Modeling MESSENGER observations of calcium in Mercury's exosphere

Authors

  • Matthew H. Burger,

    Corresponding author
    1. Planetary Magnetospheres Laboratory, Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
    2. Goddard Earth Sciences Technology and Research, Morgan State University, Baltimore, Maryland, USA
    • Corresponding author: M. H. Burger, Planetary Magnetospheres Laboratory, Solar System Exploration Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA. (matthew.burger@nasa.gov)

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  • Rosemary M. Killen,

    1. Planetary Magnetospheres Laboratory, Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
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  • William E. McClintock,

    1. Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA
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  • Ronald J. Vervack Jr.,

    1. The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
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  • Aimee W. Merkel,

    1. Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA
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  • Ann L. Sprague,

    1. Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
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  • Menelaos Sarantos

    1. Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
    2. Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County, Catonsville, Maryland, USA
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Abstract

[1] The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MESSENGER spacecraft has made the first high-spatial-resolution observations of exospheric calcium at Mercury. We use a Monte Carlo model of the exosphere to track the trajectories of calcium atoms ejected from the surface until they are photoionized, escape from the system, or stick to the surface. This model permits an exploration of exospheric source processes and interactions among neutral atoms, solar radiation, and the planetary surface. The MASCS data have suggested that a persistent, high-energy source of calcium that was enhanced in the dawn, equatorial region of Mercury was active during MESSENGER's three flybys of Mercury and during the first seven orbits for which MASCS obtained data. The total Ca source rate from the surface varied between 1.2 × 1023 and 2.6 × 1023 Ca atoms s−1, if its temperature was 50,000 K. The origin of this high-energy, asymmetric source is unknown, although from this limited data set it does not appear to be consistent with micrometeoroid impact vaporization, ion sputtering, electron-stimulated desorption, or vaporization at dawn of material trapped on the cold nightside.

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