Contours of SO2 surface frost fraction over Io’s surface from Galileo NIMS observations. SO2 frost is omnipresent over Io’s surface but is concentrated in large patches at mid-latitudes.

Snapshots of Io’s atmosphere at ~20 minute intervals starting ~2.5 hours prior to eclipse. Circumplanetary flow develops due to the day-to-night pressure gradient. Prior to eclipse, the winds are supersonic in all directions away from the subsolar point. In eclipse, the atmosphere collapses and the day-to-night pressure gradient is weakened such that the flow becomes subsonic. Only ~1 hour post-eclipse, the circumplanetary flow pattern has re-established itself.

Temperature of the SO2 frost solid as a function of depth. Outside of eclipse, the atmosphere exhibits a retrograde rotation; however, in eclipse the rotation direction reverses and becomes prograde. This reversal is due to the depth-integrated energy content stored in the solid.

Io has one of the most dynamic atmospheres in the solar system due in part to an orbital resonance with Europa and Ganymede that causes intense tidal heating and volcanism. The volcanism serves to create a myriad of volcanic plumes across Io’s surface that sustain temporally varying local atmospheres. The plumes primarily eject sulfur dioxide (SO₂) that condenses on Io’s surface during the relatively cold night. During the day, insolation warms the surface to temperatures where a global partially collisional atmosphere can be sustained by sublimation from SO₂ surface frosts. Both the volcanic and sublimation atmospheres serve as the source for the Jovian plasma torus which flows past Io at ~57 km/s. The high energy ions and electrons in the Jovian plasma torus interact with Io’s atmosphere causing atmospheric heating, chemical reactions, as well as altering the circumplanetary winds. Energetic ions which impact the surface can sputter material and create a partially collisional atmosphere. Simulations suggest that energetic ions from the Jovian plasma cannot penetrate to the surface when the atmospheric column density is greater than 10¹⁵ cm⁻². These three mechanisms for atmospheric support (volcanic, sublimation, and sputtering) all play a role in supporting Io’s atmosphere but their relative contributions remain unclear.

In the present work, the Direct Simulation Monte Carlo (DSMC) method is used to simulate the interaction of Io’s atmosphere with the Jovian plasma torus and the results are compared to observations. These comparisons help constrain the relative contributions of atmospheric support as well as highlight the most important physics in Io’s atmosphere.  These rarefied gas dynamics simulations improve upon earlier models by using a three-dimensional domain encompassing the entire planet computed in parallel. The effects of plasma heating, planetary rotation, inhomogeneous surface frost, molecular residence time of SO₂ on the exposed non-frost surface, and surface temperature distribution are investigated.

• 2012 Graduate Student Poster Session
• 3D DSMC Simulations of Io’s Unsteady Sublimation-Driven Atmosphere and Its Sensitivity to the Lower Surface Boundary Conditions
• Background Information on Io
• Controlling Parameters
• DSMC Simulation of the Plasma Bombardment on Io’s Sublimated and Sputtered Atmosphere
• Loki – A Lava Lake in Rarefied Circumplanetary Cross Flow
• Modeling the Sublimation-driven Atmosphere of Io with DSMC
• Modeling of SO2 IR Radiation in 19.3um from the Sublimation Atmosphere
• Modeling the Sublimation Atmosphere of Io

Atmosphere never reaches steady state during eclipse. Outside of eclipse, a high translational temperature region exists at the dawn terminator due to circumplanetary flow creating a non-equilibrium region.

Snapshots of Io’s atmosphere at ~20 minute intervals starting ~2.5 hours prior to eclipse. Prior to eclipse, the atmosphere is in a quasi-steady. The only exception is the dawn atmospheric enhancement. In eclipse, the atmosphere collapses rather slowly due to the high thermal inertia of the SO2 surface frost. The temperature drops ~5 K over 2 hours and corresponds to a column density reduction of ~20x. After eclipse, there is a blow-off of material that adsorbed to the cooler eclipse surface.

Atmosphere never reaches steady state during eclipse. Dawn atmospheric enhancement appears just before eclipse, disappears during eclipse, and is further enlarged after eclipse.

A snapshot of SO₂ column density for a subsolar longitude of 180 degrees W. The SO₂ column density peaks near the subsolar point and falls off exponentially toward the terminator. Inhomogeneities in the SO₂ column density due to the underlying SO₂ surface frost fraction are more visible on the nightside where the surface temperature is nearly constant.

A snapshot of the circumplanetary winds caused by the day-to-night pressure gradient. Color contours show the Mach number which peak near the terminator while streamtraces indicate the direction of the winds.