Can we acknowledge superior civilizations by their industrial air pollution? In all probability not.

The hunt for aliens goes hand in hand with the hunt for habitable planets. Astronomers are looking for exoplanets with atmospheric chemicals that could be a sign of an advanced civilization. These chemicals, known as technosignatures, are found on Earth and are the result of burning fossil fuels. A team of researchers has been studying polycyclic aromatic hydrocarbons and seeing if they can detect them.

Over the decades, researchers have developed various methods for hunting advanced civilizations. From scanning stars for abnormal radio signals or laser pulses to searching for evidence of water, the techniques have so far yielded no positive results. Initiatives like SETI (Search for Extraterrestrial Intelligence) have used some of the world's most powerful radio telescopes to search for signals. At the same time, the habitable zones of exoplanets were examined for signs of water, suggesting the existence of life.

The Allen Telescope Array searches for extraterrestrial techno-signals. Photo credit: Seth Shostak, SETI Institute

A team of researchers led by Dwaipayan Dubey investigated the feasibility of using polycyclic aromatic hydrocarbons (PAHs) as an alternative way to continue the search. PAHs hit the headlines when they were discovered in a Martian meteorite. Their discovery attracted widespread attention because the hydrocarbons are known to be a byproduct of life, and the discovery that they were buried in Martian meteorites suggested a form of life at some point in Mars' history. The debate is still ongoing, but the team believes that searching for hydrocarbons in the planet's atmosphere could lead to advanced civilizations.

In 1996, a team of scientists led by Dr. David McKay of NASA's Johnson Space Center offers possible evidence of life on Mars. The evidence came from their studies of a Martian meteorite found in Antarctica called Alan Hills 84001. The researchers found chemical and physical traces of possible life, including carbonate globules resembling terrestrial nanobacteria (electron micrograph pictured) and polycyclic aromatic hydrocarbons. In the earth's rock, the chemical traces would be considered degradation products of bacterial life. The results became the subject of controversy as non-biological explanations for the results were found. Today they are no longer considered definitive proof of life on Mars. Photo credit: NASA Johnson Space Center

There are PAH sources in space, for example in the interstellar medium, but they are mostly associated with the activities of biological organisms. The team is focusing its attention on hydrocarbons that have available absorption cross sections in the atmosphere of exoplanets like Earth. An absorption cross section is a measure of the probability that an absorption process such as particle scattering will be detected by the 8m Habitable Worlds Observatory. The chemicals selected are naphthalene, anthracene, phenanthrene and pyrene.

A future interstellar probe mission aims to travel beyond the heliosphere into the local interstellar medium to understand where our home came from and where it is going. Photo credit: John Hopkins Applied Physics Laboratory.

Based on the evidence of PAH concentrations on Earth, the team knew that they had declined slightly since the Industrial Revolution. Learning from this, they ran simulations at various concentrations in the hope of demonstrating the detection capabilities of an Earth-like civilization. The work also examined the architecture of telescopes, and although large mirrors help improve resolution and light-gathering ability, the result was less positive.

The analysis relied on a large telescope mirror being able to resolve details in the spectral signature of four molecules. However, they found that telescopes with 6 m, 8 m or 10 m apertures would have an insufficient signal-to-noise ratio to resolve the necessary details. The team's final conclusion was that detecting PAH signatures between 0.2 and 0.515 m is not possible with large ground-based telescopes.

This is a great example of work that does not lead to a positive result, but even a negative result in scientific research is valuable. Now more research and laboratory measurements are needed to improve the molecules' detectability and perhaps help us find our first strange neighbor.

Source: Polycyclic aromatic hydrocarbons as extraterrestrial atmospheric technosignatures

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