A PICTURE OF THE MOONS ATMOSPHERE

Atomic sodium is a useful tracer of the tenuous lunar atmosphere becau se of its high efficiency in scattering sunlight at the D, (5896 angst roms) and D2 (5890 angstroms) wavelengths. In 1988, Earth-based instru ments revealed the presence of sodium at a density of less than 50 ato ms per cubic centimeter at lunar altitudes below 100 kilometers. Teles copic observations that are made with a coronograph technique to block out the disk of the moon allow a true picture of the circumlunar atmo sphere to be obtained and show the presence of sodium out to a distanc e of several lunar radii. The distribution of sodium has a solar zenit h angle dependence, suggesting that most of the sodium that reaches gr eat altitudes is liberated from the moon's surface by solar photons (b y heating or sputtering) or by solar wind impact, in contrast to a sou rce driven by uniform micrometeor bombardment.