The five man-made greenhouse gases identified by astrobiologist Edward Schwieterman of the University of California, Riverside, and his colleagues could be detected in relatively low concentrations in exoplanet atmospheres using the NASA/ESA/CSA James Webb Space Telescope and future space telescopes.
Diagram of the technological features of various planets, including artificial atmospheric gases. Image courtesy of Sohail Wasif / University of California, Riverside.
“For us, these gases are bad because we don’t want them to accelerate warming,” Dr Schwietermann said.
“But they could be useful to a civilization wanting to halt an impending ice age, or to terraform uninhabitable planets in its own system, as humanity has proposed for Mars.”
“These gases are not known to occur in large quantities in nature, so they have to be manufactured.”
“Finding them would therefore be evidence of the presence of intelligent, technological life forms. Such evidence is called a technosignature.”
The five gases proposed by the authors are used on Earth for industrial purposes, such as making computer chips.
These include fluorinated versions of methane, ethane and propane, as well as gases made of nitrogen and fluorine, or sulfur and fluorine.
One advantage is that it’s a very effective greenhouse gas — sulfur hexafluoride, for example, has a warming power 23,500 times that of carbon dioxide — and even a relatively small amount could heat a frozen planet to the point where liquid water could remain on the surface.
Another advantage of the proposed gas, at least from an alien perspective, is that it is extremely long-lived, surviving in an Earth-like atmosphere for up to 50,000 years.
“You won’t need to refill it very often to maintain a comfortable climate,” Dr. Schwieterman said.
Others suggest that refrigerant chemicals such as CFCs are technology signature gases because they are almost entirely man-made and visible in Earth’s atmosphere.
But unlike the chemically inert fully fluorinated gases discussed in the new paper, CFCs damage the ozone layer and may not be advantageous.
“If other civilizations had oxygen-rich atmospheres, they would have also had an ozone layer that they wanted to protect,” Dr Schwietermann said.
“CFCs will be broken down in the ozone layer while also catalyzing its destruction.”
“CFCs degrade easily and have a short lifespan, making them difficult to detect.”
Finally, for fluorinated gases to have an effect on climate, they need to absorb infrared radiation.
This absorption creates an infrared signature that can be detected by space telescopes.
Using current and planned technology, scientists may be able to detect these chemicals in nearby exoplanetary systems.
“In an Earth-like atmosphere, only one in a million molecules could be any of these gases and be detectable, and that concentration would be enough to even alter the climate,” Dr Schwietermann said.
To reach this calculation, the astrobiologists simulated a planet in the TRAPPIST-1 system, located about 40 light-years from Earth.
They chose this system because it contains at least seven rocky planets and is one of the best-studied planetary systems other than Earth.
Although it is not possible to quantify the likelihood of discovering man-made greenhouse gases in the near future, we are confident that, if they exist, they could be detected during missions currently planned to characterize the planet’s atmosphere.
“If telescopes are already characterizing planets for other reasons, there would be no need for extra effort to look for these technical features,” Dr Schwietermann said.
“And when you find them, it’s amazing.”
Team work Published in Astrophysical Journal.
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Edward W. Schwietermann others2024. Artificial greenhouse gases as a technological feature of exoplanets. ApJ 969, 20; doi: 10.3847/1538-4357/ad4ce8
Source: www.sci.news
