THEORETICAL OVERVIEW AND MODELING OF THE SODIUM AND POTASSIUM ATMOSPHERES OF MERCURY

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 Mercury is given. Information for these four facto rs, which control the spatial distribution of these two alkali-group g ases about the planet, is incorporated in numerical models. The spatia l nature and relative importance of the initial source atom atmosphere and the ambient (ballistic hopping) atom atmosphere are then examined and are shown to be controlled and coupled to a great extent by the e xtremely large and variable solar radiation acceleration experienced b y sodium and potassium as they resonantly scatter solar photons. The l ateral (antisunward) transport rate of thermally accommodated sodium a nd potassium ambient atoms is shown to be driven by the solar radiatio n acceleration and, over a significant portion of Mercury's orbit abou t the Sun, is sufficiently rapid to be competitive with the short phot oionization lifetimes for these atoms when they are located on the sun lit surface near or within about 30 degrees of the terminator. The lat eral transport rate is characterized by a migration time determined by model calculations for an ensemble of atoms initially starting at a p oint source on the surface (i.e., a numerical spacetime dependent Gree n's function). Four animations for the spacetime evolution of the sodi um (or potassium) atmosphere produced by a point source on the surface are presented on a videotape format (ApJ, 441, Part 1, No. 2, Videota pe, Segment 3). For extended surface sources for sodium and potassium, the local column density is determined by competition between the pho toionization lifetimes and the lateral transport times of atoms origin ating from different surface source locations. Sodium surface source f luxes (referenced to Mercury at perihelion) that are required on the s unlit hemisphere to reproduce the typically observed several megarayle ighs of D-2 emission-line brightness and the inferred column densities of 1-2 x 10(11) atoms cm(-2) range from similar to 2-5 x 10(7) atoms cm(-2) s(-1). The sodium model is applied to study observational data that document an anticorrelation in the average sodium column density and solar radiation acceleration. Lateral transport driven by the sola r radiation acceleration is shown to produce this behavior for combina tions of different sources and surface accommodation coefficients. The best model fits to the observational data require a significant degre e of thermal accommodation of the ambient sodium atoms to the surface and a source rate that decreases as an inverse power of 1.5 to 2 in he liocentric distance.