Evolution of Atmosphere and Hydrosphere

The story of Earth’s atmosphere and oceans is really the story of how a hot, lifeless planet became one with air we can breathe and seas we can sail. The present atmosphere is mostly nitrogen and oxygen, but it did not start that way. It took three distinct stages, spanning billions of years, to get from a primordial haze of hydrogen and helium to the oxygen-rich air we know today.

The Three Stages of Atmospheric Evolution

The book lays out the evolution of the present atmosphere in three clear stages:

• First stage — loss of the primordial atmosphere. The very early atmosphere, made of hydrogen and helium, was stripped away entirely. The culprit was the solar wind — the stream of charged particles blowing outward from the young Sun. This did not happen only to Earth; all the terrestrial planets lost their original atmospheres the same way.
• Second stage — contribution from Earth’s hot interior. As the planet cooled, gases and water vapour escaped from the solid interior. This outpouring of gases is called degassing. Continuous volcanic eruptions fed water vapour and gases into the atmosphere, and this process started the evolution of the present atmosphere.
• Third stage — modification by the living world. Through the process of photosynthesis, life changed the composition of the atmosphere, most importantly by adding oxygen.

The Early Atmosphere and the Birth of Oceans

The early atmosphere, after degassing began, was very different from today's. It largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia, and very little free oxygen. Notice what is missing — oxygen. There was almost none of it in the early air.

The formation of the oceans followed a logical chain of events:

1. As the earth cooled, the water vapour in the atmosphere began to condense.
2. The carbon dioxide in the atmosphere dissolved in rainwater, which further lowered temperatures.
3. Lower temperatures caused more condensation and more rain.
4. The rainwater falling onto the surface collected in depressions, giving rise to oceans.

The book gives a striking figure here: the oceans formed within 500 million years of the formation of the earth. That makes the oceans about 4,000 million years old — nearly as old as the planet itself.

The Role of Life and the Rise of Oxygen

Life began to evolve around 3,800 million years ago. For a long time, life was confined to the oceans. Then, somewhere around 2,500 to 3,000 million years before the present, the process of photosynthesis evolved. …