Lunar and Planetary Laboratory
University of Arizona, Tucson, AZ 85721
dhunten
Lunar and Planetary Laboratory
University of Arizona, Tucson, AZ 85721
sprague
fax 520-621-4933
ABSTRACT
(Adv.Space Res., 19 (10), 1551-1560, 1997)
The lunar and mercurian atmospheres have similarities and
important differences. Both are thin in the same sense
that collisions between atoms are rare. Collisions with the
surface are frequent and modify the distribution of atoms.
True sources are dependent upon mechanisms to deliver
fresh material to the surface
layers. These are: volatilization following meteoritic impact
gardening and outgassing. Both mechanisms occur at both bodies
but the relative importance of each component varies with
temperature, meteoritic flux rate and magnetic field strength.
True losses: photoionization and electric field sweeping,
direct escape of neutrals, and storage in permanent shadows also
occurs at both bodies but the relative importance of
each mechanism varies with gravity, electric field configuration and
insolation. Recycling of source atoms in the ambient population
is the mechanism that gives the atmosphere its characteristic
configuration; that which is measured at the telescope (in the case of
Na and K), from orbit (in the case of lunar Ar, He)
or from a flyby (in the case of mercurian H, He, O). Recycling
depends upon mechanisms that lift atoms from the surface
(particle or photon sputtering, thermal desorption,
impact, or some other, unidentified means). Different lifting
and sticking mechanisms are characterized by different energetics.
These energetics are not fully understood and are a subject of
both laboratory and theoretical study.