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What gave the Earth its blue colour?

Seen from space, the Earth is blue. The Earth has been blue for over 4 billion years because of the liquid water on its surface. How has the Earth managed to sustain liquid water on its surface for such a long time?

There is only one known planet with permanent bodies of liquid water at its surface: ours. Earth sciences allow us to explain why the Earth has almost always been blue: it's neither too warm nor too cold. If the Earth was first red and black, it has been blue for more than 4 billion years, with rare exceptions when it got too cold and turned into a white snowball.

This incredible characteristic is due to the interactions of the water cycle with plate tectonics and the greenhouse effect, as well as the configuration of the solar system. Today, Earth's average surface temperature is about 15°C, colder than Venus 4650C and warmer than Mars (-60 °C on average) . On Earth, at sea level, water freezes below 0°C and boils at 100°C. Earth's surface is thus kept within a temperature range that might seem large to us, but is actually fairly narrow when compared with other planets, and it has remained so for billions of years.

Greenhouse gases play their part

The average temperature on a planet's surface depends on the interaction of three parameters that can vary widely from one planet to the next:

The energy arriving from the sun.
The albedo of the surface, meaning how much it reflects solar radiation away.
Greenhouse gases, which trap solar radiation within Earth's atmosphere. Without greenhouse gases, Earth's surface would be at a temperature around -15°C and probably devoid of liquid water.

Interactions between sunlight, albedo, and greenhouse gases have maintained a fairly constant energy balance since the first oceans appeared on Earth.

Early in the Earth's history, the young sun was less bright and our planet received less energy from it. However, levels of greenhouse gases such as CO₂ and methane were much higher than today, which maintained surface temperatures high enough for water to be liquid.

The greenhouse effect decreased over time because CO₂ can be removed from the atmosphere by two processes. First, the acidifying effect of CO₂ dissolved in surface waters causes rocks to dissolve, which releases calcium. Calcium combines with the dissolved CO₂ to form carbonate rocks such as limestone, one of the main carbon sinks.

The second sink is organic carbon stored in sedimentary rocks. Organisms on land and in the ocean use CO₂ to build organic matter during photosynthesis, a portion of which is deposited at the bottom of the ocean when the organisms die. There, the organic matter is incorporated into sedimentary rocks, where it can be stored for millions of years.

Although carbon sinks store CO₂ away from the atmosphere, volcanoes and oceanic ridges deliver CO₂ back into the atmosphere. This delivery is sustained through plate tectonics. Over long timescales, plate tectonics helps maintain Earth's surface temperature in the range that allows surface waters to be liquid. The presence of liquid water and plate tectonics are thus intimately linked. How does that happen?

The ocean floor is composed of oceanic plates. They move away from oceanic ridges, the chain of submarine volcanoes that runs across the planet, and then down toward the depths of the Earth through subduction. During the hundreds of millions of years that they traverse the oceans, oceanic plates become hydrated: their minerals incorporate water, which modifies their mechanical properties. As they are subducted, oceanic plates eventually dehydrate; the released water eventually produces magmas that form granites, the bedrock of the continents. Without liquid water, there would be no tectonics and therefore, no continents!

Due to this recycling of older oceanic plates into the mantle, new plates are constantly being formed from material erupted at oceanic ridges. As this material rises through the mantle and to the ocean floor, it cools and releases CO₂, helping sustain greenhouse gas concentrations. Water remains liquid and the Earth remains blue, as it has been for several billion years.

What gave the Earth its blue colour?

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