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Monte Carlo Modeling of Sodium in Mercury's Exosphere During the First Two MESSENGER FlybysWe present a Monte Carlo model of the distribution of neutral sodium in Mercury's exosphere and tail using data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during the first two flybys of the planet in January and September 2008. We show that the dominant source mechanism for ejecting sodium from the surface is photon-stimulated desorption (PSD) and that the desorption rate is limited by the diffusion rate of sodium from the interior of grains in the regolith to the topmost few monolayers where PSD is effective. In the absence of ion precipitation, we find that the sodium source rate is limited to approximately 10(exp 6) - 10(exp 7) per square centimeter per second, depending on the sticking efficiency of exospheric sodium that returns to the surface. The diffusion rate must be at least a factor of 5 higher in regions of ion precipitation to explain the MASCS observations during the second MESSENGER f1yby. We estimate that impact vaporization of micrometeoroids may provide up to 15% of the total sodium source rate in the regions observed. Although sputtering by precipitating ions was found not to be a significant source of sodium during the MESSENGER flybys, ion precipitation is responsible for increasing the source rate at high latitudes through ion-enhanced diffusion.
Document ID
Document Type
Reprint (Version printed in journal)
Burger, Matthew H.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Killen, Rosemary M.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Vervack, Ronald J., Jr.
(Johns Hopkins Univ. Laurel, MD, United States)
Bradley, E. Todd
(University of Central Florida Orlando, FL, United States)
McClintock, William E.
(Colorado Univ. Boulder, CO, United States)
Sarantos, Menelaos
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Benna, Mehdi
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Mouawad, Nelly
(Maryland Univ. College Park, MD, United States)
Date Acquired
August 25, 2013
Publication Date
May 20, 2010
Publication Information
Publication: Icarus
Volume: 209
Issue: 1
ISSN: 0019-1035
Subject Category
Lunar And Planetary Science And Exploration
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