,,- University of Virginia, Laboratory for Astrophysics & Surface Physics, Materials Science and Engineering, Charlottesville, United States of America (cdukes@virginia.edu)
Without continuous diffusion from the Hermean subsurface to the planetary epiregolith, Mercury’s Na exosphere cannot be sustained [1-4]. Meteoritic impacts and solar wind sputtering excavate Na from regolith minerals and replenish the supply, redepositing it across the planetary surface. These excavated, adsorbed Na atoms may be subsequently re-ejected into the exosphere by lower energy (< 10 eV) release processes such as thermal desorption and photodesorption. However, this “surface reservoir” appears unsustainable, with theoretical sublimation fluxes via thermal desorption surpassing the expected available atomic concentrations on Mercury’s sunlit hemisphere [3, 5-6]. With no additional source of Na diffusing from the subsurface, exospheric Na would be depleted, inconsistent with current observation [e.g., 7-8]. Therefore, sustained diffusion of Na from the subsurface is necessary to maintain the observed exospheric abundances, and the rate at which this subsurface Na migrates to the surface is critically important.
We have measured the rate of Na diffusion through a porous regolith analog using X-ray photoelectron spectroscopy (XPS). For each measurement, Na vapor was deposited onto the underside of a puck-shaped quartz glass frit, after which the Na concentration on the top side was monitored over time while the bottom of the frit was held at a constant temperature between 300 and 700 K. The high-purity quartz frits used in these experiments are commercial porous filters composed of sintered quartz-glass beads, with puck thickness ranging from 2 to 3 mm and pore sizes ranging from 16 to 40 microns. (Fig. 1). For an initial Na surface concentration of 3.1 at-% on the underside of the frit, we found the rate of diffusion through the frit at 700 K to be 6.4 x 10-7 at-% s-1, or roughly 1.3 x 109 Na cm-2 s-1.
Fig 1. Front face of disc-shaped frit, where sintered glass particles form irregular channels of 16-40 microns. Image field width is 519 microns; irregular particles create channels. Na is deposited at RT on the bottom face of the frit, diffusing through the channels to reach the front face. The frit front Na concentration is monitored as a function of time, at constant temperature.
Fig. 2. (Left) Elemental concentrations vs. time on the top surface of the porous 16-40 um pore dia. quartz frit at 700 K after Na vapor deposition onto the bottom surface. The profiles show adventitious carbon and minor N, in addition to SiO2. (Right) Zoomed region of the left-hand plot showing a steady increase in Na on the frit top over time. During ~1900 thru ~2400 minutes, the temperature was maintained but XPS spectra were not acquired.
References:
[1] Sprague 1990, Icarus, 84, 1, 93-105.
[2] Killen and Morgan 1993, J. Geophys. Res., 98, E12, 23589–23601.
[3] Gamborino et al. 2019, Ann. Geophys., 37, 455–470.
[4] Verkercke et al. 2024, Geophys. Res. Lett., 51, e2024GL109393.
[5] Killen et al. 2007, Space Sci Rev 132, 433–509.
[6] Leblanc & Johnson 2003, Icarus, 164, 261-281.
[7] Schmidt et al. 2020, Planet. Sci. Journal, 1, 14.
[8] Millano et al. 2021, Icarus, 355, 114179.
How to cite: Dukes, C., Woodson, A., and Lin, L.: Laboratory Measurement of Na Diffusion Through Hermean Regolith Analogs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15951, https://doi.org/10.5194/egusphere-egu26-15951, 2026.
