ORIGIN AND CHARACTER OF THE LUNAR AND MERCURIAN ATMOSPHERES

D.M.HUNTEN

Lunar and Planetary Laboratory
University of Arizona, Tucson, AZ 85721
dhunten


ANN L. SPRAGUE

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.