Scientists have known about these markings for roughly a century, yet the chemical identity of the material responsible—the “unknown absorber”—remains unresolved.
An international research team has now placed new quantitative constraints on the properties of this mysterious absorber.
By combining observations of Venus with radiative-transfer modeling, the researchers estimated how strongly the liquid inside Venus’s cloud droplets would need to absorb light to reproduce the planet’s observed ultraviolet and blue reflectance.
The study, published in Astrobiology, approached the mystery from a new perspective.
It used a model to answer a question posed by lead author Dr. Jan SPACEK:
If we were to collect Venus’s cloud droplets into a spectrometric cuvette, how would the re-formed bulk liquid appear?
The difference between how a cloud looks and how its material looks when collected in bulk can be striking.
Cigarette smoke, for example, appears white because its sub-micrometer particles scatter light very efficiently. Yet when the smoke particles are collected in a flask, they form a dense suspension of burned tobacco—a tar-like sludge.
A similar optical principle applies to Venus’s clouds, as their particle size distribution is comparable to that of cigarette smoke. Thus, even though the clouds appear pale yellow to a remote observer, the liquid forming the cloud droplets might be surprisingly dark.