Observations of the minor species Al and Fe in Mercury's exosphere

We report here on the first observational evidence of Al and Fe in the exosphere of Mercury, based on measurements of resolved emission lines of these metals with Keck-1/HIRES. Al emission was observed on two separate runs, in 2008 and 2013, with tangent column densities of 3.1 ± 1.0 and 4.0 ± 1.5 × 10⁷ Al atoms cm^-2 at altitudes of 1185 and 1870 km (1.5 and 1.75 R_M). The Al radiative intensity was seen to increase where the slit crossed the planetary penumbral shadow, and then decrease monotonically with altitude. Fe emission has been observed once, in 2009, indicating an extended source. We also present observed 3-σ Ca⁺ upper limits near Mercury's equatorial anti-solar limb, from which an abundance limit of 4.0 × 10⁶ cm^-2 at 1650 km altitude is derived for the Ca ion.

A simple model for zenith column abundances of the neutral species yields 1.9-5.2 × 10⁷ Al cm^-2, and 8.2 × 10⁸ Fe cm^-2. The observations appear to be consistent with production of these species by impact vaporization, with a large fraction of the Al ejecta in molecular form, and that for Fe in mixed atomic and molecular forms. The scale height of the Al gas is consistent with a kinetic temperature of 6100-8000 K. The apparent high temperature and low density of the Al gas would suggest that it may be produced by dissociation of molecules.