Early Atmosphere and Oceans

Formation of the Early Atmosphere

Outgassing

After the Earth formed and it continued to cool, it didn’t have an atmosphere like we have today.

As Earth’s inside was still hot and molten, volcanoes erupted all the time. These eruptions released gases trapped inside the planet. This process is called outgassing, and it is a process that still happens today.

The main gases released were:

Water vapor (H₂O)
Carbon dioxide (CO₂)
Nitrogen (N₂)

Along with small amounts of methane, ammonia, and sulfur gases.

This mix of gases formed Earth’s first atmosphere, but it had no oxygen.

Formation of the Early Oceans

Rain

At the same time as the early atmosphere was forming, the water vapor in the atmosphere began to condense—just like how steam turns back into water on a cold surface. Eventually, it started to rain. It rained for millions of years, filling low areas in the crust to form the first oceans.

2. Comets

It is also believed that some of the water was delivered to Earth by constant bombardment of comets. Comets are icy bodies left over from the formation of the early solar system that contain large amounts of water-ice. As these comets rained down on the early Earth, much of the ice melted and the water was incorporated into the early oceans.

Why are the Oceans Salty?

The first rainwater was fresh, but over time the oceans became salty because of:

Erosion of Rocks

Rainwater is slightly acidic (because it absorbs CO₂ from the air).
As it flowed over rocks and soil, it dissolved minerals—especially salts like sodium and chloride.
These minerals were carried into rivers and streams, which flowed into the oceans.

2. Hydrothermal Vents

Deep underwater, hydrothermal vents (cracks in the ocean floor) released minerals from inside Earth.
These also added salts and other chemicals to the ocean.

The oceans are salty today because:

Water evaporates from the oceans (forming clouds), but salt stays behind.
Over billions of years, salts built up, making the oceans as salty as they are today.

Fun Fact:

If all the salt in the ocean were dried and spread out, it would form a layer more than 500 feet deep across the whole planet!

The Great Oxygenation Event (GOE)

About 2.4 to 2.0 billion years ago

The Great Oxygenation Event was the time in Earth’s history when oxygen started building up in the atmosphere for the first time.

Tiny, single-celled organisms called cyanobacteria (also known as blue-green algae) learned how to do photosynthesis—the process of using sunlight, water, and carbon dioxide to make food.

A waste product of photosynthesis is oxygen (O₂). So, as cyanobacteria spread across the oceans, they began producing tons of oxygen and releasing it into the water and air.

At first, the oxygen didn’t go into the atmosphere. Instead, it reacted with things in the ocean, especially iron. The oxygen combined with iron to form iron oxide (rust), which settled to the seafloor. This created banded iron formations, which we still find in rocks today. Only after the iron was used up did oxygen start accumulating in the atmosphere.

Why was the GOE a Big Deal?

It Changed Earth's Atmosphere

Before the GOE, Earth’s atmosphere had no oxygen.
Afterward, it started to slowly fill with oxygen.

2. It Killed off Many Ancient Organisms

Oxygen was toxic to many early life forms that didn’t need it.
Many of them died off in what was basically Earth’s first mass extinction.

3. It Made Complex Life Possible

Oxygen in the air eventually allowed for more complex life like animals to evolve—millions of years later.

4. Ozone Formed

The buildup of oxygen in the atmosphere led to the development of the ozone layer, which protected life from the harmful ultraviolet (UV) rays from the sun. This eventually allowed life to move from the oceans onto land.

Gigantism

During a time called the Carboniferous Period (about 359 to 299 million years ago), Earth was covered with lush forests and giant insects roamed the land and air. For example, there were dragonflies with wingspans over 2 feet wide!

So, why were insects so big back then?

More Oxygen in the Air

During the Carboniferous period, Earth’s atmosphere had much more oxygen than today—around 30-35% (compared to today’s 21%).
Insects breathe through tiny holes in their bodies called spiracles, which lead to a system of air tubes (tracheae).
They don’t have lungs, so their oxygen intake depends on how much oxygen is in the air.
With higher oxygen levels, their bodies could support larger sizes and still get enough oxygen to their tissues.