Carl Sagan detected life on Earth 30 years ago – here’s how his experiment is helping us search for alien species today
It's been 30 years since a group of scientists led by Carl Sagan found evidence for life on Earth using data from instruments on board the Nasa Galileo robotic spacecraft. Yes, you read that correctly.
Among his many pearls of wisdom, Sagan was famous for saying that science is more than a body of knowledge – it is a way of thinking.
In other words, how humans go about the business of discovering new knowledge is at least as important as the knowledge itself. In this vein, the study was an example of a “control experiment” – a critical part of the scientific method. This can involve asking whether a given study or method of analysis is capable of finding evidence for something we already know.
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Suppose one were to fly past Earth in an alien spacecraft with the same instruments on board as Galileo had. If we knew nothing else about Earth, would we be able to unambiguously detect life here, using nothing but these instruments (which wouldn't be optimised to find it)? If not, what would that say about our ability to detect life anywhere else?
Galileo launched in October 1989 on a six-year flight to Jupiter. However, Galileo had to first make several orbits of the inner Solar System, making close flybys of Earth and Venus, in order to pick up enough speed to reach Jupiter.
In the mid-2000s, scientists took samples of dirt from the Mars-like environment of Chile's Atacama desert on Earth, which is known to contain microbial life. They then used similar experiments as those used on the NASA Viking spacecraft (which aimed to detect life on Mars when they landed there in the 1970s) to see if life could be found in Atacama.
They failed – the implication being that had the Viking spacecraft
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