Something mysterious floats amidst the clouds of Venus –strange, dark patches, called “unknown absorbers” because they absorb large amounts of solar radiation. What these dark patches are is an enigma, though astronomers as renowned as Carl Sagan have suggested they could be extraterrestrial microorganisms.
The planet’s hot, harsh atmosphere is thick with carbon dioxide and sulfuric acid. Atmospheric gases circulate amid cloud layers according to patterns that scientists don’t fully understand. Scientists have speculated that the patches might be forms of sulfur, ferric chloride or, as Sagan speculated, even microscopic life.
“These are questions that haven’t been fully explored yet and I’m shouting as loud as I can saying that we need to explore them,” says Sanjay Limaye, an atmospheric scientist from the University of Wisconsin, Madison and a former chair of NASA’s Venus Exploration Analysis Group. A more exciting explanation for the dark patches is that they are evidence of microbial life. “It’s a possibility we can’t overlook,” says Limaye.
Mars, for example, has geological features that suggest it once had — and still has — subsurface liquid water, an almost sure prerequisite for life. Scientists have also eyed Saturn’s moons Titan and Enceladus as well as Jupiter’s moons Europa, Ganymede and Callisto as possible havens for life in the oceans under their icy crusts.
On Earth, terrestrial microorganisms — mostly bacteria — are capable of being swept into the atmosphere, where they have been found alive at altitudes as high as 41 kilometers (25 miles) by scientists using specially equipped balloons, according to study co-author David J. Smith of NASA’s Ames Research Center.
“Venus has had plenty of time to evolve life on its own,” explains Limaye, noting that some models suggest Venus once had a habitable climate with liquid water on its surface for as long as 2 billion years. “That’s much longer than is believed to have occurred on Mars.”
There is also a growing catalog of microbes known to inhabit incredibly harsh environments on our planet, including the hot springs of Yellowstone, deep ocean hydrothermal vents, the toxic sludge of polluted areas, and in acidic lakes worldwide.
“On Earth, we know that life can thrive in very acidic conditions, can feed on carbon dioxide, and produce sulfuric acid,” says Rakesh Mogul, a professor of biological chemistry at California State Polytechnic University, Pomona, and a co-author on the new paper. He notes that the cloudy, highly reflective and acidic atmosphere of Venus is composed mostly of carbon dioxide and water droplets containing sulfuric acid.
Like Earth, the Venusian weather is driven by solar radiation and is deeply influenced by changes in the reflective properties or albedo of the clouds that envelop the planet. And now a better picture of that weather and how it is influenced by changes in the reflectivity of the clouds has emerged, thanks to an international team of researchers using a suite of satellites to assess in ultraviolet light the long-term variations in Venus’ albedo, or how much energy is reflected back to space.
“The difference between Earth and Venus is that on Earth most of the energy from the sun is absorbed at ground level while on Venus most of the heat is deposited in the clouds,” explains Limaye, a University of Wisconsin–Madison planetary scientist and a co-author of the new study.
What is curious about Venus’ clouds—other than that they are unlike anything on Earth—is that in those clouds are mysterious dark patches, dubbed “unknown absorbers” by scientists as the tiny particles that make up the patches soak up most of the ultraviolet and some of the visible light from the sun and thus affect the planet’s albedo and energy budget.
The patches were first observed by ground-based telescopes more than a century ago. They ebb and flow over time, changing their distributions and contrasts.
“The particles that make up the dark splotches, have been suggested to be ferric chloride, allotropes of sulfur, disulfur dioxide and so on, but none of these, so far, are able to satisfactorily explain their formation and absorption properties,” explains Yeon Joo Lee, a researcher in the Center for Astronomy and Astrophysics at the Technical University of Berlin, and the senior author of the new report.
On the other hand, Limaye notes observations that the particles are about the same size and have the same light-absorbing properties as microorganisms found in Earth’s atmosphere, and scientists, beginning with the noted biophysicist Harold Morowitz and Sagan, have long speculated about the possibility that the shadowy patches in the clouds of Venus are, in fact, microscopic life.
Whatever their composition, Venus’ “unknown absorbers,” according to the new measurements of the planet’s albedo, by a team of scientists led by Lee, has shown that the unknown absorbers are affecting Venus’s weather by studying more than a decade of data from Venus Express, Akatsuki, Messenger and the Hubble Space Telescope. The researchers found a relationship between Venus’ clouds and its winds. The clouds absorb solar radiation, which causes temperature changes that affect wind patterns. The unknown absorbers seem to play a role in this process by affecting the planet’s albedo.
“It is hard to conceive of what would cause a change in the albedo without a change in the absorbers,” said Sanjay Limaye, a planetary scientist at the University of Wisconsin-Madison and paper co-author, who suggests that because it is difficult to explain the absorbers’ changes inorganically, there’s the possibility that they might be microorganisms similar to those in Earth’s atmosphere. “Since there are few species which have physical, chemical and spectral properties that are consistent with the composition of the Venus clouds, they may have evolved independently on Venus.”
Limaye suggests that liquid water may have survived on Venus for as long as two billion years, while at the same time volcanoes erupted and hydrothermal vents may have existed. “If similar conditions elsewhere led to the evolution of life, why not on Venus?” Limaye said.
“This new result is very important because every piece of information we obtain from atmospheric levels from the upper clouds and above will help resolve the enigma” of the planet’s mysterious atmosphere, says Emilie Royer, a research scientist at the Planetary Science Institute who was not involved in the study.
Limaye says scientists will need constant, systematic monitoring of the planet’s cloud cover over multiple 11-year solar cycles to solve the mystery. He also suggests aerial platforms that can sample Venus’s cloud layer, search for bio-signatures and take other measurements. “Understanding the nature of the absorbers on Venus is the key,” he says.
Between 2006 and 2017, Venus’ albedo, the measure of ultraviolet light reflected back to space, halved before beginning to rebound. Those changes to the planet’s albedo sparked big variations in the amount of solar energy absorbed by the clouds and, consequently, the circulation of Venus’ atmosphere. In particular, the albedo changes help explain variations in the vigorous activity of the planet’s upper atmosphere, which exhibits what scientists call “super-rotation,” a phenomenon driven by winds exceeding 200 miles per hour.
Takeshi Horinouchi of Japan’s Hokkaido University, also a co-author of the new Astronomical Journal report and an expert on Venusian weather, says the new results of the changes in the planet’s albedo provide a link between solar heating and the powerful gusts that underpin the dynamics of the planet’s upper atmosphere.
“What really struck me about this paper is that it shows that Venus’ climate has decadal-long climate variations, just like the Earth,” says Venus expert Mark Bullock of the Southwest Research Institute and who was not involved in the new study. “Even more amazing, the strength of the climate oscillation on Venus is much greater than Earth’s long-term variations.”
“That is a striking result,” Limaye adds. “It suggests that something is changing. We can see the change in brightness. If the albedo is changing, something is driving those changes. The question is, what is the cause?”
The ebb and flow of the mysterious dark splotches at the tops of Venus’ clouds, the “unknown absorbers,” are near the top of the list of suspects and could, in fact, be playing a major role in those changes, say Lee and Limaye. Haze above the clouds and composed of smaller particles, they add, can make Venus appear even brighter.
Weather and climate, be it on Earth or on Venus, are driven by solar radiation, including the ultraviolet radiation that we can’t see. And clouds and their changing ability to reflect light have a huge influence.
“Is solar ultraviolet light impacting Venus’ cloud cover? Are cosmic rays—subatomic particles from space that continuously rain down on all the planets—affecting cloud cover by triggering cloud nucleation? Would the planetary sulfuric dioxide affect the formation of sulfuric acid cloud?,” asks Lee.
“I cannot say that there is microbial life in Venus’ clouds,” says Limaye. “But that doesn’t mean it’s not there either. The only way to learn is to go there and sample the atmosphere.”
Image at top of page shows the Moon, Venus, Jupiter, Earth from the ISS
The Daily Galaxy, Sam Cabot, via University of Wisconsin-Madison