Early Oceans On Earth Had Surprising Colors, Scientists Find
When they first formed, they weren’t blue at all. They were green.
We all know Earth has gone through some massive changes over the last few billion years. The continents have shifted, climates have swung from scorching to frozen, and the landscapes we see today would be almost unrecognizable compared to those in our planet’s distant past.
But here’s something that might surprise you: the deep blue oceans we’re so familiar with today didn’t always look like that. When they first formed, they weren’t blue at all. They were green.
That’s the fascinating discovery from a recent study by researchers at Nagoya University in Japan. Their findings suggest that early Earth was once covered in green water, challenging what many of us picture when we think about our planet’s early days.
So, what caused this unusual color? It comes down to microscopic life, specifically cyanobacteria.
Cyanobacteria are ancient bacteria that had a massive impact on Earth's history. Around 2.4 billion years ago, they kickstarted a process called oxygenic photosynthesis. By using sunlight to create energy, they also began releasing oxygen into the atmosphere.
This set the stage for what scientists call the Great Oxidation Event, a major turning point that allowed oxygen-dependent life forms to eventually evolve.
The green color likely came from pigments used by these cyanobacteria during photosynthesis. Instead of the clear blues we see in oceans today, early seas were filled with these microbes, giving the water a green appearance.
Beyond just changing how we think about Earth’s past, this discovery could also help scientists search for life on other planets. By understanding how microbial life transformed Earth, researchers may be better equipped to spot signs of similar activity elsewhere in the universe.
When oceans first formed, they weren’t blue at all. They were green.
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According to the researchers, Earth’s early oceans appeared green because they reflected light in the green part of the spectrum. This was due to high concentrations of ferrous iron, which was released into the water from hydrothermal vents on the seafloor.
But this vibrant green hue didn’t last forever. As cyanobacteria began producing oxygen through photosynthesis, they triggered the Great Oxidation Event—a major atmospheric shift that had a profound impact on the chemistry of the oceans. The oxygen released by these microbes converted ferrous iron (Fe²⁺) into ferric iron (Fe³⁺), which behaves differently when it comes to light absorption.
Ferric iron particles absorb more blue and red wavelengths, allowing green light to pass through more freely. This made Earth’s ancient oceans appear visibly green when seen from above or afar.
These findings, led by Taro Matsuo and his team at Nagoya University, could have exciting implications for the search for extraterrestrial life. If green seas on early Earth were a sign of microbial activity, similar signatures on distant planets might offer clues about their habitability.
“Remote-sensing data show that waters rich in iron hydroxide, such as those around Iwo Island in the Satsunan archipelago, appear noticeably brighter than typical blue oceans,” Matsuo explained.
“This leads us to think that green oceans might be observable from a longer distance, making them easier to detect.”
In other words, spotting a planet with green-tinted seas could one day be a signal that life, at least microbial life, might be present, or once was.
Damjan
