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).

The relative atomic mass, AT, is the average mass of the atoms of the element as it occurs on Earth. It is thus an average over all the isotopes of the element. The scale is fixed by giving the carbon isotope ll C a relative atomic mass of 12.0. By convention, the Solar System abundance is normalized to 1012 atoms of hydrogen, whereas the CI chondrite abundance is normalized to 106 atoms of silicon. To directly compare chondrite abundance to Solar System abundance (by number), you would multiply chondrite abundance by 35.8.

Introduction

This is one version of the Greek creation myth. It considers that we started with ‘nothing’ and evolved fairly rapidly towards the environment which we experience today. In fact, this is a feature of nearly all creation myths - the Sun, the Earth, its inhabitants, and by inference, the planetary system around us, all formed soon after a divine event had acted to add purpose to the pre-existing nothingness, or chaos.

The details of the traditional scientific view are somewhat different. The Universe was created about 14 Ga ago, in the Big Bang (the exact age is unclear although somewhere between 13.7 and 13.9 Ga is the current consensus). Clumps of material then formed into galaxies, and galaxies spawned stars. From that time until the present day, the cycle of stellar birth and death has continued remorselessly. Our own Solar System formed around 4.56 Ga ago from materials that had been cycled in and out of stars several times (see Box 8.1).

There have been many different theories of how our Solar System formed. These can be split into theories suggesting that the processes that formed the Sun and the planets took place simultaneously in a single integrated event, versus theories suggesting that the planetary system was added to a pre-existing Sun, some time after the Sun's formation. These two approaches are referred to as monistic (single event) and dualistic (two separate events). An example of a dualistic theory of Solar System formation, would be the theory that another star passed close to the Sun, causing matter to be pulled from the Sun into a single filament, which then broke up along its length to form individual planets.