Venus's Clouds May Contain Potent Light-Absorbing Compounds
Recent research has provided new insights into the mysterious dark and bright patterns observed in Venus's upper clouds, suggesting that highly efficient organic compounds may be responsible for the planet's ultraviolet light absorption. A study published on August 27, 2026, in Astrobiology by Jan Spacek and colleagues offers quantitative constraints on the so-called 'unknown absorber' in Venus's atmosphere.
The study modeled how cloud-droplet liquid would appear in bulk, determining that a decadic absorption coefficient of approximately 1,278 cm⁻¹ at 375 nm is necessary to match ultraviolet and blue reflectance observations. This suggests that the absorber must be either extremely efficient, highly concentrated, or both, potentially involving porphyrinoid-like organic pigments at concentrations around 10 g/L.
While the study aligns these properties with conjugated organics, it does not propose specific biological compounds like chlorophyll. The researchers note that simple organics in concentrated sulfuric acid would typically form dark mixtures inconsistent with the observed absorption patterns, indicating that the actual absorber may be chemically well-defined and resistant to degradation.
Previous studies have largely ruled out other candidate substances. For instance, a 2025 study in Geophysical Research Letters concluded that OSSO (cis-/trans-S₂O₂) concentrations are about 1,000 times too low to account for the unknown UV absorption. Similarly, sulfur allotropes present in the upper cloud were found to be insufficient in concentration to explain the absorber.