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Pathways for Energization of Ca in Mercury's ExosphereWe investigate the possible pathways to produce the extreme energy observed in the calcium exosphere of Mercury. Any mechanism must explain the facts that Ca in Mercury's exosphere is extremely hot, that it is seen almost exclusively on the dawnside of the planet, and that its content varies seasonally, not sporadically. Simple diatomic molecules or their clusters are considered, focusing on calcium oxides while acknowledging that Ca sulfides may also be the precursor molecules. We first discuss impact vaporization to justify the assumption that CaO and Ca-oxide clusters are expected from impacts on Mercury. Then we discuss processes by which the atomic Ca is energized to a 70,000 K gas. The processes considered are (1) electron-impact dissociation of CaO molecules, (2) spontaneous dissociation of Ca-bearing molecules following impact vaporization, (3) shock-induced dissociative ionization, (4) photodissociation and (5) sputtering. We conclude that electron-impact dissociation cannot produce the required abundance of Ca, and sputtering cannot reproduce the observed spatial and temporal variation that is measured. Spontaneous dissociation is unlikely to result in the high energy that is seen. Of the two remaining processes, shock induced dissociative ionization produces the required energy and comes close to producing the required abundance, but rates are highly dependent on the incoming velocity distribution of the impactors. Photodissociation probably can produce the required abundance of Ca, but simulations show that photodissociation cannot reproduce the observed spatial distribution.
Document ID
20160010401
Document Type
Reprint (Version printed in journal)
Authors
Killen, Rosemary M. (NASA Goddard Space Flight Center Greenbelt, MD United States)
Date Acquired
August 17, 2016
Publication Date
December 30, 2015
Publication Information
Publication: Icarus
Volume: 268
Subject Category
Lunar and Planetary Science and Exploration
Report/Patent Number
GSFC-E-DAA-TN34134
Funding Number(s)
CONTRACT_GRANT: NNX07AR78G
Distribution Limits
Public
Copyright
Other
Keywords
Sulfides
Mercury
Exosphere