Long before Earth became the familiar “pale blue dot” described by Carl Sagan it may have shimmered with a very different hue green. According to a groundbreaking study published in Nature, Earth’s oceans during the Archaean era around 2.4 billion years ago were likely emerald green thanks to chemical conditions vastly different from those we know today.
Back then, Earth’s atmosphere lacked oxygen, and hydrothermal vents at the ocean floor were constantly releasing ferrous iron (iron in its reduced, soluble form) into the waters. With no oxygen to rust it out, this iron built up in the seas, changing not just the chemistry but also the optical appearance of the oceans.
As life slowly evolved, cyanobacteria ancient microbes capable of photosynthesis began producing oxygen. This oxygen reacted with the ferrous iron, converting it into ferric iron, which is insoluble. The resulting iron hydroxide particles scattered in the water absorbed red and blue wavelengths of light, allowing green light to pass through. The result? Oceans that looked green from above so if satellites or cameras had existed then, Earth might have resembled a glowing pale green dot.
To back up this theory, scientists genetically engineered modern cyanobacteria to produce a pigment called phycoerythrobilin, which absorbs green light. These engineered microbes thrived under green light, mirroring how ancient cyanobacteria might have flourished in iron-rich, green-tinted oceans.
Researchers also found that even 5 to 20 meters below the surface, the green light remained dominant, due to how iron particles filtered sunlight. This has intriguing implications beyond Earth: planets that appear green from space might also be early in their evolutionary journey, possibly harboring primitive, photosynthetic life.
“Our findings reimagine the green color as a marker of a unique evolutionary stage on habitable planets,” the study concluded—suggesting that the search for alien life may benefit from looking for worlds that aren’t blue, but green.
