Venus looks like a pale yellow world in visible light, but ultraviolet images tell a very different story.
They reveal striking dark and bright patterns moving through the planet’s upper clouds. Scientists have observed these patterns for about a century, but they still do not know what chemical is creating them.
Researchers call the mysterious material the “unknown absorber” because it absorbs ultraviolet and blue light.
Now, an international team has placed new limits on what this substance could be, and the findings suggest it may be an exceptionally powerful light absorber.
The study, published in Astrobiology, combined observations of Venus with computer models to estimate how strongly the liquid inside the planet’s cloud droplets would absorb light if it could be collected and examined in a laboratory.
Venus’s clouds are largely made of sulfuric acid droplets. Although they look relatively bright from space, that does not necessarily mean the liquid inside them would also look bright.
The researchers compare this effect with cigarette smoke. Smoke can appear white because its tiny particles scatter light very efficiently. But if those particles are collected together, they form a much darker, tar-like material. Venus’s cloud particles have similar light-scattering properties, meaning their actual liquid contents could be surprisingly dark.
The team used a model that accounts for the complicated way sunlight is repeatedly scattered and absorbed by Venus’s cloud droplets and atmosphere. This allowed them to translate observations from space into measurements that could be compared with laboratory experiments.
The results showed that the mysterious material must absorb light extremely strongly. Between wavelengths of 365 and 455 nanometers, its estimated absorption reached about 1,278 per centimeter at 375 nanometers.
That means the unknown substance must either be an unusually powerful absorber, exist at a very high concentration in the clouds, or both.
Certain carbon-based molecules could potentially meet these requirements. Highly efficient pigment-like molecules, for example, might need concentrations of around 10 grams per liter. However, the researchers emphasize that this does not mean Venus contains chlorophyll or other biological pigments. In chemistry, “organic” simply means carbon-based and does not necessarily indicate life.
There is another complication. Simple organic compounds exposed to concentrated sulfuric acid can turn into dark, complex, tar-like mixtures. These materials usually absorb light broadly across the visible spectrum and appear brown or black.
Venus’s mysterious absorber behaves differently. Its ability to absorb light falls sharply between 365 and 455 nanometers. If an organic compound is responsible, the researchers say it may therefore need to be a relatively well-defined chemical that can survive the harsh sulfuric acid environment without becoming a tar-like mixture.
The findings do not provide evidence for life in Venus’s clouds or prove that the unknown absorber is organic. Instead, they give scientists much clearer requirements for testing possible candidates.
Future missions could help solve the mystery directly. Instruments being developed to explore Venus’s clouds may search for complex organic molecules and fluorescence from cloud particles.
By sampling the clouds themselves, scientists could finally identify the substance behind Venus’s century-old ultraviolet mystery—and learn much more about the chemistry of Earth’s mysterious planetary neighbor.
