A. A. Pavlov¹, A. K. Pavlov^2,3, V. M. Ostryakov³, G. I. Vasilyev², P. Mahaffy¹ and A. Steele⁴

¹Planetary Environments Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
²A. F. Ioffe Physico-Technical Institute of Russian Academy of Sciences, St. Petersburg, Russia
³St. Petersburg State Polytechnical University, St. Petersburg, Russia
⁴Geophysical Laboratory, Carnegie Institute of Washington, Washington, District of Columbia, USA

¹³C/¹²C and ¹⁵N/¹⁴N isotopic ratios are pivotal for our understanding of the Martian carbon cycle, history of the Martian atmospheric escape, and origin of the organic compounds on Mars. Here we demonstrate that the carbon and nitrogen isotopic composition of the surface rocks on Mars can be significantly altered by the continuous exposure of Martian surface to cosmic rays. Cosmic rays can effectively produce ¹³C and¹⁵N isotopes via spallation nuclear reactions on oxygen atoms in various Martian rocks. We calculate that in the top meter of the Martian rocks, the rates of production of both ¹³C and ¹⁵N due to galactic cosmic rays (GCRs) exposure can vary within 1.5–6 atoms/cm³/s depending on rocks’ depth and chemical composition. We also find that the average solar cosmic rays can produce carbon and nitrogen isotopes at a rate comparable to GCRs in the top 5–10 cm of the Martian rocks. We demonstrate that if the total carbon content in a surface Martian rock is <10 ppm, then the “light,” potentially “biological” ¹³C/¹²C ratio would be effectively erased by cosmic rays over 3.5 billion years of exposure. We found that for the rocks with relatively short exposure ages (e.g., 100 million years), cosmogenic changes in ¹⁵N/¹⁴N ratio are still very significant. We also show that a short exposure to cosmic rays of Allan Hills 84001 while on Mars can explain its high-temperature heavy nitrogen isotopic composition (¹⁵N/¹⁴N). Applications to Martian meteorites and the current Mars Science Laboratory mission are discussed.

Reference
Pavlov AA, Pavlov AK, Ostryakov VM, Vasilyev GI, Mahaffy P and Steele A (in press) Alteration of the carbon and nitrogen isotopic composition in the Martian surface rocks due to cosmic ray exposure. Journal of Geophysical Research: Planets
[doi:10.1002/2014JE004615]
Published by arrangement with John Wiley & Sons

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Bruce G. Bills, Sami W. Asmar, Alexander S. Konopliv, Ryan S. Park, Carol A. Raymond

Jet Propulsion Laboratory, California Institute of Technology,Pasadena, CA 91109

We examine the gravity and topography of the asteroid 4 Vesta, as recently revealed by the Dawn mission. The observed gravity is highly correlated with the observed topography, and suggests little lateral variation in density. The variance spectra of both gravity and topography follow power laws which are very similar to those seen for the Moon, Mars, Venus, and Earth. A significant way in which Vesta differs from these larger silicate bodies is that both gravity and topography are significantly anisotropic, with more north-south variation than east-west variation. Rapid rotation plausibly contributes to this anisotropy, but only at harmonic degree two. The remainder of the anisotropy appears related to the large impacts which formed the Rheasilvia and Veneneia basins. We note that, as usual, gravitational inverse problems are non-unique. While the observed gravity and topography of Vesta do not preclude existance of a metallic core, they certainly do not require it.

Reference
Bills BG, Asmar SW, Konopliv AS, Park RS and Raymond CA (in press) Harmonic and statistical analyses of the gravity and topography of Vesta. Icarus
[doi:10.1016/j.icarus.2014.05.033]
Copyright Elsevier

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H. Jönsson¹, N. Ryde¹, G. M. Harper², M. J. Richter³ and K. H. Hinkle⁴

¹Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, SE-221 00 Lund, Sweden
²School of Physics, Trinity College, Dublin 2, Ireland
³Physics Department, University of California, Davis, CA 95616, USA
⁴National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ 85726, USA

The origin of “cosmic” fluorine is uncertain, but there are three proposed production sites/mechanisms for the origin: asymptotic giant branch (AGB) stars, ν nucleosynthesis in Type II supernovae, and/or the winds of Wolf-Rayet stars. The relative importance of these production sites has not been established even for the solar neighborhood, leading to uncertainties in stellar evolution models of these stars as well as uncertainties in the chemical evolution models of stellar populations. We determine the fluorine and oxygen abundances in seven bright, nearby giants with well determined stellar parameters. We use the 2.3 μm vibrational-rotational HF line and explore a pure rotational HF line at 12.2 μm. The latter has never been used before for an abundance analysis. To be able to do this, we have calculated a line list for pure rotational HF lines. We find that the abundances derived from the two diagnostics agree. Our derived abundances are well reproduced by chemical evolution models including only fluorine production in AGB stars and, therefore, we draw the conclusion that this might be the main production site of fluorine in the solar neighborhood. Furthermore, we highlight the advantages of using the 12 μm HF lines to determine the possible contribution of the ν process to the fluorine budget at low metallicities where the difference between models including and excluding this process is dramatic.

Reference
Jönsson H, Ryde N, Harper GM, Richter MJ and Hinkle KH (2014) Fluorine in the Solar Neighborhood: Is It All Produced in Asymptotic Giant Branch Stars? The Astrophysical Journal Letters 789:L41.
[doi:10.1088/2041-8205/789/2/L41]

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