Earth's Early Atmosphere

Keywords

Atmosphere, photosynthesis, carbon dioxide, nitrogen, methane, ammonia, water vapour, algae, oceans, primordial.

Introduction

Earth is known as having an oxygen-rich atmosphere where we can breathe, and sustain life. But it's not always been that way. Having had five extinction points, the atmosphere has taken a bit of a battering over the last 4.6 billion years.

The Original Atmosphere

Scientists today think that the atmosphere when it was first created was very different. So much so that there was very little oxygen in it. Throughout the Earth's history, there has been an ebb and flow of gases in the atmosphere, but they have contained:

• carbon dioxide (CO2) - we know this one is a cause of climate change, but is the most likely to have been the main gas in early atmospheres.
• nitrogen (N2) - over a long period of time, nitrogen in the atmosphere would have built up.
• methane (CH4) and ammonia (NH3) - these were probably only around in small amounts.
• water vapour (H2O) - during the first period, this would have been in a gaseous form, and later cooled when the planet cooled to possibly form lakes and oceans.

If you take a look at other planets, they have similar properties to the early Earth atmosphere. Jupiter still has an atmosphere that is similar to the Sun, and the gas giants further afield also have similar atmospheres.

Interesting fact: Earth has had three atmospheres: the first (primordial) one was predominantly hydrogen-based, with trace amounts of other gases; the second was mainly nitrogen and carbon-dioxide, while the third one is what we have now, with 78% nitrogen and 21% oxygen.

Introduction of Oxygen

Oxygen wasn't really present in the early atmosphere, so how did it get there?

Well, at around the time that first single-cell organisms appeared that reacted with the Sun (somewhere around the 2.7 billion year ago mark), organisms such as algae formed, and this algae photosynthesised, which reacts with carbon dioxide to create glucose and oxygen. Algae creates around 60% of the world's oxygen in our current day atmosphere.

Loss of Carbon Dioxide

Where did all the carbon dioxide go when this happened? Well, through the algae. It uses the carbon dioxide to create oxygen. At the same time, the oceans were now in full swing, and they dissolve carbon dioxide through movement in waves. Some of the carbon dioxide formed into compounds such as calcium carbonate, which in turn became the shells and skeletons of some creatures that later formed in the oceans.

We know this, as sedimentary rocks formed and layered up through the millions of years, and studies have shown this through research.