Upper limits on the lunar atmosphere determined from solar-wind measurements

Ionization of the lunar atmosphere by solar photons and solar-wind particles will create perturbations in the solar wind. Observations by lunar-orbiting Explorer 35 show that any such perturbations are generally <10% in the magnetic field (the practical threshold of detectability) but that they may occasionally be >10%. The perturbations occasionally observed may be due to transient increases in the atmosphere or to some other cause. Upper limits on the steady-state atmosphere, which correspond to a perturbation of 10%, can be determined if the coupling between the ionized atmosphere and the solar wind is known. Upper limits are given here for two coupling limits: a single-fluid limit, in which the ions instantly join the solar-wind flow, and a two-fluid limit, in which the ions are coupled to the wind by the steady solar-wind electric and magnetic fields. The results give upper limits on the surface density of between 10³ and 105 cm−3 for H, H2, He, Ne, A, O, O2, N, N2, H2O, OH, NO, N2O, CO, CO2, and CH4.