A recent paper suggests that if astrobiologists want to make an educated guess about what life on Venus might look like, they should look to a weird microbe called A. ferrooxidans, found here onEarth.
Earthly life-forms are the only examples we have of what life looks like, so astrobiologists often study them for clues about how alien ones might evolve. If we want a good example of what life on Venus might look like, however, we need to consider that like on that planet may live in its atmosphere, and not on the ground. But any life in Venus’s clouds would have to find a way to keep its organelles in and deadly venusian acids out, while surviving with almost no water.
Grzegorz Słowik, a planetary scientist at Maria Curie-Skłodowska University in Poland, and his colleagues recently put forward a contender in the search for a terrestrial analog for venusian life: a bacteria called Acidithiobacillus ferrooxidans. They published their work in November 2024 in the International Journal of Astrobiology.
Related: Could the clouds of Venus support life?
An Earth analog for alien microbes
A few studies have offered tantalizing hints that alien life might be drifting among the clouds of Venus. Some hard-to-explain chemical imbalances exist in the planet’s atmosphere, which might suggest that life is affecting the planet’s chemistry (although these imbalances may not be as pronounced as initially reported, or attributable to life at all). And isolated stripes within the planet’s lower cloud layers (between 30 and 32 miles [48–51 kilometers] above the surface) absorb ultraviolet (UV) light in the same wavelengths as certain types of chlorophyll — and the way these stripes change size and shape throughout the venusian year reminds some astrobiologists of marine algal blooms here on Earth.
Airborne life isn’t a totally farfetched idea: Biologists have found bacteria floating in Earth’s atmosphere at an altitude of 25 miles (41 km), more than three times higher than commercial airliners fly. And the venusian skies offer a better chance at survival than the ground. The surface of Venus is a hellscape with temperatures around 900 degrees Fahrenheit (485 degrees Celsius) and pressures 92 times greater than the surface of Earth. That’s hot enough to melt lead or tin, and heavy enough to crush a nuclear submarine. But a few dozen miles above the surface, temperatures are a sweltering but technically livable 140 F (60 C), and reach about the same pressure as sea level on Earth.
No problem — except that Venus’ clouds are made of very concentrated sulfuric acid.
Enter A. ferrooxidans, a deeply weird microbe that has evolved to live in extremely acidic environments. A. ferrooxidans thrives in conditions about as acidic as soda, vinegar, or lemon juice, but it’s been found surviving in environments with a pH comparable to human stomach acid.
“The fact that A. ferrooxidans thrives at pH as low as 1.3 suggests it could potentially survive and even metabolize inorganic sulfur compounds found in Venus’s clouds,” write Słowik and his colleagues in their paper.
Meet the acidophiles
A. ferrooxidans turns up in sulfur-rich rocks and soil and toxic, uranium-laden drainage waters from mines (in fact, A. ferrooxidans seems to help make those drainage waters toxic by dissolving sulfur-rich minerals). It gets energy by munching on iron and sulfur compounds, which makes it a life-form called a chemotroph. More specifically, it is a chemolithoautotroph, an organism that survives by breaking down inorganic chemicals in rocks or grains of soil or dust. (In this case, “inorganic” simply means molecules that don’t contain carbon-hydrogen bonds.)
“Understanding how chemotrophic organisms like A. ferrooxidans metabolize these compounds could give us a better understanding of Venus’ potential to support life — or how life might have evolved there in the past,” Duke University planetary scientist Aleksandra Stryjska, a coauthor of the recent paper, tells Astronomy. And understanding how A. ferrooxidans survives in such painfully acidic conditions could shed light on how any life-form could survive in the sulfuric acid clouds of Venus.
And there’s another thing that makes A. ferrooxidans so intriguing: It absorbs UV light in a very similar set of wavelengths to the unknown absorber in Venus’ atmosphere.
“This is intriguing because the UV absorption feature on Venus has been speculated to be due to a photosynthetic organism,” says Stryjska. “If the UV absorber is indeed something like A. ferrooxidans, it might indicate a form of life that utilizes chemical energy (such as sulfur or iron compounds) rather than sunlight, which fits with Venus’ highly acidic and energy-rich environment.”
Alien analogs
Słowik and his colleagues aren’t suggesting that colonies of A. ferrooxidans are floating around in the clouds of Venus. Instead, they argue that this microbe, and other acidophiles (microbes that live in acidic environments), could offer clues about the types of traits life could evolve to survive in such a hostile environment. They suggest that biologists should experiment with different strains of A. ferrooxidans in the lab, testing them under conditions more like Venus, until they eventually breed a strain that could survive there.
“If A. ferrooxidans were to survive on Venus, it would likely need to be adapted to the high acidity, lack of oxygen, and low levels of available water,” says Stryjska.
But the goal wouldn’t be to seed Venus with terrestrial life — just to see what venusian life might look like and what might make it tick.
“Understanding how chemotrophic organisms like A. ferrooxidans metabolize [iron and sulfur] compounds could give us a better understanding of Venus’ potential to support life — or how life might have evolved there in the past,” says Stryjska. “How did the chemistry of Venus’ atmosphere evolve to support or inhibit life? Did it start with more habitable conditions, or has it always been extreme?”
Studying an analog like A. ferrooxidans could also give astrobiologists clues about how to recognize the chemical signatures of potential life in the atmosphere of Venus, or another world — if it’s there.
What if there is no analog?
“It’s an open question whether Venus’ clouds have the necessary conditions to support even [extreme] life,” says Stryjska.
A key question, which won’t be settled without sending more spacecraft to study the venusian atmosphere up close, is exactly how acidic the planet’s clouds really are. So far, no mission has measured this clouds directly; instead, planetary scientists have estimated the clouds’ acidity based on other measurements of their chemical makeup. But MIT planetary scientist Janusz Petkowski, who was not involved in the recent study, tells Astronomy that he and his team of researchers expect the clouds to be several orders of magnitude more acidic than any environment on Earth — so acidic that it has to be measured on something called the Hammett scale, which extends the familiar pH scale.
On the pH scale, which runs from 1 to 14, water is a neutral 7. Stomach acid rates a 1. And on the Hammett scale, Venusian clouds rate somewhere around –12, according to Petkowski’s calculations.
“Where we have very acidic conditions here on Earth, it is always water that dominates the chemistry of these reservoirs, not acid. In the clouds of Venus, you have traces of water dissolved in an acid,” says Petkowski. “Concentrated sulfuric acid prevents any Earth-like organism to be there, because you would have to adapt to sulfuric acid as a solvent.”
In other words, life on Venus might have to use chemicals like sulfuric acid in the same way that life on Earth uses water. And to fully understand it, researchers might have to dream up alternative biochemistries in the lab.
Related: How to build aliens in the lab
Beyond the clouds
Regardless, A. ferrooxidans may offer some important hints about life on Venus and elsewhere, according to Stryjska.
“Since meteorites can contain organic compounds and possibly viable microbes, A. ferrooxidans could be used to understand how life might survive the extreme conditions of space travel — such as radiation, vacuum, and temperature fluctuations — while embedded within a meteorite,” says Stryjska. So, “A. ferrooxidans could also be an analog for studying life in environments where chemical energy rather than solar energy is the primary energy source.”
That could mean places like hydrothermal vents on the sea floor of Europa and underground ecosystems on Mars.
Of course, whether we will ever find life in these places is yet to be seen, but understanding how it might develop and survive is an important first step.
