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 space probe and only combining instruments while ignoring existing knowledge made this possible
By: Gareth Dorian, PhD Research Fellow in Space Sciences, University of Birmingham
Earth and Moon as photographed by the Galileo probe from a distance of 6 million km. NASA
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 space probe. Yes, you read that right. 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, the way humans go about the business of discovering new knowledge is at least as important as the knowledge itself. In this spirit, the study was an example of a "control experiment" - a critical part of the scientific method. Among other things, the control experiment examines whether a given study or analysis method is able to find evidence for something we already know.
We assume aliens approaching the Earth and have on board the same instruments as were on Galileo, and ask: if we knew nothing else about the Earth, would we be able to unequivocally discover life here, using only these instruments (which were not designed in advance to find life )? If not, what does that say about our ability to discover life anywhere else?
Galileo was launched in October 1989 for a six-year flight towards Jupiter. It first had to make several orbits around the inner solar system, passing close by Earth and Venus, in order to gather enough speed to reach Jupiter.
In the mid-2000s, scientists took samples of soil from Chile's Martian-like Atacama Desert environment, which is known to harbor microbial life. They then used experiments similar to those used by NASA's Viking spacecraft (designed to discover life on Mars when they landed there in the 70s) to see if life could be found in the Atacama.
They failed - meaning that if Viking spacecraft had landed on Earth in the Atacama Desert, and performed the same experiments as they did on Mars, they would probably have missed signs of life, even though it was known to be present.
Galileo was equipped with a variety of instruments designed to study the atmosphere and space environment of Jupiter and its moons. These included observation cameras, spectrometers (which break down light into wavelengths) and a radio experiment.
It is important to note that the authors of the study did not presuppose any characteristics of life on Earth, but tried to draw their conclusions only from the data. The near-infrared mapping spectrometer (NIMS) discovered water in a gaseous state scattered in the earth's atmosphere, ice at the poles and vast areas of liquid water "of oceanic dimensions". He also recorded temperatures ranging from -30 degrees Celsius to 18 degrees Celsius.
A vision for life? Not yet. The study concluded that the discovery of liquid water and a weather system based on the water cycle is necessary but not sufficient.
NIMS also discovered high concentrations of oxygen and methane in Earth's atmosphere, compared to other known planets. Both of these gases are highly reactive gases that would react quickly with other chemicals and dissipate within a short period of time. The only way such concentrations of these species could be maintained was if they were continuously replenished by some means – again implying, but not proving, life. Other instruments on the spacecraft revealed the presence of an ozone layer, which protects the surface from harmful ultraviolet radiation from the sun.
One could imagine that a simple look at the camera would be enough to see life. But the images showed oceans, deserts, clouds, ice and darker areas in South America that only with prior knowledge, we of course know to be rainforests. However, if data from spectrometry is combined, a pronounced absorption of red light above the dark areas is revealed, which the study concluded is "strongly suggestive" of light being absorbed by photosynthetic vegetation. No minerals were known that absorbed light in such a precise manner.
The highest resolution images taken, as determined by the geometry of the transition, were of the central Australian deserts and the Antarctic ice sheets. Therefore none of the photos taken showed cities or clear examples of agriculture. The spacecraft also passed by Earth at its closest distance during the day, so city lights at night were not visible either.
More interesting was the plasma radio wave experiment. The universe is full of natural radio emission, but most of it is a ribbon. That is, the emission from a given natural source occurs over many frequencies. Artificial radio sources, on the other hand, are produced in a narrow band: an everyday example is the fine tuning of analog radio required to find a station among background noise.
An example of natural radio emission from the aurora in Saturn's atmosphere can be heard below. The frequency changes quickly - unlike a radio station.
Galileo discovered consistent narrowband radio emission from Earth at fixed frequencies. The study concluded that this could only come from a technological civilization, and was only detectable in the last century. If our alien spacecraft had made a similar pass by Earth at any time during the billions of years before the 20th century, then it would have found no unequivocal evidence of any civilization on Earth at all.
It is therefore not surprising that so far no evidence of extraterrestrial life has been found. Even a spacecraft flying only thousands of kilometers away from human civilization on Earth will not necessarily detect it. Such control experiments are therefore critical in guiding the search for life elsewhere.
In the current era, humanity has so far discovered more than 5,000 planets around other stars, and we have even discovered the presence of water in the atmospheres of some of the planets. Sagan's experiment shows that these discoveries are not enough by themselves.
To conclude with certainty about life on another planet would require a combination of mutually supporting evidence, such as absorption of light by photosynthesis-like processes, narrow-band radio emission, moderate temperatures and weather, and chemical traces in the atmosphere that are difficult to explain by non-biological means. As we enter the era of instruments like the James Webb Space Telescope, it remains as important as it was 30 years ago.
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