Microbial Colors in Clouds May Indicate Life on Other Planets

Scientists have for the first time measured the colors of microbes residing in high-altitude clouds, providing insights that could aid the search for extraterrestrial life.

Microorganisms have been found in Earth’s atmosphere at densities reaching up to 100,000 per cubic meter, contributing to cloud formation.

These tiny life forms produce pigments to shield themselves from intense ultraviolet radiation present at high altitudes.

Thus, if similar airborne organisms are present in the atmospheres of other planets, they might be detectable from afar by studying the light wavelengths, or spectra, reflected by those planets. Ligia Coelho from Cornell University in New York notes.

“Essential pigments are robust and surprisingly universal biosignatures,” Coelho explains. “Ultraviolet light is a common stressor for life on any planet with a star, suggesting that reflective pigments serving similar roles could evolve elsewhere.”

To investigate the colors of airborne microorganisms on Earth, Coelho’s team cultured microbes collected by Brent Kritner from the University of Florida and colleagues. Kritner’s team employed helium balloons to collect microorganisms attached to sticky rods at altitudes between 3 and 38 kilometers above the Earth.

Subsequently, Coelho’s team analyzed the reflectance spectra of the colored compounds produced by these microbes, observing a spectrum of colors from yellow to orange to pink, manifested by carotenoid pigments like beta-carotene, commonly found in carrots.

Finally, the team simulated how these spectra might alter across various planetary conditions, including wetter and drier environments.

“For the first time, we possess actual reflectance spectra of pigmented microorganisms in the atmosphere, which can serve as reference points for modeling and detecting life forms within clouds,” stated Coelho.

Astronomers are actively searching for signs of life beyond our solar system by analyzing light reflected from planets, which reveals the chemical footprints of gases—like oxygen and methane—that may be produced by biological activities, as well as indicators of surface life such as green chlorophyll generated by vegetation and microorganisms.

Up until recent findings, clouds surrounding exoplanets were perceived as obstructions, hindering the identification of atmospheric and surface-level biosignatures.

“Our planetary simulations indicate that when exoplanetary clouds are rich in these microorganisms, their spectra can change in identifiable ways,” Coelho elaborates.

Forthcoming space telescopes, such as NASA’s proposed Habitable World Observatory, could bolster efforts to search for life in other star systems.

Nevertheless, even with advancements in technology, the concentrations of airborne microorganisms need to be significantly high to be detected from extensive distances. “The concentrations of these organisms present in Earth’s atmosphere are currently below our detection limits,” Coelho remarked.

“According to the expected resolution of NASA’s Habitable World Observatory (which we modeled in this study), we would require microbial cell densities akin to those found in oceanic algal blooms, which are typically detectable from space.”

Claire Fletcher, a researcher from the University of New South Wales, suggests that it may be advantageous to search for carotenoids produced by microbes in the stratosphere alongside chlorophyll from plant life. “However, while we assume that life on these exoplanets will mirror that of Earth, this assumption may not hold true,” she cautions.

Peter Tuthill, a professor at the University of Sydney, expresses skepticism regarding the utility of the stratospheric biosignatures identified in the study for extraterrestrial life detection. “I appreciate the fact that we don’t need to engineer devices to detect biosignatures amidst noise from distances of 20 parsecs,” he remarks.