A number of bacteria, fungi and viruses live in the clouds above the Earth’s surface
George Pachantouris/Getty Images
The colors of microbes that live in clouds high in the sky have been measured for the first time, giving scientists clues that could help us find life on other planets.
A wide range of microorganisms have been found that live high in the earth’s atmosphere in concentrations of up to 100,000 microbes per cubic meters and they are known to play a role in cloud formation.
These organisms produce pigments to protect themselves from the strong ultraviolet light at extreme altitudes.
If similar airborne life forms exist in the atmospheres of other planets, we could therefore detect them from a distance by analyzing the wavelengths or spectra of light that these planets reflect, says Ligia Coelho at Cornell University in the state of New York.
“Biopigments are a powerful and surprisingly universal biosignature,” says Coelho. “Since UV is a universal stressor for life on any planet with a star, it is likely that reflective pigments that serve the same function could also evolve elsewhere.”
To learn more about the colors of airborne microbes on Earth, Coelho cultivated microbes collected by Brent Christner at the University of Florida and his colleagues. Christner’s team used a helium balloon to trap the microbes on sticky rods between 3 and 38 kilometers above the Earth’s surface.
Coelho’s team then measured the reflectance spectra of the colored compounds produced by the microbes. They generated a range of yellow, orange and pink colors, created by carotenoid pigments such as beta-carotene, which is also found in carrots.
Finally, the team modeled how these spectra would vary on worlds with different environmental conditions — such as wetter or drier planets.
“For the first time, we now have real reflectance spectra of pigmented microorganisms from the atmosphere that can be used as reference data to model and detect life in clouds,” says Coelho.
Astronomers are already looking for evidence of life outside our solar system by analyzing the light reflected by planets. This can reveal chemical signatures of gases in the atmosphere, such as oxygen and methane, which could be produced by living organisms, or signs of surface life, such as green chlorophyll produced by vegetation or microbes.
Until now, clouds enveloping exoplanets have been considered an obstacle because they hide both atmospheric and surface-level biosignatures.
“Our planetary simulations show that if a planet’s clouds had high concentrations of these microorganisms, their spectra would potentially change in a detectable way,” says Coelho.
Future space telescopes such as NASA’s proposed Habitable Worlds Observatory could improve our ability to look for life in other star systems.
But even with more advanced instruments, concentrations of airborne microbes must be very high for us to detect them from such great distances. “The concentrations of these microorganisms in the Earth’s atmosphere are far below current detection thresholds,” says Coelho.
“Based on the resolution expected for NASA’s Habitable Worlds Observatory, the one we modeled in this study, we would need microbial cell densities comparable to those found in a marine algal bloom that can actually be tracked from space.”
Clare Fletcher of the University of New South Wales, Australia, says it may be useful to look for carotenoids produced by stratospheric microbes as well as chlorophyll produced by plants. “But that assumes that life on these exoplanets would be similar to life on Earth, which may not be the case,” she says.
Peter Tuthill of the University of Sydney, Australia, says he is skeptical that the stratospheric biosignatures identified by the study will be useful in the search for life on other planets. “I’m happy that I don’t have to design the instrument to pick that biosignature out of the noise at a distance of 20 parsecs,” he says.
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