What have been the crucial stages in the history of the Earth’s atmosphere?

 

 

The previous lecture described the origins of our Sun and solar system about 4.5 billion years ago. Now we cross from astronomy to geology as we focus on our own planet, the Earth. We’ll look at the Earth in its early days. What was it like? How friendly an environment was it for life?

 

How different was it from today’s Earth?

 

The early Earth would have seemed an unlikely home for life. The earliest era (strictly “eon”) of Earth history is known as the Haden (or “hellish”) eon. It lasted from about 4.5 to about 3.8 billion years ago. It is the of four eons in the Earth’s history. (The others are the Archaean eon, from 3.8 to 2.5 billion years ago; the Proterozoic eon, from 2.5 billion years ago to 550 million years ago; and the Phanerozoic eon, or “eon of multi-celled organisms,” from 550 million years ago to today.) As the young Earth formed through accretion, it heated up and melted. It was heated by three different forces. The was repeated violent collisions.

 

The early solar system was much like a cosmic demolition derby. The second force was radioactivity, because the early solar system contained many radioactive materials created in supernovae. The third force was pressure. As each planet grew in size through accretion, pressure and temperature built up at its centre. Then what took place was a sorting of elements by their density in a process known as “differentiation.” Heavy materials sank to the centre, and lighter materials rose to the surface. Differentiation gave the Earth the internal structure it has to the present day. Metals (mainly iron and nickel) sank to the centre to form the core, with a solid centre and a liquid outer layer.  

 

The core generated the Earth’s magnetic eld. Lighter materials formed the semi molten 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 Haden eon, the Earth cooled. Eventually, water vapor rained down to form the 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 Haden 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. So, the appearance of free oxygen must mean that 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 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 Haden eon. The oldest Earth rocks date to about 4 billion years ago. Our understanding of the Haden 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 centre (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 build-up 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 different from today’s Earth. How did its surface change to create the familiar geography that has done so much to shape human history? The next lecture will tackle this question by describing the fundamental geological paradigm of plate tectonics.