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Investigating potential sources of Mercury's exospheric Calcium: Photon‐stimulated desorption of Calcium Sulfide
Author(s) -
Bennett Chris J.,
McLain Jason L.,
Sarantos Menelaos,
Gann Reuben D.,
DeSimone Alice,
Orlando Thomas M.
Publication year - 2016
Publication title -
journal of geophysical research: planets
Language(s) - English
Resource type - Journals
eISSN - 2169-9100
pISSN - 2169-9097
DOI - 10.1002/2015je004966
Subject(s) - mercury (programming language) , exosphere , thermal desorption , desorption , analytical chemistry (journal) , chemistry , ionization , photoionization , impact crater , physics , ion , astrobiology , environmental chemistry , adsorption , organic chemistry , computer science , programming language
Ground‐based and MErcury Surface, Space ENvironment, GEochemistry, and Ranging observations detected Ca 0 and Ca + in the exosphere of Mercury as well as unexpectedly high levels of sulfur on Mercury's surface. The mineral oldhamite ((Mg,Ca)S) could be a predominant component of the Mercury surface, particularly within the hollows identified within craters, and could therefore serve as a source of the observed exospheric calcium. Laboratory measurements on the photon‐stimulated desorption (PSD) of CaS powder (an analog for oldhamite) at a wavelength of λ = 355 nm have been conducted, utilizing resonance‐enhanced multiphoton ionization time‐of‐flight mass spectrometry to determine the yields and velocity distributions of Ca 0 . The desorbing Ca 0 could be fit using two Maxwell‐Boltzmann components: a 600 (±30) K thermal component and a 1389 (±121) K nonthermal component, the latter accounting for ~25% of the observed signal. Cross sections for PSD using 3.4 eV photons were found to be 1.1 (±0.7) × 10 −20 cm 2 for Ca 0 and 3.2 (±0.9) × 10 −24 cm 2 for Ca + . Adopting these cross sections, a Monte Carlo model of the release of Ca 0 by PSD from the Tyagaraja crater finds the neutral microexosphere created from this process to be substantial even if only 1% CaS is assumed in the hollows. Diffuse reflectance UV‐visible measurements were made on the CaS powder to determine a bandgap, E g , of 2.81 (±0.14) eV via the Tauc method.