Io's surface and environs: A magmatic‐volatile model

A new model is proposed to account for the observed properties of Io. The crust is probably siliceousbut the outermost visible surface is composed primarily of several allotropes of elemental sulfurplus frosts of SO 2 H 2 S and S 2 O. The 4µm feature in Io's IR reflectance is identified as the (ν 1 + ν 3 ) band of SO 2 frost. A possible source of the S is dissociation of iron sulfide brought up from the core by mantle convection. The SO 2 is produced in the volcanoes and the H 2 S by interaction of magnetospheric protons with the surface S. Delayed luminescence from S 2 O could cause the post‐eclipse brightening. The orbital torus is sustained by evaporation from molten ejecta from the volcanoes. Several additional diagnostic absorption bands and two additional minor torus species are predicted by this model.