Venus clouds may hide dark liquid, study finds

September 12, 2026 at 05:00 AM2 min

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An international team has placed new numerical limits on the unknown absorber in Venus's clouds. Using observations and radiative-transfer modeling, they estimated how strongly the cloud droplet liquid must absorb ultraviolet and blue light. The study, published in Astrobiology, suggests the pale yellow clouds could contain surprisingly dark concentrated material.

Key Facts

The study was published in the journal Astrobiology.
Lead author Jan Spacek is from the Foundation for Applied Molecular Evolution, USA.
Dr. Yeon Joo Lee of the Institute for Basic Science in South Korea performed the radiative-transfer model calculations.
Venus's cloud particle size distribution is comparable to that of cigarette smoke.

The Unknown Absorber

Venus appears pale yellow in visible light, but ultraviolet observations reveal dark and bright features in its upper sulfuric acid clouds. Scientists have observed these patterns for about a century, yet the substance producing them, known as the unknown absorber, has never been identified. An international team has now placed new numerical limits on what this material must be like. Using observations of Venus and radiative-transfer modeling, they estimated how strongly the liquid inside cloud droplets would need to absorb ultraviolet and blue light.

Laboratory Analogy

Lead author Jan Spacek asked what Venus's cloud material might look like if the droplets could be collected into a spectrometric cuvette and studied as a bulk liquid. Cigarette smoke offers a familiar example: smoke can appear white because its sub-micrometer particles scatter light extremely effectively, but collected particles form a tar-like sludge. Venus's clouds may behave according to a similar optical principle because their particle size distribution is comparable to that of cigarette smoke. As a result, clouds that look pale yellow from far away could contain liquid that appears surprisingly dark when concentrated.

Radiative-Transfer Modeling

The researchers combined observations of Venus with a radiative-transfer model that tracks how light is repeatedly scattered and absorbed by cloud droplets and atmospheric molecules. They converted the astronomical measurements into the absorption coefficient of the liquid making up the cloud droplets. Dr. Yeon Joo Lee of the Planetary Atmospheres Group within the Institute for Basic Science in South Korea performed the radiative-transfer model calculations. Lee noted that Venus's 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.