These clues might point to life on other worlds

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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.

an image of the planet Venus against a black background, showing swirling light-colored clouds — NASA’s first close-up of the swirling clouds that obscure the surface of Venus. It was taken by the Mariner 10 on Feb. 5, 1974. Scientists have recently found that those clouds contain the chemical signatures of gas that might have been made by microbial life. NASA

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.

Lindsay Hays, a NASA program scientist, describes what researchers mean when they talk about looking for a “biosignature” of life, especially extraterrestrial life.

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.

NASA rovers are scouting rocks on the Martian surface. They hope to confirm potential signs that this planet once hosted life — even if that life was only one-celled microbes.

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.