Spectroscopic studies of Venusian atmosphere by BepiColombo's MERTIS instrument during the two Venus flybys
Authors: Arnold Gabriele , Haus Rainer , Helbert Joern , D'Amore Mario , Maturilli Alessandro , Hiesinger Harald 
Published: September 2022

&lt;p&gt;&lt;strong&gt;Introduction&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;The MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) onboard BepiColombo is designed to study and map Mercury surface mineralogy and temperatures. It combines an imaging push-broom infrared spectrometer (TIS, 7-14 &amp;#181;m) with a radiometer (TIR, 7-40 &amp;#181;m) that share the same in-flight optics, electronics, and calibration unit [1]. During the two Venus flybys of BepiColombo spacecraft in October 2020 and August 2021, MERTIS-TIS acquired several hundred thousand spectra in the mid-infrared range with spectral resolution down to 90 nm [2, 3, 4]. The MERTIS measurements were the first spectrally resolved observations of Venus in the mid-IR range since the Venera 15 mission in 1983 [8]. We report on the results of MERTIS mid IR measurements and show average brightness temperature spectra. Basing on a multi-channel radiative transfer simulation and retrieval algorithm we extract atmospheric temperature profiles and cloud parameters of Venus&amp;#8217; upper troposphere and mesosphere.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Method&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;A multi-channel radiative transfer simulation and retrieval algorithm has been adapted to investigate MERTIS-TIS data and to extract atmospheric temperature altitude profiles and cloud mode factors MF&lt;sub&gt;1/2 &lt;/sub&gt;of Venus&amp;#8217; atmosphere. The Radiative Transfer Model (RTM) includes the discrete ordinate package DISORT [5]. It considers absorption, emission, and multiple scattering by gaseous and particulate atmospheric constituents and makes simultaneous use of information from different spectral ranges of an individual spectrum [6,7]. Mesospheric temperature profiles are retrieved using Smith&amp;#8217;s relaxation method [8]. It employs initial and measured brightness temperatures in the 15 &amp;#181;m CO&lt;sub&gt;2&lt;/sub&gt; band. The results of the MERTIS measurements were compared with mid-infrared data recorded by the FS-1/4 spectrometer (PMV) aboard the Venera-15 mission in 1983 [8].&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Results&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;We show that Venusian mesospheric parameters like temperature profiles and cloud properties can reliable determined from MERTIS flyby data (Fig. A). The short-wave edge of the 15-&amp;#181;m CO&lt;sub&gt;2&lt;/sub&gt; band was used to determine atmospheric temperature profiles in an altitude range of about 58-77 km (Fig. B). The retrieved temperature profiles well agree both with VIRA [9] and corresponding profiles obtained from Venera 15 PMV data.&amp;#160;&lt;/p&gt;
&lt;p&gt;Atmospheric cloud factors were investigated from MERTIS spectral data around 10 &amp;#181;m. They are also well coinciding with earlier analyses.&amp;#160;&amp;#160; Still ongoing refinements of calibration will allow the use of data between 7 and 10 &amp;#956;m for the analysis of further atmospheric properties.&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;Fig. MERTIS-TIS FB2, A: Comparison of measured and simulated MERTIS brightness temperature spectra, B: Comparison of initial and retrieved temperature profiles.&amp;#160;&quot; width=&quot;11,2'&quot; height=&quot;7,7'&quot; /&gt;&lt;img src=&quot;&quot; alt=&quot;Fig. MERTIS-TIS FB2, A: Comparison of measured and simulated MERTIS brightness temperature spectra, B: Comparison of initial and retrieved temperature profiles.&quot; width=&quot;470&quot; height=&quot;322&quot; /&gt;&lt;/p&gt;
&lt;p&gt;Fig. MERTIS-TIS FB2, A: Comparison of measured and simulated MERTIS brightness temperature spectra, B: Comparison of initial and retrieved temperature profiles.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;
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&lt;p&gt;[2] Maturilli, A., et al., 52&lt;sup&gt;nd&lt;/sup&gt; LPSC, 1435, (2021).&lt;/p&gt;
&lt;p&gt;[3] Helbert, H., et al., 52&lt;sup&gt;nd&lt;/sup&gt; LPSC, 1521, (2021).&lt;/p&gt;
&lt;p&gt;[4] Arnold, G., et al., SPIE Optics+Photonics, IR Remote Sensing and Instrumentation XXIX, 1183009, (2021).&lt;/p&gt;
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&lt;p&gt;[6] Haus, R., Kappel, D., Arnold, G., Icarus 232C, 232-248, (2014)&lt;/p&gt;
&lt;p&gt;[7] Haus, R., Kappel, D., Arnold, G., Planet. Space Sci. 89, 77-101, (2013).&lt;/p&gt;
&lt;p&gt;[8] Smith, W.L., Appl. Opt.9(9), 1993-1999, (1970).&lt;/p&gt;
&lt;p&gt;[8] Oertel, R. et al., Adv. Space Res. 5(9), 25-36, (1985).&lt;/p&gt;
&lt;p&gt;[9] Zasova, L.V. et al., Cosmic Res. 44(4), 364-383, (2006).&lt;/p&gt;