A variety of bacteria, fungi and viruses live in the clouds above the Earth's surface.
George Pachanturis/Getty Images
For the first time, the colors of microbes living in clouds high in the sky have been measured, giving scientists a clue that could help us find life on other planets.
A wide range of microorganisms have been found living high in the Earth's atmosphere, with concentrations of up to 100,000 microbes per cubic meter. they are known to play a role in cloud formation.
These organisms produce pigments to protect themselves from strong ultraviolet radiation at extreme altitudes.
If similar aerial life forms exist in the atmospheres of other planets, we could detect them from afar by analyzing the wavelengths or spectra of the light those planets reflect, he says. Ligia Coelho at Cornell University in New York State.
“Biopigments are a powerful and surprisingly versatile biosignature,” says Coelho. “Since UV is a universal stressor for life on any planet with a star, it is likely that light-reflecting pigments serving the same function could emerge in other places.”
To learn more about the colors of airborne microbes on Earth, Coelho cultured microbes collected Brent Christner at the University of Florida and his colleagues. Christner's team used a helium balloon to trap 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 created 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 change 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 for modeling and detecting life in clouds,” says Coelho.
Astronomers have already searching for evidence of life beyond our solar system by analyzing the light reflected by the planets. This can reveal chemical signatures of gases in the atmosphere, such as oxygen and methane, which can be produced by living organisms, or signs of surface life, such as green chlorophyll produced by vegetation or microbes.
Until now, clouds enveloping exoplanets were considered a nuisance because they obscure the biosignatures of both the atmosphere and surface.
“Our planetary modeling shows that if there were a high concentration of these microorganisms in the clouds of a planet, their spectra could potentially change in noticeable ways,” says Coelho.
Future space telescopes such as NASA's Proposed Habitable Worlds Observatory could expand our ability to search for life in other star systems.
However, even with better tools, concentrations of airborne microbes would have to be very high for us to detect them over such long distances. “The concentration of these microorganisms in the Earth's atmosphere is well below current detection thresholds,” says Coelho.
“Based on the resolution expected for NASA's Habitable Worlds Observatory, which we simulated in this study, we would need microbial cell densities comparable to those found in ocean algae blooms that can actually be detected from space.”
Claire Fletcher from the University of New South Wales in Australia say that searching for carotenoids produced by stratospheric microbes, as well as chlorophyll produced by plants, could be useful. “However, life on these exoplanets is expected to be similar to life on Earth, which may not be the case,” she says.
Peter Tuthill from the University of Sydney, Australia, says he is skeptical that the stratospheric biosignatures identified in the study will be useful in the search for life on other planets. “I'm glad I won't have to design a device that can pick out this biosignature from noise at 20 parsecs,” he says.
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