Abstract
Abstract. The optical spectroscopy measurements of sodium in Mercury's exosphere near the subsolar point by MESSENGER Mercury Atmospheric and Surface Composition Spectrometer Ultraviolet and Visible Spectrometer (MASCS/UVVS) have been interpreted before with a model employing two exospheric components of different temperatures. Here we use an updated version of the Monte Carlo (MC) exosphere model developed by Wurz and Lammer (2003) to calculate the Na content of the exosphere for the observation conditions ab initio. In addition, we compare our results to the ones according to Chamberlain theory. Studying several release mechanisms, we find that close to the surface, thermal desorption dominates driven by a surface temperature of 594 K, whereas at higher altitudes micro-meteorite impact vaporization prevails with a characteristic energy of 0.34 eV. From the surface up to 500 km the MC model results agree with the Chamberlain model, and both agree well with the observations. At higher altitudes, the MC model using micro-meteorite impact vaporization explains the observation well. We find that the combination of thermal desorption and micro-meteorite impact vaporization reproduces the observation of the selected day quantitatively over the entire observed altitude range, with the calculations performed based on the prevailing environment and orbit parameters. These findings help in improving our understanding of the physical conditions at Mercury's exosphere as well as in better interpreting mass-spectrometry data obtained to date and in future missions such as BepiColombo.
Highlights
The Hermean particle environment is a complex system consisting of a surface-bounded exosphere and a magnetosphere that contains volatile and refractory species from the regolith as well as backscattered solar wind and interplanetary dust (Killen et al, 2007)
We find that the Na observation can be explained by two combined processes: a low-energy process, thermal desorption– evaporation (TD), that dominates at low altitudes and is driven by the high surface temperature and a comparably high-energy process, meteorite impact vaporization (MIV), that is responsible for the Na observed at high altitudes
The results using the Chamberlain model with the modification for radiation pressure are plotted: the curve with square symbols corresponds to a surface temperature of 594 K, and the curve with circle symbols corresponds to an assumed temperature of 2500 K, with the transversal column density (TCD) at the surface of this component adjusted to match the observations
Summary
The Hermean particle environment is a complex system consisting of a surface-bounded exosphere (i.e., a collisionless atmosphere down to the planet’s surface) and a magnetosphere that contains volatile and refractory species from the regolith as well as backscattered solar wind and interplanetary dust (Killen et al, 2007). By the end of the 1970s Mariner 10 made the first observations of the composition of the exosphere around Mercury and found hydrogen and helium (Broadfoot et al, 1976) It was only during the year 1985, and further on, that many ground-based observations identified the presence of sodium in the Hermean exosphere and found that Na emissions are temporally and spatially variable (e.g., Potter et al, 2007; Leblanc and Johnson, 2010), often enhanced near north and south poles, have a moderate north–south asymmetry (e.g., Potter and Morgan, 1985; Sprague et al, 1998; Schleicher et al, 2004), are concentrated on the dayside (Killen et al, 2007; Mouawad et al, 2011), and are correlated with in situ magnetic field observations (Mangano et al, 2015). 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
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