Do studies of the Venusian mesosphere provide important information about the current state of the entire Venusian atmosphere? Why does this information about the structure of dense clouds, its vertical thermal characteristics, temperature fields, and dynamic and meteorological processes provide? Does it help to understand more deeply the different evolutionary paths of the climate of Earth and Venus? Why were the last measurements obtained in 1983 during the Venera-15 mission? Why do our results not agree well with the findings of the Venera-15 mission? Why has the stability of the Venusian atmosphere changed on time scales of several decades?

With Venus and Earth we have two planets in the solar system that formed in the same region of the solar nebula have similar size, but evolved dramatically different. While Earth developed into a habitable planet, Venus took a divergent development path. Understanding the divergent evolution of Earth and Venus especially in the context of potentially Earth like exoplanets is major challenges in planetary and solar system research. Any knowledge increase in this context will bring forward our understanding of possible feedback mechanisms in the sensitive Earth’s climate system1 . Analyses of the thermal structure and cloud formation patterns are of central importance for assessing dynamical and meteorological processes and the environmental conditions prevailing on Venus. Especially the transition region between troposphere and mesosphere at about 60-70 km altitude is important for climate tracking, since it displays a strong variability with latitude and local time. An overview of the most important past experiments on remote sensing of temperature altitude profiles and cloud features in the atmosphere of Venus has been given by Haus et al.2 . Much more information originated from ESA’s Venus Express mission (VEX, 2006- 2014, Svedhem et al.3 ). In particular, the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS, Drossart et al.4 ) and the Venus Radio science experiment (VeRa, Tellmann et al.5 ) on VEX provided large data amounts on global scales. Latest data came from the Japanese Akatsuki mission (2016-2018, Nakamura et al.6 ). After its successful launch in October 2018 from the European Spaceport in Kourou, French Guiana, the ESA-JAXA mission BepiColombo is currently on its nominal 7-year journey to Mercury (Benkhoff et al.)7 . The interplanetary cruise includes nine flybys for gravitational assists: one at Earth, two at Venus and six at Mercury. The Earth flyby and the first Venus flyby (FB1) took place in April and October 2020, respectively. The second Venus flyby (FB2), on which the present work focuses, took place on August 10, 2021 with a closest approach (CA) at 13:51 UTC at a minimum altitude of 552 km (Mangano et al.)8 . In this paper, thermal infrared spectra obtained by the infrared (IR) grating spectrometer (TIS) of the MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) instrument during the second Venus flyby of the BepiColombo mission are presented and analyzed. The MERTIS observations provide not only temperature profiles but also independent information on SO2 and H2SO4 cloud aerosol properties. The retrieved temperature profile at latitude 10°N well agrees both with the corresponding VIRA (Venus International Reference Atmosphere) profile and the profile obtained from the directly comparable Venera-15 PMV (Profile Measuring Instrument for Venus) data 37 years ago. Retrieved cloud properties (mode factors MF1/2) and cloud top altitudes zt also well coincide. Results Our Venus observations were performed with MERTIS, which is an instrument designed to operate in Mercury environment. As described in (Hiesinger et al.)9 MERTIS combines a push-broom IR grating spectrometer (TIS) with a radiometer (TIR). TIS operates between 7 and 14 µm and will record dayside radiance spectra of Mercury to infer surface emissivity characteristics, whereas TIR is going to measure the surface temperature (80–700 K) at night- and dayside in the spectral range from 7 40 µm. TIR is implemented by an in-plane separation arrangement. In this configuration the two radiometer detector lines form the slit of the TIS channel which is an imaging spectrometer (Hiesinger et al.)9 . It uses the first European-built space-qualified uncooled microbolometer array. The optical design of MERTIS combines a three mirror anastigmat (TMA) with a modified Offner grating spectrometer. A pointing device allows viewing the planet (planetbaffle), deep space (space-baffle), and two black bodies at 300 K and 700 K temperature, respectively. The observations of Venus with MERTIS have been challenging. The planet Mercury exhibits dayside surface temperatures up to 700 K. Maximum infrared brightness temperatures that can be observed over the dense atmosphere of Venus are in the order of 220–260 K. This pushes the instrument to its sensitivity limits. Moreover, the team had to devise a new observation and calibration strategy to make these observations possible at all. During cruise the ESA Mercury Planetary Orbiter (MPO) and the JAXA Mercury Magnetospheric orbiter (MMO) fly in a composite configuration with a propulsion element, the Mercury Transfer Module (MTM) and a sunshade cone (MOSIF) to protect the MMO. In this configuration the nadir panel (z-axis) of the MPO points towards the MTM. Therefore, most instruments cannot operate during cruise. MERTIS has a viewport through the space baffle which is used in nominal operations for deep space calibration (Hiesinger et al.)9 . During the Venus flyby this port was used for planetary observations. It means however that the instrument had to operate in a way that it uses the deep space calibration port as its main observation port while maintaining the standard sequence that includes regular observations of the internal calibration targets. Calibration using deep space, which is the key to obtain the thermal radiation coming from the instrument itself, was obtained before and after the flyby. The whole observing timeline and the instrument observation procedure had been tested during several instrument checkouts. MERTIS theoretically has an accuracy in the range of 0.03 K based on its internal calibration when using the MERTIS planetary baffle, which is optimized to reject all out-of-field radiation from a source larger than the field of view of the instrument. For the Venus observations, however, the instrument had to observe using the space port. This port has a simpler, asymmetrically shaped baffle, which means that stray light from the extended disk of Venus must be considered as an additional source of uncertainty. Since no in-flight calibration of the space port could be performed for the Venus flyby, we chose the data clustering described in the paper to minimize this error for temperature retrieval. The closer BC flyby of Venus, FB2, was explicitly suited for developing this methodology because nadirequivalent transmitted atmospheric columns could be directly compared. A more detailed analysis of FB1 in a follow-up publication may be able to place additional constraints on the effect of stray light for a larger interval of nadir observation geometries. In this paper, we focus on the comparison with the atmospheric models and previous mid-IR nadir studies of the Venusian mesosphere as the PMV measurements. We discuss these results in comparison with recent highprecision temperature profiles obtained from radio occultation measurements5,10–12. Measurements with the MERTIS hyperspectral channel (MERTISTIS) are the first spectrally resolved observations of Venus in the thermal spectral range longward of 5 µm since the Venera-15 Fourier spectrometer experiment FS-1/4 in 1983 (Oertel and Moroz, 1984; Oertel et al.)13,14. This instrument (also named PMV, Profile Measuring Instrument for Venus) obtained spectra in the 6.0-36.5 μm range at spectral resolution of 6.3 cm−1 . The MERTIS resolution of 90 nm corresponds to 18 cm−1 near 7 µm and 5 cm−1 near 14 µm. This offers the great opportunity to further test and adapt software and to compare retrieval results for atmospheric parameters of Venus. This paper elaborates on the similarities of MERTIS and PMV data. It is shown that the radiation measurements performed by the two experiments are well eligible to reliably retrieve mesospheric parameters of Venus like temperature profiles and cloud properties, and that the results largely agree. This contributes to our understanding of the atmosphere of Venus and its stability on timescales of decades. It is also one main goal of the present paper to demonstrate the capabilities of an instrument like MERTIS using an uncooled microbolometer for hyperspectral observations of relative cool objects as in case of Venus’ mesosphere.Present results create the prerequisites to investigate MERTISflyby 1 data that will be done in near future. The observation geometry of FB1 is more challenging due to the spaceship’s larger distance to Venus. As a consequence, FB1 observations permitted much larger latitude coverage from 50°S to 85°N. Observations MERTIS data analyses that are presented here are based on Venus flyby 2 measurements. Planetary radiation has been recorded through the MERTIS space baffle that was designed to perform deep-space calibration measurements at Mercury. Due to the spacecraft trajectory and the comparatively low distance to Venus (6300 km on average) most data were acquired at latitudes between 8 and 14°N and at local times between 7 and 15 h. Therefore, FB2 data contain limited information on Venus thermal structure on global scale from the outset, but nevertheless their analysis is important to validate the above discussed approach (direct comparison with PMV, preparation of FB1 data analyses). During FB2 the spacecraft approached the planet from its night side, and the closest approach (CA) occurred near the evening terminator of the planet, but slightly shifted toward late afternoon side. Then the spacecraft moved away from the planet on the dayside. MERTIS observations were performed from −4 min to +23 min around CA, from distances ranging from 13,000 km to 900 km. This covered local times between 6.0 and 18.0 h at low latitudes. MERTIS obtained around 2,200 spectrometer channel measurements as well as about 300 measurements with the radiometer channel. The TIS channel was working at full spatial resolution without any spatial binning providing 100 pixels across the track, and >900,000 single spectra of Venus could be recorded in spite of the non-standard operation procedure.Venus observations present a special challenge for the instrument, because observed brightness temperatures are much lower than Mercury dayside temperatures. The calibration procedure was enhanced to take into account deep space pre- and post- Venus spectra and adjust the image offset and stripe noise accordingly with this special observation. MERTIS has two internal blackbodies, one at instrument temperature (equipped with two redundant PT1000 temperature sensors) and the second actively heated to 700 K. During the observations the instrument every 60 s takes observations of both blackbodies to verify the calibration. During the Venus flyby the first blackbody exhibits temperatures between 282.4 K and 286.9 K as measured by the temperature sensors embedded in the blackbody surface. This is slightly higher but still comparable with range of temperatures observed at Venus. The regular calibration measurements show a difference of