The second Venus flyby of BepiColombo mission reveals stable atmosphere over decades

Understanding the divergent evolutionary paths of Earth and Venus, two similarly sized planets formed in the same region of the solar nebula, is crucial for planetary science and our understanding of potentially habitable exoplanets. While Earth became habitable, Venus took a drastically different path. Analyzing the thermal structure and cloud formation patterns on Venus, particularly the troposphere-mesosphere transition region (60-70 km altitude), is key to understanding these differences. This region exhibits strong variability in latitude and local time, making it crucial for climate tracking. Previous remote sensing studies, such as those conducted by Venera-15, Venus Express (VIRTIS and VeRa), and Akatsuki, have provided valuable data. However, the last measurements of this kind were obtained in 1983 during the Venera-15 mission. This paper aims to fill this data gap by presenting new observations from the BepiColombo mission, which is currently on its way to Mercury. The second Venus flyby (FB2) of BepiColombo, which occurred on August 10, 2021, at a minimum altitude of 552 km, offered a unique opportunity to acquire new data on the Venusian atmosphere.

Haus et al. (reference 2) provide an overview of past experiments on remote sensing of Venus' atmospheric temperature and cloud features. The ESA's Venus Express mission (2006-2014) contributed significantly, with its Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) and Venus Radio science experiment (VeRa) providing extensive global-scale data (references 3, 4, 5). The Japanese Akatsuki mission (2016-2018) also contributed latest data before this study (reference 6). This study builds upon this existing knowledge by utilizing data from the BepiColombo mission, adding a new, independent data point to this long-term investigation.

This research utilized data from the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) instrument onboard the BepiColombo spacecraft during its second Venus flyby. MERTIS, primarily designed for Mercury observations, was repurposed for these Venus observations, necessitating a novel observation and calibration strategy. The instrument's thermal infrared grating spectrometer (TIS) operating between 7 and 14 µm was used to collect data. The challenges of observing the relatively cool Venusian atmosphere (220-260 K) with an instrument designed for the much hotter Mercury surface (up to 700 K) are discussed. The data acquisition involved using the deep space calibration port instead of the planetary baffle, which introduced additional uncertainty due to stray light. A data clustering method was implemented to minimize this error during temperature retrieval. The study also compares the MERTIS data with data from the Venera-15 Profile Measuring Instrument for Venus (PMV) from 1983, which obtained spectra in the 6.0-36.5 µm range. The comparison focuses on the retrieval of mesospheric temperature profiles and cloud properties. The radiative transfer model used for data analysis included the DISORT package, which accounts for absorption, emission, and multiple scattering by atmospheric constituents. A four-modal cloud model, with H2SO4 aerosols, was employed. The retrieval technique utilized Smith's relaxation method to obtain temperature profiles, with the VIRA model providing initial temperature structure. The iterative retrieval algorithm optimizes parameters to best fit the measured and simulated radiance spectra.

The analysis of MERTIS data from the BepiColombo's second Venus flyby revealed several key findings: 1. **Stable Mesospheric Temperature and Cloud Properties:** The retrieved temperature profiles and cloud properties (mode factors MF1/2 and cloud top altitudes) from MERTIS data at 10°N latitude showed excellent agreement with both the Venus International Reference Atmosphere (VIRA) model and the Venera-15 PMV data from 1983. This demonstrates the remarkable stability of the Venusian atmosphere's equatorial region over several decades. The agreement between MERTIS and PMV data is particularly significant given that the instruments differ significantly in spectral resolution and the observation conditions. 2. **Successful Adaptation of MERTIS:** Despite being designed for Mercury observations, MERTIS successfully acquired useful data on the Venusian atmosphere. This highlights the instrument's versatility and potential for future use in similar missions. 3. **Methodological Validation:** The consistency of temperature profiles and cloud parameters retrieved using two different spectral ranges within PMV data further validated the methodology. The near-identical retrievals indicate the robustness of the retrieval algorithm even when dealing with limited spectral coverage. The close agreement between MERTIS and VIRTIS data also provides further confidence in the analysis method. 4. **Data Limitations:** Because of the spacecraft trajectory during FB2, most MERTIS data were acquired over a narrow latitude band around 10°N. This limits the study's ability to make conclusions about global atmospheric dynamics and necessitates further analysis of the FB1 data, which covered a wider range of latitudes. The challenges associated with operating MERTIS outside its design parameters caused noise in single spectra, so spectra averaging was required for analysis. 5. **Comparison with Previous Measurements:** The MERTIS observations complement existing radio occultation data from Venus Express and Akatsuki, providing independent confirmation of the atmosphere's stability, particularly in the equatorial region. Radio occultation data is sensitive to smaller-scale variations, unlike MERTIS, which is sensitive to overall profiles and cloud properties.

The findings of this study significantly advance our understanding of the Venusian atmosphere. The remarkable agreement between MERTIS data and previous measurements from Venera-15, obtained almost four decades earlier, strongly supports the long-term stability of the equatorial mesospheric region. This stability has implications for our understanding of climate dynamics on Venus and provides a valuable baseline for future studies. The successful adaptation of MERTIS for Venus observations underscores the feasibility of repurposing instruments designed for one planetary body to study another. This reduces the need for entirely new instrumentation and promotes more cost-effective missions. This study's limitations, primarily the narrow latitudinal coverage of the FB2 data, highlight the need to analyze data from FB1 to obtain a more comprehensive picture of the Venusian atmosphere's global dynamics. Future research should focus on analyzing the FB1 data, which sampled a wider range of latitudes, and incorporating these results with the findings from this study to provide a more complete picture of Venusian atmospheric stability and variability.

This paper demonstrates the successful retrieval of mesospheric temperature profiles and cloud properties from MERTIS data acquired during BepiColombo's second Venus flyby. The results confirm the remarkable long-term stability of the Venusian atmosphere, specifically in the equatorial region, over four decades. This research underscores the potential of using uncooled microbolometers for hyperspectral observations of relatively cool planetary atmospheres, opening possibilities for future missions. The limited latitudinal coverage of this study highlights the necessity of further analyses of the FB1 data and the importance of incorporating multiple, independent datasets for comprehensive planetary atmospheric studies. The findings will also greatly benefit the upcoming EnVision and VERITAS missions to Venus.

The primary limitation of this study stems from the spacecraft trajectory during the second Venus flyby (FB2), which resulted in most observations being concentrated over a narrow latitude belt (around 10°N). This limits the ability to draw conclusions about the global dynamics of the Venusian atmosphere. The use of the deep-space calibration port instead of the planetary baffle introduced additional uncertainty due to stray light. Data clustering helped mitigate this issue, but the limitation is noteworthy. Finally, the noise present in individual spectra necessitated averaging which reduced the sensitivity to finer details.