Wh. Smyth et Ml. Marconi, THEORETICAL OVERVIEW AND MODELING OF THE SODIUM AND POTASSIUM ATMOSPHERES OF THE MOON, The Astrophysical journal, 443(1), 1995, pp. 371-392
A general theoretical overview for the sources, sinks, gas-surface int
eractions, and transport dynamics of sodium and potassium in the exosp
heric atmosphere of the Moon is given. These four factors, which contr
ol the spatial distribution of these two alkali-group gases about the
Moon, are incorporated in numerical models. The spatial nature and rel
ative importance of the initial source atom atmosphere (which must be
nonthermal to explain observational data) and the ambient (ballistic h
opping) atom atmosphere are examined. The transport dynamics, atmosphe
ric structure, and lunar escape of the nonthermal source atoms are tim
e variable with season of the year and lunar phase because of their de
pendence on the radiation acceleration experienced by sodium and potas
sium atoms as they resonantly scatter solar photons. The dynamic trans
port time of fully thermally accommodated ambient atoms along the surf
ace because of solar radiation acceleration (only several percent of s
urface gravity) is larger than the photoionization lifetimes and hence
unimportant in determining the local density, although for potassium
the situation is borderline. The sodium model was applied to analyze s
odium observations of the sunward (D-1 + D-2) brightness profiles acqu
ired near last quarter by Potter and Morgan (1988b), extending from th
e surface to an altitude of 1200 km, and near first quarter by Mendill
o, Baumgardner, and Flynn (1991), extending in altitude from similar t
o 1430 to similar to 7000 km. The observations at larger altitudes cou
ld be fitted only for source atoms having a velocity distribution with
a tail that is mildly nonthermal (like an similar to 1000 K Maxwell-B
oltzmann distribution). For both the lower and higher altitude observa
tions, a number of equally good fits were achieved for differing amoun
ts of ambient atom atmosphere as determined by different combinations
of (1) the shape of the velocity distribution for the lower speed sour
ce atoms and (2) the gas-surface sticking and thermal accommodation co
nditions for the ambient atoms. For cases considered here, the sodium
flux for source atoms ranged for the Mendillo et al. (1991) observatio
ns from 3.5 x 10(5) atoms cm(-2) s(-1) for a dominant ambient atom atm
osphere near the surface to 2.1 x 10(6) atoms cm(-2) s(-1) for no ambi
ent atom atmosphere, while the flux values for the observations of Pot
ter and Morgan (1988b) were similar to 40% lower. Solar wind sputterin
g appears to be a viable source atom mechanism for the sodium observat
ions with photon-stimulated desorption also possible but highly uncert
ain, although micrometeoroid impact vaporization appears to have a sou
rce that is too smalt and too hot, with likely an incorrect angular di
stribution about the Moon.