Chen-Yen Hsu (Institute of Astronomy, National Central University, Taoyuan City, Taiwan); Wing-Huen Ip (Institute of Astronomy, National Central University, Taoyuan City, Taiwan)
Mercury, the closest planet to the Sun, possesses a quite thin exosphere. Na, K, Ca and some other elements in the exosphere had been detected from ground-based observations (Bida et al., 2000). Afterwards, the MASCS spectrometer onboard the MESSENGER spacecraft investigated the components and confirmed that the surrounding Ca atoms tend to be extremely hot. The Ca atoms can reach to a temperature of 12,000 – 20,000 K, even ~70,000 K (Killen et al., 2005; Killen, 2015), much higher than the surface temperature of Mercury. The “Ca corona” has a specifically temporal and spatial distribution. It appears on the dawnside and varies seasonally. These features give an idea that the hot Ca atoms are generated by the surface impact processes of the 2P/Encke meteor stream (Killen and Hahn, 2015). One of the mechanisms having been proposed is that Ca-bearing molecules are vaporized by the impact and then the Ca atoms become the products from the dissociation of CaO (Killen and Hahn, 2015; Killen, 2015). In this study, we simulate the distribution of the Ca-exosphere through numerical methods on the basis of the equation of motion with the CaO photodissociation rate of 7.5×10^(-5) s^(-1) at perihelion and Ca atoms production rate ~80 atoms cm^(-2) s^(-1) (Killen, 2015). Compared to the observational results, we try to find out some more details about the generation of the hot Ca atoms.