Lead-melting temperatures on the surface of Venus should make life as we know it impossible there. But scientists have detected some curious chemicals in clouds above the planet’s surface. At those heights, temps and pressures would be similar to those on Earth.

These clouds sit 48 to 60 kilometers (30 to 37 miles) above the surface. Potential traces of oxygen, methane and phosphine gas have shown up in them. This has led some scientists to suspect the clouds might host microbes. But those single-celled life forms would have to survive harsh conditions when they occasionally sank into lower, hotter layers of the atmosphere.

In 2021, researchers at the Massachusetts Institute of Technology, in Cambridge, suggested how microbes might survive in clouds high above Venus. They would just need to tolerate briefly drying out whenever they sank into hotter cloud layers. Later, when swept back into cooler, wetter layers, the cells could revive. Noelle Bryan, Sara Seager and their colleagues proposed this idea.

Bryan had reasons for thinking this might be possible. Years before, she had collected microbes from Earth’s stratosphere. This hostile zone sits dozens of kilometers (miles) above our planet’s surface. The microbes she found seemed to survive extreme drying while up there — much as the Venus microbes would have to survive drying out.

What Bryan learned about those high-flying cells might now guide the search for alien life in solar systems light-years away.

For instance, some bacteria on Earth produce colorful pigments. These molecules can protect the cells from harmful ultraviolet (UV) radiation. Finding them on other worlds might point to life there. Extraterrestrial cells might use similar pigments to protect themselves from UV too.

Future telescopes looking at planets orbiting distant stars might be able to detect the colors of those pigments. This could reveal microbial life living in the clouds of faraway worlds, says Lígia Coelho. An astrobiologist, she works at Cornell University in Ithaca, N.Y.

Coelho recently analyzed pigments in Bryan’s stratospheric bacteria. The pigments absorbed or reflected specific wavelengths of light, she found. She hopes to look for similar light and color patterns in distant planets.

Scouting for tiny Martians

The recent discovery of microbes in Earth’s stratosphere is probably most relevant to imagining life on Mars. After all, cells high above Earth endure conditions similar to those on the Red Planet’s surface.

Andrew Schuerger has spent years hunting Earthly microbes that might be able to survive on Mars. He works at the University of Florida in Gainesville. He recently teamed up with Rachel Harris. She’s an astrobiologist at NASA and Harvard University in Cambridge, Mass. Last year, the duo found a microbe that seems to check many of the boxes needed to make it on Mars.

Its name is a mouthful: Methanosarcina barkeri (Meh-THAAN-oh-sar-SEE-nuh BARK-er-eye). It gets its energy by turning carbon dioxide and hydrogen into methane.

M. barkeri normally lives in sewage, landfills, rice paddies and lake bottoms. Those buried habitats would hardly mimic Mars’ thin, dry air. But unlike microbes in Earth’s stratosphere, M. barkeri can live without oxygen, which is absent on Mars. And even at low, Mars-like pressures, M. barkeri can still make methane, Schuerger and Harris have shown.

That’s big news, because Mars orbiters and rovers have detected methane gas seeping into Mars’ atmosphere.

Unfortunately, methane-making critters on Mars would likely be hard to spot. They might need to hide deep underground to survive the planet’s low atmospheric pressure.

But finding that M. barkeri is active at low pressures means life might be able to survive quite close to the Red Planet’s surface, Schuerger says. A rover might only need to drill 20 centimeters (8 inches) down to reach single-celled Martians — if they exist.

Any pockets of microbes near Mars’ surface might be quite small and short-lived, says Lynn Rothschild. She’s an astrobiologist at NASA Ames Research Center. It’s in Mountain View, Calif. She imagines microbes using Mars’ surface the way high-flying species seem to use Earth’s stratosphere. It would be a hostile place they briefly tolerate while moving from one home to another.

If she’s right, then windstorms on Mars might sweep up not only dust, but also Martian life. Microbes could be “floating around, just looking for a place to land [where] they can survive” and grow, she says. If so, then a rover might catch life the way a fence catches blowing leaves.

The stratosphere is both beautiful and deadly — a boundary between Earth and space, between life and death.

Hover within it, some 30 kilometers (almost 20 miles) above the ground, and you’d be nearly twice as high as any rain cloud. You would actually see the edges of Earth curve beneath you. And the sky above? It would look black — like outer space.

The air pressure here is just one percent of that at sea level. You’d never survive in such thin air. As you gasped for oxygen, this super-low pressure would cause your blood to boil inside you.

With temps as low as −60° Celsius (−76° Fahrenheit) and intense, DNA-damaging ultraviolet (UV) radiation, the conditions here would kill most life as we know it. Yet some living things persist in the stratosphere. A surprising number of one-celled microbes somehow survive its extremes.

“If you took a microbe from [Earth’s stratosphere] and you put them on the surface of Mars, they wouldn’t even know the difference.”

Brent Christner

“If you took a microbe from those altitudes and you put them on the surface of Mars, they wouldn’t even know the difference,” says Brent Christner. A microbiologist, he works at the University of Florida in Gainesville.

His team began looking for life in the atmosphere back in 2008. They were hunting for the upper limits of Earth’s habitable zone. This, they hoped, might reveal whether life could persist on the cold, near-airless, radiation-pummeled surface of Mars.

Noelle Bryan, Christner’s graduate student at the time, sent sampling balloons as high as 38 kilometers (24 miles) into the air. What she found utterly shocked her: “We did not hit an altitude where we couldn’t find something [alive].”

In 2025, Christner’s team reported something that proved even more surprising. These microbes weren’t ones we usually imagine living in extreme conditions, such as volcanic hot springs. In fact, they were some of the same microbes found living on our crops, in our gardens and even on our skin.

Normally Earth-dwellers, these microbes seem to live a secret double life — sailing around the world at up to two or three times the height of a passenger jet.

For them, Christner says, the atmosphere “is like a highway.” It allows them “to move globally in periods of weeks.” From there, they can cross oceans to find new homes.

These discoveries could reshape what we know about how pathogens spread across our planet. They also could transform where we look for life on other worlds — from Mars to exoplanets light-years away.

Hunting for microbes in the sky

There’s a long history of looking for high-flying life.

In 1935, the U.S. Army Air Corps launched a helium balloon over the Black Hills in South Dakota. It reached 22 kilometers (14 miles) high, a world record. Two pilots went along in a pressurized gondola. They recorded air temperatures, magnetic fields, radiation levels and more. They also dropped a sterile sampling device designed to collect airborne microbes on its way down. After falling halfway, it closed and parachuted the rest of the way to Earth.

Scientists later grew 10 types of bacteria and fungi from that sample. But this was before scientists discovered DNA. So they never learned much more than the shapes of these mystery cells, the conditions they had survived and what nutrients they needed to grow.

Nearly four decades later, Soviet scientists launched rockets with sticky microbe collectors 48 to 77 kilometers (30 to 48 miles) into the mesosphere. It’s the layer just above the stratosphere. Those collectors parachuted back with a few living cells stuck on them. These, too, grew in the lab.

“But there’s just no way to know what [the Soviets had] sampled,” Christner says. The microbes could have been contamination from the ground. He doesn’t believe that the scientists actually found something alive at that altitude.

More recent U.S. studies started in the early 2000s.

NASA was flying a plane 20 kilometers (12 miles) up to collect cosmic dust. Its goal was to study the history of our solar system. Microbiologist Dale Griffin then worked for the U.S. Geological Survey in St. Petersburg, Fla. He arranged for sticky microbe-catchers to fly on several missions between 2003 and 2008.

Griffin grew several types of bacteria from those samples. He also performed a simple test on them, called genetic barcoding. It looks at a short part of a single gene. From this, biologists can get a general idea of a life form’s family or genus. And these microbes, he realized, were related to several known from remote islands and volcanic soils.

Perhaps, Griffin thought, eruptions lofted them high into the air.

The Russian and NASA findings were based on what they learned from growing sampled microbes in the lab. Yet scientists can’t grow 99 percent of the microbes that exist most places. They just can’t recreate the environments these cells need to grow.

As such, those early tests couldn’t measure how many living cells were in the stratosphere. They usually couldn’t identify the species. The scientists didn’t even test whether these microbes could survive stratosphere-like temps, dryness or UV radiation.

And they never learned the lifestyles of these microbes on Earth’s surface — whether they lived on plants, slurped raw sewage or grew in the armpits of NFL linebackers.

At last: Tallying living space microbes

Christner was at Louisiana State University in Baton Rouge (LSU) when he and Bryan began their search for life in the stratosphere. They planned to start low and sample higher and higher. They would try to grow the cells they collected, as others had. But they would also do something new: count the total number of living cells at these altitudes.

They expected that at some height, the number of living cells would drop to zero. This would mark the outer limit of life on Earth.

Bryan built her first collectors from Styrofoam and balsa wood, purchased at hobby and hardware stores. She dangled these homemade gizmos from helium balloons launched in Texas and Louisiana. Each collector had a control chamber that didn’t open during the flight. This helped her estimate how much her samples were contaminated with ground-level germs. Eventually, she got that contamination down to almost zero.

In 2013, Bryan started using a more advanced system. It was developed by LSU engineer T. Gregory Guzik. Its sturdy circuits allowed it to reach even greater heights. (In the mid-stratosphere, electricity can arc from one wire to another, damaging circuits.)

Bryan and Guzik used this system in a key series of balloon launches over Fort Sumner. This little town crouches amid a dry plain of grass and yucca in eastern New Mexico. The 19th-century gunslinger Billy the Kid is buried in a cemetery at one end of town.

The samples that Bryan collected here would be the final ones for her PhD research. They would also reveal some major surprises.

On the morning of August 21, 2013, Bryan, Guzik and several others wore hard hats as they emerged from a hangar at the local airport. They grasped the tether of a round, white helium balloon the size of a small elephant. It bobbed and tugged impatiently as they walked into an open field.

Then, they let it go.

A downward-pointing camera captured video as it surged upward.

At one point, a large jet streaked by far below, tiny and silent.

Later, the balloon was tossed by violent winds as it entered the stratosphere. At 18 kilometers (11 miles), an electric motor opened one of two collectors. It exposed a sticky surface to the air.

Minutes later, as the balloon ascended past 23 kilometers (14 miles), the collector closed. An electric heating coil severed the tether. This sent the balloon’s payload parachuting back to Earth. Using a GPS tracker, Bryan drove to where the collector had landed between a yucca and a prickly pear cactus.

In all, the team launched six balloons over three weeks. Some reached as high as 29 kilometers (18 miles). A seventh launch, on a larger NASA balloon, reached 38 kilometers (24 miles).

Microbes in living color

Bryan spent months growing her samples. The one from August 21 spawned a glistening orange colony of bacteria. She named it L6-1.

Samples from other launches grew into yellow, pink and black colonies. The colors likely came from protective pigments that shield the microbes against harmful UV light.

Bryan assumed that these still-unidentified cells would turn out to be tough cookies. For example, some bacteria can form dry spores that survive boiling. But as she isolated and barcoded the cells’ DNA, a very different picture emerged.

These cells “should have been nothing but spore-formers,” Bryan says. They “should have been nothing but extremophiles.” But only one out of a dozen or so bacteria were.

L6-1 and several others belonged to a genus often found on plants: Curtobacterium. Others belonged to groups that live in soil. The fact that these critters came from such everyday places posed a mystery that Bryan desperately wanted to solve.

Radiation? No biggie       

Bryan spent most of 2014 and 2015 running lab experiments. From these, she estimated that at 24 kilometers (15 miles) up, each cubic meter (m³) of air held about 100,000 living cells. At 36 kilometers (22 miles), the air still held nearly 8,000 cells/m³.

In 2016, Bryan exposed her stratospheric cells to UV-C radiation to see how tough they were. These wavelengths of ultraviolet light kill cells better than the UV-A and UV-B types that cause sunburns. UV-C is absent at Earth’s surface, because the ozone layer blocks it. But this radiation is plentiful in the mid-stratosphere.

In these experiments, Bryan compared her stratospheric germs to a bacterium called Deinococcus radiodurans (Dye-noh-KOK-us Ray-dee-oh-DUR-uns). It is said to be the toughest organism on Earth. It can survive roughly 1,000 times the gamma radiation needed to kill a human. It’s also uniquely resistant to UV-C radiation.

Surprisingly, her L6-1 cells tolerated UV-C just as well as D. radiodurans did. Some of her other plant and soil bacteria from the stratosphere weren’t far behind. Bryan was shocked to see this. “I [fudged] up,” she remembers thinking.

Bryan repeated the experiments over many months, looking for mistakes she might have made. But her results never changed.

As the team realized the results were actually correct, it “all sort of came together,” Christner recalls. These plant-dwelling bacteria had a secret talent: They could survive, at least briefly, on the edge of space.

Were these cells infectious?

Bryan got her PhD in 2017. (She now works at Mass General Brigham in Boston, Mass.) Around that same time, Christner moved to the University of Florida. There, he began looking into a strange question.

Bryan’s DNA barcoding had showed that L6-1 and several of the other stratospheric bacteria were closely related to a germ that sickens crops. It’s called Curtobacterium flaccumfaciens. From the 1920s to the 1970s, it had browned and wilted Midwestern bean fields. After disappearing for a while, C. flaccumfaciens reemerged around 2003.

Christner and Bryan wondered: Did L6-1 also cause disease?

“Plant pathogens in the stratosphere — that was the furthest thing that
we expected to run into.”

Brent Christner

To find out, Christner teamed up with two plant pathologists. Brian Kvitko works at the University of Georgia in Athens. Robert Harveson is at the University of Nebraska in Scottsbluff. Last year, they reported that L6-1 really does cause bean plants to yellow and wilt. So do two other Curtobacterium microbes that Bryan collected from as high as 29 kilometers (18 miles).

Adam Ellington was a graduate student in Christner’s lab in Florida. In 2020, he sequenced the full genome of L6-1. This showed it to be a new species. It also was a genetic match for an unidentified germ that Harveson had isolated from diseased millet years earlier.

Christner’s group described it in Microbiology Spectrum in 2025. They named it C. aetherium.

“Plant pathogens in the stratosphere — that was the furthest thing that we expected to run into,” Christner says.

Harveson, a Curtobacterium expert, had long thought this germ spread through infected seeds. Learning that at least some members of its genus could survive high in the atmosphere stunned him. This discovery, he says, could “change our thought processes about movement of diseases.”

Life finds a way

By flying through the atmosphere, a microbe could cross oceans and drop into new ecosystems. A pathogen like C. aetherium could escape regions where plants had become resistant to it and land on new hosts.

Leaf-dwelling bacteria are known to experience frequent drying. This can damage their DNA in a manner similar to radiation. So some of these microbes have evolved ways to repair their DNA fairly well. Such tricks might help them survive other harsh conditions, too.

Leaf bacteria can also enter the air with surprising ease.

One 1982 study showed that during each hour on a warm day, 20 billion or so cells per hectare (50 billion per acre) enter the air from an alfalfa field. Breezes and rising warm air may loft most of these. Bacteria may even encourage this by clumping onto tiny leaf hairs where air currents can more easily lift them.

Once microbes are aloft, big thunderstorms, dust storms, hurricanes and volcanic eruptions can lift them into the upper atmosphere. Smoke plumes from large wildfires can also ferry billions of live cells into the air.

Ellington’s work suggested that C. aetherium could have started as one of these leaf-squatters. Over time, as it got lofted higher, it could have evolved new traits to survive extreme altitudes.

He learned this by studying a Curtobacterium species that has never been found in the atmosphere. When exposed to UV-C, it quickly adapted. This indicates that some plant-dwelling bacteria already have a flair for evolving strong radiation resistance.

Ellington also found that UV-C radiation turns on several DNA-repair enzymes in C. aetherium. One of them repaired an unusual type of UV damage. This type occurs only when a cell is so dehydrated that its DNA folds into a somewhat different structure. Cells drifting in the stratosphere are very dried out. So their DNA might actually fold this way, Ellington notes. Having this enzyme might allow them to repair a type of DNA damage that often happens up there.

Still, many mysteries remain.

Bryan was able to grow only 12 or so species of bacteria from the stratosphere. For every type that did grow in the lab, 100 to 1,000 others might have also been present and alive. They might not have grown for Bryan simply because she didn’t provide them with the right conditions.

Burak Erkorkmaz wonders about these other microbes.

“The next question would be to understand the whole community” up there, he says. He’s a microbiologist at the University of Gothenburg in Sweden. He’d like to take those thousands of live cells per cubic meter of stratospheric air and sequence all of their genomes. It would provide a fuller picture of the diversity of species there. It also could hint at whether they, too, carry genes for surviving high altitudes.

Erkorkmaz and other scientists have already started doing this with airborne cells caught a few meters above the ground.

Most aerial microbes likely never reach the stratosphere. They probably travel at lower altitudes, where conditions are milder. But finding even a few microbes alive in the stratosphere has huge implications. It might hint at whether microbes can survive in other worlds (see sidebar).

Many years may pass before these lessons from Earth’s stratosphere are applied to the search for extraterrestrial life. But for Bryan, the excitement of it began years ago when she made her first shopping trips to buy materials for her germ catchers.

“It was space exploration,” she says. “It was uncharted territory.”