When describing an image captured by Voyager 1, Carl Sagan, the host of the well-known television program Cosmos, once referred to Earth as a “pale blue dot.” According to a recent study, our oceans formerly shimmered in green, yet humans have only ever seen Earth as blue from space.
According to a study that was published in the journal Nature, the oceans on Earth were probably bathed in green light 2.4 billion years ago, during the Archaean epoch. Reduced iron, or iron deposited without oxygen, was poured into the water by hydrothermal vents erupting in the ocean floors prior to the emergence of life on Earth, filling the seas with ferrous iron.
With no oxygen in the atmosphere, the oceans lacked the reflective quality of today’s blue waters. However, with the arrival of cyanobacteria, oxygen started appearing in the water owing to the rise of photosynthesis. The oxygen transformed ferrous iron into ferric iron, which is insoluble and forms rust-like particles.
The ferric iron, suspended in water as iron hydroxide, owing to its insoluble nature, created a powerful optical effect. It absorbed red and blue wavelengths but allowed the green light to pass. Consequently, the oceans wore a green hue and if cameras existed, Earth would have looked emerald green from up above.
Theory proved
In order to validate their hypothesis, the scientists genetically modified contemporary cyanobacteria to employ phycoerythrobilin, a pigment that absorbs green light. Under green light, the altered microorganisms flourished, partially simulating a natural event that might have occurred billions of years ago.
“By mimicking previous natural selection, genetic engineering of extant cyanobacteria indicates that cyanobacteria that developed a green-specialized phycobilin known as phycoerythrobilin may have thrived in green-light conditions.”
The results demonstrated that iron hydroxide particles produced a sustained green-light window even at depths of five to twenty meters.
The study’s main conclusions include the possibility that early photosynthetic life could exist on pale-green dot worlds that are visible from space.
The report emphasized, “Our findings…envision the green color as a sign of the distinct evolutionary stage of inhabited planets.”