Thermal structure and aerosol content in the martian atmosphere from ACS-TIRVIM on board ExoMars/TGO.

The ExoMars Trace Gas Orbiter (TGO), a mission by ESA and Roscosmos, was launched inMarch 2016 and reached its final, near-circular 400 km orbit around Mars in March, 2018. Onboard TGO, the Atmospheric Chemistry Suite (ACS) is a set of three spectrometers includinga thermal-infrared channel, TIRVIM. This Fourier-transform spectrometer covers the range600–6000 cm−1 (1.7–17μm) with a spectral resolution of 1.2 cm−1. We focus here on nadirobservations in the range 600-1300 cm−1, which covers absorption by CO2 (centered at 667cm−1), water ice clouds (centered at 820 cm−1) and dust (centered at 1100 cm−1).The advantage of the TIRVIM data set over previous instruments comes from theTGO orbit, which is not sun-synchronous and was designed to sample a completedaily cycle every 55 days. Hence, TIRVIM has the capacity to uniquely study boththe diurnal and seasonal variability of the thermal structure, dust and ice cloudopacity. We have developed a radiative transfer model coupled to a retrieval algorithm thatexploits TIRVIM spectra to simultaneously retrieve vertical profiles of the temperature from 5 to 45 km, surface temperature, and integrated optical depth of dust and water ice clouds.This algorithm was tested and validated against synthetic observations generated undervarious conditions (local time, seasons, latitude, aerosol load,...). We have thenapplied this algorithm to the first 45 days of TIRVIM data acquired in March-April,2018. The retrieved temperature profiles have been validated against thousandsof co-located measurements acquired by the Mars Climate Sounder around 3amand 3pm, a limb-viewing radiometer onboard Mars Reconnaissance Orbiter. Theagreement between the two data sets is very satisfactory. In particular, the amplitudeof the diurnal thermal tide as seen by MCS is very well captured by TIRVIM atpressures greater than 10 Pa. However, TIRVIM underestimates it in the range 1–10 Pa,which is due to a much poorer vertical resolution in this region (as expected froma nadir-viewing sounder). We will present these results with an emphasis on thediurnal cycle of the atmospheric temperature and the aerosols load, along withcomparisons with predictions from the LMD Mars GCM. Analysis of TIRVIM dataacquired during the dust storm in June-July 2018 is in progress and will also bepresented. Acknowledgements: The ACS experiment is led by IKI in Moscow. We acknowledgesupport by CNES and Roscosmos. [ABSTRACT FROM AUTHOR]