When Earth was ruled by purple bacteria, its bio-signature would still have been recognisable, say astrobiologists who think similar signs might be visible on other planets.
In the 1980s, when NASA built the Galileo probe destined for Jupiter, the plan was to launch it in the cargo bay of the space shuttle complete with a powerful booster rocket that would send it directly on its way to the giant planet. But after the Challenger tragedy in 1986, the safety review that followed concluded that it would not be a good idea to place an unlit rocket inside any future shuttle. And since no other rocket was powerful enough to lift the space probe and its booster, NASA had to find another way of getting Galileo to Jupiter.
The solution was to send Galileo around Venus, back around Earth and back to Venus again before catapulting it on its way towards Jupiter. This new mission profile gave the mission scientists an idea. Galileo, they realized, would be the first spacecraft to fly past Earth on its way to somewhere else. And that gave them a unique opportunity to use Galileo’s powerful suite of instruments to look for signs of life on the home planet.
Astrobiologists have always been keenly interested in finding signs of life on other planets. The new mission would provide a powerful control experiment of their capabilities. In the event, Galileo gathered a great deal of evidence that pointed to something interesting happening on the surface of Earth. The results, said the Galileo team, “are strongly suggestive of life on Earth.”
One of the more interesting features was in the spectrum of light reflected from the surface. The team noted that a pigment on the surface strongly absorbed light in the red part of the spectrum. This has since become known as “the red edge” and astrobiologists think that if life on other planets is anything like that on Earth, then a similar feature ought to be visible in the light reflected from life-bearing exoplanets too.
So what kind of signature might this exovegetation produce? Today, we get an answer thanks to the work of Esther Sanromá at the Universidad de La Laguna in Spain and a few pals who have calculated what Earth’s signature would have looked like during the Archaen era 3 billion years ago when the planet was probably ruled by purple bacteria.
At that time, the Sun was only about 80 per cent as bright as it is today and Earth was very different place. The atmosphere was dominated by nitrogen, carbon dioxide and water vapor.
Life had sprung into existence just 800 million years earlier and the first photosynthetic life was a purple bacteria that did not produce oxygen as a by-product—hence the lack of oxygen in the atmosphere.
Since these bacteria absorb light, this ought to have been visible in the spectrum of light reflected from the surface. So what kind of “edge” would this have produced?