Ferric Chloride: The Venusian Unknown UV Absorber?
Abstract
Near-ultraviolet imaging of the planet Venus reveals inhomogeneous absorption features in the otherwise bright clouds. This absorption has been studied for nearly 100 years, but identification of the near-UV absorber remains one of the largest open questions in Venusian research. Based on a multiple scattering radiative transfer model, the observed absorption between 300 and 600 nm can be produced by 1.0-1.2 wt% ferric chloride (FeCl 3 ) in the mode 1 (∼0.2 μm radius) sulfuric acid cloud droplets-less than the in-cloud iron mass loading measured during the Venera-12 mission. We present a novel chemical network within a global chemistry-climate model, which provides the required source of FeCl 3 in the upper cloud region from the reaction of Fe-containing molecules, produced by the ablation of cosmic dust particles, with atmospheric HCl. A residence lifetime of ∼3,000 years is required to provide sufficient optical absorption from a purely meteoric source of FeCl 3 . Iron chloride has been proposed to cause the strong absorption seen in Venus's clouds. Using computer modeling of light in the atmosphere, we test the effect of iron chloride in the droplets that make up the Venusian clouds. We find that a modest concentration of 1.0%-1.2% (by mass) in the smallest cloud droplets can reproduce the shape and strength of the absorption recorded by the MESSENGER spacecraft. We present chemical reactions that produce iron chloride from the reaction of atmospheric hydrochloric acid gas and iron atoms produced from vaporized cosmic dust, and estimate that the iron chloride would take approximately 3,000 years to concentrate enough to match observations of the clouds.
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BibTeX
@article{egan2026ferric,
title = {{Ferric Chloride: The Venusian Unknown UV Absorber?}},
author = {Joanna V Egan and Wuhu Feng and Alexander D James and James Manners and Daniel R Marsh and Sébastien Lebonnois and Franck Lefèvre and Aurélien Stolzenbach and John M C Plane},
journal = {Journal of Geophysical Research. Planets},
year = {2026},
month = sep,
doi = {10.1029/2025je009634},
url = {https://hal.science/hal-05752314v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Ferric Chloride: The Venusian Unknown UV Absorber?
AU - Joanna V Egan
AU - Wuhu Feng
AU - Alexander D James
AU - James Manners
AU - Daniel R Marsh
AU - Sébastien Lebonnois
AU - Franck Lefèvre
AU - Aurélien Stolzenbach
AU - John M C Plane
PY - 2026
DA - 2026/09/16
JO - Journal of Geophysical Research. Planets
DO - 10.1029/2025je009634
UR - https://hal.science/hal-05752314v1
AB - Near-ultraviolet imaging of the planet Venus reveals inhomogeneous absorption features in the otherwise bright clouds. This absorption has been studied for nearly 100 years, but identification of the near-UV absorber remains one of the largest open questions in Venusian research. Based on a multiple scattering radiative transfer model, the observed absorption between 300 and 600 nm can be produced by 1.0-1.2 wt% ferric chloride (FeCl 3 ) in the mode 1 (∼0.2 μm radius) sulfuric acid cloud droplets-less than the in-cloud iron mass loading measured during the Venera-12 mission. We present a novel chemical network within a global chemistry-climate model, which provides the required source of FeCl 3 in the upper cloud region from the reaction of Fe-containing molecules, produced by the ablation of cosmic dust particles, with atmospheric HCl. A residence lifetime of ∼3,000 years is required to provide sufficient optical absorption from a purely meteoric source of FeCl 3 . Iron chloride has been proposed to cause the strong absorption seen in Venus's clouds. Using computer modeling of light in the atmosphere, we test the effect of iron chloride in the droplets that make up the Venusian clouds. We find that a modest concentration of 1.0%-1.2% (by mass) in the smallest cloud droplets can reproduce the shape and strength of the absorption recorded by the MESSENGER spacecraft. We present chemical reactions that produce iron chloride from the reaction of atmospheric hydrochloric acid gas and iron atoms produced from vaporized cosmic dust, and estimate that the iron chloride would take approximately 3,000 years to concentrate enough to match observations of the clouds.
SN - 2169-9097
SN - 2169-9100
ER -