M. Laneuville^a, M.A. Wieczorek^a, D. Breuer^b, J. Aubert^a, G. Morard^c, T. Rückriemen^b

^aInstitut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, Case 7011, 35 rue Hélène Brion, 75205 Paris Cedex 13, France
^bInstitute of Planetary Research, German Aerospace Center (DLR), 6 Rutherfordstraße 2, 12489 Berlin, Germany
^cInstitut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Universités – UPMC Univ Paris 06, UMR CNRS 7590, Muséum National d’Histoire Naturelle, IRD UMR 206, 4 Place Jussieu, F-75005 Paris, France

The Moon does not possess an internally generated magnetic field at the present day, but extensive evidence shows that such a field existed between at least 4.2 and 3.56 Ga ago. The existence of a metallic lunar core is now firmly established, and we investigate the influence of inner core growth on generating a lunar core dynamo. We couple the results of a 3-D spherical thermochemical convection model of the lunar mantle to a 1-D thermodynamic model of its core. The energy and entropy budget of the core are computed to determine the inner core growth rate and its efficiency to power a dynamo. Sulfur is considered to be the main alloying element and we investigate how different sulfur abundances and initial core temperatures affect the model outcomes. For reasonable initial conditions, a solid inner core between 100 and 200 km is always produced. During its growth, a surface magnetic field of about 0.3 μT is generated and is predicted to last several billion years. Though most simulations predict the existence of a core dynamo at the present day, one way to stop magnetic field generation when the inner core is growing is by a transition between a bottom–up and top–down core crystallization scheme when the sulfur content becomes high enough in the outer core. According to this hypothesis, a model with about 6 to 8 wt.% sulfur in the core would produce a 120–160 km inner core and explain the timing of the lunar dynamo as constrained by paleomagnetic data.

Reference
Laneuville M, Wieczorek MA, Breuer D, Aubert J, Morard G and Rückriemen T (in press) A long-lived lunar dynamo powered by core crystallization. Earth and Planetary Science Letters
[doi:10.1016/j.epsl.2014.05.057]
Copyright Elsevier

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Ilya G. Usoskin^a and Gennady A. Kovaltsov^b

^aReSoLVE Center of Excellence and Sodankylä Geophysical Observatory (Oulu unit) University of Oulu, Finland
^bIoffe Physical-Technical Institute, St.Petersburg, Russia

A mysterious increase of radiocarbon ¹⁴C ca. 775 AD in the Earth’s atmosphere has been recently found by Miyake et al. (Nature, 486, 240, 2012). A possible source of this event has been discussed widely, the most likely being an extreme solar energetic particle event. A new exotic hypothesis has been presented recently by Liu et al. (Sci. Rep., 4, 3728, 2014) who proposed that the event was caused by a cometary impact on Earth bringing additional ¹⁴C to the atmosphere. Here we calculated a realistic mass and size of such a comet to show that it would have been huge (≈100 km across and 10¹⁷-10²⁰ gram of mass) and would have produced a disastrous geological/biological impact on Earth. The absence of an evidence for such a dramatic event makes this hypothesis invalid.

Reference
Usoskin IG and Kovaltsov GA (in press) The carbon-14 spike in the 8th century was not caused by a cometary impact on Earth. Icarus
[doi:10.1016/j.icarus.2014.06.009]
Copyright Elsevier

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Shinya Wanajo¹, Yuichiro Sekiguchi², Nobuya Nishimura³, Kenta Kiuchi², Koutarou Kyutoku⁴ and Masaru Shibata²

¹iTHES Research Group, RIKEN, Wako, Saitama 351-0198, Japan
²Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan
³Astrophysics, EPSAM, Keele University, Keele ST5 5BG, UK
⁴Department of Physics, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA

Recent studies suggest that binary neutron star (NS-NS) mergers robustly produce heavy r-process nuclei above the atomic mass number A ~ 130 because their ejecta consist of almost pure neutrons (electron fraction of Y_e < 0.1). However, the production of a small amount of the lighter r-process nuclei (A  90-120) conflicts with the spectroscopic results of r-process-enhanced Galactic halo stars. We present, for the first time, the result of nucleosynthesis calculations based on the fully general relativistic simulation of a NS-NS merger with approximate neutrino transport. It is found that the bulk of the dynamical ejecta are appreciably shock-heated and neutrino processed, resulting in a wide range of Y_e (0.09-0.45). The mass-averaged abundance distribution of calculated nucleosynthesis yields is in reasonable agreement with the full-mass range (A  90-240) of the solar r-process curve. This implies, if our model is representative of such events, that the dynamical ejecta of NS-NS mergers could be the origin of the Galactic r-process nuclei. Our result also shows that radioactive heating after ~1 day from the merging, which gives rise to r-process-powered transient emission, is dominated by the β-decays of several species close to stability with precisely measured half-lives. This implies that the total radioactive heating rate for such an event can be well constrained within about a factor of two if the ejected material has a solar-like r-process pattern.

Reference
Wanajo S, Sekiguchi Y, Nishimura N, Kiuchi K, Kyutoku K and Shibata M (2014) Production of All the r-process Nuclides in the Dynamical Ejecta of Neutron Star Mergers. The Astrophysical Journal Letters 789:L39.
[doi:10.1088/2041-8205/789/2/L39]

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