Desorption of alkali atoms and ions from oxide surfaces: Relevance to origins of Na and K in atmospheres of Mercury and the Moon

This paper begins with a brief survey of the literature dealing with the adsorption and desorption of alkalis on oxide surfaces. Emphasis is on desorption phenomena: thermal desorption, electron- and photon-stimulated desorption, and ion-induced desorption (sputtering). Then the relevance of these data to the desorption of alkalis from mineral surfaces and to the origins of alkali vapors in tenuous planetary atmospheres is discussed. The data presented for Na and K indicate that desorption processes initiated by thermal or electronic excitations do not depend strongly on whether the Na returns to the surface or diffuses up through the regolith, and that neutral yields dominate ion yields in all cases. Although the desorbed neutral energy distributions are not well approximated by Maxwell-Boltzmann distributions, the mean energies of the desorbed neutral Na and K are seen to be consistent with the temperatures extracted for the “hot” component of the lunar atmosphere. This suggests that the “hot” component may be produced by electronically stimulated desorption (e.g., electron-stimulated desorption and/or photon-stimulated desorption). If this is the case, a possible “size effect” may be operative, in which desorbed neutral K atoms are somewhat more energetic than desorbed Na. In such desorption processes a low-energy component may be generated by scattering of desorbing atoms in the porous regolith; thermal desorption can also generate low-energy atoms. The data further indicate that thermal desorption should be rapid in the equatorial regions of Mercury, possibly depleting this region of alkalis, whereas thermal desorption should be less efficient on the Moon. Surface charging may be important at the surface of the Moon, by accelerating the solar electrons to energies above the threshold for initiating alkali desorption. Suggestions are made for future laboratory work.