LECTURE

oxygen was continually being freed by some process occurring on or near the Earth’s surface. That process, as we will see later, was photosynthesis: Free oxygen was produced by plant-like organisms, a rst hint of how living organisms could transform the Earth’s surface.

How do we know about the early history of the Earth? The Earth has changed so much that we have little direct evidence from the Hadean eon. The oldest Earth rocks date to about 4 billion years ago. Our understanding of the Hadean eon depends on a combination of theoretical inference and the indirect evidence described in the previous lecture.

At present, we cannot drill deeper than about 7 miles into the Earth, which is just 0.2% of the distance to the center (4,000 miles). To understand what’s inside the Earth we have to use indirect methods. One of the most important techniques depends on seismology, the study of earthquakes. Different types of seismic waves travel in different ways and at different speeds through different types of rock, so careful comparisons of their movements, using seismographs placed at many different parts of the Earth, can tell us much about what is inside the Earth. Seismographs have played a similar role in study of the Earth’s interior to spectroscopes in studies of the stars.

How can we know what was in the ancient atmosphere billions of years ago? Capturing the gases released by volcanoes today can tell us much about the constituents of the Earth’s early atmosphere, as we know that volcanoes generated much of that atmosphere. We can observe and date the buildup of free oxygen in the atmosphere from the appearance of increasing amounts of rusted iron in the Archaean eon.

This lecture has described the early Earth and some of the more important changes it underwent during its early years through accretion, differentiation, and early changes in its atmosphere. We saw that the early Earth was very

Capturing the gases released by volcanoes today can tell us much about the constituents of the Earth’s early atmosphere, as we know that volcanoes generated much of that atmosphere.