A Century-Old Mystery in Venus’s Clouds Just Got Even Stranger

Venus's clouds appear pale yellow, but the liquid inside their droplets might be much darker. A new study has calculated how strongly this liquid would need to absorb light. This helps explain mysterious ultraviolet markings that have puzzled scientists for about a century.

These dark and bright patterns show up in ultraviolet images of Venus. They move with the planet’s upper sulfuric acid clouds. Something in these clouds absorbs ultraviolet and blue light, but scientists still don't know what this "unknown absorber" is.

Instead of trying to identify the absorber directly, a global team asked a different question. They wondered what the liquid inside Venus’s cloud droplets would look like if it could be collected and studied in a lab.

Jan Spacek, the lead author from the Foundation for Applied Molecular Evolution, explained their model. He said it's like asking what would happen if they could put the cloud material into a lab device. This is important because light absorption in a liquid can be linked to how much light-absorbing material is in it.

Venus's Clouds May Hide Dark Liquid

The difference between how a cloud looks and how its collected material looks is key. Tiny particles can scatter light so well that a cloud seems much brighter than the material itself.

Think of cigarette smoke. It looks white because its tiny particles scatter light. But if you collect those particles, you get a dense, dark, tar-like substance. Venus’s cloud droplets are similar in size to cigarette smoke particles. This suggests their pale look could hide a much darker liquid.

To figure out how strongly the liquid would need to absorb light, researchers combined observations of Venus with a special model. This model accounted for how sunlight is scattered and absorbed by cloud droplets and gases in the atmosphere.

Dr. Yeon Joo Lee from the Institute for Basic Science (IBS) in South Korea worked on the model. She noted that Venus’s cloud particles scatter sunlight very well. This means the brightness seen from space can't be directly compared to how a liquid absorbs light in a lab. The model helps estimate how strongly the cloud droplet liquid itself must absorb light.

The Mystery Absorber Must Be Unusually Strong

The researchers converted Venus observations into a lab measurement called an absorption coefficient. This shows how strongly a material absorbs light.

The model found that the required absorption was very strong, especially near the ultraviolet end of the spectrum. This was true for wavelengths from 365 to 455 nanometers. At 375 nanometers, the calculated absorption coefficient was about 1,278 inverse centimeters.

This sets a high bar for whatever substance causes the effect. The unknown absorber must either absorb light very well, be present in a very high amount, or both.

One idea is a group of carbon-based molecules called conjugated organics. In this case, "organic" just means it contains carbon, not that it came from life.

Molecules that absorb light as well as efficient porphyrinoid pigments would need to be at concentrations of about 10 grams per liter. This is about 1.3 ounces per gallon. The researchers are not saying that chlorophyll or other biological pigments are in Venus’s clouds. They are just using these as examples of substances that absorb light very well.

Sulfuric Acid Challenges Organic Explanations

The way Venus’s absorption spectrum is shaped also creates a challenge for organic explanations.

When simple organic compounds are put into concentrated sulfuric acid, they can form complex, dark, tar-like mixtures. These mixtures usually absorb light across the whole visible spectrum, making them look brown or black.

This doesn't match the pattern seen on Venus. The modeled absorption drops sharply between 365 and 455 nanometers.

Spacek explained that if the light absorption is from conjugated organic matter, the sharp absorption profile suggests a specific absorber. This absorber would resist turning into the tar-like mixture usually seen with organics in concentrated sulfuric acid.

So, if the absorber is organic, it would need to stay chemically distinct in Venus’s extreme sulfuric acid clouds. It couldn't just become a broadly absorbing mixture.

Janusz J. Petkowski from Wroclaw University of Science and Technology noted that by adding more limits on the unknown absorber, the mystery might have become even more puzzling.

Inorganic explanations also have their own problems. Paul B. Rimmer from the University of Cambridge said the model puts tough limits on any proposed absorber. Many suggested inorganic candidates would need to be present in very high amounts to match the required absorption.

Future Missions Could Test Candidates

These findings don't prove that Venus’s clouds contain life, nor do they show the unknown absorber is organic. Instead, they set clear requirements for any proposed explanation. These include how well the material absorbs light, its concentration, where it is in the atmosphere, and if it fits with realistic cloud particle sizes.

These requirements can now be tested in lab experiments. Eventually, they can be tested with direct measurements inside Venus’s clouds.

The Morning Star Missions to Venus initiative is working on ways to study Venusian cloud chemistry directly. This includes looking for complex organic molecules and measuring things related to the unknown absorber. An instrument called the Autofluorescence Nephelometer is designed to look for fluorescence from organic molecules in cloud particles. It is planned for a Rocket Lab mission to Venus.

The study, published in Astrobiology, narrows down one of Venus’s oldest mysteries without solving it. Scientists still don't know what creates the planet’s ultraviolet markings. But now they have clearer limits on what the responsible material would have to be.

Deep Dive & References

A Model of UV–Blue Absorbance in Bulk Liquid of Venusian Cloud Aerosols Is Consistent with Efficient Organic Absorbers at High Concentrations - Astrobiology, 2025