First observation of the magnetic dipole CO2 main isotopologue absorption band at 3.3 µm in the atmosphere of Mars by ACS ExoMars

The CO2-dominated atmosphere of Mars is an ideal natural laboratory to study the spectroscopy of this gas. The Atmospheric Chemistry Suite (ACS) package onboard the ExoMars 2016 Trace Gas Orbiter (TGO) sounds the atmosphere in solar occultation, allowing, in case of a very clear atmosphere, reaching an optical path of 300-400 km at an effective pressure of a few millibars. During the first year of ACS observations, the focus of attention was kept on the spectral range covering the fundamental methane absorption band, 2900-3300 cm–1. No methane was detected, while a further improvement of the data processing led to the identification of weak periodic absorption lines, missing from spectroscopic databases. The observed frequencies of the lines match theoretically computed positions of the Q, P and R branches of the magnetic dipole 01111-00001 absorption band of the main CO2 isotopologue, never measured or computed before. We will report the first observational evidence of a magnetic dipole CO2 absorption. The data analysis was supported by RSF (project No. 20-42-09035).