Investigation of ferric chloride as the cause of the Venusian NUV absorption

Understanding the composition and distribution of the unknown UV absorber (Figure 1) in the Venusian atmosphere has been an open question in planetary science for close to 100 years. Ferric chloride (FeCl₃) has been proposed as the cause of the absorption [1], but the absorption spectrum of FeCl₃ generally used in the literature was measured in ethyl acetate [2], which is not present on Venus and produces an absorption spectrum with little similarity to the Venusian absorber [3].

Figure 1: A false colour image of Venus. Regions of UV absorption appear orange. The cause of this absorption is unknown. Image credit: NASA/JPL-Caltech. 

 We have measured the absorption spectrum of FeCl₃ in sulphuric acid with small quantities of HCl added, and we found that it is much more similar in shape to the observed spectrum of the unknown absorber than prior FeCl₃ spectra available in the literature (Figure 2). 

Figure 2: Comparison of the FeCl₃ spectra in ethyl acetate [2] and sulphuric acid (this work) with a spectrum of the unknown UV absorber recorded by MASCS/MESSENGER [3].

When added to sulphuric acid, a mixture of ferric chloride and ferric sulphate ions are observed. We estimated the molar partitioning of the species in the mixtures by performing least squares fitting to reproduce each measured spectrum from spectra of pure ferric sulphate (measured for Fe₂(SO₄)₃ in 75-78 wt% aqueous H₂SO₄) and pure ferric chloride (measured for FeCl₃ in 5-37 wt% aqueous HCl).

The fraction of the observed absorption that can be reproduced by FeCl₃ was investigated using three models: the global Planetary Climate Model for Venus (PCM-Venus) to model the photochemistry and 3D transport of the candidates in the atmosphere [4], a 1D sectional aerosol model to predict agglomeration and sedimentation as a transport mechanism of FeCl₃ particles [5], and the 1D multiple scattering radiative transfer model SOCRATES [6]. 

The required concentrations of FeCl₃ in the different cloud modes to reproduce observations taken by MESSENGER/MASCS during its June 2007 Venus flyby [3] were estimated for different cloud modes using SOCRATES. The full absorption can be explained by approximately 1.5 - 2 wt% FeCl₃ in the mode 1 cloud droplets (Figure 3). Some contribution from ferric sulphate ions is also expected, but absorbs in the same wavelength region as SO₂, so its precise contribution could not be reliable constrained.

Figure 3: Comparison of MASCS/MESSENGER measured reflectance (crosses) [3] with the modelled spectrum with SO₂ only (blue line), and with 1.2 – 2.0 wt% FeCl₃ in mode 1 cloud droplets (orange and red lines).

Iron chemistry was added into PCM-Venus in order to predict the abundance of gas-phase FeCl₃ produced by the reaction of gas-phase HCl with iron produced by the ablation of cosmic dust particles around 115 km. Mean gas and dynamical profiles from the PCM were used to initialise the agglomeration and sedimentation model, which was then run for many Venus years to reach steady state. Agglomeration and sedimentation modelling of FeCl₃ suggests that the PCM-modelled FeCl₃ column abundance above 60 km can account for more than 40% of the observed absorption. The flux of meteoric iron into the Venusian atmosphere has not been measured, and may be significantly higher than assumed in this work [7].

   

References 

[1] Zasova et al. 1981, https://doi.org/10.1016/0273-1177(81)90213-1 

[2] Aoshima et al. 2013, http://doi.org/10.1039/C3PY00352C

[3] Pérez-Hoyos et al. 2018, https://doi.org/10.1002/2017JE005406 

[4] Martinez et al. 2024, doi.org/10.1016/j.icarus.2024.116035 

[5] Frankland et al. 2017, https://doi.org/10.1016/j.icarus.2017.06.005 

[6] Manners et al. 2022, SOCRATES Technical Guide, available at: https://code.metoffice.gov.uk/trac/socrates 

[7] Carrillo-Sánchez et al. 2020, https://doi.org/10.1016/j.icarus.2019.113395