[P-PS01] Outer Solar System Exploration Today, and Tomorrow

[PPS01-06] Radiative transfer simulation including a non-LTE model for terahertz observations of Ganymede's atmosphere

*Takayoshi Yamada¹, Ladislav Rezac², Richard Larsson², Paul Hartogh², Naohiro Yoshida³, Yasuko Kasai^1,3 (1.National Institute of Information and Communications Technology, 2.Max Planck Institute for Solar System Research, 3.Tokyo Institute of Technology)

Keywords:THz, Radiative trasfer

We are developing a teraherz radiative transfer code, named Atmospheric Teraherz Radiation Simulator (ATRASU), for simulations of sub-millimeter observations of planetary atmospheres.
Because of the tenuous Ganymede atmosphere we need to include non-local thermodynamic equilibrium (non-LTE) conditions of H₂O rotational levels to simulate observations by the Submillimeter Wave Instrument (SWI) on the JUpiter ICy moon Explorer (JUICE). The frequency windows of the JUICE/SWI are 530 to 625 GHz and 1080 to 1275 GHz with 100 kHz spectral resolution.
We developed a deterministic non-LTE solution based on the multilevel Gauss–Seidel method. The simulated energy level populations of H₂O for SWI observations start to deviate from LTE at 100 to 200 km altitude around sub-solar latitudes of 10 degrees. At sub-solar latitudes around 60 degrees the populations are in non-LTE over the entire range, starting from the surface. The difference of the simulated spectra between LTE and non-LTE conditions, and their sensitivity to various parameters, such as collisional rates for H₂O, will be presented.