LECTURE

the semimolten middle layers of the mantle. Even lighter materials such as granites formed the eggshell-thin crust, which cooled most rapidly. Gases and water vapor bubbling up through volcanoes formed the Earth’s earliest atmosphere, which was dominated by water vapor, nitrogen and carbon dioxide. (An earlier atmosphere of hydrogen and helium had probably drifted into space when the Earth was too small to hold them through its gravitational pull.)

The Earth also acquired a satellite of its own, the Moon. The fact that the Moon contains few metallic elements suggests that it was gouged out of the Earth’s upper layers by a violent collision with a Mars-sized object just after differentiation, when most metals had sunk to the core.

During the Hadean eon, the Earth cooled. Eventually, water vapor rained down to form the rst seas. As we will see, water in liquid form appears to be vital for the complex chemical reactions that gave rise to life. At the end of the Hadean eon, the Earth would still have seemed an extremely hostile environment to modern humans.

The Archaean eon, the eon of the earliest life forms, lasted from 3.8 billion to 2.5 billion years ago. There were two important changes during this era. Asteroid impacts diminished as more and more stray objects were absorbed within existing planets, and the solar system became a less violent place. However, as we will see in Lecture Seventeen, occasional impacts could still play a critical, and catastrophic, role in the Earth’s history.

The Earth’s atmosphere began to change. Most important for us, there appeared increasing amounts of free oxygen. Oxygen is an extremely reactive element that eagerly combines with other elements, a fact we observe whenever we light a re. So the appearance of free oxygen must mean that

Apollo 17 astronaut Harrison Schmitt collects lunar rock samples.

Courtesy NASA.