-free, sealed environment since recovery and return to Earth has been carried out under water-, oxygen-, CO₂– and organic-controlled conditions. Spectra of these pristine samples contain features near 3 μm wavelength similar to those observed from the lunar surface by the Chandrayaan-1 Moon Mineralogy Mapper (M³), Cassini Visual and Infrared Mapping Spectrometer (VIMS), and Deep Impact Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) High-Resolution Instrument (HRI) instruments. Spectral feature characteristics and inferred OH/H₂O concentrations are within the range of those observed by spacecraft instruments. These findings confirm that the 3 μm feature from the lunar surface results from the presence of hydration in the form of bound OH and H₂O. Implantation of solar wind H⁺ appears to be the most plausible formation mechanism for most of the observed lunar OH and H₂O.

Reference
Izawa MRM, Cloutis EA, Applin DM, Craig MA, Mann P and Cuddy M (2014) Laboratory spectroscopic detection of hydration in pristine lunar regolith. Earth and Planetary Science Letters 390:157–164.
[doi:10.1016/j.epsl.2014.01.007]
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Jessica J. Barnes^a,b, Romain Tartèse^a, Mahesh Anand^a,b, Francis M. McCubbin^c, Ian A. Franchi^a, Natalie A. Starkey^a, Sara S. Russell<sup>b

a</sup>Planetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
^bDepartment of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK
^cInstitute of Meteoritics, University of New Mexico, 200 Yale Blvd SE, Albuquerque, NM, USA

The recent discoveries of hydrogen (H) bearing species on the lunar surface and in samples derived from the lunar interior have necessitated a paradigm shift in our understanding of the water inventory of the Moon, which was previously considered to be a ‘bone-dry’ planetary body. Most sample-based studies have focused on assessing the water contents of the younger mare basalts and pyroclastic glasses, which are partial-melting products of the lunar mantle. In contrast, little attention has been paid to the inventory and source(s) of water in the lunar highlands rocks which are some of the oldest and most pristine materials available for laboratory investigations, and that have the potential to reveal the original history of water in the Earth–Moon system. Here, we report in-situ measurements of hydroxyl (OH) content and H isotopic composition of the mineral apatite from four lunar highlands samples (two norites, a troctolite, and a granite clast) collected during the Apollo missions. Apart from troctolite in which the measured OH contents in apatite are close to our analytical detection limit and its H isotopic composition appears to be severely compromised by secondary processes, we have measured up to ~2200 ppm OH in the granite clast with a weighted average δD of , and up to ∼3400 ppm OH in the two norites (77215 and 78235) with weighted averageδD values of −281±49‰ and −27±98‰, respectively. The apatites in the granite clast and the norites are characterised by higher OH contents than have been reported so far for highlands samples, and have H isotopic compositions similar to those of terrestrial materials and some carbonaceous chondrites, providing one of the strongest pieces of evidence yet for a common origin for water in the Earth–Moon system. In addition, the presence of water, of terrestrial affinity, in some samples of the earliest-formed lunar crust suggests that either primordial terrestrial water survived the aftermath of the putative impact-origin of the Moon or water was added to the Earth–Moon system by a common source immediately after the accretion of the Moon.

Reference
Barnes JJ, Tartèse R, Anand M, McCubbin FM, Franchi IA, Starkey NA, Russell SS (2014) The origin of water in the primitive Moon as revealed by the lunar highlands samples. Earth and Planetary Science Letters390:244–252.
[doi:10.1016/j.epsl.2014.01.015]
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