A. J. Evans^1,2, M. T. Zuber¹, B. P. Weiss¹ and S. M. Tikoo¹

¹Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
²Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA

While recent analyses of lunar samples indicate the Moon had a core dynamo from at least 4.2–3.56 Ga, mantle convection models of the Moon yield inadequate heat flux at the core-mantle boundary to sustain thermal core convection for such a long time. Past investigations of lunar dynamos have focused on a generally homogeneous, relatively dry Moon, while an initial compositionally stratified mantle is the expected consequence of a postaccretionary lunar magma ocean. Furthermore, recent re-examination of Apollo samples and geophysical data suggests that the Moon contains at least some regions with high water content. Using a finite element model, we investigate the possible consequences of a heterogeneously wet, compositionally stratified interior for the evolution of the Moon. We find that a postoverturn model of mantle cumulates could result in a core heat flux sufficiently high to sustain a dynamo through 2.5 Ga and a maximum surface, dipolar magnetic field strength of less than 1 μT for a 350-km core and near ~2 μT for a 450-km core. We find that if water was transported or retained preferentially in the deep interior, it would have played a significant role in transporting heat out of the deep interior and reducing the lower mantle temperature. Thus, water, if enriched in the lower mantle, could have influenced core dynamo timing by over 1.0 Gyr and enhanced the vigor of a lunar core dynamo. Our results demonstrate the plausibility of a convective lunar core dynamo even beyond the period currently indicated by the Apollo samples.

Reference
Evans AJ, Zuber MT, Weiss BP and Tikoo SM (in press) A wet, heterogeneous lunar interior: Lower mantle and core dynamo evolution. Journal of Geophysical Research: Planets 
[doi:10.1002/2013JE004494]
Published by arrangement with John Wiley & Sons

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T. Tsujimoto¹ and T. Shigeyama²

¹National Astronomical Observatory of Japan, Mitaka-shi, 181-8588 Tokyo Japan
²Research Center for the Early Universe, Graduate School of Science, University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

The origin of r-process elements remains unidentified and still puzzles us. The recent discovery of evidence for the ejection of r-process elements from a short-duration γ-ray burst singled out neutron star mergers (NSMs) as their origin. In contrast, core-collapse supernovae are ruled out as the main origin of heavy r-process elements (A > 110) by recent numerical simulations. However, the properties characterizing NSM events – their rarity and high yield of r-process elements per event – have been claimed to be incompatible with the observed stellar records on r-process elements in the Galaxy. We add to this picture with our results, which show that the observed constant [r-process/H] ratio in faint dwarf galaxies and one star unusually rich in r-process in the Sculptor galaxy agree well with this rarity of NSM events. Furthermore, we found that a large scatter in the abundance ratios of r-process elements to iron in the Galactic halo can be reproduced by a scheme that incorporates an assembly of various protogalactic fragments, in each of which r-process elements supplied by NSMs pervade the whole fragment while supernovae distribute heavy elements only inside the regions swept up by the blast waves. Our results demonstrate that NSMs occurring at Galactic rate of 12–23 Myr^-1 are the main site of r-process elements, and we predict the detection of gravitational waves from NSMs at a high rate with upcoming advanced detectors.

Reference
Tsujimoto T and Shigeyama T (2014) Enrichment history of r-process elements shaped by a merger of neutron star pairs.  Astronomy & Astrophysics 565:AL5.
[doi:10.1051/0004-6361/201322175]
Reproduced with permission © ESO

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Thomas M. McCollom¹, Bethany L. Ehlmann^3,4, Alian Wang⁵, Brian M. Hynek^1,2, Bruce Moskowitz^6,7 and Thelma S. Berquó⁸

¹Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80309, U.S.A.
²Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, U.S.A.
³California Institute of Technology, Pasadena, California 91125, U.S.A.
⁴Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, U.S.A.
⁵Deptartment of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University, St. Louis, Missouri 63130, U.S.A.
⁶Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.
⁷Institute for Rock Magnetism, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.
⁸Department of Physics, Concordia College, Moorhead, Minnesota 56562, U.S.A.

Natroalunite containing substantial amounts of Fe occurs as a prominent secondary phase during acid-sulfate alteration of pyroclastic basalts in volcanic fumaroles in Nicaragua and elsewhere, and has been observed in laboratory simulations of acid-sulfate alteration as well. Reaction path models constrained by field and experimental observations predict that Fe-rich natroalunite should also form as a major secondary phase during alteration of martian basalt under similar circumstances. Here, we evaluate the potential to use spectroscopic methods to identify minerals from the alunite group with chemical compositions intermediate between natroalunite and natrojarosite on the surface of Mars, and to remotely infer their Fe contents. X-ray diffraction and spectroscopic measurements (Raman, visible/near infrared, mid-infrared, Mössbauer) were obtained for a suite of synthetic solid solutions with a range of Fe contents ranging from natroalunite to natrojarosite. In the visible/near infrared, minerals with intermediate compositions display several spectral features not evident in end-member spectra that could be used to remotely identify these minerals and infer their composition. In addition, Raman spectra, mid-infrared spectra, and X-ray diffraction peaks all show systematic variation with changing Fe content, indicating that these methods could potentially be used to infer mineral compositions as well. The results suggest that alunite group minerals with intermediate Fe compositions may be able to account for some visible/near-infrared and Mössbauer spectral features from Mars that had previously been unidentified or attributed to other phases. Overall, our findings indicate that consideration of solid solutions may lead to new identifications of alunite group minerals on the surface of Mars, and raise the possibility that minerals with compositions intermediate between natroalunite and natrojarosite may be widely distributed on the planet.

Reference
McCollom TM, Ehlmann BL, Wang A, Hynek BM, Moskowitz B and Berquó TS (2014) Detection of iron substitution in natroalunite-natrojarosite solid solutions and potential implications for Mars. American Mineralogist 99:948-964.
[doi:10.2138/am.2014.4617]
Copyright: The Mineralogical Society of America

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