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Mercury's Seasonal Sodium Exosphere: MESSENGER Orbital ObservationsThe Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Ultraviolet and Visible Spectrometer (UVVS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft now orbiting Mercury provides the first close-up look at the planet's sodium exosphere. UVVS has observed the exosphere from orbit almost daily for over 10 Mercury years. In this paper we describe and analyze a subset of these data: altitude profiles taken above the low-latitude dayside and south pole. The observations show spatial and temporal variations, but there are no obvious year-to-year variations in most of the observations. We do not see the episodic variability reported by some ground-based observers. We used these altitude profiles to make estimates of sodium density and temperature. The bulk of the exosphere, at about 1200 K, is much warmer than Mercury's surface. This value is consistent with some ground-based measurements and suggests that photon-stimulated desorption is the primary ejection process. We also observe a tenuous energetic component but do not see evidence of the predicted thermalized (or partially thermalized) sodium near Mercury's surface temperature. Overall we do not see the variable mixture of temperatures predicted by most Monte Carlo models of the exosphere.
Document ID
20150010117
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Cassidy, Timothy A.
(Colorado Univ. Boulder, CO, United States)
Merkel, Aimee W.
(Colorado Univ. Boulder, CO, United States)
Burger, Matthew H.
(Morgan State Univ. Baltimore, MD, United States)
Killen, Rosemary M.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
McClintock, William E.
(Colorado Univ. Boulder, CO, United States)
Vervack, Ronald J., Jr.
(Johns Hopkins Univ. Laurel, MD, United States)
Sarantos, Menelaos
(Maryland Univ. Baltimore County Baltimore, MD, United States)
Date Acquired
June 8, 2015
Publication Date
November 1, 2014
Subject Category
Astrophysics
Report/Patent Number
GSFC-E-DAA-TN18236
Funding Number(s)
CONTRACT_GRANT: NAS5-97271
CONTRACT_GRANT: NNG11PL02A
CONTRACT_GRANT: NASW-00002
CONTRACT_GRANT: NNG07EK31C
CONTRACT_GRANT: NNG11HP16A
Distribution Limits
Public
Copyright
Public Use Permitted.
Keywords
Environment
Geochemistry
Exosphere
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