New research puts stringent limits on the unknown ‘material’ responsible for Venus’ dark ultraviolet markings, suggesting it must absorb sunlight with remarkable efficiency or exist in unusually high concentrations.
Venus in real colors, processed from Mariner 10 images. Image credit: Mattias Malmer / NASA.
At visible wavelengths, Venus appears serene and pale yellow. But since the 1920s, astronomers have noted high-contrast features in the ultraviolet (UV).
These features track the four-day superrotation of the planet’s upper cloud deck and vary widely over time and space.
The identity of the UV absorber remains unknown, as no proposed candidate fully matches all observational data.
In new research, Dr. Jan Spacek from the Foundation for Applied Molecular Evolution and colleagues estimated how strongly the liquid inside Venusian cloud droplets would need to absorb light to reproduce the planet’s observed ultraviolet and blue reflectance.
“Our model effectively asks what would happen if we could collect that cloud material into a cuvette and put it into a laboratory spectrometer,” Dr. Spacek said.
“This is important, as light absorption in a bulk liquid may be correlated with the concentration of light-absorbing material in the solution.”
The researchers combined observations of Venus with a radiative-transfer model that accounts for multiple scattering by cloud droplets and atmospheric molecules.
They translated the astronomical observations into a quantity routinely measured in laboratory UV-visible spectroscopy: the absorption coefficient of the bulk cloud liquid.
“The key is that Venus’ cloud particles scatter sunlight very efficiently, so the brightness observed from space cannot be directly compared with the absorption of a bulk liquid measured in the laboratory,” said Dr. Yeon Joo Lee of South Korea’s Institute for Basic Science.
“By accounting for the scattering and absorption by the cloud particles and atmosphere, the model allows us to estimate how strongly the liquid of cloud droplets itself must absorb light.”
Within the modeled 365-455 nm range, the required decadic absorption coefficient reaches approximately 1,278 cm-1 at 375 nm.
The result implies that the unknown absorber must either absorb light very efficiently, occur at a very high concentration, or both.
Highly absorbing conjugated organic molecules could satisfy this requirement. Here, organic refers to carbon-based compounds and does not imply a biological origin.
Molecules with light-absorption strengths characteristic of efficient porphyrinoid pigments would require concentrations on the order of 10 grams per liter.
The authors stress that they are not proposing chlorophyll, heme, or any specific biological pigment as the Venus absorber; these compounds serve only as familiar examples of efficient light absorbers.
The shape of the spectrum provides another important constraint.
Simple organics exposed to concentrated sulfuric acid can form dark, chemically complex ‘tar-like’ mixtures.
However, such complex mixtures tend to absorb broadly across the visible spectrum, appearing brown or black. This does not match the steep decrease in absorption inferred for Venus between 365 and 455 nm.
“If the observed light absorption is due to conjugated organic matter, the relatively sharp absorption profile is consistent with a chemically defined absorber that resists conversion into the tar-like mixture we typically observe with organics dissolved in concentrated sulfuric acid,” Dr. Spacek said.
“Paradoxically, by placing additional constraints on the unknown absorber, we might have made the mystery even more intriguing,” said Dr. Janusz Petkowski from the Wroclaw University of Science and Technology.
“The model places a demanding constraint on any proposed absorber,” said Dr. Paul Rimmer from the University of Cambridge.
“Many of the proposed inorganic candidates would need to be present at very high concentrations to match the required absorption.”
The results were published in the journal Astrobiology.
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Jan Spacek et al. A Model of UV-Blue Absorbance in Bulk Liquid of Venusian Cloud Aerosols Is Consistent with Efficient Organic Absorbers at High Concentrations. Astrobiology, published online August 25, 2026; doi: 10.1177/1531107426147750
