A new study puts strict limits on the unknown “matter” responsible for Venus’ dark ultraviolet marks, suggesting it must either absorb sunlight with remarkable efficiency or be present in unusually high concentrations.
Actual color Venus processed from Mariner 10 images. Image credit: Matthias Malmar / NASA.
At visible wavelengths, Venus appears a calm, pale yellow color. But since the 1920s, astronomers have focused on high-contrast features in the ultraviolet (UV).
These features track a four-day superrotation of the planet’s upper cloud layer and vary significantly in time and space.
The identity of the UV absorber remains unknown, as the proposed candidates do not fully match all observed data.
In a new study, Dr. Jan Spacek of the Foundation for Applied Molecular Evolution and colleagues estimated how strongly the liquid in Venus’ cloud grains would need to absorb light to reproduce the observed ultraviolet and blue reflectance of Venus’ clouds.
“Our model effectively asks what would happen if we could collect that cloud material into a cuvette and put it into a spectrometer in the lab,” Dr. Spacek said.
“This is important because light absorption in bulk liquids can be correlated with the concentration of light-absorbing substances in solution.”
The researchers combined observations from 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 with laboratory UV-visible spectroscopy: the absorption coefficient of bulk cloud liquids.
“Importantly, the brightness observed from space cannot be directly compared to bulk liquid absorption measured in the laboratory because particles in Venus’ clouds scatter sunlight very efficiently,” said Dr. Yeon-Ju Lee of the Korea Institute of Basic Science.
“By taking into account scattering and absorption by cloud particles and the atmosphere, this model allows us to estimate how strongly the cloud droplets themselves need to absorb light.”
Within the modeled range of 365 to 455 nm, the required decimal absorption coefficient amounts to approximately 1,278 cm.-1 At 375nm.
This result means that the unknown absorber must either absorb light very efficiently, be present in very high concentrations, or both.
Highly absorbent conjugated organic molecules can meet this requirement. Here, organic refers to carbon-based compounds and does not imply biogenic.
Concentrations on the order of 10 grams per liter are required for molecules with the light absorption intensity characteristic of efficient porphyrinoid pigments.
The authors emphasize that they are not proposing chlorophyll, heme, or any particular biological pigment as a Venus absorber. These compounds serve only as well-known examples of efficient light absorbers.
The shape of the spectrum provides another important constraint.
When simple organic materials are exposed to concentrated sulfuric acid, a black, chemically complex “tarry” mixture can form.
However, such complex mixtures tend to absorb broadly across the visible spectrum and appear brown or black. This is inconsistent with the sharp decrease in absorption estimated between 365 and 455 nm for Venus.
“If the observed optical absorption is due to conjugated organics, the relatively sharp absorption profile is consistent with a chemically defined absorber that resists conversion to the tar-like mixture typically observed with organics dissolved in concentrated sulfuric acid,” Spacek said.
“Paradoxically, by imposing additional constraints on the unknown absorber, we may have made the mystery even more interesting,” says Dr. Janusz Petkowski from Wrocław University of Science and Technology.
“This model imposes severe constraints on any proposed absorber,” said Dr. Paul Rimmer of the University of Cambridge.
of result Published in a magazine astrobiology.
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Jan Spacek others. A model for UV-blue absorption in the bulk liquid of Venusian cloud aerosols is consistent with efficient organic absorbers at high concentrations. astrobiologypublished online on August 25, 2026. doi: 10.1177/1531107426147750
Source: www.sci.news
