Thanks to the Marie Skłodowska-Curie fellowship, much progress could be made in the characterization of the Venus mesosphere using the SOIR instrument onboard Venus Express. Most of the goals of the proposal could be reached, or about to be reached. Hereafter is a list of all the topics that were studied, with the status of the work in case the work was not finished yet.
I used the SOIR mean carbon monoxide (CO) and carbon dioxide (CO2), and data from previous missions (Pioneer Venus) regarding helium (He) to constrain the eddy diffusion coefficient in the Venus mesosphere and thermosphere, from 85 km up to 200 km. I used a 1-D (in the vertical direction) photochemical diffusion model encompassed in a fitting routine to model the Venus atmosphere to adjust the eddy diffusion profile to best fit the SOIR mean CO and CO2 profiles, and the Pioneer Venus He and CO2 profiles. The eddy diffusion profile could be used in Global Climate Models (GCM), and photochemical models.
I also worked on the development of a specific technique to detect and derive the abundance of minor species that have been never detected in the Venus mesosphere using the SOIR/Venus Express transmittances. Those species are SO2, SO3, CS, CS2, OCS, H2S, and HOCl, the latest playing an important role in the sulfur cycle based on Venus GCMs. These species have spectral absorption signatures very close to the detection limit of the SOIR instrument, which means that the signature cannot be seen with a bare eye, as it is of the same order of magnitude as the instrument noise level. I decided to tackle the problem using a technique based on the statistical criteria. This technique was used to decide whether a detection of the targeted species was obtained, or only an upper limit was computed.
In preparation for the publication of the correlation between minor species in the Venus mesosphere detected by SOIR/Venus Express, I decided to publish the final version of the SOIR detected species density database and the temperature database that was created and validated before the start of the SOIROpenVenus grant.
In this work, I present the number density and temperature profiles of all the species that have clear spectral signatures in the SOIR spectra: CO2, CO, H2O, HDO, H35Cl, H37Cl, and HF. For each species and the temperature, I also compute the mean profile considering the whole database, for the morning and evening observations, and per latitude bin (0°-30°, 30°-60°, 60°-80°, and 80°-90°).
Another work focused on the H2O and HDO profiles that were measured simultaneously by SOIR. The D/H ratio in Venus's atmosphere is larger than the one on Earth by a factor of 140, which shows that water vapor escape from Venus was much larger than on Earth. Indeed, H2O escapes easier than HDO because of the slightly larger weight of the molecule. From the SOIR measurement, we observe that just above the cloud top at 70 km, the ratio is equal to the literature value, and this ratio increases with altitude by a factor of 100 above 100 km. In this work, I study the whole SOIR water database and try to explain the results by comparing them with a 1D vertical photochemical diffusion model, including CO2, H2O, and HDO photochemical reactions.
I also worked on the detection of minor non-sulfur species. This work is still in progress, as it is tackling the detection of never-measured minor species in the Venus mesosphere: H2CO, HCN, NH3, N2O, NO2, NO, and HO2, and O3, the latest was already detected by Evdokimova et al. (2021).
I performed a study of the correlation between the different species measured by SOIR in the Venus mesosphere, focusing on CO, H2O, HDO, HCl, HF, and temperature. I computed error-weighted regressions of couples of species column densities over each pressure region.
During the time frame of this project, four publications were accepted in peer review journal (wherein three as first author), one is currently under review, and one is in preparation. Ten conferences where attended, during which 9 presentations/posters were presented as first author, and 5 as co-author (Venus Science Today 2020 in September 2020, EGU 2022 Conference in May 2022, 2022 EPSC in October 2022, 2022 AGU in December 2022, EGU 2023 Conference in May 2023, Envision Conference in May 2023, 12th Atmospheric Limb Workshop in May 2023, 2023 IUGG annual meeting in July 2023, EANA 2023 Conference in September 2023, and 2023 DPS/EPSC conference in October 2023).
