The global distribution, abundance, and stability of SO 2 on Io

Abstract

Sulfur dioxide distribution and abundances, bolometric hemispheric albedos, and passive surface temperatures on Io are modeled and mapped globally from Voyager multispectral mosaics, Earth-based spectra, and photometric descriptions. Photometric models indicate global average values for regolith porosity of 75–95% and macroscopic roughness with a mean slope angle of ∼30°. Abundances of SO 2 suggested by observations at uv-visible wavelengths and at 4.08 μm are partially reconciled by intimate-mixing models; 30–50% SO 2 coverage of the integral disk is indicated. Three major spectral end members, with continuous mixing, are recognized from the Voyager multispectral mosaics; one of these end members is identified as SO 2. Intimate-mixing models with the three spectral end members are used to produce abundance maps for the optical surface; ∼30% of Io's total optical surface consists of SO 2. The SO 2 is concentrated in the bright equatorial band is relatively deficient in the region of Pele-type volcanic eruptions (long 240°–360°°) and the polar regions. Temperatures are computed to vary over a 40°K range, at the same illumination angle, according to variations in surface bolometric hemispheric albedo. The brightest (and locally coldest) areas correspond to areas rich in SO 2 and are concentrated in an equatorial band (±30° lat), but many small cold patches occur elsewhere. These cold patches have radiative equilibrium temperatures ≤120°K at the subsolar point, resulting in SO 2 saturation vapor pressures ≤10 −8 bar. Midlatitude areas and the region of Pele-type plume eruptions are generally warmer (due to lower albedos). These results for surface temperatures and SO 2 abundances and distribution support the regional coldtrapping model for the surface and atmospheric SO 2 presented by F. P. Fanale, W. B. Banerdt, L. S. Elson, T. V. Johnson, and R. W. Zurek (1982, In Satellites of Jupiter (D. Morrison, Ed.), pp. 756–781, Univ. of Arizona Press, Tucson), although the region of Pele-type volcanic eruptions may be better characterized by the regolith coldtrapping/volcanic-venting model of D. L. Matson and D. B. Nash (1983, J. Geophys. Res. 88, 4771–4783). The bright equatorial band is especially effective at slowing the formation of polar caps of SO 2, both by reducing the sublimation rate near the subsolar point and by coldtrapping the SO 2 in the equatorial region, so that competing processes of sputtering and volcanic resurfacing may prevent the formation of polar SO 2 caps.