Cosmic Dust as a Source for the Venusian Lower Haze

Venus is shrouded in thick sulfuric acid clouds at altitudes of 47–70 km. Below the cloud base (~47 km), high temperatures cause sulfuric acid to evaporate completely, preventing liquid particles from persisting in the lower atmosphere. However, probes from multiple missions—including VeGa, Venera, and Pioneer Venus—have consistently detected a persistent aerosol layer below the cloud layer, known as the lower haze (e.g., Knollenberg & Hunten, 1980; Titov et al., 2018). These particles are critical to cloud formation, acting as condensation nuclei for the main cloud decks (e.g., James et al., 1997). Yet, for nearly five decades, their origin and composition have remained unknown, as previous models treated the haze as sulfur allotropes with fixed boundary conditions rather than simulating its formation.

In this study, we employ the Simulator of Particle Evolution, Composition, and Kinetics (SPECK) (Karyu et al., 2025) to model the lifecycle of Venusian clouds and aerosols, focusing on the role of meteoric smoke particles (MSPs) as an involatile core component. Our approach simulates the nucleation and condensation of sulfuric acid (H₂SO₄), water (H₂O), and elemental sulfur (S₈) without prescribing particle distributions at the lower boundary. The meteoric smoke particles with olivine composition are injected at the model top at 100 km as particles with a 1-nm radius. Given the large uncertainty in the meteoric influx at Venus, we performed a sensitivity analysis varying the MSP influx from 0 to 1000 tons day^-1 around our 10 tons day^-1 baseline derived by Carrillo-Sánchez et al. (2020).

Our results demonstrate that the lower haze layer is a direct consequence of the cosmic dust input. First, MSPs are scavenged by sulfuric acid droplets at high altitudes and transported downward via sedimentation. As these droplets cross the cloud base, H₂SO₄ evaporates, leaving behind residual MSP cores. Then, these involatile particles coagulate to form the observed lower haze. We find that an MSP influx of 3 to 300 tons day^-1 accurately reproduces the effective radii (0.08–0.21 µm) reported by PV and Venera probes. This flux range encompasses terrestrial MSP injection flux between 5 and 60 tons day^-1.

Furthermore, these MSP-derived aerosols may be a source of iron for the unknown UV absorber. Iron species, such as FeSO₄ (Jiang et al., 2024) and FeCl₃ (Egan et al., 2025), potentially formed in H₂SO₄ droplets, have been shown to have a similar UV absorption feature as the Venus cloud top. The MSP influx delivers metal species, including iron, to the atmosphere, which accumulate as the lower haze. A fraction of these iron-bearing particles can be transported upward by diffusion, where they are incorporated into the cloud droplets to form the UV-absorbing compounds. The EnVision mission will be an ideal opportunity to check this hypothesis by monitoring the behavior and distribution of the UV-absorbing materials.

The detail of this research can be found in Karyu et al. (2026) (https://doi.org/10.1038/s41550-026-02843-4).

 

References: Knollenberg & Hunten, 1980, J. Geophys. Res.; Titov et al., 2018, Space Sci. Rev.; James et al., 1997, Icarus; Karyu et al., 2025, Earth Space Sci; Carrillo-Sánchez et al., 2020, Icarus; Jiang et al., 2024, Sci. Adv.; Egan et al., 2025, ACS Earth Space Chem.; Karyu et al., 2026, Nat. Astron.