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Planet Earth viewed from space

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. Sagan was famous for saying that science is more than a body of knowledge – it is a way of thinking.

The study was an example of a ‘control experiment’ – a critical part of the scientific method which involves asking whether a given study or method of analysis can find evidence for something we already know.

After its launch in October 1989, Galileo had to first make several orbits of the inner Solar System, with close flybys of Earth and Venus, to pick up enough speed to reach Jupiter.

Suppose one were to fly past Earth in an alien spacecraft with the same instruments on board as Galileo on its six-year flight to Jupiter. If we knew nothing else about Earth, would we be able to unambiguously detect life here, using nothing but these instruments? If not, what would that say about our ability to detect life anywhere else?

Dr Gareth Dorrian - Post Doctoral Research Fellow in Space Science, University of Birmingham

Suppose one were to fly past Earth in an alien spacecraft with the same instruments on board as Galileo on its six-year flight to Jupiter. If we knew nothing else about Earth, would we be able to unambiguously detect life here, using nothing but these instruments? If not, what would that say about our ability to detect life anywhere else?

In the mid-2000s, scientists took samples of dirt from the Mars-like environment of Chile’s Atacama Desert, which is known to contain microbial life. They then used similar experiments as those used on the NASA Viking spacecraft - which landed on Mars in the 1970s - to see if life could be found in Atacama.

They failed – the implication being that had the Viking spacecraft landed on Earth in the Atacama Desert, and performed the same experiments as they did on Mars, they might well have missed signatures for life, even though it is known to be present.

Galileo was kitted out with a variety of instruments designed to study the atmosphere and space environment of Jupiter and its moons. These included imaging cameras, spectrometers and a radio experiment.

Importantly, the study authors did not presume any characteristics of life on Earth, but attempted to derive their conclusions just from the data. The near infra-red mapping spectrometer (NIMS) detected gaseous water throughout the terrestrial atmosphere, ice at the poles and large expanses of liquid water ‘of oceanic dimensions’. It also recorded temperatures ranging from -30°C to +18°C.

Evidence for life? Not yet. The study concluded that the detection of liquid water and a water weather system was a necessary, but not sufficient argument.

NIMS also detected high concentrations of oxygen and methane in the Earth’s atmosphere, as compared to other known planets - suggesting continuous replenishment by some means, but not proving, life. Other instruments an ozone layer shielding the surface from damaging UV radiation from the Sun.

Camera images showed only oceans, deserts, clouds, ice, and darker regions in South America which, with prior knowledge, we know to be rain forests. However, combined with more spectrometry, a distinct absorption of red light was found to overlay the darker regions, which suggested photosynthetic plant life. No minerals were known to absorb light in exactly this fashion.

The highest resolution images taken - dictated by the flyby geometry - were of the deserts of central Australia and the ice sheets of Antarctica. No images showed cities or examples of agriculture. Galileo also flew by the planet at closest approach during the daytime, so lights from cities at night were not visible.

Of greater interest though was Galileo’s plasma wave radio experiment. The cosmos is full of natural radio emission and the emission from a given natural source occurs across many frequencies. Artificial radio sources, by contrast, are produced in a narrow band – just think of tuning an analogue radio to find a station amidst the static.

Galileo detected consistent narrowband radio emission from Earth at fixed frequencies. The study concluded this could only have come from a technological civilisation and be detectable within the last century. If our alien spacecraft had made the same flyby of Earth at any time before the 20th century it would have found no definitive evidence life on Earth.

It is perhaps no surprise then that, as yet, no evidence for extra-terrestrial life has been found. Even a spacecraft flying within a few thousand kilometres of human civilisation on Earth is not guaranteed to detect it. Control experiments like this are therefore critical in informing the search for life elsewhere.

In the present era, humanity has now discovered over 5,000 planets around other stars, and even detected water in the atmospheres of some planets. Sagan’s experiment shows this is not enough by itself.

A strong case for life elsewhere will likely require a combination of mutually supporting evidence. As we move into the era of instruments such as the James Webb space telescope, Sagan’s experiment remains as informative now as it was 30 years ago.

A version of this article appears in The Conversation