Spatial variations of the sodium/potassium ratio in Mercury’s exosphere uncovered by high-resolution spectroscopy

Hybrid Simulations of Solar Wind Proton Precipitation to the Surface of Mercury

“…Mercury's Na exosphere shows temporal and spatial variability, reported by both ground-based telescopes and MESSENGER spectrometer observations (Cassidy et al, 2015(Cassidy et al, , 2016Doressoundiram et al, 2010;Killen et al, 2001;Leblanc et al, 2008Leblanc et al, , 2009Leblanc & Johnson, 2010;Mangano et al, 2013Mangano et al, , 2015Massetti et al, 2017;Orsini et al, 2018;Potter & Morgan, 1990;Potter et al, 1999;Schmidt et al, 2010). The temporal variability has a broad time-range from tens of minutes to days (e.g., Cassidy et al, 2015;Mangano et al, 2013;Massetti et al, 2017;Leblanc et al, 2009;Potter & Morgan, 1990;Schmidt et al, 2010).…”

“…Mercury has a very thin, collisionless, and surface‐bound gaseous exosphere mainly composed of H, He, Na, Mg, Al, K, Ca, Mn, Fe, and perhaps O. These elements have been observed through measurements of neutrals using Earth‐based ground telescopes (e.g., Bida & Killen, ; Bida et al, ; Doressoundiram et al, ; Killen et al, ; Leblanc et al, ; Leblanc et al, ; Mangano et al, , ; Potter & Morgan, , ; Sprague et al, ) and direct in situ measurements by the Mariner 10 and MESSENGER spacecraft (e.g., Broadfoot et al, ; Merkel et al, ; McClintock et al, , ; Vervack et al, ). They have also been observed indirectly through MESSENGER observations of planetary ions in Mercury's magnetosphere (e.g., Raines et al, ; Vervack et al, ; Zurbuchen et al, , ).…”

“…Since sodium (Na) is a volatile element and therefore is released from the surface more easily compared to other elements, is highly affected by solar radiation pressure, and has strong emission lines, Na has been one of the most observed species in the exosphere of Mercury (Baumgardner et al, ; Cassidy et al, , ; Doressoundiram et al, ; Killen & Ip, ; Killen et al, ; Killen et al, ; Leblanc et al, , ; Leblanc & Johnson, ; Mangano et al, , ; Massetti et al, ; McGrath et al, ; Mouawad et al, ; Orsini et al, ; Potter & Morgan, , , ; Potter et al, , ; Schleicher et al, ; Yoshikawa et al, ). In contrast to other neutral elements, the Na exosphere possesses noticeable features including high peaks at relatively high latitudes on the dayside (e.g., Leblanc et al, ; Mangano et al, , ; Massetti et al, ; Orsini et al, ; Potter et al, ), a dawn‐dusk asymmetry with a slightly denser exosphere at dawn compared to dusk (e.g., Cassidy et al, ; Leblanc & Johnson, ; Potter et al, ; Sprague et al, ; Schleicher et al, ), and an extended comet‐like tail on the nightside of Mercury (e.g., Baumgardner et al, ; Kameda et al, , ; McClintock et al, ; Potter et al, ; Potter & Killen, ; Schmidt et al, ).…”

Variations in the abundances of potassium and thorium on the surface of Mercury: Results from the MESSENGER Gamma‐Ray Spectrometer

“…Doressoundiram et al [2010] measured Na and K simultaneously at Mercury over an extended time period in 2006 and found that the Na/K ratio varied with true anomaly. They also observed the dawn enhancement of K first noted by Sprague [1992], again indicating a strong mobilization of K by thermal processes.…”

“…[47] The observations of Potter and Morgan [1997] showed that K in Mercury's exosphere is spatially and temporally variable, with indications that K might be sputtered from surface materials at high latitudes. Doressoundiram et al [2010] measured Na and K simultaneously at Mercury over an extended time period in 2006 and found that the Na/K ratio varied with true anomaly. They also observed the dawn enhancement of K first noted by Sprague [1992], again indicating a strong mobilization of K by thermal processes.…”