The Red Edge and the Search for Alien Life

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 when the space shuttle Challenger blew up 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 realised, 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 vapour.

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?

Sanromá and co point out that instead of reducing water to create oxygen as today’s photosynthetic organisms do, purple bacteria probably reduced hydrogen or hydrogen sulphate. Since this requires less energy than reducing water, these organisms would have absorbed light at longer, lower energy wavelengths, perhaps as long as 1025 nanometres, which is in the near infrared.

“Thus, their color is distinctly diﬀerent from that of land plants that dominate the Earth today,” say Sanromá and co.

This would have created a similar effect to the red edge observed by Galileo. “Purple bacteria show a reﬂectance spectrum with a sharp increase in reﬂectivity similar to the red edge of leafy plants, but shifted redwards,” conclude Sanromá and co.

They go on to model how that might have been visible in various potential scenarios such as with varying amounts of cloud cover and whether the bacteria were confined to the ocean or were also found elsewhere.

They conclude that the biosignature of purple bacteria would have been observable in most scenarios with the technology we have available today.

That has interesting implications for astrobiologists studying exoplanets. It means that not only should it be possible to spot the red edge or infrared edge associated with different types of life, it ought to be possible to distinguish an exoplanet with early Earth-like life from one that has mature Earth-like life.

So if we can spot signs of life, we just might be able to determine how old it is.

The study of exoplanet signatures is a new field that is likely to get significant attention in the coming years and decades. Given the rate at which astronomers are finding exoplanets—they have over 1000 on their books with more than 2000 awaiting confirmation—one of the major drivers of future research will be the search for evidence of life on them.

Indeed, the growing number of papers on alien atmospheres and surfaces is proof of this. (We looked at one on this blog only last week.)

And while the evidence from these methods might not be conclusive of life, Galileo has already shown that the evidence can be strongly suggestive. For many astrobiologists, it is only a matter of time.

Ref: arxiv.org/abs/1311.1145: Characterizing The Purple Earth: Modelling The Globally-Integrated Spectral Variability Of The Archean Earth