Introduction

Earth at the close of the Archean, 2.5 billion years ago, was a world in which life had arisen and plate tectonics dominated the evolution of the crust and the recycling of volatiles. Yet oxygen (O2) still was not prevalent in the atmosphere, which was richer in CO2 than at present. In this last respect, Earth's atmosphere was somewhat like that of its neighbors, Mars and Venus, which today retain this more primitive kind of atmosphere.

Speculations on the nature of Mars and Venus were, prior to the space program, heavily influenced by Earth-centered biases and the poor quality of telescopic observations (Figure 15.1). Forty years of US and Soviet robotic missions to these two bodies changed that thinking drastically. The overall evolutions of Mars and Venus have been quite different from that of Earth, and very different from each other. The ability of the environment of a planet to veer in a completely different direction from that of its neighbors was not readily appreciated until the eternally hot greenhouse of Venus' surface and the cold desolation of the Martian climate were revealed by spacecraft instruments.

However, robotic missions also provided evidence that Mars once had liquid water flowing on its surface. It is tempting, then, to assume that the early Martian climate was much warmer than it is at present, warm enough perhaps to initiate life on the surface of Mars. The difficulty of sustaining a warm Martian atmosphere in the face of the faint-young-Sun problem of Chapter 14 remains a daunting puzzle, one that is highly relevant to the broader question of habitable planets beyond our solar system. What is the range of distances from any given star for which liquid water is stable on a planetary surface and life can gain a foothold?