New insights into the mesosphere of Venus from MERTIS measurements during the two BepiColombo flybys

<p>Analyses of measurements made by MERTIS¹ (<strong>ME</strong>rcury <strong>R</strong>adiometer and <strong>T</strong>hermal <strong>I</strong>nfrared <strong>S</strong>pectrometer) during the BepiColombo mission's close flyby 2 of Venus (FB2) have already demonstrated the instrument's capacity to explore the planet's mesosphere at near equatorial latitudes. The MERTIS instrument was designed to study the hot surface of Mercury. It performed well beyond its design limits when analyzing the Venusian mesosphere because of the much lower radiances. MERTIS&#8217; measurements are the first spectrally resolved observations of Venus in the thermal spectral range longward of 5 &#181;m since the Venera-15 Fourier spectrometer experiment in 1983². It could be shown that MERTIS FB2 data enable reliable retrievals of mesospheric temperature profiles and cloud parameters between 60 and 75 km altitude. They are in good agreement with the results of the Venera-15 mission. This indicates the stability of the Venusian atmosphere on time scales of decades^3,4.</p> <p>In this paper we discuss preliminary results from MERTIS measurements of the first flyby (FB1) in October 2020. During the first flyby the spacecraft approached the planet from the solar direction over the dayside. The closest approach (CA) occurred at about 11,000 km distance above the evening terminator of the planet, and then the spacecraft moved away from the planet to the anti-solar direction. In this time the apparent size of Venus increased from slightly larger than one MERTIS pixel (0.7 mrad) to more than 1 degree. MERTIS performed close-up dayside observations from early morning to late afternoon via noon time on Venus at latitudes between 50&#176;S and 85&#176;N and obtained about 785,000 hyperspectral observations with the spectrometer channel. Thus, FB1 observations permit much larger latitude coverage from 50&#176;S to 85&#176;N compared to FB2. To process the FB1 data in terms of both a reasonable signal-to-noise ratio and comparable observing conditions, individual spectra were averaged over 5&#176; latitude belts and 0.5 h local time intervals. We further excluded extreme observation geometries for latitudes northward of 80&#176;N and southward of 40&#176;S as well as very weak spectra. As a result, we are able to generate a reliable data base for use in radiative transfer analyses for the Venusian mesosphere. We present preliminary results on the temperature fields of the mesosphere as a function of local time, altitude, and latitude.</p> <p>&#160;</p> <p>Hiesinger, H. et al. Space Sci. Rev. <strong>216</strong>,<strong> </strong>6 (2020).</p> <p>Oertel, D. et al. Adv. Space Res. <strong>5,</strong> 25-36 (1985).</p> <p>Arnold, G. et al., SPIE Optic+Photonics, San Diego, Proceedings Volume 12233, doi.org/10.1117/12.2632548 (2022).</p> <p>Helbert, J. et al. submitted to Nature Astronomy (2023).</p>