THE CASE FOR AN EQUABLE CLIMATE IN THE ARCHEAN

There is ample evidence that the Archean Earth possessed liquid water. The existence of metamorphosed sedimentary rocks from this period, as discussed in chapter 11, require erosion by liquid water and deposition in a lake or marine environment. The presence of life itself, recorded through isotopic signatures and fossil evidence, also implies liquid water. As discussed in chapter 12, we know of no living thing today that can get by without water. Many don't require oxygen (and are poisoned by it), but all require liquid water.

Figure 14.1 summarizes constraints arguing for Earth's mean temperatures being above the melting point of water during the Archean. In chapter 15, we explore the case for a Martian climate, at the time of Earth's Archeon eon, which was warmer than at present (either continuously or episodically). In total, the evidence on Earth and Mars points to planetary climates at least as warm as those experienced today. Surprisingly, as we now show, such climates impose rather strong constraints on the nature of the Archean atmospheres of the Earth and Mars – provided our understanding of the evolution of the Sun is correct.

THE FAINT EARLY SUN

Simple reasoning about the physics of hydrogen fusion indicates that the Sun was cooler in the past than it is at present.