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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 variation but there is little or no year-to-year variation; we do not see the episodic variability reported by ground-based observers. We used these altitude profiles to make estimates of sodium density and temperature. The bulk of the exosphere is about 1200 K, 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
20170002395
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in 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)
Sarantos, Menelaos
(NASA Goddard Space Flight Center Greenbelt, 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)
Date Acquired
March 21, 2017
Publication Date
October 25, 2014
Publication Information
Publication: Icarus
Publisher: Elsevier Inc.
Volume: 248
ISSN: 0019-1035
Subject Category
Astrophysics
Lunar And Planetary Science And Exploration
Report/Patent Number
GSFC-E-DAA-TN40264
Funding Number(s)
CONTRACT_GRANT: NAS5-03127
Distribution Limits
Public
Copyright
Other
Keywords
Exosphere
Mercury
Sodium

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