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

Cosmic dust injects around 28 ± 16 tons per day of material in the atmosphere of Earth; this dust ablates between 80 and 105 km, generating neutral and ionized metal layers. These layers have been detected by space-based spectroscopy, as well as ground-based lidar observations and rocket-borne mass spectrometry. The same cosmic dust is estimated to deliver 31 ± 18 t d⁻¹ of material in the atmosphere of Venus [Carrillo-Sanchez et al., 2020], which 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 metal layers (Na, Mg, Fe and Si) on Venus, and we analyse the probability of their spectroscopic detection. For this investigation, we used the Venus Planetary Climate (PCM) Model; the metals are injected in the upper atmosphere of Venus through an orbitally varying Meteoric Input Function (MIF) under development for Venus with detailed neural and ion-molecule atmospheric chemistry for Fe, Mg, Na and Si. The model is run for over one Venus year.  

This work indicates there is pronounced diurnal variability in the metal layers on Venus. Our analysis shows neutral metal layers peak in concentration on the night side, with a maximum before the morning terminator – where the metals have had the longest time to accumulate. The latitudinal variability in the metal column density is highly correlated with the circulation on Venus, governed by strong meridional and zonal winds. The metal layers peak at different altitudes, resulting in varying latitudinal concentrations depending on the metal analysed. For example, Mg peaks 10 kg higher than Fe and tends to concentrate in the northern hemisphere, while Fe tends to concentrate in the southern hemisphere; this is caused by varying wind direction and speed at different altitudes. 

In conclusion, our study suggests the metal layers are present and potentially observable in the atmosphere of Venus. In particular, we focus on the Na layer as it should be detectable from a terrestrial telescope observing solar-pumped resonance fluorescence at 589 nm; Na should produce a particularly strong signal at the morning terminator in the northern hemisphere, due to the variability highlighted above. We outline a VLT ESO proposal we submitted with the aim of detecting sodium on the morning terminator on Venus, along with any available preliminary results.   

Finally, we emphasize the importance of detecting metal layers in the CO₂-rich atmosphere of Venus, in contrast with the O₂–rich atmosphere of the Earth. Knowledge of the metal layers on Venus is a helpful tool to probe the atmosphere of Venus exoplanet analogues, which appear to be abundant in the universe. In general, the distribution and behaviour of meteoric sodium in exoplanetary atmospheres is a field worth exploring, as indicated by the growing detections of Na in the atmospheres of Hot Jupiters and Hot Neptunes.