While ground-based and Earth-orbiting telescopes have recorded a wealth of information concerning the giant planets, the vast majority of what we know about these worlds comes from spacecraft observations of flyby, and more recently, orbiting spacecraft. The advantages of spacecraft observations for understanding the atmospheres of the giant planets are enormous. By recording the strength of the signal from the spacecraft as it travels behind the planet (or emerges from behind) the number density of molecules in the atmosphere may be almost directly determined and, using simple assumptions, the temperature-pressure profiles may be extracted with a high degree of precision. Such radio occupation measurements have been made for all the giant planets and provide the bedrock of the models of atmospheric structure. Another advantage is that the spacecraft can get very close to the planets and thus record images at much higher spatial resolution than can usually be achieved from the ground, or indeed from Earth orbit, especially for the more distant worlds of Neptune and Uranus. Reflectance and thermal emission spectra may be recorded either at high spatial resolution, allowing studies of the spatial variation of atmospheric constituents or, by averaging a large number of spectra or averaging over large solid angles, with high sensitivity at the expense of spatial resolution. In addition, these thermal emission spectra can be recorded at a wide range of emission angles, thus allowing for much better determination of the vertical profiles by increasing the vertical spread of the weighting functions. Similarly, reflectance spectra can be measured with a wide range of incident solar, and reflected zenith angles, which not only allows for much better vertical discrimination of cloud structure, but also allows better estimates of the aerosol properties since the reflected intensity and polarization may be sampled over a wide range of phase angles (the angle between the incident and reflected beam). In contrast, ground-based and Earth-orbiting telescope observations are limited to phase angles close to zero. A final advantage is that if the FOVs of the instruments are small enough, limb-sounding may be performed, which offers significant advantages in terms of vertical resolution and enhanced sensitivity to trace species, as we saw in Chapter 6.
A number of spacecraft have now visited the giant planets, each armed with a wide selection of remote-sensing instruments which will now be reviewed. The angular resolution of these instruments is typically not as fine as that of terrestrial instruments, but since they are so much closer to their targets, both their spatial resolution and sensitivity are usually significantly better. In order to compare between the spatial resolutions quoted in the following sections and the terrestrial observations described previously, Table 7.4 converts the angular resolutions possible by terrestrial and space-based observatories to maximum possible spatial resolution (at opposition) for the four giant planets. Table 7.5 converts a range of possible angular resolutions for flyby and orbiting spacecraft observations to spatial resolution, dependent on the distance from the target.
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