What our clouds might tell us about Venus

Researchers in the emerging field of aerobiology are using microorganisms swept up in the Earth’s atmosphere to probe the prospects for life in the clouds of Venus and on planets circling other stars.

These organisms aren’t just in breezes blowing dust across the ground. Viable organisms have been found all the way into the stratosphere and may extend to the edge of space.

“We don’t know where Earth’s biosphere stops above our heads,” says David Smith, an aerobiologist at NASA’s Ames Research Center, who was one of several scientists to address the topic yesterday at the virtual meeting of the American Geophysical Union. “It looks like everywhere we sample in the atmosphere we find biomarkers of life.”

Not that it’s always easy to obtain these samples.

At low elevations, they can be acquired by putting fog collectors on mountaintops, using them to sift water droplets out of scudding clouds.

Microbes in these droplets initially swept off the Earth’s surface by wind, says Kevin Dillon, a microbiologist at Rutgers University. Once aloft, they use the clouds like a highway to hitch a ride from one part of the globe to another.

Some of these airborne microbes aren’t even dormant, says Diana Gentry an astrobiologist at NASA Ames. Instead, they are metabolically active – though, she notes, researchers haven’t yet observed them being so active that they reproduce when aloft.

Higher up, Smith says, researchers use a plane capable of soaring at an altitude of 12,000 metres or employ balloons capable of going higher yet. These collect biological “debris” and in some instances even complete microorganisms that can be nursed back to life.

But so far, Gentry says, all of the viable organisms found that far up are dormant: not surprising, because the stratosphere is cold, dry, and exposed to harsh solar radiation. “We’ve only seen inactive life,” she says.

On Earth, there is as yet no evidence that life can linger in the atmosphere indefinitely. “Any microbes that get lifted up come back down,” Smith says. But that doesn’t mean it has to be the same on Venus, where the atmosphere is different and much thicker than ours.

Furthermore, says Noam Izenberg of Johns Hopkins University, planetary scientists are starting to realise that Venus’s surface may not always have been the inferno it presently is.

Rather, it may have spent billions of years at the right temperature for surface water. This means that before the oceans vanished and life was snuffed out lower down, it might have found its way into the clouds that now shroud the planet’s surface.

“One thing that makes me excited is the possibility of life today in these clouds,” Izenberg says.

Planetary scientists hope to see a mission head to Venus to sample these clouds in the not-too-distant future. That wouldn’t just be something to please Venus enthusiasts; it would also be useful to scientists studying exoplanets, Gentry says, because Venus-like worlds may prove more common than Earthlike ones.

Meanwhile, the more scientists learn about life in the Earth’s atmosphere, the more they hope to understand the range of what might be possible elsewhere.

“We are looking for the limits of life on Earth,” Dillon says.