Wednesday, 17 April 2024
WorldCarl Sagan discovered life on Earth 30 years ago, his experiments are...

Carl Sagan discovered life on Earth 30 years ago, his experiments are helpful in the search for aliens today.

Birmingham. It has been 30 years since a group of scientists led by Carl Sagan found evidence of life on Earth using data from instruments aboard NASA’s Galileo robotic spacecraft. Yes, you read it right. Among his many pearls of wisdom, Sagan was famous for saying that science is more than a storehouse of knowledge – it is a way of thinking. In other words, how humans discover new knowledge is at least as important as the knowledge itself. Thus, the study was an example of a “control experiment” – an important part of the scientific method.

This may involve asking whether a given study or method of analysis is capable of finding evidence for something we already know. Suppose someone were to fly by Earth in an alien spacecraft with instruments similar to Galileo’s. If we knew nothing else about Earth, would we be able to unambiguously detect life here, using these instruments (which would not be adapted to find it)? If not, what would it say about our ability to detect life elsewhere? Galileo departed in October 1989 on a six-year flyby of Jupiter.

However, to gain enough speed to reach Jupiter, Galileo first had to make several orbits of the inner Solar System, flying close to Earth and Venus. In the mid-2000s, scientists took dust samples from the Mars-like environment of Chile’s Atacama Desert on Earth, which is known to harbor microbial life. They then used experiments similar to those used on the NASA Viking spacecraft (which was intended to detect life there when it landed on Mars in the 1970s) to see if life could be found in the Atacama. They failed – the implication is that if the Viking spacecraft had landed on Earth in the Atacama Desert and conducted the same experiments they did on Mars, they might have missed the detection of life, even if it were known. That it exists.

galileo results
Galileo was equipped with a variety of 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 a radio experiment. Importantly, the study authors did not infer any characteristics of life on Earth from the beginning, but simply attempted to draw their conclusions from the data. The Near Infra-Red Mapping Spectrometer (NIMS) instrument detected gaseous water distributed throughout the terrestrial atmosphere, ice at the poles, and large expanses of liquid water “of oceanic dimensions”.

The temperature here was recorded from -30°C to +18°C. Proof for life? not yet. The study concluded that detecting liquid water and hydrometeorological systems was a necessity, but not a sufficient argument. NIMS also detected higher concentrations of oxygen and methane in Earth’s atmosphere compared to other known planets. Both of these are highly reactive gases that react rapidly with other chemicals and dissipate in a short period of time. The only way to maintain such concentrations of these species was to constantly replenish them with some medium – again suggesting, but not proving, life.

Other instruments on the spacecraft detected the presence of the ozone layer, which protects the surface from harmful UV radiation from the Sun. One can imagine that a simple glance through a camera could be enough to see life. But the images showed oceans, deserts, clouds, snow, and deep areas in South America that, only with prior knowledge, we know for certain to be rain forests. However, once combined with more spectrometry, a distinct absorption of red light was found to cover the darker areas, leading the study to conclude that the light was being absorbed by photosynthetic plant life. No mineral was known to absorb light in this way.

The highest resolution images determined by flyby geometry were of the deserts of central Australia and the ice sheets of Antarctica. Therefore none of the images taken showed clear examples of cities or agriculture. The spacecraft flew closest to the planet even during the day, so even the city lights were not visible at night. However, Galileo’s plasma wave radio experiment was more interesting. The universe is full of natural radio emissions, although most of it is broadband. That is to say, emissions from a given natural source occur at multiple frequencies. In contrast, artificial radio sources are built into a narrow band: an everyday example is the careful tuning of an analog radio required to find a station among the static.

Galileo detected continuous narrowband radio emissions at certain frequencies from Earth. The study concluded that it could only come from a technological civilization, and would only be detected within the last century. If our alien spacecraft had flown by Earth any time in the few billion years before the 20th century, it would have found no definite evidence of any 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 is not guaranteed to detect it.

Therefore such control experiments are important in informing the search for life elsewhere. In the current era, humanity has discovered more than 5,000 planets around other stars, and we have also detected the presence of water in the atmospheres of some planets. Sagan’s experiment shows that this in itself is not enough. A strong case for life elsewhere would require a combination of mutually supporting evidence, such as light absorption by processes such as photosynthesis, narrowband radio emissions, modest temperatures and weather, and chemical traces in the atmosphere that can be detected by non-biological means. It is difficult to explain. As we move into the age of instruments like the James Webb Space Telescope, Sagan’s experiment remains as informative now as it was 30 years ago.

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