Investigating the Origin of Venus’ Clouds Using a Cloud Microphysics Model

Sulfuric acid clouds on Venus play a pivotal role in atmospheric radiation, chemistry, and material transport. Therefore, understanding the mechanisms underlying cloud formation on Venus is essential for gaining a better insight into the planet’s climate and atmospheric processes. Cloud formation on Venus begins with the nucleation process, which provides cloud condensation nuclei (CCN) necessary for subsequent condensational growth. Elemental sulfur is frequently assumed to be the primary CCN substance, as its vapor can be readily produced through photochemical reactions and solidifies upon condensation. Meteoric dust has been proposed as CCN as well and cloud droplets could also form by homogeneous nucleation.

Cloud microphysics models are effective tools for exploring the mechanisms of cloud formation and have been widely applied in studies of Venus. Previous modeling studies that assumed elemental sulfur as CCN have successfully reproduced observed cloud structures [1,2,3,4]. However, these studies have typically simplified the CCN production process by directly injecting particles with predefined sizes ranging from 0.01 to 0.1 µm, rather than explicitly calculating the CCN production rate based on nucleation theory. In addition, the elemental sulfur CCN are also provided from the lower model boundary at ~40 km altitude in the previous studies, despite uncertainties about the stability of elemental sulfur as a solid phase at these altitudes. Consequently, the fundamental initial step of cloud formation on Venus remains poorly understood.

In this study, we perform 1D cloud microphysics simulations incorporating elemental sulfur vapor and its nucleation process to investigate the origin of Venus’ clouds.  A cloud microphysics model used here is the Simulator of Particle Evolution, Composition, and Kinetics (SPECK) [5]. SPECK accurately calculates condensation processes and is particularly suitable for aerosols with diverse compositions. Thus, it effectively simulates particle evolution from nucleation through condensation and coagulation, tracking interactions among particles with different composition. Our model includes three condensable vapor species: sulfuric acid, water, and elemental sulfur (S₈). The homogeneous nucleation of sulfuric acid occurs via binary nucleation with water [6], while the nucleation of S₈ is computed using a classical homogeneous nucleation theory. The size bins of the model range from 1 nm to 30 µm, and homogeneously nucleated particles are introduced into the smallest bin size of 1 nm. The model also considers the heterogeneous nucleation of sulfuric acid and water on the formed elemental sulfur particles. The vertical model domain spans altitudes from 40 km to 100 km, encompassing the entire cloud structure from the lower clouds to the upper haze. In addition to homogeneously nucleated particles, our model incorporates meteoric smoke particles (MSPs) as CCN with a radius of 1 nm. MSPs, assumed to consist of olivine, are introduced at the top of the model domain since the production of MSP is expected to occur around 115 km [7]. A parameter study is conducted with respect to the meteoric dust ablation flux ranging from 1 t d^-1 to 1000 t d^-1.

Figure 1. (a) Homogeneous nucleation rate of sulfuric acid and water (blue solid line) and S₈ (red-dashed line). (b) Heterogeneous nucleation rate of sulfuric acid onto S₈ particles.

 

Our results indicate that different nucleation processes dominate at different altitudes. Specifically, homogeneous nucleation of elemental sulfur prevails below 70 km altitude, whereas homogeneous nucleation of sulfuric acid dominates above 80 km (Figure 1a). The S₈ particles are eventually activated through heterogeneous nucleation and become coated by sulfuric acid solution (Figure 1b). This suggests that cloud particles below 70 km and haze particles above 70 km have distinct origins. Parameter studies varying the MSP injection flux by three orders of magnitude resulted in negligible differences in the upper haze structure, consistent with previous findings [2]. Additionally, we confirmed that elemental sulfur particles evaporate below the cloud base due to higher temperatures. This result raises questions about the previous assumption that elemental sulfur serves as CCN around the cloud base, highlighting the possibility that alternative CCN substances such as minerals [8] or salts [9] may be more suitable.

 

[1] Imamura & Hashimoto (2001), JAS

[2] Gao et al. (2014), Icarus

[3] McGouldrick & Barth (2023), PSJ

[4] Karyu et al. (2024), PSJ

[5] Karyu et al. (2025), ESS, under review

[6] Määttänen et al. (2018), JGR

[7] Carillo-Sanchez et al. (2020), Icarus

[8] Krasnopolsky (2017), Icarus

[9] Rimmer et al. (2020), PSJ