Carl Sagan, the host of the popular TV show Cosmos, once described Earth as a 'pale blue dot' when explaining an image taken by Voyager 1. Humans have only observed Earth as blue from space but a new study claims that our oceans shimmered in green in the past.
The study published in the journal , argues that during the Archaean era, around 2.4 billion years ago, Earth's oceans were likely bathed in green light. Before life thrived on Earth, the hydrothermal vents erupting in the ocean floors, pumped reduced iron (iron deposited in the absence of oxygen) into the water, 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.
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Theory proved
To confirm their theory, the researchers genetically engineered modern cyanobacteria to use a green-absorbing pigment called phycoerythrobilin. The modified microbes grew better under green light which somewhat mimicked the natural phenomenon that may have transpired billions of years ago.
"Genetic engineering of extant cyanobacteria, simulating past natural selection, suggests that cyanobacteria that acquired a green-specialised phycobilin called phycoerythrobilin could have flourished under green-light environments."
The findings showed that even at depths of five to 20 meters, iron hydroxide particles created a persistent green-light window.
One of the major implications of the study is that pale-green dot worlds viewed from space could be good candidate planets to harbour early photosynthetic life.
"Our findings...envision the green colour as a sign of the distinct evolutionary stage of inhabited planets," the study highlighted.
