Escape rates and variability constraints for high‐energy sodium sources at Mercury

Abstract

[1] We present a 3-D time-dependent modeling of Mercury's extended sodium exosphere in the region of the distant tail. Recent simulations in support of the MESSENGER and BepiColombo missions have shown spatial distributions in Mercury's exosphere can be linked to specific source processes. Our model builds upon these efforts by concentrating on the escaping component of Mercury's surface-bound exosphere to assess the wide-field (7°) data described in the studies by Baumgardner et al. (2008) and Schmidt et al. (2010). Escape rates of several source processes in Mercury's exosphere are determined, and the effects of orbital motion, surface-gas interactions, variable source rates, and spatially heterogeneous sources are explored. Our simulations demonstrate that both photon-stimulated desorption and micrometeoroid impacts can result in a $20% loss of Mercury's sodium atmosphere, depending on orbital phase, and are jointly responsible for the observed comet-like tail as driven by solar radiation pressure. For a constant supply of both these sources into the exosphere, the modeled average column density over Mercury's disc decreases by $1/3 at 70°true anomaly (the orbital position for peak escape), consistent with observations over many orbits.

Monte Carlo modeling of north‐south asymmetries in Mercury's sodium exosphere

“…Simulations using 1 Earth day time constant better approximate the removal of ion‐implanted Na proposed by Killen and Morgan [] and result in 50% greater escape. PSD determined in these simulations may be lower depending on Na depletion rates but still exceed those previously derived [ Schmidt et al ., ] as a consequence of lower ejection energies.…”

“…The present study adapts the Schmidt et al . [] model to investigate if ion sputtering and PSD could produce north‐south asymmetries in the Na tail.…”

“…The thermal accommodation of atoms bouncing off the surface is treated as a free parameter. The present study adapts the Schmidt et al [2012] model to investigate if ion sputtering and PSD could produce north-south asymmetries in the Na tail.…”

Lunar exospheric argon modeling

“…The high g-value leads to a tail of neutral sodium atoms in the anti-sunward direction many planetary radii long both at the Moon (Mendillo et al, 1991;Matta et al, 2009;Line et al, 2012) and at Mercury (Potter et al, 2002;McClintock et al, 2008;Schmidt et al, 2012). As an example, the g-value for the combined D2 + D1 lines of Na ($5900 Å) at 1 AU was $0.8 s À1 for the observations of Mendillo et al (1991).…”

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

“…Mercury possesses a tenuous surface-bound exosphere which is thought to be replenished primarily through meteoritic impact-induced vaporization, photon-stimulated desorption (PSD), ion-stimulated desorption (ISD), electron-stimulated desorption (ESD), and volatile evaporation. The precise contribution of each of these processes is currently unknown [e.g., Killen et al, 2007;Domingue et al, 2007Domingue et al, , 2014McLain et al, 2011;Schriver et al, 2011;Schmidt et al, 2012;Burger et al, 2012]. Neutral calcium (Ca 0 ) has been identified as a known component of the exosphere of Mercury since observations made with the Keck telescope during the summer of 1998 successfully detected the Ca-I emission line at 422.7 nm [Bida et al, 2000].…”