Investigating potential sources of Mercury's exospheric Calcium: Photon-stimulated desorption of Calcium Sulfide

Abstract

Ground‐based and MErcury Surface, Space ENvironment, GEochemistry, and Ranging observations detected Ca0 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 Ca0. The desorbing Ca0 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 cm2 for Ca0 and 3.2 (±0.9) × 10−24 cm2 for Ca+. Adopting these cross sections, a Monte Carlo model of the release of Ca0 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, Eg, of 2.81 (±0.14) eV via the Tauc method.

Mercury's low-reflectance material: Constraints from hollows

“…Calcium or magnesium sulfides are most commonly-cited as candidates for the relatively volatile substance [Blewett et al, 2011[Blewett et al, , 2013Helbert et al, 2012], on the basis of the high concentration of sulfur detected at Mercury's surface [Nittler et al, 2011;Evans et al, 2012], expectations for mineralogy under Mercury's highly reducing conditions [Burbine et al, 2002], and correlations between sulfur, calcium and magnesium abundance in heavily-cratered terrains [Weider et al, 2015]. This hypothesis has recently gained further support from observations of anomalously high exospheric calcium above the extensively-hollowed Tyagaraja crater [Bennett et al, 2016]. Laboratory experiments with magnesium, calcium and manganese sulfides do indicate that sulfides can be volatilized at the daytime temperatures experienced at Mercury's surface, and that extreme thermal cycling on the planet's surface could account for the lack of characteristic absorption bands [Helbert et al, , 2013], but have not provided a spectral match to BCFDs, LRM, or indeed any surface unit on Mercury [Blewett et al, 2013;Izenberg et al, 2014].…”

High‐Resolution Potassium Observations of the Lunar Exosphere

“…We interpret this finding as the likely consequence of meteoroid impacts. To understand the discrepancy between the waxing and waning phases, we simulated the line widths that an Earth observer would see with a Monte Carlo exosphere model, previously validated in Collier et al () and Bennett et al (). The version of the model used here includes the effect of radiation pressure on potassium trajectories.…”

Analysis of MESSENGER high‐resolution images of Mercury's hollows and implications for hollow formation

“…This set of processes also has the attraction that partial or total removal of the low‐reflectance graphite phase would leave behind higher‐reflectance remnant material, thus providing an explanation for the high reflectance of hollows, although consideration of color trends within hollows may point to the volatile phase having a redder spectrum than graphite [ Thomas et al ., ]. The loss of graphite might be in addition to loss of sulfides [ Blewett et al ., , ; Vaughan et al ., ; Helbert et al ., ; Vilas et al ., ; Bennett et al ., ]. Shadow‐length depth measurements were performed at 2518 hollows locations within 565 high‐resolution (pixel size <20 m) images. The mean depth is 24 m, with a standard deviation of 16 m. These findings are consistent with prior reports derived from fewer measurements made from shadow lengths on lower‐resolution images.…”

“…However, the environment at the planet's surface offers a number of possibilities, including sublimation driven by solar heating or by contact with impact melt or volcanic eruptive products; destruction by solar ultraviolet flux, solar wind ions, or magnetospheric ions; and heating and vaporization by micrometeoroid bombardment. On the basis of the high surface abundance of sulfur, sulfide minerals have been considered as leading candidates for the component subject to loss [ Blewett et al ., , ; Vaughan et al ., ; Helbert et al ., ; Bennett et al ., ]. If this is the case, then the formation of hollows on Mercury could represent a greatly magnified version of the highly surficial sulfur loss proposed to have occurred on asteroid Eros in response to space weathering [ Nittler et al ., ; Killen , ; Kracher and Sears , ; Loeffler et al ., ].…”