Simultaneous mapping of SO 2, SO, NaCl in Io’s atmosphere with the Submillimeter Array

Abstract

Many of the key properties of Io’s atmosphere, such as its spatial distribution, temperature, column density and composition, are still not fully assessed despite decades of extensive observations. The contribution of the possible gas sources to the atmospheric replenishment are then still unclear. This paper presents disk-resolved observations performed with the Submillimeter Array (SMA) at 345 GHz of atmospheric rotational lines of the main atmospheric species SO 2, and, for the first time, of the minor species SO and NaCl. All these species appear concentrated on the anti-jovian hemisphere, but do not share the same spatial distribution. The obtained maps and line-averaged fluxes are compared to realistic models simulating gas sources including volcanic plume outgassing, SO 2 frost sublimation and photolysis. Arguments in favor of each sources are examined and compared to observations, putting constraints on their relative roles for each species. While sublimation clearly appears as the favored major source for SO 2, SO 2 photolysis may account for most of the production of SO. Using constraints on the volcanic plumes distribution from Galileo results, we find that direct volcanic input can only contribute for a minor fraction of atmospheric SO 2, but represent a more significant source for SO atmosphere, and is likely to be the only source for NaCl. Temperature and column densities findings are also presented for SO 2, and compare well to previously published observations and atmospheric models.