A team led by the University of Wisconsin-Madison has revisited an old theory that the atmosphere of Venus may contain extraterrestrial microbial life. 

According to planetary scientist Sanjay Limaye of the University of Wisconsin-Madison’s Space Science and Engineering Center, some models suggest that Venus had a habitable climate with liquid water on its surface for as long as two billion years.  

“That’s much longer than is believed to have occurred on Mars,” Limaye said in a statement. “Venus has had plenty of time to evolve life on its own.” 

Terrestrial microorganisms—mostly bacteria—are capable of being swept into the atmosphere on Earth  where they have been found alive at altitudes as high as 25 miles by scientists using specially equipped balloons. 

There is also a growing catalog of microbes that inhabit extremely harsh environments on Earth, including in the hot springs of Yellowstone, deep ocean hydrothermal vents, the toxic sludge of polluted areas and in acidic lakes around the globe. 

According to Rakesh Mogul, a professor of biological chemistry at California State Polytechnic University, Pomona, the cloudy, highly reflective and acidic atmosphere of Venus is mostly made of carbon dioxide and water droplets that contain sulfuric acid. 

“On Earth, we know that life can thrive in very acidic conditions, can feed on carbon dioxide, and produce sulfuric acid,” Mogul, who is also a co-author on the new paper, said in a statement. 

The habitability of Venus’ clouds was first raised in 1967 by biophysicist Harold Morowitz and astronomer Carl Sagan. Decades later, the planetary scientists David Grinspoon, Mark Bullock and their colleagues expanded on the idea. 

A series of space probes to Venus between 1962 and 1978 showed that the temperature and pressure conditions in the lower and middle portions of the atmosphere with altitudes between 25 and 27 miles would not preclude microbial life, while the surface conditions of the planet remained inhospitable. 

After hearing that bacteria on Earth with light-absorbing properties were similar to those of unidentified particles that make up the unexplained dark patches observed in the clouds of Venus, Limaye wanted to revisit the decades old theory. The particles that make up the dark patches have nearly the identical dimensions as some bacteria on Earth. 

However, the instruments used to sample Venus’ atmosphere are not able to distinguish between materials of an organic or inorganic nature.  

Spectroscopic observations, particularly in the ultraviolet, show that the dark patches are composed of concentrated sulfuric acid and other unknown light-absorbing particles. 

“Venus shows some episodic dark, sulfuric rich patches, with contrasts up to 30-to-40 percent in the ultraviolet, and muted in longer wavelengths,” Limaye said. “These patches persist for days, changing their shape and contrasts continuously and appear to be scale dependent.” 

According to Limaye, it is not known when Venus’ liquid water evaporated. Extensive lava flows in the last billion years likely either destroyed or covered up the earlier terrestrial history. 

“To really know, we need to go there and sample the clouds,” Mogul said. “Venus could be an exciting new chapter in astrobiology exploration.” 

Limaye said the Venus Atmospheric Maneuverable Platform—a spacecraft that flies like a plane but floats like a blimp—could stay aloft in the Venus’ cloud layer for up to a year to gather data and samples. It could include instruments like Raman LiDAR,a meteorological and chemical sensor, and spectrometers, as well as a microscope capable of identifying living microorganisms. 

The researchers are now having ongoing discussions about possible NASA participation in Russia’s Roscosmos Venera-D mission, now slated for the late 2020s. Plans for Venera-D could include an orbiter, a lander and a NASA-contributed surface station and maneuverable aerial platform.