Venus's Yellow Clouds May Hide a Surprisingly Dark Liquid

Venus looks pale yellow in ordinary visible light, but ultraviolet images reveal dark and bright patterns sweeping across its upper sulfuric acid clouds. Scientists have observed these ultraviolet features for about a century, yet the substance responsible, called the "unknown absorber," has never been identified. A new study published in the journal Astrobiology has placed the sharpest numerical limits yet on what that substance must be like.

Lead author Dr. Jan Spacek of the Foundation for Applied Molecular Evolution in the US and colleagues, including Dr. Yeon Joo Lee of the Institute for Basic Science in South Korea, asked what Venus's cloud droplets would look like if collected into a laboratory container rather than viewed as a cloud from space. They point to cigarette smoke as a comparison: it looks pale from a distance because its tiny particles scatter light efficiently, but the same material collected in a flask forms a dark, tar-like sludge. Venus's cloud particles are a similar size to smoke particles, so a pale-yellow cloud could still be made of a much darker liquid.

To estimate how dark that liquid must be, the researchers combined telescope observations of Venus with a radiative-transfer model that tracks how light scatters and is absorbed inside the clouds. Across ultraviolet and blue wavelengths of 365 to 455 nanometers, they calculated that the liquid inside the droplets would need a decadic absorption coefficient of about 1,278 per centimeter at 375 nanometers -- a demanding requirement that means the absorber must be highly effective at absorbing light, present at high concentration, or both.

One class of substances that could satisfy this requirement is highly light-absorbing carbon-based molecules, similar in efficiency to natural pigments such as chlorophyll or heme, at a concentration of roughly 10 grams per liter. The researchers stressed they are not suggesting any biological pigment is present on Venus; those compounds are only reference points for how strongly a molecule can absorb light. Simple organic compounds dissolved in concentrated sulfuric acid usually form dark, tar-like mixtures that absorb light broadly across the visible spectrum, but Venus's absorption drops sharply between 365 and 455 nanometers, a pattern that does not fit that kind of mixture.

The findings do not identify the unknown absorber, and they do not show that life exists in Venus's clouds. Instead, the study sets out specific requirements -- how strongly the material must absorb light, how concentrated it must be and where it sits in the atmosphere -- that any future candidate, organic or inorganic, will have to meet.