John Maurice Campbell Plane

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 .

The kinetics of I+ + O3, IO+ + O3, and IO+ + O are measured using a selected-ion flow tube apparatus over the range of 160–300 K. Recommended rate constants (cm3 s–1) are: I+ + O3 k = 8.9 ± 1 × 10–10 (T/200 K)−0.15; IO+ + O3k = 8 ± 2 × 10–11; IO+ + O k = 1.6 ± 0.5 × 10–10 (T/200 K)−2 cm3 s–1. The IO+ + O3 reaction yields primarily IO2+ + O2, with an upper limit on I+ + 2O2 of 10–11 cm3 s–1. The results are supported by density functional calculations. A previously reported implementation of iodine chemistry using the Whole Atmosphere Community Climate Model is updated with these values and the chemistry of iodine in the upper atmosphere is discussed.

The upper haze region of the Venusian atmosphere (∼70–100 km) experiences localized cooling, probably due to gravity waves, creating transient cold pockets. Water ice clouds may form in these cold pockets when sulfuric acid droplets undergo hygroscopic growth and subsequent freezing. However, the freezing of sulfuric acid droplets under conditions relevant for the Venusian upper haze region has not been previously investigated (∼2 O ice nucleation in H ₂ SO ₄ solution droplets using a cryomicroscope, where droplet emulsions (micron‐scale droplets) were created using a microfluidic device. Crystallization was observed down to ∼160 K at conditions close to those predicted by the water activity criterion, but crystallization was increasingly restricted by slow crystal nucleation and growth rates as the solutions became ultra‐viscous or glassy at lower temperatures. Combining our measurement with satellite retrievals of temperature and water suggests that H ₂ O ice may form homogeneously in liquid aqueous H ₂ SO ₄ droplets or heterogeneously in glassy aqueous H ₂ SO ₄ droplets. Resulting H ₂ O ice crystals are unlikely to grow larger than ∼1 μm and will therefore persist in the upper haze layer of Venus forming detached layers. In extreme temperature minima, CO ₂ ice will likely nucleate heterogeneously on H ₂ O ice, followed by rapid growth, sedimentation, and sublimation, resulting in downward transport of H ₂ SO ₄ , H ₂ O, and other materials. Overall, our findings highlight the potential for cirrus‐like clouds to form on Venus, but the extent to which they form strongly depends on temperature retrievals that carry a high degree of uncertainty.

The thermal structure and water cycle in the lower atmosphere of Mars are thought to be controlled by dust particles supplied from the surface, acting as cloud condensation nuclei (CCN) for water ice clouds. However, the presence of water ice clouds in the mesosphere (Bernal et al., 2021) suggests that additional CCN sources may be required. Meteoric smoke particles (MSPs) are thought to be these sources, which are composed of carbonates formed by the reaction of atmospheric molecules with metal atoms ablated from meteoroids as they enter the atmosphere (Plane et al., 2018).Due to their small size and difficulty in observing them, the distribution and function of MSPs as CCN remain unclear. Previous model studies have investigated cloud formation associated with MSPs (Hartwick et al., 2019). However, due to computational constraints in the GCM, these studies assumed that MSPs are supplied only from the upper boundary of the model, located at approximately 60 km altitude. As a result, they could not explicitly represent the 60–100 km altitude region, where meteoric metal ablation and subsequent chemical reactions responsible for MSP formation are expected to occur.Here we focus on meteoric Mg species commonly observed in the Martian atmosphere (Crismani et al., 2023) and investigate their role in mesospheric cloud formation using two 1-D models: a photochemical model, PROTEUS (Nakamura et al., 2023), to evaluate the formation process of MSPs, and a cloud microphysics model, SPECK (Karyu et al., 2025), to track the evolution of the particle size distribution.Clouds formed when CCN was supplied according to the altitude distribution of carbonates calculated by PROTEUS, but no clouds formed when CCN was supplied only from the upper boundary at an altitude of 200 km. This implies that nm-sized MSPs are not subject to sedimentation effects and that it is important to track the chemical reactions involved in the formation of MSPs.These results support MSPs as effective condensation nuclei for Martian mesospheric water ice clouds and provide a quantitative link between meteoric metal chemistry and cloud microphysics.

The atmosphere of the planet Venus contains an extensive sulfur cycle, with permanent sulfuric acid (H2SO4) clouds and haze layers spanning altitudes from 48 km to ~90 km. H2SO4 is formed from SO2 near the cloud top. Observations of SO2 in the Venusian atmosphere show concentrations of ~100 ppm below the sulfuric acid clouds and 0.1 – 1 ppb at the cloud top. Known chemistry is insufficient to produce the observed decrease in SO2 and no chemical models have successfully reproduced the three orders-of-magnitude decrease. The precise cause of the SO2 decrease remains unknown. Many chemical processes have been proposed that could contribute to the removal of SO2, including conversion to other sulfur reservoir species such as polysulfur (S2, S3, S4, S8, etc.) or polysulfur oxides (SnO); oxidative uptake to droplets by H2O2 or O3; increased uptake due to buffering of the pH of the cloud droplets by dissolved salts, and reaction with NOx species.Two possible routes for SO2 + NOx chemistry have been postulated: reactive uptake [1] and the lead chamber process [2]. [1] reported single droplet levitation experiments which showed droplet growth only when SO2 and NO2 were both present. [2] proposed that NO and NO2 could catalyse SO2 conversion to SO3, leading to conversion to sulfuric acid by reaction with water, in a process analogous to the Lead Chamber process previously used to produce sulfuric acid for industrial use on Earth.We present a newly developed droplet chemistry module in the Venus Planetary Climate Model, which extends the gas phase chemistry scheme to permit aqueous and droplet surface reactions. Gases condense to the cloud droplets according to their Henry’s law solubilities, and react with other species, which are then released from the droplets when they exceed their solubility limits. We use this new scheme to test both reactive uptake of SO2 and NO2 and the “lead chamber” process. In both of these cases, we predict the increases required to the reaction rates or the NOx reactant concentrations to fully explain the observed SO2 decrease through the clouds.References:Ubukata, S., et al., Uptake of SO2 into sulfuric acid droplets through the oxidation by NO2 under Venus-analogous conditions. ACS Earth and Space Chemistry, 2025. 9(6): p. 1525–1533.Sill, G.T., The clouds of Venus: Sulfuric acid by the lead chamber process. Icarus, 1983. 53(1): p. 10–17.

This study provides a comprehensive description of the meteoric cycle on Venus, considering the temporal, latitudinal, and orbital variations of meteoroid mass influx before atmospheric entry, the ablation rates and the subsequent ionization of meteoric elements in the upper atmosphere, along with the accretion of micrometeorites at the surface. The study integrates three models: (i) the NASA-GSFC Meteoroid Input Function for Venus, which characterizes the size, velocity, and radiant distributions of the Jupiter-family comets (JFCs), main-belt asteroids, Halley-type comets (HTCs), and Oort-cloud comets (OCCs); (ii) the University of Leeds Chemical Ablation Model, which computes ablation rates of 12 meteoric species based on entry parameters; and (iii) the Electron Linear Density model, which estimates the ionization probabilities from hyperthermal collisions with atmospheric molecules. The total meteoroid mass influx is shown to peak during Venus’ ecliptic crossings (∼31.8 × 10 ³ kg Earth-day ⁻¹ at the second crossing) and reaches a minimum around the minimum and maximum vertical displacements (∼24.2 × 10 ³ kg Earth-day ⁻¹ ). The JFCs dominate pre-atmospheric mass input and largely drive variability along Venus’ orbit, while ablation is primarily contributed by the HTCs and OCCs. Ablation exhibits a pronounced diurnal asymmetry at equator, with enhanced injection on the dayside hemisphere around a height of 118 km. Neutral species account for 83.1% of ablated species, while ionization dominates above ∼119 km, particularly for Na and K. JFCs supply ∼90% of micrometeorites deposited at the surface, being the maximum accretion rates of organics at the second crossing between ∼0.3 × 10 ³ kg Earth-day ⁻¹ (CI-chondrite) and ∼1.7 × 10 ³ kg Earth-day ⁻¹ (cometary enrichment).

We introduce the Whole Atmosphere Community Climate Model with Regional Refinement (WACCM‐RR) and study the impact of directly resolving gravity waves (GWs) in the mesosphere and lower thermosphere (MLT; 70–140 km). Two simulations of 2010 are compared: “Non‐RR” is a standard WACCM case with a horizontal resolution of ∼1° (111 km) globally; and “RR” is a WACCM‐RR case with a ∼1/8° (14 km) nested grid over the contiguous United States (CONUS) and a 1° grid outside the nest. More GWs are resolved in RR and can be connected to tropospheric sources. Enhancements of downward fluxes of heat, O, NO, and CO are seen near the RR mesopause, doubling those of Non‐RR, which typically reduce these quantities in the lower thermosphere and increase them in the mesosphere. Temperature increases of 10–15 K, O by factors of 1.5–3, NO by 1.5–2.5, occur around 10 ⁻³ hPa (95 km), and a 10%–20% CO decrease is seen as much of it is transported below 10 ⁻² hPa (80 km). Enhanced upward CO ₂ fluxes are observed, with CO ₂ increasing by 50% at 10 ⁻⁵ hPa (125 km). Additionally, O ₃ mixing ratios double at the secondary maximum where a doubling of O is seen. Compared to vertical heat flux estimates from Na LiDAR within CONUS, larger RR fluxes remain smaller than observed. Despite this, WACCM‐RR enables MLT GW studies and may address long‐term discrepancies between models and observations at significantly lower cost than global high‐resolution simulations.

Mercury is a persistent pollutant with significant public health impacts in polar regions where fish consumption drives human exposure. Atmospheric oxidation pathways control where mercury deposits globally, but the lack of molecular-level observations of oxidized mercury products has hindered the validation of proposed chemical mechanisms. Here, we show the in-situ online detection of individual mercuric halides (HgCl₂, BrHgCl, HgBr₂, ClHgI, BrHgI, and HgI₂) in the polar boundary layer using atmospheric pressure chemical ionization mass spectrometry. Our observations identify HgBr₂ as the dominant oxidized mercury species at both poles, while HgCl₂ and other halides were also observed in Antarctica. The observed speciation diverges from current model predictions, which favor HgCl₂ and HOHgBr as dominant oxidized forms. Our results show that real-time molecular measurements can substantially advance global mercury monitoring and improve the chemical models used to assess environmental policies and predict deposition patterns.

The Mesosphere and Lower Thermosphere (MLT, ~70–120 km) is a key transition region governing the coupling between the lower atmosphere and near-Earth space. Despite its central role in atmospheric chemistry and dynamics, the MLT remains one of the least observed domains, leading to large uncertainties in composition, temperature, density, and winds, particularly near the mesopause and below the turbopause. A long-standing “holy grail” of MLT research is the direct, global, and time-resolved measurement of atomic oxygen, the dominant energy carrier controlling the chemistry and thermal balance of the region, which has remained inaccessible until recent advances in terahertz (THz) receiver technology.Keystone is one of the four ESA Earth Explorer 12 candidate missions and is currently undergoing Phase-0 science and system studies. Its primary scientific objective is to provide comprehensive, global, and time-resolved measurements of MLT chemistry, temperature, and dynamics, enabling improved understanding of vertical coupling and wave–mean flow interactions involving gravity waves, tides, and planetary waves from diurnal to seasonal timescales. The mission’s core payload is a high-spectral-resolution supra-THz (1–5 THz) radiometer, complemented by infrared and UV–visible limb instruments. Keystone will retrieve vertical profiles of key neutral species, including direct global measurements of atomic oxygen, together with temperature profiles and mesospheric winds derived from Doppler-shift spectroscopy. These simultaneous observations of neutral dynamics and composition also support improved understanding of the drivers of ionospheric variability, including the neutral wind dynamo governing electrodynamics in the E-region.Beyond its fundamental science goals, Keystone addresses an important societal challenge. Improved constraints on MLT density and temperature provide physically consistent lower-boundary conditions for thermospheric density models used in satellite drag prediction. By reducing uncertainties propagated upward into the thermosphere, such constraints are expected to yield order-tens-of-percent improvements in residual drag and orbit propagation accuracy, supporting safer and more sustainable operation of the increasingly congested low and very-low-Earth-orbit environment.

The influx of anthropogenic metals into the atmosphere is expected to increase substantially due to the rapid growth of the space industry. More than 20 elements from re-entering spacecraft have been identified in sulphuric acid droplets in the Junge layer, with several estimated to surpass the background level from cosmic dust. While the atmospheric impact of these particles is uncertain, they have been widely hypothesised, including ozone destruction, increased polar stratospheric cloud formation, harmful surface deposition, a perturbed radiative balance and in turn, changes to global circulation.The spacecraft ablation process and subsequent formation of space debris particles (SDPs) are not well defined. The dominant constituent of spacecraft is aluminium. If vaporised, aluminium is expected to undergo a series of reactions to form aluminium hydroxide (Al(OH)3). The initial form and size of the particles will strongly influence the coagulation, global transport, and atmospheric lifetime of the particles. Constraining these factors is vital to accurately assessing the impact SDPs have on the atmosphere.This work provides an update on the work presented at EGU2025 (Egan et al., Modelling impacts of ablated space debris on atmospheric aerosols, EGU25-4460), using the Whole Atmosphere Community Climate Model with the Community Aerosol and Radiation Model for Atmospheres (WACCM-CARMA) to simulate the production and transport of SDPs. This work investigates the sensitivity of the initial particle radius to the transport, lifetime and surface deposition of particles.