Exploring the variability of the meteoric metal layers in the Venusian atmosphere

The rate of injection of cosmic dust into the Earth’s atmosphere is estimated to be 28 ± 16 tonnes per day. As this dust ablates between altitudes of 80 and 105 km, it generates neutral and ionized metal layers. These layers have been characterized through space-based spectroscopy, ground-based lidar observations, and rocket-borne mass spectrometry. A cosmic dust input rate of 31 ± 18 t d⁻¹ is estimated for Venus [Carrillo-Sanchez et al., 2020]; this material should ablate and form metal layers between 105 and 120 km. However, these layers have not yet been observed on Venus.   

In this study, we model the latitudinal and diurnal variability of the Venusian metal layers (mainly Na, Mg, Fe and SiO), and assess the probability of their spectroscopic detection. For the simulations, the whole atmosphere Venus Planetary Climate (PCM) Model was used, with the metals being injected into the upper atmosphere through an orbitally varying Meteoric Input Function (MIF). The model also incorporates detailed neural and ion-molecule atmospheric chemistry networks for Fe, Mg, Na and Si, and was run for over two Venus years.   

A pronounced diurnal variability in the Venusian metal layers is predicted: the neutral metal atom layers peak on the night side, with maxima occurring around the morning terminator, when neutral metals have had the longest time to build up through the neutralization of their corresponding metal ions. Latitudinal variability in metal column density is highly correlated with Venusian circulation, driven by strong meridional and zonal winds. The Fe, Na, Mg and SiO layers peak at different altitudes, which results in species-dependant latitudinal distributions shaped by the horizontal winds.  

The metal layers are potentially observable in the atmosphere of Venus. In particular, the predicted Na layer should be detectable from terrestrial telescopes, either via solar-pumped resonance fluorescence or via occultation at 589 nm. Venusian Na should produce a particularly strong signal at the morning terminator in the northern hemisphere, consistent with the variability highlighted above. Different observational techniques will be discussed.    

Finally, we emphasize the significance of detecting metal layers in Venus’s CO₂-rich atmosphere, in contrast to Earth’s O₂–rich atmosphere. Knowledge of the metal layers on Venus should provide a useful framework for probing the atmospheres of Venus-like exoplanets. In general, the distribution and behaviour of meteoric sodium in exoplanetary atmospheres merits further exploration, as indicated by the growing number of sodium detections in the atmospheres of Hot Jupiters and Hot Neptunes.