Methane in the lunar exosphere: Implications for solar wind carbon escape

A positive identification of methane in the lunar exosphere has been made in data from the neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. Like argon-40, methane is adsorbed on the lunar surface during nighttime. However, higher activation energies for methane delay its desorption at sunrise by about an hour local time, creating a postsunrise bulge with peak concentration of approximately 400-450moleculescm(-3) at a reference altitude of 12km, which is just above the highest topographic feature on the Moon. The rate of escape of carbon as methane derived from the LADEE data is estimated to be in the range 1.5-4.5x10(21)s(-1). A lower bound for solar carbon escape derived separately from Apollo sample analyses is 3.4 x 10(21)s(-1).