Introduction

Having dealt with some of the tools and key concepts to which we will return as we develop the history of Earth and the other planets, we are ready now to consider that history. Five centuries after the beginning of the European Renaissance, humanity's explorations of Earth and the cosmos have exposed an intriguing, perhaps profound, paradox. Earth and the other planets of the solar system seem to be explainable as manifestations of common physical processes that have operated over very small and very large scales, to produce a range of cosmic phenomena. In this sense we are neither special nor particularly important in the grand scheme of things.

On the other hand, in our own solar system, we now understand Earth as the one planet with a uniquely stable climate on its surface, equable for liquid water over the billions of years required to bring forth intelligent life. Although Mars may have come close to this state at one time, the surface appears lifeless today. Europa may have a habitable oceanic environment beneath its icy crust. An intriguing possibility is that Saturn's moon Titan may have had a stable “hydrosphere” over its history, but one in which methane substitutes for water: whether such an environment could be habitable for a very exotic form of life is not known (Chapter 12). Other solar systems may be common and life may flourish elsewhere, but it is also possible, with what we know today, that we are a rare or even unique speck in the cosmos. We will know more over the decades to come, but for now we seek to understand how this planet Earth came to be, and how physical processes have operated to make it habitable for billions of years.