Our blue-green Earth might have actually been a different color, thanks to this molecule.
The notion of our beautiful, fragile planet as a "pale blue dot" is an image that has been popularized by scientists for many years now. After all, seeing Earth from space -- from a different perspective -- helped to further nurture the nascent environmental movement beyond mere conservation and into a more planetary, wider view of things.
But perhaps that view of a blue jewel floating out in the dark, interstellar reaches might be a relatively recent one. According to a new study from NASA, Earth may have actually been purple for the first 2 billion years of its existence -- thanks to a purple-tinted molecule called retinal.NASA's research postulates that being a simpler molecule, retinal was more abundant earlier on in the Earth's history, and likely preceded -- or at least co-evolved with -- chlorophyll as the dominant molecule that allowed organisms to absorb sunlight. As Shiladitya DasSarma, study co-author and a molecular biology professor at the University of Maryland, tells 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.
However, that apparently changed 2.4 billion years ago, when the level of free oxygen in our atmosphere dramatically rose, precipitating what is now called the Great Oxygenation Event, likely brought on by the proliferation of cyanobacteria. These blue-green algal organisms are able to photosynthesize – meaning they can transform sunlight and carbon dioxide into energy, and produce oxygen as a ‘waste’ product – by using chlorophyll, a green pigment.
The findings could have interesting implications in our search for potentially habitable planets, as the telltale colours of a faraway planet's biosignature can be used to deduce whether it harbours Earth-like conditions for supporting life. As Astrobiology Magazine explains:
Because vegetation on Earth absorbs red light, but reflects infrared light, viewing vegetation using a spectroscope reveals a dramatic dip in reflected light at red wavelengths, a sudden decrease that is called the ‘red edge’. It has been suggested that when probing the spectrum of light reflected from potentially habitable exoplanets, scientists could search for a red edge in the planet’s light, which would be a biosignature indicative of vegetation using chlorophyll, or its extraterrestrial equivalent.
Intriguingly, since retinal pigments absorb green and yellow light, and reflect or transmit red and blue light, then retinal-based life would appear purple in color. [..] Because retinal is a simpler molecule than chlorophyll, then it could be more commonly found in life in the Universe, and therefore a ‘green edge’ in a planet’s spectrum could potentially be a biosignature for retinal-based life.