Introduction

The previous chapter dealt with objects with definite surfaces - the terrestrial planets. We turn now to objects for which there is no discernible surface and where the greater part of the object (possibly all) is fluid (i.e. gas or liquid). These are the giant planets: Jupiter, Saturn, Uranus and Neptune (Figure 6.1).

We start by considering the overall structure of these planets. Much of the detailed evidence has come from instruments on board spacecraft, and it is hoped that even more information will be gathered by future missions. However, Earthbased instruments are by no means obsolete in this field and observations by space telescopes (in orbit around the Earth) have provided much valuable data. Groundbased observations were necessary as a starting point for data collection by the spacecraft. One advantage of Earth-based and space-telescope observations is that they can be used to study changes in a planet's appearance over a long time (in the case of Jupiter, hundreds of years), whereas fly-by and lander spacecraft observe for only a limited time.

The first fly-bys of Jupiter and Saturn were achieved by two probes of NASA's Pioneer series in the 1970s (Appendix Table A7). Much more data came from NASA's Voyager probes, of which Voyager 2 is the only spacecraft to have visited Uranus or Neptune (Box 6.1). In addition there have been three giant planet orbiters (Galileo, Cassini and Juno, Box 6.1) plus a useful fly-by of Jupiter by NASA's Plutobound New Horizons probe in 2007.

We have a fairly accurate picture of the composition and structure of the outer layers or atmospheres of these planets, because we can detect and positively identify molecules in them. Our knowledge of the interiors is less certain and is based on indirect measurements and modelling. As none of the four planets has an accessible surface (if they have any surfaces at all), we do not know where the base of the atmosphere is. The radii of the planets are therefore often defined as the distance from the centre of the planet to the 1 bar pressure level (1 bar being approximately the pressure of the Earth's atmosphere at sea-level).