Identifying the main sources of thermospheric heating on the giant planets remains a major unsolved problem in planetary science today. The known sources of heating cannot explain their high temperatures. So some other process must be playing the dominant role. In the upper thermospheres and ionospheres of the giant planets, the collision rate becomes too low to maintain a Boltzmann population of energy levels vacated by spontaneous and induced emission, or visited by cascade following fluorescent or electron-impact excitation. This departure from local thermodynamic equilibrium (LTE) means that the observed spectrum will bear the signature of the physical and chemical processes operating in these upper atmospheres. Observations of outer-planet IR emission spectra, particularly the H2 quadrupole lines, are needed to constrain the processes and parameters of non-LTE emission models, in order to infer the structure and energy balance of these upper atmospheres and ionospheres. Such observations will insure that the important chemical processes taking place in these atmospheres will be included in the emission model, and will then allow the dominant source of thermospheric/ionospheric heating to be identified.
Dr. Laurence Trafton will obtain these needed observations in a comparative study of outer planet thermospheres that involves surveying the near-and mid-infrared H2 quadrupole line emission over multiple vibrational levels. The results will be applied to constraining the non-LTE emission model developed by Dr. Don Shemanski at the University of Southern California (USC) and in preliminary modeling of the structure and energy balance of these upper atmospheres. A comparative, parametric study of processes in our own giant planets is needed in order to extrapolate to the environments of other bodies. Comparative planetology can provide insights into atmospheric processes of extra-solar giant planets. Another broader impact of this program will be to provide baseline reference data for future studies of solar cycle and seasonal effects on these planets. Finally, by supporting the development of the USC emission model, the proposed observations will help to support the Cassini Orbiter UVIS mission, which will be taking place during the grant period, because the UVIS team will be applying the USC emission model to the analysis of their data. ***
