Turns out Earth once used to be purple in colour

If you thought Earth was a mixture of green, blue, brown and white colors since the beginning, you might have been wrong as a new study suggests that Earth was in fact purple once.

A new NASA-supported study indicates that there is a possibility that our home planet Earth was a shade of purple thanks to a purple-tinged molecule known as ‘retinal’.

Science says that due to photosynthesis, a process through which plants convert energy from sun into chemical energy to produce food and release oxygen, the plants are usually green. The green color comes from a pigment called chlorophyll that absorbs sun energy for converting the carbon dioxide and water into sugars.

However, the new study published in the International Journal of Astrobiology claims that early forms of life may have been able to produce metabolic energy using retinal that most probably surpassed chlorophyll in becoming the dominant sunlight-absorbing molecule. Retinal-containing proteins such as ‘bacteriorhodopsin’ absorb sunlight in the range that is inaccessible to chlorophyll, explained Big Think.

As the retinal pigments absorb green and yellow light and give out red and blue, life based on retinal would look purple. This theory makes it possible to say that there was once a stage where there was ‘Purple Earth’. That age would date somewhere around 2.4 to 3.5 billion years ago, before the ‘Great Oxygenation Event’ that was likely due to rise of chlorophyll-based photosynthesis and created an abundance of oxygen in air.

Author Shiladitya DasSarma told Astrobiology Magazine, “Retinal-based phototrophic metabolisms are still prevalent throughout the world, especially in the oceans, and represent one of the most important bioenergetic processes on Earth.”

What’s more is that if Earth had a retinal stage and because retinal is a simpler molecule as compared to chlorophyll, then it stands to reasons that we should take this into account while looking for new inhabitable or habitable planets. It is completely possible that retinal-based life could be more widespread throughout the universe and a ‘green edge’ in a planet’s spectrum could potentially be a biosignature for retinal-based life.