How Carl Sagan’s Experiment from 30 Years Ago is Aiding in the Search for Extraterrestrial Life Today

Carl Sagan and his team of scientists discovered evidence of life on Earth three decades ago by analyzing data collected by instruments on the Nasa Galileo robotic spacecraft.

Among his many pearls of wisdom, Sagan was famous for saying that science is more than a body of information—it’s a way of thinking.

In other words, how people seek to find new information is at least as important as the information itself. In this sense, the study was an example of a “control experiment” – a critical part of the scientific method. This may include the question of whether a particular study or analytical method can find evidence for something already known.

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Suppose someone were to fly past Earth in an alien spacecraft with the same instruments as Galileo. If we knew nothing else about Earth, would we be able to unequivocally detect life here using only these instruments (which would not be optimized to find it)? If not, what would that say about our ability to detect life elsewhere?

Galileo was launched in October 1989 on a six-year flight to Jupiter. However, Galileo first had to make several orbits of the inner solar system and make close passes of Earth and Venus to gain enough speed to reach Jupiter.

In the mid-2000s, scientists sampled dirt from a Martian-like environment in Chile’s Atacama Desert on Earth, which is known to harbor microbes. They then used experiments similar to those used by NASA’s Viking spacecraft (which was designed to detect life on Mars when they landed there in the 1970s) to see if life could be found in the Atacama.

They failed – if the Viking spacecraft landed on Earth in the Atacama Desert and did the same experiments they did on Mars, they might have missed out on life, even though it is known to be present.

Galileo’s results

Galileo was equipped with various instruments designed to study the atmosphere and space environment of Jupiter and its moons. These included imaging cameras, spectrometers (which break up light by wavelength), and radio experimentation.

Importantly, the authors of the study did not assume any characteristics of life on Earth ab initio (from the beginning), but tried to draw their conclusions from the data alone. The Near-Infrared Imaging Spectrometer (NIMS) detected gaseous water distributed throughout Earth’s atmosphere, polar ice and large regions of liquid water known as “oceans.” It also recorded temperatures 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 basis.

NIMS also found high levels of oxygen and methane in Earth’s atmosphere compared to other known planets. Both are highly reactive gases that react quickly with other chemicals and break down in a short time.

The only way to maintain the concentrations of these species was if they were constantly replenished by some means – which again suggests, but does not prove, life. Other spacecraft instruments observed the ozone layer, which protects the surface from the Sun’s harmful UV radiation.

One could imagine that just looking through the camera might be enough to detect life. But the images showed oceans, deserts, clouds, ice, and darker areas in South America, which of course we know to be rainforests only by prior knowledge. However, after combining more spectrometry, it was found that the absorption of red light clearly covered the darker areas, which the study said “strongly” indicates absorption of light by photosynthetic plants. No minerals were known to absorb light in exactly this way.

The highest resolution images dictated by overflight geometry were the deserts of central Australia and the glaciers of Antarctica. Consequently, none of the images taken showed cities or clear examples of agriculture. The spacecraft also flew by the planet closest during the day, so at night the lights of the cities were also not visible.

Of greater interest, however, was Galileo’s plasma wave radio experiment. The cosmos is full of natural radio radiation, but most of it is broadband. In other words, the emission from a given natural source occurs at several frequencies. Artificial radio sources, on the other hand, are produced in a narrow band: an everyday example is the careful tuning of an analog radio, which is required to find a station in the middle of static electricity.

Below is an example of natural radio emission from the aurora borealis in Saturn’s atmosphere. The frequency changes quickly – unlike a radio station.

Galileo detected a steady narrowband radio transmission from Earth on fixed frequencies. The study concluded that this could only have come from a technological civilization and would only be noticeable in the last century. If our alien spaceship had flown past Earth at any point in the few billion years before the 20th century, it would have seen no definitive evidence of civilization on Earth.

It is perhaps no surprise that so far no evidence of extraterrestrial life has been found. Even a spacecraft flying within a few thousand kilometers of human civilization on Earth cannot detect it. Control experiments like this are therefore critical in informing the search for life elsewhere.

In modern times, humanity has now discovered more than 5,000 planets around other stars, and we have even detected the presence of water in the atmosphere of some planets. Sagan’s experiment shows that this alone is not enough.

A strong case for life elsewhere is likely to require a combination of mutually supportive evidence, such as absorption of light in processes such as photosynthesis, narrow-band radio emission, modest temperatures and weather, and chemical signatures in the atmosphere that are difficult to explain by non-biological means. . As we move into the age of instruments like the James Webb Space Telescope, Sagan’s experiment remains as informative as it was 30 years ago.

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