Io's volcanic and sublimation atmospheres

Abstract

Computer simulations of SO 2 and H 2S frost sublimation and SO 2 day and nightside volcanic atmospheres on Io are presented. The models include a surface temperature which is a function of distance from the volcanic vent or subsolar point, sublimation and condensation with a sticking coefficient of unity, and a crude radiative transfer approximation. The full three-dimensional, axisymmetric gas dynamic equations are solved numerically using a time explicit, finite volume formulation. Neither a SO 2 frost sublimation atmosphere nor a SO 2 Pele-type eruption centered on the dayside extends into the nightside. However, if the volcanic emissions contain a gas with a lower condensation rate than SO 2, for example H 2S, then a Pele-type volcanic eruption centered on the dayside can reach the terminator with the pressure required by the Pioneer 10 radio occultation measurements. Also, the atmosphere generated by a SO 2 Pele-type volcano located at a distance of ∼400 km from the terminator can extend to the terminator with the required Pioneer 10 radio occultation pressure. An extended nightside atmosphere could be formed by horizontal flow from multiple volcanoes located on the nightside. Both sublimation and volcanic atmospheres produce horizontal supersonic winds away from the subsolar point or the volcanic vent. The sublimation atmosphere is driven mainly by horizontal pressure gradients determined by surface temperatures. The volcanic atmosphere is driven by pressure gradients determined by the source rate. The near surface momentum flux for a dayside Pele-type volcanic atmosphere is consistent with observations of winds blowing vent emissions at 700 km from the main vent. The volcanic atmosphere features a shock that forms as the gas falls supersonically toward Io's surface. The volcanic nightside shock comes closer to Io's surface than the dayside shock. No shock develops in the sublimation atmosphere. Sublimation and condensation produce patterns of surface deposits which are characteristic of the two types of atmospheres. A sublimation atmosphere develops a resurfacing band pattern. Sublimation removes mass from an equatorial band of ∼±27° of Iographic latitude, while condensation deposits mass poleward of ±27° latitude. The maximum condensation rate per Io rotation period occurs at ∼50° Iographic latitude and the rate decays to zero at ∼80° latitude. Volcanic atmospheres have condensation deposits in the form of rings that fall within the particulate plume and are caused by plume shock conditions. The volcanic model is quantitatively consistent with Voyager observations of ring deposits. Volcanic eruptions expose SO 2 and other possible gaseous constituents of the plume such as sodium to sputtering by corotating torus ions in times very short (∼ 1 hr) compared to diffusion and dissociation times. Thus, the source of neutrals to the Jovian magnetosphere reflects the volcanic source composition and meets the constraints imposed by multiple torus observations, particularly the ratio of oxygen to sulfur. Furthermore, the elevation of the exobase above the surface in the vicinity of volcanic vents may locally increase the neutral sputtering rate if the diversion of magnetospheric plasma flow around Io is considered.