  Events following the giant impact formation of the Moon are thought to have
led to volatile depletion and concurrent mass-dependent fractionation of the
isotopes of moderately volatile elements (MVE). The detailed processes and
conditions surrounding this episode remain obscured and are not unified by a
single model for all volatile elements and compounds. Using available data,
including new Zn isotope data for eight lunar samples, we demonstrate that the
isotopic fractionation of MVE in the Moon is best expressed by non-ideal
Rayleigh distillation, approaching the fractionation factor {\alpha} using the
reduced masses of the evaporated isotopologues. With these calculations a best
fit for the data is obtained when the lunar MVE isotope data is normalized to
ordinary or enstatite chondrites ({\Delta}Moon-OC,EC), rather than a bulk
silicate Earth (BSE) composition. This analysis further indicates that the
parent body from which the Moon formed cannot have partitioned S into its core
based on S isotope compositions of lunar rocks. The best fit between
{\Delta}Moon-OC,EC and modelled non-ideal Rayleigh fractionation is defined by
a slope that corresponds to a saturation index of 90 +/- 4 %. In contrast, the
older Highland suite is defined by a saturation index of 75 +/- 2 %, suggesting
the vapor phase pressure was higher during mare basalt eruptions. This provides
the first tangible evidence that the Moon was veiled by a thin atmosphere
during mare basalt eruption events spanning at least from 3.8 to 3 billion
years ago and implies that MVE isotope fractionation dominantly occurred after
the Moon had accreted.
