Observed Latitudinal, Longitudinal and Temporal Variability of Io's Atmosphere Simulated by a Purely Sublimation Driven Atmosphere

Abstract

How much of Io's SO$_2$ atmosphere is driven by volcanic outgasing or sublimation of SO$_2$ surface frost is a question with a considerable history. We develop a time dependent surface temperature model including thermal inertia and the exact celestial geometry to model the radiation driven global structure and temporal evolution of Io's atmosphere. We show that many observations can be explained by assuming a purely sublimation driven atmosphere. We find that a thermal diffusivity $\alpha=2.41\times10^{-7}$ m$^2$s$^{-1}$ yields an averaged atmospheric SO$_2$ column density decreasing by more than one order of magnitude from the equator to the poles in accordance with the observed spatial variations of Io's column densities. Our model produces a strong day-night-asymmetry with modeled column density variations of almost two orders of magnitude at the equator as well as a sub-anti-Jovian hemisphere asymmetry, with maximum dayside column densities of $3.7\times10^{16}$ cm$^{-2}$ for the sub-Jovian and $8.5\times10^{16}$ cm$^{-2}$ for the anti-Jovian hemisphere. Both are consistent with the observed temporal and large-scale longitudinal variation of Io's atmosphere. We find that the diurnal variations of the surface temperature affect the subsurface structure up to a depth of 0.6m. Furthermore, we quantify seasonal effects with Io having a northern summer close to perihelion and a northern winter close to aphelion. Finally, we found that at Io's anomalous warm polar regions a conductive heat flux of at least 1.2 Wm$^{-2}$ is necessary to reach surface temperatures consistent with observations.