This book is about the physical chemistry of planetary processes. Although in detail each planetary body in the Solar System looks very different, all of the planets and moons have reached their current states as a result of the same fundamental laws of nature, which are codified into the sciences that we know as physics and chemistry. A real understanding of the nature and evolution of the bodies that make up the Solar System requires that we immerse ourselves in physics and chemistry, and that we come to think of planetary processes as specific applications of these sciences. These applications can be more complex, in the sense of the number of variables involved, than those that physicists and chemists deal with when working under controlled laboratory conditions. Perhaps for this reason students of geological and planetary sciences tend to view these sciences as separate or “stand alone”. This is not so, however. Using an analogy that most of us are likely to be familiar with (and that, admittedly, may be a bit stretched), the sciences that we know as geology and planetary science (and their “sub-fields” such as petrology, mineralogy, or oceanography, to name just a few) are the “user interface”, the set of graphics and icons and mnemonics that we see on our computer screens. This user interface is supported and made possible by a rich and complex operating system (e.g. Linux, Windows, Mac, according to our tastes).