Thermally driven atmospheric escape: Monte Carlo simulations for Titan's atmosphere

摘要

Recent models of Titan's upper atmosphere were used to reproduce the Cassini measurements of the density vs. altitude by allowing a net upward flow at the exobase. Large mass flow rates were extracted and interpreted due to thermally driven escape to space at a rate that is orders of magnitude larger than the Jeans escape rate. This process is referred to as slow hydrodynamic escape. Direct simulation Monte Carlo (DSMC) models are used here to describe the transition region of Titan's atmosphere where the gas changes from being dominated by collisions to being dominated by ballistic transport. When normalized at an altitude below the exobase to the densities and temperatures calculated in the recent continuum descriptions of the Cassini ion neutral mass spectrometer data, these simulations show no evidence for slow hydrodynamic escape. In addition, above the nominal exobase there is no evidence for the proposed enhancement in the tail of the molecular speed distribution that would be required at these temperatures to give the suggested escape rates. Even simulations at Titan for artificially small Jeans parameters do not give thermal escape rates that deviate enormously from the Jeans estimate. Therefore, we conclude that the suggested upward flow rates extracted from the 1NMS data, if confirmed, must be due to mass loss by non-thermal processes and/or global transport.