In the dark forest, have we been exposed?

In 1920, General Motors hired American inventor Charles Franklin Kettering to improve chlorofluorocarbons, which were first synthesized by Frederick Swart in the 1990s, making them stable, "safe" and effective refrigerants.

This type of refrigerant was later called Freon, and was widely used in refrigeration equipment such as air conditioners and refrigerators in the following decades. Due to its excellent physical properties, Freon is also widely used as a dispersant for spray facilities such as perfumes and insecticides, and as a cleaning agent for precision equipment.

People at that time did not realize how this chemical substance, which had never existed in nature, could change the earth.

In 1957, Joe Farman, who was working at Cambridge University, went to the Halley Bay Observatory with the Antarctic scientific expedition team. His task was to monitor the ozone content in the air. Thereafter, he would go to Antarctica every year to record this data.

At that time, exploring the seventh continent was a very valuable task. Twelve countries, including the United Kingdom, established multiple observation stations in Antarctica, and monitoring ozone content was only a very small task.

However, in the spring of 1981, the results of the new measurements attracted the attention of Joe Farman and his colleagues, because the data showed that the ozone layer over Antarctica was much smaller than in the past. In order to eliminate the errors caused by measurement errors, they recalibrated the instrument and continued to observe. However, the data in the following years showed that the ozone over Antarctica continued to decrease.

Changes in the ozone hole from 1979 to 2019 / European Environment Agency

Joe Farman realized that things might be getting serious.

According to statistics, by October 1984, the ozone layer area over Antarctica had decreased by 40% compared to the average level, and there was a huge ozone hole in the sky. And according to past records, the reduction of the ozone layer actually began in 1977. In May of the following year, Nature magazine published an article by Joe Farman's team, and the ozone hole issue officially entered the public's field of vision.

Satellite monitoring of the annual decrease in Antarctic ozone thickness/TMOS

It was a very quick process for researchers to turn their attention to Freon, but at the beginning no one could have imagined that this synthetic substance would cause a disaster half a century later.

Freon is chemically very stable and can exist safely in the troposphere for nearly a century. However, it takes less than 15 years for it to slowly climb to the calm stratosphere above the troposphere. Here, Freon releases chloride ions under the action of ultraviolet rays and becomes a catalyst for the decomposition of ozone. During the decomposition process, one Freon molecule can promote the annihilation of nearly 100,000 ozone molecules.

Although most countries have signed the Montreal Protocol in August 1987, banning the use of CFCs and other Freon-like substances that are prone to cause ozone destruction, the hydrofluorocarbons used as alternatives are greenhouse gases. In fact, to this day, humans have not yet found any completely non-toxic and non-flammable compounds to replace Freon as a refrigerant.

What’s even more terrible is that the CFCs emitted previously are still floating in the atmosphere, and they may be detected by extraterrestrial civilizations as a mark of human civilization on Earth.

As a substance that will never be produced spontaneously in nature, complex organic substances such as Freon in the atmosphere will become a very obvious sign of civilization, which is even more certain than the faint phosphine gas on Venus. After all, only those civilizations that have undergone rampant industrialization will leave these residues in the atmosphere of their home planet.

Of course, the consequences of this blind ignorance may be common. After all, in the history of civilization, widespread rational cognition always lags behind. It is for this reason that using non-natural substances as life marks may also be a new way to find extraterrestrial life.

A recent article published on the ArXiv board explores the possibility of using the James Webb Telescope (JWST) to search for industrial pollutants in the atmosphere of exoplanets. This article discusses the performance of substances such as chlorofluorocarbons and potent greenhouse gases in the atmosphere. Of course, there are still many limitations for the James Webb telescope to search for substances such as chlorofluorocarbons, such as the stars near the planets are too bright and will drown out the signals in the atmosphere.

But for M-type stars, which are dim but long-lived red dwarfs, telescopes may have a good chance of seeing whether these products of industrial civilization exist in the atmospheres of nearby planets.

For example, TRAPPIST-1, a red dwarf star 40 light years away in the constellation Aquarius, is located near the solar system. There are seven Earth-like planets around it, and at least three of them are in the habitable zone. If the James Webb telescope can notice something in the atmosphere of a planet, it may open a new milestone in the exploration of extraterrestrial civilizations.

Interestingly, if a civilization on one of the TRAPPIST-1 planets really launched a space telescope of the James Webb Telescope class, they probably wouldn't be able to see CFCs in the Earth's atmosphere because the sun is too bright. But if they could launch an observation device with higher detection accuracy, perhaps the story would be very different.

The possibility of using pollutants to detect the existence of extraterrestrial civilizations is not a new topic. In addition to chlorofluorocarbon derivatives, nitrogen dioxide (NO2) is also a pollutant with a technological imprint. This by-product from fossil fuel consumption will only be enriched in the atmosphere of civilizations with highly developed industrial civilizations.

When industrial civilization reached its later stages, some civilized individuals finally realized the seriousness of atmospheric pollution and turned to developing sustainable energy sources, such as solar energy. The solar panels they laid on the ground on a large scale may be rich in a series of minerals, and the light reflected by these substances will also be visible in the planetary spectrum.

In 2020, NASA awarded a grant to the University of Rochester in New York to build a database to monitor possible biosignatures in planetary atmospheric spectra - carbon dioxide, chlorofluorocarbons, or light reflected from solar panels.

Although we have tried our best to proactively search for the existence of aliens, perhaps our Earth, which has been polluted by industrial civilization, has already appeared clearly in the telescope of some advanced civilization. Perhaps they are also trying to get in touch with humans, or perhaps they have made other decisions...

Source: China National Astronomical Service

Author: Mao You, a doctoral student at the National Astronomical Observatory, is currently using the Sky Eye to observe the distribution of neutral hydrogen in the universe. He often gets frustrated because his cat at home doesn't understand his science popularization.

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