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Modeling the Sublimation-driven Atmosphere of Io with DSMC

Introduction: In 1979 the primary Ionian dayside species, sulfur dioxide, was detected by the Voyager IR spectrometer (IRIS) [1]; however, the dominant mechanism (volcanic activity or sublimation from surface frost) of atmospheric production still remains in question. The local dayside SO2 vertical column abundance varies between 10 and 6×10 cm, depending on latitude and whether any surface frosts, volcanoes, or lava lakes are present. There have been several attempts to model Io’s rarefied atmosphere as a continuum; this is a poor approximation at high altitudes and away from the subsolar region. We simulate the atmosphere with a non-continuum method and attempt to reconcile the results with observations. Constraining Observations: In the last few years there have been many important observations which could help develop an understanding of the underlying physics. We concentrate on Lyman-α images which were used to map the dayside column densities on Io [2], disk-integrated observations of the 19.3 μm SO2 band [3], surface temperature data from the Galileo photopolarimeter-radiometer (PPR) [4], and Galileo near-infrared mapping spectrometer (NIMS) data which were used to derive the fractional abundance of SO2 surface frost [5]. Model: We present a fully three-dimensional model of Io’s sublimation atmosphere computed using the DSMC method [6]. The atmospheric density is controlled by the local vapor pressure and local areal coverage of SO2 surface frost. The gas densities span the range from highly collisional (near the peak surface frost temperature at low altitudes) to free molecular flow (at high altitudes and on the nightside). Previous models of Io’s atmosphere have been either onedimensional, in which only the vertical structure and temporal evolution are investigated, or twodimensional, which are axi-symmetric about the subsolar point. Our model uses the DSMC method which is valid throughout the entire atmosphere. Our threedimensional simulation is able to model asymmetries in atmospheric parameters such as density, temperature, and flow velocity due to planetary rotation and an inhomogeneous frost distribution. The atmosphere is most sensitive to the surface frost temperature distribution, Tfrost, due to the exponential dependence of vapor pressures on this parameter. Tfrost is matched to the nightside cooling rate of surface temperature data from the Galileo PPR [4]. Results: The effects on Io’s atmosphere of planetary rotation, heating due to plasma bombardment, inhomogeneous surface frost, SO2 residence time on rock, subsolar temperature, and volcanic plumes are examined. The subsolar temperature, Tss, is parameterized to be either 120 K or 115 K. Overall flow features. Circumplanetary flow develops as the high vapor pressure region near the peak Tfrost pushes gas toward the low vapor pressure region on and near the nightside. The vapor pressure drops rapidly near the terminator as the surface cools. The flow is forced to be nearly parallel to the surface by the high day-to-night pressure gradient. The winds are not directly away from the region of origin because the inhomogeneous surface frost alters the pressure field leading to a slight curvature of the streamtraces. Weak irregular flows up to 50 m/s do develop where strong frost gradients occur. These weak flows are more visible on the nightside where the surface temperature is nearly constant and the flow is nearly free molecular. The pressure gradient between the warmest point on Io and the terminator is strong enough to drive the flow supersonic. When the supersonic gas slows to subsonic speeds on and near the nightside, a C-shaped shock is formed.