Physics and Chemistry of Sulfur Lakes on Io

Abstract

A model for a convecting sulfur lake, heated from below by a silicate magma chamber, is constructed and applied to major hot spot regions on Jupiter's satellite Io. We use a two-layer parametrized convection scheme for sulfur and silicates based on a local boundary layer analysis to calculate temperature profiles in the system and the maximum flux which can be extracted from the silicate magma in steady state. The results indicate that the highest-component temperature of some observed hot spots (J. S. Pearl and W. M. Sinton, 1982, In The Satellites of Jupiter (D. Morrison, Ed.), pp. 724–755. Univ. of Arizona Press, Tucson) is consistent with a convecting molten sulfur system, and the total flux from the most energetic spot, Loki Patera, is close to the maximum which can be extracted from molten silicates by convection. Simple hydrodynamic models of evaporative outflow from sulfur lakes indicate that the intermediate-component temperature of hot spots such as Loki can be identified with the evaporative sulfur flux which condenses in the atmosphere and over a wide area surrounding the lake(s). The ratio of warm to hot component fluxes for Loki and other hot spots is consistent with this interpretation, and evaporation sets a strong constraint on the maximum surface temperature for a steady-state lake. The Voyager IRIS continuum spectrum can be fitted by a sulfur lake model in which sulfur vapor condensing on the shore is assumed to radiate as a blackbody. The lifetime of such a lake, in steady state, based on evaporation and silicate cooling time scales is 1–100 years, implying long-term Earth-based observations could detect variations in the Loki thermal output. The model provides a useful interpretive tool for possible variability because it gives predictions for the relative thermal fluxes at different wavelengths. The sodium-sulfur phase diagram is also presented and used to show the evaporated lakes may leave behind a sodium-rich residue which could supply the torus with sodium. Finally, uncertainties in the model are assessed, including the lack of sulfur emission features in the Loki spectrum, and the alternative possibility that the SO_2 plume observed at Loki could be supplying the excess thermal flux.

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