With the exception of Jupiter, none of the atmospheres of the giant planets has been directly sampled. Even with Jupiter the only in situ measurements that are available were made from the Galileo entry probe, which sampled a single, probably not very representative, region of the planet, namely a 5 ^m hotspot. Hence, the bulk of our knowledge of the composition, cloud structure, and dynamics of these planets has not come from direct measurements, but instead has come indirectly from analyzing features in their electromagnetic spectrum, measured by ground-based telescopes, Earth-orbiting telescopes, and from specific flyby and orbiting spacecraft missions. There are two main components of the observed spectra that provide atmospheric information: reflected sunlight from the cloud and haze layers, and thermal emission from the atmosphere itself.
In this chapter we will examine how electromagnetic radiation interacts with molecules and thus how the spectra of these planets are formed through the process of radiative transfer. Once a satisfactory radiative transfer model has been developed (sometimes called a forward model), synthetic spectra of the planets may be generated from initial assumptions of the mean atmospheric vertical profile and compared with observations. Differences between observed and modeled spectra then may be used to revise the atmospheric profile assumptions used to generate the synthetic spectra and thus improve the fit. The process of revising the atmospheric profiles to improve the spectral fit is known as the inverse model or retrieval model and will be discussed in Chapter 7.
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