The Planetary Spectrum Generator PSG is a suite of atmosphere and radiative transfer models that allow you to synthesize and retrieve (exo-)planet atmospheric spectra. It can simulate several different observation facility and covers wavelengths from the UV to radio waves. It makes use of several atmospheric models and material spectroscopic databases
The tool is hosted at NASA’s Goddard Space Flight Center and can be used from the web-interface free of charge.
Find the NASA-PSG via this link.