Fr.: queue de Mercure
A narrow and elongated structure of glowing → sodium gas associated with Mercury. Mercury's thin atmosphere contains small amounts of sodium that glow when excited by radiation from the Sun. Solar photons also liberate these molecules from Mercury's surface and pushes them away. Because Mercury's gravity is too weak to hold a permanent atmosphere, when atoms are evaporated from the planet's surface, some of the atoms form a tail that points away from the Sun. In particular, the yellow glow from sodium → D line is relatively bright. First predicted in the 1980s, the tail was first discovered in 2001 (A.E. Potter et al., 2001). Many tail details were revealed in multiple observations by NASA's robotic → MESSENGER spacecraft that orbited Mercury between 2011 and 2015.