Study reveals Earth’s oceans were once green, long before life began, know why

We usually imagine Earth as a beautiful blue planet, especially after seeing space images like the famous “Pale Blue Dot” captured by Voyager 1, a term made popular by astronomer Carl Sagan. But according to a new scientific study, Earth may not have always looked this way. Billions of years ago, our oceans might have appeared green instead of blue.

The research, published in the journal Nature, says that Earth's oceans looked very different during a time called the Archaean era — which was about 2.4 billion years ago. Back then, there wasn’t any oxygen in the atmosphere, and life as we know it today hadn’t yet developed. The ocean floors had many hydrothermal vents, which are cracks in the seabed that release hot, mineral-rich water. These vents pumped large amounts of ferrous iron (iron that forms without oxygen) into the ocean water.

Since there was no oxygen in the atmosphere, this iron built up in the water and changed how light behaved in the oceans. That’s where things get interesting. When cyanobacteria — tiny photosynthetic organisms- started producing oxygen, it began reacting with the ferrous iron in the water. This reaction created ferric iron, which is not soluble in water and formed particles like rust, known as iron hydroxide.

These rust-like particles floated in the water and created a unique optical effect. They absorbed red and blue wavelengths of light but allowed green light to pass through. As a result, the oceans glowed with a green hue. If we had cameras back then and looked at Earth from space, it would’ve appeared as a glowing green planet instead of blue.

The researchers also found that this green effect wasn’t just at the surface — it could be seen at ocean depths of 5 to 20 meters as well. One exciting part of this discovery is that it could help scientists look for life on other planets. If we spot planets that look pale green from far away, they might be going through a similar stage in evolution as early Earth — possibly hosting simple life forms like early cyanobacteria.

The study suggests that the green glow could be a helpful sign for finding planets where life is just beginning.