Measurement of the radius of Mercury by radio occultation during the MESSENGER flybys |
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| Authors: | Mark E Perry Daniel S Kahan Olivier S Barnouin Carolyn M Ernst Sean C Solomon Maria T Zuber David E Smith Roger J Phillips Dipak K Srinivasan Jürgen Oberst Sami W Asmar |
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| Institution: | aPlanetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 21044, USA;bJet Propulsion Laboratory, Pasadena, CA 91109, USA;cDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA;dDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA;ePlanetary Science Directorate, Southwest Research Institute, Boulder, CO 80302, USA;fGerman Aerospace Center, Institute of Planetary Research, D-12489 Berlin, Germany |
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| Abstract: | The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft completed three flybys of Mercury in 2008–2009. During the first and third of those flybys, MESSENGER passed behind the planet from the perspective of Earth, occulting the radio-frequency (RF) transmissions. The occultation start and end times, recovered with 0.1 s accuracy or better by fitting edge-diffraction patterns to the RF power history, are used to estimate Mercury's radius at the tangent point of the RF path. To relate the measured radius to the planet shape, we evaluate local topography using images to identify the high-elevation feature that defines the RF path or using altimeter data to quantify surface roughness. Radius measurements are accurate to 150 m, and uncertainty in the average radius of the surrounding terrain, after adjustments are made from the local high at the tangent point of the RF path, is 350 m. The results are consistent with Mercury's equatorial shape as inferred from observations by the Mercury Laser Altimeter and ground-based radar. The three independent estimates of radius from occultation events collectively yield a mean radius for Mercury of 2439.2±0.5 km. |
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| Keywords: | Mercury MESSENGER Occultation RF Radius |
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