Discovery of Sodium in the Atmosphere of Mercury

The spectrum of Mercury at the Fraunhofer sodium D lines shows strong emission features that are attributed to resonant scattering of sunlight from sodium vapor in the atmosphere of the planet. The total column abundance of sodium was estimated to be 8.1 x 1011 atoms per square centimeter, which corresponds to a surface density at the subsolar point of about 1.5 x 105 atoms per cubic centimeter. The most abundant atmospheric species found by the Mariner 10 mission to Mercury was helium, with a surface density of 4.5 x 103 atoms per cubic centimeter. It now appears that sodium vapor is a major constituent of Mercury's atmosphere.