Christoph Burkhardt^a, Remco C. Hin^a,1, Thorsten Kleine^b, Bernard Bourdon^c

^aInstitute of Geochemistry and Petrology, Clausiusstrasse 25, ETH Zürich, CH-8092 Zürich, Switzerland
^bInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm Klemm-Strasse 10, 48149 Münster, Germany
^cLaboratoire de Géologie de Lyon, ENS Lyon and Université Claude Bernard Lyon 1, 46 Allée d’Italie, F-69364 Lyon, France
¹School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK.

Mass-dependent Mo isotope fractionation has been investigated for a wide range of meteorites including chondrites (enstatite, ordinary and carbonaceous chondrites), iron meteorites, and achondrites (eucrites, angrites and martian meteorites), as well as for lunar and terrestrial samples. Magmatic iron meteorites together with enstatite, ordinary and most carbonaceous chondrites define a common δ^98/95Mo value of−0.16±0.02‰ (relative to the NIST SRM 3134 Mo standard), which is interpreted to reflect the Mo isotope composition of bulk planetary bodies in the inner solar system. Heavy Mo isotope compositions for IAB iron meteorites most likely reflect impact-induced evaporative losses of Mo from these meteorites. Carbonaceous chondrites define an inverse correlation between δ^98/95Mo and metal content, and a positive correlation between δ^98/95Mo and matrix abundance. These correlations are mainly defined by CM and CK chondrites, and may reflect the heterogeneous distribution of an isotopically light metal and/or an isotopically heavy matrix component in the formation region of carbonaceous chondrites. Alternatively, the elevated δ^98/95Mo of the CM and CK chondrites could result from the loss of volatile, isotopically light Mo oxides, that formed under oxidized conditions typical for the formation of these chondrites.
The Mo isotope compositions of samples derived from the silicate portion of differentiated planetary bodies are heavy compared to the mean composition of chondrites and iron meteorites. This difference is qualitatively consistent with experimental evidence for Mo isotope fractionation between metal and silicate. The common δ^98/95Mo values of −0.05±0.03‰ of lunar samples derived from different geochemical reservoirs indicate the absence of significant Mo isotope fractionation by silicate differentiation or impact metamorphism/volatilization on the Moon. The most straightforward interpretation of the Mo isotope composition of the lunar mantle corresponds to the formation of a lunar core at a metal–silicate equilibration temperature of . The investigated martian meteorites, angrites and eucrites exhibit more variable Mo isotope compositions, which for several samples extend to values above the maximumδ^98/95Mo=+0.14‰ that can be associated with core formation. For these samples post-core formation processes such as partial melting, metamorphism and in the case of meteorite finds terrestrial weathering must have resulted in Mo isotope fractionation. Estimates of the metal–silicate equilibration temperatures for Mars () and the angrite parent body () are thus more uncertain than that derived for the Moon. Although the Mo isotope composition of the bulk silicate Earth has not been determined as part of this study, a value of −0.16‰<δ^98/95Mo<0 can be predicted based on the chondrite and iron meteorite data and by assuming a reasonable temperature range for core formation in the Earth. This estimate is in agreement with four analyzed basalt standards (−0.10±0.10). Improved application of mass-dependent Mo isotope fractionation to investigate core formation most of all requires an improved understanding of potential Mo isotope fractionation during processes not related to metal–silicate differentiation.

Reference
Burkhardt C, Hin RC, Kleine T and Bernard Bourdon B (2014) Evidence for Mo isotope fractionation in the solar nebula and during planetary differentiation. Earth and Planetary Science Letters 391:201–211.
[doi:10.1016/j.epsl.2014.01.037]
Copyright Elsevier

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Edgard G. Rivera-Valentin^a,b and Amy C. Barr^a,b

^aBrown University, Department of Geological Sciences, 324 Brook St., Box 1846, Providence, RI 02912, United States
^bCenter for Lunar Origin and Evolution, Southwest Research Institute, Boulder, CO 80302, United States

Remote sensing data suggest Mercury’s surface has compositional variations spatially associated with crater and basin ejecta, the so-called “Low-Reflectance Material” (LRM), which has been suggested to be enriched in a subsurface native darkening agent that is excavated and redeposited onto the surface. This unit may record the evidence of impact-induced mixing of Mercury’s outer layers during its early history. Here, we develop a fully three-dimensional Monte Carlo model of impact cratering, excavation, and ejecta blanket deposition on a global scale for Mercury.
New dynamical simulations of the early evolution of the asteroid belt hint at the presence of additional asteroids in a region interior to the present-day belt, known as the “E-belt”. We use Monte Carlo methods to show that the predicted bombardment from this population matches the observed spatial crater densities on Mercury. Impacts large enough to pierce through the crust create surface ejecta deposits rich in mantle material. Later impacts onto enriched ejecta deposits redistribute mantle material away from the basins. For the suggested average mercurian crustal thickness of 50 km, the surface has, on average, ~0.4% mantle material by volume; the most enriched areas have ~30% mantle by volume.
The regional coverage of impact-induced compositional changes is strongly dependent on the thickness to the subsurface source. Because observations indicate LRM covers ~15% of Mercury’s surface, our model suggests the darkening agent is ~30 km deep. Considering the current estimated average mercurian crustal thickness of 50 km, this implies the darkening agent is likely located within a chemically distinct lower crust.

Reference
Rivera-Valentin EG and Barr AC (2014) Impact-induced compositional variations on Mercury. Earth and Planetary Science Letters 391:201–211.
[doi:10.1016/j.epsl.2014.02.003]
Copyright Elsevier

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S. N. Borovikov¹ and N. V. Pogorelov^1,2

¹Center for Space Physics and Aeronomic Research, The University of Alabama in Huntsville, Huntsville, AL 35805, USA
²Department of Space Sciences, The University of Alabama in Huntsville, Huntsville, AL 35805, USA

Recent observations from the Voyager 1 spacecraft show that it is sampling the local interstellar medium (LISM). This is quite surprising because no realistic, steady-state model of the solar wind (SW) interaction with the LISM gives an inner heliosheath width as narrow as ~30 AU. This includes models that assume a strong redistribution of the ion energy to the tails in the pickup ion distribution function. We show that the heliopause (HP), which separates the SW from the LISM, is not a smooth tangential discontinuity, but rather a surface subject to Rayleigh–Taylor-type instabilities which can result in LISM material penetration deep inside the SW. We also show that the HP flanks are always subject to a Kelvin–Helmholtz instability. The instabilities are considerably suppressed near the HP nose by the heliospheric magnetic field in steady-state models, but reveal themselves in the presence of solar cycle effects. We argue that Voyager 1 may be in one such instability region and is therefore observing plasma densities much higher than those in the pristine SW. These results may explain the early penetration of Voyager 1 into the LISM. They also show that there is a possibility that the spacecraft may start sampling the SW again before it finally leaves the heliosphere.

Reference
Borovikov SN and Pogorelov NV (2014) Voyager 1 near the heliopause.  The Astrophysical Journal Letters 783:L16.
[doi:10.1088/2041-8205/783/1/L16]

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