Venus looks pale yellow in ordinary visible light, but the view changes dramatically in ultraviolet wavelengths. There, dark and bright features sweep across the planet's 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.
Now, an international team of researchers has placed new numerical limits on what this mysterious material must be like. Using observations of Venus together with radiative-transfer modeling, the scientists estimated how strongly the liquid inside the planet's cloud droplets would need to absorb ultraviolet and blue light to match what spacecraft and telescopes observe. The study, published in Astrobiology, examined the problem in an unusual way.
Jan Spacek asked what Venus's cloud material might look like if the droplets could somehow be collected into a spectrometric cuvette and studied as a bulk liquid. That distinction matters because a cloud can look very different from the material that actually makes up its particles. Cigarette smoke offers a familiar example.
Smoke can appear white because its sub-micrometer particles scatter light extremely effectively. But if those particles are collected inside a flask, they form a dense suspension of burned tobacco -- 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. "Our model effectively asks what would happen if we could collect that cloud material into a cuvette and put it into a laboratory spectrometer," said lead author Jan Spacek of the Foundation for Applied Molecular Evolution, USA. "This is important, as light absorption in a bulk liquid may be correlated with the concentration of light-absorbing material in the solution." Measuring How Strongly the Mystery Material Absorbs Light To make that comparison possible, 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 then converted the astronomical measurements into something commonly used in laboratory UV-visible spectroscopy: the absorption coefficient of the liquid making up the cloud droplets. "The key is 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," said Dr. Yeon Joo Lee of the Planetary Atmospheres Group within the Institute for Basic Science (IBS), S.
Extract — continue reading at the source.
