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Mercury's Na Exosphere from MESSENGER DataMESSENGER entered orbit about Mercury on March 18, 2011. Since then, the Ultraviolet and Visible Spectrometer (UWS) channel of MESSENGER's Mercury Atmospheric and Surface Composition Spectrometer (MASCS) has been observing Mercury's exosphere nearly continuously. Daily measurements of Na brightness were fitted with non-uniform exospheric models. With Monte Carlo sampling we traced the trajectories of a representative number of test particles, generally one million per run per source process, until photoionization, escape from the gravitational well, or permanent sticking at the surface removed the atom from the simulation. Atoms were assumed to partially thermally accommodate on each encounter with the surface with accommodation coefficient 0.25. Runs for different assumed source processes are run separately, scaled and co-added. Once these model results were saved onto a 3D grid, we ran lines of sight from the MESSENGER spacecraft :0 infinity using the SPICE kernels and we computed brightness integrals. Note that only particles that contribute to the measurement can be constrained with our method. Atoms and molecules produced on the nightside must escape the shadow in order to scatter light if the excitation process is resonant-light scattering, as assumed here. The aggregate distribution of Na atoms fits a 1200 K gas, with a PSD distribution, along with a hotter component. Our models constrain the hot component, assumed to be impact vaporization, to be emitted with a 2500 K Maxwellian. Most orbits show a dawnside enhancement in the hot component broadly spread over the leading hemisphere. However, on some dates there is no dawn/dusk asymmetry. The portion of the hot/cold source appears to be highly variable.
Document ID
Document Type
Conference Paper
Killen, Rosemary M.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Burger, M. H.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Cassidy, T. A.
(Colorado Univ. Boulder, CO, United States)
Sarantos, M.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Vervack, R. J.
(Johns Hopkins Univ. Laurel, MD, United States)
McClintock, W. El
(Johns Hopkins Univ. Laurel, MD, United States)
Merkel, A. W.
(Johns Hopkins Univ. Laurel, MD, United States)
Sprague, A. L.
(Arizona Univ. Tucson, AZ, United States)
Solomon, S. C.
(Lamont-Doherty Geological Observatory Palisades, NY, United States)
Date Acquired
August 26, 2013
Publication Date
September 14, 2012
Subject Category
Space Sciences (General)
Report/Patent Number
Meeting Information
Division for Planetary Sciences Meeting(Reno, NV)
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