Molecular gas species in the lunar atmosphere

There is good evidence for the existence of very small amounts of methane, ammonia and carbon dioxide in the very tenuous lunar atmosphere which consists primarily of the rare gases helium, neon and argon. All of these gases, except⁴⁰Ar, originate from solar wind particles which impinge on the lunar surface and are imbedded in the surface material. Here they may form molecules before being released into the atmosphere, or may be released directly, as is the case for rare gases. Evidence for the existence of the molecular gas species is based on the pre-dawn enhancement of the mass peaks attributable to these compounds in the data from the Apollo 17 Lunar Mass Spectrometer. Methane is the most abundant molecular gas but its concentration is exceedingly low, 1 × 10³ mol cm^?3, slightly less than³⁶Ar, whereas the solar wind flux of carbon is approximately 2000 times that of³⁶Ar. Several reasons are advanced for the very low concentration of methane in the lunar atmosphere.