When the James Webb Space Telescope takes aim at the atmosphere of exoplanets, it will use spectroscopy to identify chemical elements. In particular, it will look for methane, which planetary scientists have recognized as a compelling biosignature.
Finding large amounts of methane in an exoplanet's atmosphere, they say, could be the most reliable sign of life. There are abiotic sources of methane, but for the most part it is a waste product.
Just the other day, NASA announced that over 30 years of research, astronomers have discovered more than 5,000 exoplanets. This threshold was reached on March 21, when 65 more objects that are outside the solar system were added to the Exoplanet Archive.
Methane is interesting because it does not last long in the atmosphere. Photochemical reactions destroy it, so the discovery of a large amount of gas means that there is a stable source of replenishment.
However, scientists need a way to work with the future detection of methane in exoplanet atmospheres. The researchers decided to create "a special assessment of the planetary conditions required for methane to be considered a good biosignature."
Maggie Thompson, a graduate student in the Department of Astronomy and Astrophysics at the University of California, Santa Cruz, raised the issue in her research letter. His team studied abiotic sources of methane to understand how they might explain the presence of the gas in the exoplanet's atmosphere. These are volcanoes, mid-ocean ridges, hydrothermal vents and tectonic subduction zones. Collisions with comets and asteroids can also produce methane.
According to the scientists, “Methane is only part of the puzzle, but to determine if there is life on a planet, you have to consider its geochemistry, how it interacts with its star, and the many processes that can affect the planet’s atmosphere in geological terms. time scales." Maggie Thompson argues that known abiotic processes, unlike the terrestrial biosphere, cannot easily create atmospheres rich in CH4 and CO2 and limited in CO.
Earth's history gives scientists some clues about the presence of methane in exoplanet atmospheres. It is known that in the Earth had an atmosphere even richer in methane than it is now, and its source was life. "If you find a lot of methane on a rocky planet, you generally need a massive source to explain it," said paper co-author Joshua Crissansen-Totton, a postdoctoral fellow at the University of California. “We know that biological activity creates a lot of methane on Earth, and it probably happened on the early Earth as well, because methane production is a pretty simple metabolic thing.”
Methane-producing microorganisms called methanogens were one of the earliest forms of life on Earth, occurring between 4.11 and 3.78 billion years ago. They were so efficient at producing methane that the early Earth probably had a hazy, methane-filled atmosphere similar to Saturn's moon Titan.
Finding abiotic sources of methane is potentially much easier. Volcanoes emit not only methane, but also carbon monoxide into the atmosphere. On the other hand, biological activities are likely to consume carbon monoxide. Researchers have found that non-biological processes cannot easily create a habitable planetary atmosphere rich in methane and carbon dioxide and virtually free of carbon monoxide.
The authors note that the detection of methane in the atmosphere of an exoplanet is just the beginning. They believe its presence is a strong sign of life on a rocky planet orbiting a sun-like star if the atmosphere also contains carbon dioxide. If there is more methane than carbon dioxide, it is also a more reliable indicator of life if the planet is not too rich in water.
“With upcoming technological advances in exoplanet observations to characterize potentially habitable ones, it is important to consider possible biosignature gases and sources of false positive detections,” the research letter says. Scientists recognize that different abiotic sources can replenish atmospheric methane in different planetary environments. But for a planet to produce a methane flux comparable to Earth's, those same abiotic sources would also generate "observable contextual clues" that would point to false identification of biosignatures.
If the current study is focused on the most obvious false positives of methane as a biosignature, then future work should attempt to anticipate and quantify the most unusual mechanisms of its non-biological production.Since 2012, the instruments of the Curiosity rover, which landed in Gale Crater, have recorded methane bursts six times. In 2021, researchers at the California Institute of Technology were able to determine the approximate location of the source of the gas. The scientists modeled the movement of the methane particles by taking into account the speed and direction of the wind at they were detected. After analyzing each outbreak, they were able to map out the area with the most likely sources of methane.
Water has previously been found on Mars, but it has also been found on the Moon and Mercury, and now water is no longer considered an important biosignature.
bbabo.Net