Venus appears pale yellow in visible light, but ultraviolet images reveal dramatic dark and bright patterns that move together with the planet’s upper sulfuric acid clouds. Scientists have known about these markings for about a century, but the chemical nature of the material responsible for them — the “unknown absorber” — has still not been identified.
The Mysterious Light Absorber
As phys.org reports, an international team of researchers has established new quantitative constraints on the properties of this mysterious absorber. By combining observations of Venus with a radiative-transfer model, the researchers estimated how strongly the liquid inside droplets in the planet’s clouds would have to absorb light in order to reproduce the observed ultraviolet and blue reflectance of the planet.
The published study approached the mystery from a new angle. A model was used to answer a question posed by lead author Dr. Jan Spacek: “If we collected Venus cloud droplets into a spectroscopic cuvette, what would the reconstructed bulk liquid look like?”
From Bright Clouds to Dark Liquid
The difference between how a cloud appears and how its material looks in bulk can be striking. Cigarette smoke, for example, appears white because its submicron particles scatter light very efficiently. But when smoke particles are collected in a flask, they form a dense suspension of burned tobacco — something resembling a tar-like paste.
A similar optical principle applies to the clouds of Venus because the size distribution of their particles is comparable to that of cigarette smoke. Therefore, even though the clouds appear pale yellow to a distant observer, the liquid forming the cloud droplets may actually be extremely dark.
“Our model essentially asks: what would happen if we could collect this cloud material into a cuvette and place it in a laboratory spectrometer?” said lead author Spacek of the Foundation for Applied Molecular Evolution in the United States. “This is important because light absorption in a bulk liquid can be related to the concentration of light-absorbing material in solution.”
The researchers combined observations of Venus with a radiative-transfer model that accounts for multiple scattering by cloud droplets and atmospheric molecules. They converted astronomical observations into a quantity normally measured in laboratory ultraviolet-visible spectroscopy: the absorption coefficient of the cloud liquid.
By taking into account scattering and absorption by cloud particles and the atmosphere, the model with this coefficient makes it possible to estimate how strongly the liquid inside the cloud droplets itself must absorb light.
Constraints on the Properties of the Light Absorber
Within the modeled wavelength range of 365–455 nm, the required decadic absorption coefficient reaches approximately 1.278 cm⁻¹ at 375 nm. This means that the unknown absorber must either absorb light very efficiently, be present 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 necessarily imply a biological origin. Molecules with light-absorption properties characteristic of efficient porphyrin pigments would require concentrations on the order of 10 grams per liter. The authors emphasize that they are not proposing chlorophyll, heme, or any specific biological pigment as the Venus absorber; these compounds are used only as familiar examples of efficient light absorbers.
The shape of the spectrum provides another important constraint. Simple organic substances exposed to concentrated sulfuric acid can form dark, chemically complex “tar-like” mixtures. However, such complex mixtures generally absorb light broadly across the visible spectrum, appearing brown or black. This does not match the sharp decrease in absorption observed on Venus between 365 and 455 nm.
“If the observed light absorption is caused by conjugated organic matter, then the relatively sharp absorption profile is consistent with a chemically defined absorber that is resistant to transformation into the tar-like mixture we typically observe when organics are dissolved in concentrated sulfuric acid,” Spacek said.
Scientists note that many of the proposed inorganic candidates would also have to be present at very high concentrations to produce the required absorption.
The Need for Direct Experimental Verification
The results do not show that life exists in the clouds of Venus and do not prove that the absorber is organic. Instead, the study establishes quantitative requirements that any candidate — organic or inorganic — must satisfy, including absorption efficiency, concentration, atmospheric distribution, and compatibility with the actual distribution of cloud-particle sizes.
These constraints can now be tested experimentally and, ultimately, through direct investigation. The Morning Star Missions initiative to Venus is developing in-situ methods for studying the chemistry of the Venusian clouds, including searches for complex organic molecules and measurements related to the unknown absorber.
An autofluorescence nephelometer, designed to search Venusian cloud particles for fluorescence expected from the presence of organic molecules, is planned for a Rocket Lab mission to Venus.
