Inner Solar System
The atmosphere of planets
ISO spectrometers have also concentrated on our closest neighbours, the planets in the Solar System. In several cases, such as in the giant planets and even in Mars, the study of the atmospheres' chemical constituents has produced unexpected surprises.
The infrared wavelength region is useful for investigating comets because they are cold. The thermal emission of the comet nucleus and dusty atmosphere peaks at infrared wavelengths. In this emission region the chemical composition of the comet can be studied. The volatile molecular species sublimated from cometary nucleus ices can be identified through their fundamental bands of vibration, and are therefore a subject for high-resolution spectroscopic observations.
Infrared observations from the ground are limited to a few atmospheric windows, which is an annoying restriction for investigating changes in the comet on its approach to the sun. Infrared spectra of comets have been obtained for Comet Halley, but these were limited in spectral coverage, resolution or both.