First observation of the magnetic dipole CO2 absorption band at 3.3 μm in the atmosphere of Mars by the ExoMars Trace Gas Orbiter ACS instrument

The atmosphere of Mars is dominated by CO2, making it a natural laboratory for studying CO2 spectroscopy. The Atmospheric Chemistry Suite (ACS) on board the ExoMars Trace Gas Orbiter uses solar occultation geometry to search for minor atmospheric species. During the first year of ACS observations, the attention was focused on the spectral range covering the methane nu(3) absorption band, 2900-3300 cm(-1), which has previously been observed on Mars. No methane was detected by ACS; instead, an improvement of the data processing has led to the identification of 30 weak absorption lines that were missing from spectroscopic databases. Periodic series of absorptions up to similar to 1.6% deep are observed systematically around the position of the methane Q-branch when the line of sight penetrates below 20 km (creating an optical path length of 300-400 km, with an effective pressure of a few millibar). The observed frequencies of the discovered lines match theoretically computed positions of the P-, Q-, and R-branches of the magnetic dipole and electric quadrupole 01111-00001 (nu(2) + nu(3)) absorption bands of the main CO2 isotopologue; neither band has been measured or computed before. The relative depths of the observed spectral features support the magnetic dipole origin of the band. The contribution of the electric quadrupole absorption is several times smaller. Here we report the first observational evidence of a magnetic dipole CO2 absorption.