Io meteorology: How atmospheric pressure is controlled locally by volcanos and surface frosts

Abstract

The hydrodynamic model of A. P. Ingersoll, M. E. Summers, S. G. Schilpf, 1985, Icarus 64, 375–390 is modified to include the effects of nonuniform surface properties as revealed in recent observational studies. The observations indicate that SO_2 frost is concentrated in a band within 30° latitude of the equator covering 270° of longitude, and that the darker surface at midlatitude is warmer than the frost at times of maximum insolation. The approach to the hydrodynamics has been to make the model simpler and more versatile. Solutions are now obtained in closed analytic form. We calculate atmospheric pressures, horizontal winds, sublimation rates, and condensation rates for a wide variety of surface conditions-patchy and continuous frost cover, volcanic venting (treated as a source of mass distributed over the plume area), discontinuities of surface temperature, subsurface cold trapping, and insolation propagation into the frost. The principal new concept is the horizontal averaging length L, which is equal to √2πH/α where H is the atmospheric scale height and α is the sticking coefficient. If frost is present, either on the surface or just below it, then each area of dimension L (of order 100 km) determines its own atmospheric pressure. Away from the volcanic plumes, the pressure follows the local vapor pressure. Inside the plumes the pressure is higher, e.g., a volcanic source of 10^6 kg/sec spread over an area 300 km in radius raises the local pressure by an amount equal to the vapor pressure of a frost at 124 K. The key unknowns for Io are the strength of the volcanic sources and the temperature of frost near the subsolar point. If the frost is cold (below 110 K) then the only substantial atmospheres are beneath the plumes. If the frost is warm (e.g. 125 K) then there is also a substantial atmosphere near the subsolar point. The equatorial frost deposits are then losing net mass at a substantial rate (e.g., 0.1 cm/year for a subsolar frost temperature of 125 K).

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