Extended sodium exosphere of the Moon

Sodium emission has been observed up to 1200 km above the subsolar limb of the Moon at third quarter. The intensity of the emission is far in excess of that predicted from the scale height and surface density derived previously from measurements of sodium emission at low altitudes above the lunar surface. This can be explained by assuming that there are both “hot” and “cold” components of the lunar sodium exosphere. The “hot” sodium atoms which produce the extended component of the exospheric emission are those which have been freshly introduced to the lunar exosphere by meteoric impact or solar wind sputtering, and the “cold” sodium atoms are those which have collisionally accommodated to the lunar surface. From the density and apparent temperature of “hot” component we derive a sodium supply rate of 0.72*105 atoms cm−2 s−1 in agreement with rates calculated for impact vaporization and charged particle sputtering of 0.97*105 atoms cm−2 s−1 and 0.5*105 atoms cm−2 s−1, respectively. Since some fraction of the sodium evolved by both impact vaporization and sputtering will possess escape velocity, there may be a sodium coma.