In the depths of Venus' hellish atmosphere, a layer of mysterious haze could be home to life, a new study finds.
Venus is the hottest planet in the solar system, a broiling world where temperatures can reach 870 degrees Fahrenheit (465 degrees Celsius), more than hot enough to melt lead. And this scorching heat causes any water on the surface of Venus to boil away, leaving the rock extraordinarily dry.
Previous research has suggested that Venus might once have been a habitable desert world as recently as 1 billion years ago. But over time, escalating temperatures caused steam to build up in the atmosphere of Venus, which trapped heat that vaporized still more water, leading to a runaway greenhouse effect that boiled all the oceans off the planet. Nowadays the Venusian atmosphere is about 50 times as arid as the driest place on Earth, the Atacama Desert, at midday in summer, researchers noted.
If any life on Venus survived this runaway greenhouse effect, a possible haven for it may be the planet's clouds, a region spanning from about 30 to 37 miles (48 to 60 kilometers) in altitude, according to the new research.
Although such a possibility might seem far-fetched, "Earth itself has an aerial biosphere," study lead author Sara Seager, a planetary scientist at the Massachusetts Institute of Technology, told Space.com. "Microbial life does not stay up there permanently, but it can get transported up and stay a week or two before coming back down again."
In the skies of Venus, the researchers determined, life would likely need to shelter within cloud droplets. But such cloud droplets repeatedly merge and grow, eventually getting large enough they fall out of the atmosphere. And although such a tumble is rarely a problem for airborne microbes on Earth, it would likely prove deadly to Venusian life.
Now Seager and her colleagues have discovered a scenario in which hypothetical microbes could survive in the skies of Venus, escaping a fate where they rain down onto the planet's hellish surface to die.
Prior work found that below the lowest clouds of Venus, at altitudes of 29.5 to 20.5 miles (47.5 to 33 kilometers), is a layer of mysterious haze. Scientists don't know what particles make up the haze, just that they measure between 0.4 to 4 microns wide. (In comparison, the average human hair is about 100 microns wide.)
"People don't really know why this haze layer is there," Seager said. "There's a lot we don't know about Venus."
The researchers suggested that Venusian microbes could fall into this haze layer and stay there, suspended in the air, just as similarly-sized haze particles have. In order to survive the heat of the lower atmosphere, the microbes would have to go dormant, becoming inactive, dessicated spores that could better resist their hazardous surroundings.
Updrafts could pull spores up from this haze depot to more temperate layers of the atmosphere. The spores could then attract droplets of fluid around themselves, much as dust and potentially microbes do in Earth's atmosphere, and return to active life. During their time aloft, the microbes could grow and reproduce, potentially living in these droplets for hours to months to even years. Eventually, the droplets would grow too large and fall into the lower atmosphere, beginning the cycle anew.
"People talked before about life in Venus' atmosphere, but they never thought through what that would mean," Seager said.
If it were permanently confined to an aerial biosphere, life on Venus would have to get all its nutrients from the atmosphere and would have to be photosynthetic, like plants on Earth.
Although the scientists assumed the spores would passively stay in temperate layers until their droplets grew enough to fall downward, microbes could also potentially employ active means of remaining aloft. Seager noted that renowned astronomer Carl Sagan proposed a number of ways life could potentially stay aloft in Jupiter's atmosphere, such as sucking in gases that could help them stay buoyant.
If just 1% of one-thousandth of the mass of Venus' lower haze layer is dried spores, there could be 5,500 tons (5,000 metric tons) of microbes there, the researchers calculated. Assuming these desiccated microbes weigh about 0.4 trillionths of a gram each, as they might on Earth, there could be about 10^22 spores (a 1 with 22 zeroes behind it) within the haze of Venus. In comparison, scientists estimate that Earth has about 10^24 cells in its aerial biosphere, and more than 10^30 free-living single-celled microbes altogether.
Life in the Venusian atmosphere would still prove extremely harsh compared to its Earthly counterpart. In addition to the extraordinarily dry air constantly threatening to dehydrate any microbes, Venusian life would also have to contend with the local clouds, which are rich in sulfuric acid.
"There are a lot of challenges to life on Venus," Seager said.
Although microbes can survive in hyper-acidic environments on Earth, such as the Dallol Geothermal Area in Ethiopia, Venusian clouds are more than 100 billion times more acidic than that environment, the researchers said. Still, any Venusian microbes might potentially be able to survive these deadly conditions by forming sulfur shells around themselves, the scientists suggested.
And thinking about how life might survive on Venus could shed light on how it might exist elsewhere. "Could some exoplanets have life in their clouds? Could planets like Jupiter or Neptune have life in their clouds?" Seager said. Future missions to Venus could help shed light on just how extreme life could get across the universe.
The scientists detailed their findings online Aug. 13 in the journal Astrobiology.
Follow Charles Q. Choi on Twitter @cqchoi. Follow us on Twitter @Spacedotcom and on Facebook.