The problem
What makes parts of Venus’s clouds dark in ultraviolet light? Some of the absorbing chemicals are still unidentified.[1]
Venus’s clouds show extra near-ultraviolet absorption whose chemical cause and persistence remain unknown. What mixture of substances could produce it under cloud conditions? A model with ferric chloride in sulfuric-acid droplets can reproduce the retrieved spectrum. Reflectance depends on absorption and repeated scattering, as well as droplet sizes and locations, so that fit does not uniquely identify a molecule. A proposed iron cycle must also replenish the absorber as its chemistry changes.[1] [2]
Scope
The chemical identity and maintenance of the unidentified near-ultraviolet absorber in Venus’s clouds.
What remains open
An iron-containing compound could fit the light pattern, but its proposed recycling has not been confirmed.[2]
Why it matters
Cloud absorption changes where sunlight heats Venus.[1]
The identity connects cloud chemistry to the vertical deposition of solar energy.[1]
Definitions
2 terms
ferric chloride
An iron-and-chlorine compound, FeCl3. Its behavior dissolved in acidic cloud droplets is modeled as a possible source of Venus’s extra near-ultraviolet absorption.[2]
reflectance
The proportion of incoming light returned by a surface or atmosphere. Cloud absorption, scattering and droplet properties together affect Venus’s observed reflectance.[2]
Sources
- Venus Cloud Research: Progress and Perspectives
Notes on Venus Cloud Research: Progress and Perspectives
Review separates known cloud chemistry from proposed near-ultraviolet absorbers.
- Ferric chloride: the Venusian unknown UV absorber?
Notes on Ferric chloride: the Venusian unknown UV absorber?
Accepted July 22, 2026, in JGR: Planets; models ferric chloride with long-lived recycling in cloud droplets.