Astronomers use the color reflected by the surface of an exoplanet to assess their chances of sheltering life. The pale blue hue of the Earth is the most sought after. Scientists who have studied the distant past of our planet suggest that another, rather unusual, color could be of great interest.
Looking at the Earth from space, two colors stand out: blue, reflected by our liquid oceans, and green, indicating the presence of plants. These two colors are important clues to life on our planet, to the point where researchers looking for extraterrestrial life track down the combination of these hues on the surface of other exoplanets in the hope of discovering a revealing sign of a habitable planet.
However, an analysis published by US researchers proposes to add another color to this list of signs of life: purple. At first glance, the addition may seem strange. Purple is not a color whose hues are commonly observed in nature. However, this has not always been the case. According to the scientists, billions of years ago, some of the first forms of life on Earth could have had this color in common. Because of their number, they would have given our Earth a purple hue making it unrecognizable to us.
For researchers, a deeper knowledge of these variations of “styles” in the appearance of the Earth throughout history would open up better prospects for the search for life elsewhere in the Universe.
All the colors we can see with the naked eye exist thanks to the properties of light. White light, such as that produced by our Sun, is composed of a multitude of electromagnetic waves, some covering the entire spectrum of colors. Shorter waves draw toward the violet, the longer ones tend towards the red and all the other colors are between these two extremes.
If an object has a color, it is composed of materials that can absorb certain wavelengths and reflect others. In the case of the color of a tree leaf, the green comes from the fact that chlorophyll mainly absorbs red and blue wavelengths in order to produce its energy. The green thus is reflected, giving it its color to our eyes.
It is the same for the blue of the ocean or that of the sky, whose transparency has the property of easily diffuse the blue wavelength, which, over a thickness of several kilometers, gives it this color.
On Earth there are very ancient forms of life, called archaea, unicellular beings resembling bacteria, but forming a large family that is evolutionarily separated from any other living being. Although it is very difficult to evaluate the order of appearance of the very first life forms on Earth, the archaea remain very old and could have appeared on Earth more than 3 billion years ago.
Some archaea produce their energy in the same way as plants, absorbing sunlight. However, contrary to the use that plants make of chlorophyll, archaea rather use a molecule called retinal, which absorbs mainly the wavelength corresponding to green and yellow, giving the organism that possesses a purple hue .
This molecule is much less effective than chlorophyll for the production of energy and does not lead to oxygen production. However, its method is much simpler in its chemical reactions and could have been used by organisms on Earth at the same time or even before the arrival of chlorophyll and oxygen production, 2.4 billions of years ago.
These life forms still exist today, giving a purple color to certain extreme environments. If they had been in sufficiently large numbers, several billion years ago, they could have given the surface of the Earth a lavender hue, very different from that of today.
Although it will be difficult to prove that the Earth has ever been so different, the presence, even today, of life forms using retinal as a mechanism for energy production shows that this mode of survival is ideal for many species. .
As increasingly high-performing telescopes are under construction and researchers become more and more inventive in their quest for extraterrestrial life, this work suggests that observing the visible signs of life elsewhere should not be based solely on research resemblances to the present Earth, but also on the Earth of the past.