Photon‐Stimulated Desorption of MgS as a Potential Source of Sulfur in Mercury's Exosphere

Abstract

Mercury has a relatively high sulfur content on its surface, and a signal consistent with ionized atomic sulfur (S+) was observed by the fast ion plasma spectrometer (FIPS) instrument on the MESSENGER spacecraft. To help confirm this assignment and to better constrain the sources of exospheric sulfur at Mercury, 193 nm photon‐stimulated desorption (PSD) of neutral sulfur atoms (S0) from MgS substrates was studied using resonance‐enhanced multiphoton ionization (REMPI) spectroscopy and time‐of‐flight (TOF) mass spectrometry. Though the PSD process is inherently nonthermal, the measured velocities of ejected S0 were fit using flux‐weighted Maxwellian distributions with translation energies <E> expressed as translational “temperatures” Ttrans = <E>/μkB. A bimodal distribution consisting of both thermal (Ttrans = 300 K) and suprathermal (Ttrans > 1,000 K) components in roughly a 2:1 ratio was found to best fit the data. The PSD cross‐section was measured to be approximately 4 × 10−22 cm2 and, together with the velocity distributions, was used to calculate the PSD source rate of S0 into the exosphere of Mercury. Exosphere simulations using the calculated rates demonstrate that PSD is likely the primary source of S0 in Mercury's exosphere at low (<1,000 km) altitudes.

Particles and Photons as Drivers for Particle Release from the Surfaces of the Moon and Mercury

“…The inset shows the final E (t r ) energy as a function of excited state resonance time, t r . Figure reproduced from Schaible et al (2020) with permission in the range of about 4-10 eV. Photo-excitations invoke an electronic transition, which must be localized at the surface, leading to an anti-bonding state that results in the release of the excited atom (Na, K, .…”

“…4). Experimental data for H 2 O desorption can be fit using this form with a bi-modal distribution consisting of a low temperature thermal component (matching the substrate temperature) and a high temperature 'suprathermal' component due to direct desorption along the normal component (Bennett et al 2016;DeSimone and Orlando 2014;Zimmermann and Ho 1994;Schaible et al 2020). Since this distribution function includes the intrinsic physics of the desorption process, this is often used as an experimental fitting procedure in addition to the Weibull distributions discussed below.…”

“…Therefore, for interpreting Ca observations in the exosphere also PSD must be considered, at least at provinces with a significant oldhamite fraction on the surface. Schaible et al (2020) studied PSD of S from powder samples of MgS. They measured cross sections for PSD using 6.42 eV photons for neutral S 0 as Q i = 4 • 10 −22 cm 2 .…”

Volatiles and Refractories in Surface-Bounded Exospheres in the Inner Solar System

“…Theoretical estimates of the S column density in Mercury's exosphere (6 × 10 7 cm −2 from Wurz et al 2010; 10 9 cm −2 from Berezhnoy 2018; 2 × 10 10 cm −2 from Morgan and Killen 1997; and 2 × 10 13 cm −2 from Sprague et al 1995) are inconsistent. Recent laboratory experiments suggest that photon-stimulated desorption of S from MgS, a proxy for the global form of S on Mercury's surface, may provide a global, additional source of S at low altitudes of Mercury's exosphere (Schaible et al 2020). Doressoundiram et al (2009) reported upper limits for the Mercury's exosphere of silicon (Si) of 5 × 10 10 cm −2 ) from the European Southern Observatory -New Technology Telescope in La Silla, Chile.…”

“…Also needed are experiments that refine the yields, cross sections, and threshold energy for photon-stimulated desorption (e.g. Schaible et al 2020) and electron-stimulated desorption (e.g. McLain et al 2011).…”