A cold‐pole enhancement in Mercury's sodium exosphere

Abstract

The ultraviolet and visible spectrometer component of the Mercury Atmospheric and Surface Composition Spectrometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft characterized the local‐time distribution of the sodium exosphere over the course of its orbital mission. The observations show that the sodium exosphere is enhanced above Mercury's cold‐pole longitudes. Based on previously published sodium exosphere models, we infer that these regions act as night side surface reservoirs, temporary sinks to the exosphere that collect sodium atoms transported antisunward. The reservoirs are revealed as exospheric enhancements when they are exposed to sunlight. As in the models the reservoir is depleted as the cold poles rotate from dawn to dusk, but unlike the models the depletion is only partial. The persistence of the reservoir means that it could, over the course of geologically long periods of time, contribute to an increase in the bulk concentration of sodium near the cold‐pole longitudes.

Mercury's subsolar sodium exosphere: an ab initio calculation to interpret MASCS/UVVS observations from MESSENGER

“…Subsequent in situ observations made by MESSENGER provided a close-up look at the Hermean exosphere for over 10 Mercury years, including observations of the sodium exosphere. These in situ observations show, in contrast with some ground-based observations, that sodium has little or no year-to-year variation (Cassidy et al, 2015) and separately show a dawn-dusk asymmetry (Cassidy et al, 2016). Due to the significant solar radiation pressure on the Na atoms in the exosphere, which can be up to half of Mercury's surface gravitational acceleration (Smyth, 1986;Ip, 1986), the sodium exosphere exhibits many interesting effects, including the formation of an extended Na corona and a Na tail-like structure (Potter et al, 2007;Wang and Ip, Published by Copernicus Publications on behalf of the European Geosciences Union.…”

“…The limitation of Chamberlain theory (Chamberlain, 1963) in this context is that it was originally developed for Earth's exosphere where the only controlling factors considered are the gravitational attraction and the "thermal energy conducted from below" (also known as exobase). For the case of Mercury, the only layer below the exosphere is the surface.…”

“…They do so by converting the UVVS emission radiance to line-ofsight column density N (cm −2 ) using the approximation N = 10 9 4π I /g, where 4π I is the radiance in kiloröntgen and g is the rate at which sodium atoms scatter solar photons in the D1 and D2 lines. Cassidy et al (2015) analyzed the UVVS limb scan data by fitting the Chamberlain model (Chamberlain, 1963) to estimate the temperature and density of the near-surface exosphere, including the effects of radiation acceleration and photon scattering. The authors concluded that none or little evidence of thermal desorption of sodium was found.…”

Evidence Connecting Mercury's Magnesium Exosphere to Its Magnesium‐Rich Surface Terrane

“…The link shows that for exospheric Mg, a significant, if not dominant, component of its source is directly coupled to the local crustal composition, and this result supports theoretical and experimental arguments that impact vaporization is able to directly source material from the regolith of rocky, airless bodies (Leblanc & Johnson, ). Because exospheric Mg is detected over all terranes and not just within the bounds of the Mg‐rich terrane, a global surface reservoir, such as is the case for Na (Cassidy et al, ; Leblanc & Johnson, ), could still play a role in sourcing some fraction of the overlying Mg exosphere. However, the year‐to‐year regularity and lack of short‐term temporal variations argue that for the observations analyzed here, sputtering is not an important contributor (but could be at higher latitudes).…”

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

“…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, ). The high‐latitude Na enhancements, which often appear at both hemispheres and are known as “two peaks” or “double peaks” (e.g., Mangano et al, ; Massetti et al, ; Orsini et al, ; Potter et al, , ), are hypothesized to be related to the sputtering of neutral Na induced by the incidence of the solar wind plasma to the surface of Mercury through magnetospheric cusps (e.g., Killen et al, ; Killen et al, ; Leblanc & Johnson, ; Mangano et al, , ; Massetti et al,…”