Unravelling the isotopic imprints of Martian CO2 and H2O using solar occultation observations from the Atmospheric Chemistry Suite onboard ExoMars Trace Gas Orbiter
Keywords: ExoMars TGO
Isotopic ratios
Mars' atmosphere
Mars' atmospheric escape

Numerous lines of geomorphological and mineralogical evidence suggest that liquid water was once abundant on Mars’ surface, which might also be indicative of a more hospitable environment for life to grow. The presence of large amounts of liquid water on the surface require climatic conditions very different from those enabled by the dry and thin atmosphere Mars has today. In fact, it is still not well understood how the past Martian atmosphere was able to produce sufficient greenhouse warming to sustain liquid water on the surface, nor what drove the transition of the climate to the one we observe today. Enrichment in the heavy isotopes of atmospheric species such as hydrogen, nitrogen or the noble gases with respect to Earth suggest that this transition was driven by the escape of a large portion of the atmosphere to space. Isotope ratios can provide constraints on the amount of atmosphere that existed in the past, but require a very detailed knowledge of the present day isotopic ratios and the relative rates by which the different isotopologues of each species escape to space.
To this aim, we monitor the isotopic composition of H, C and O in carbon dioxide and water vapour using sensitive infrared solar occultation measurements made by the mid-infrared channel of the Atmospheric Chemistry Suite (ACS MIR) onboard the ExoMars Trace Gas Orbiter (TGO), which makes measurement in a spectral range between 2.3 and 4.2 µm. The modelling and inversion of the spectral data is performed using the NEMESIS radiative transfer and retrieval algorithm, which is adapted to model the particularities of the measurements of ACS MIR, and retrieve the isotopic ratios along with other atmospheric parameters such as the pressure and temperature profiles.
The developed retrieval scheme is applied in three instances to constrain different aspects of the isotopic composition of the Martian atmosphere. First of all, eight solar occultation observations made during the first months of operations of the ExoMars TGO are used to measure, for the first time, the vertical distribution of the oxygen isotope ratios in Hsub>2O (18O/16O and 17O/16O). These results show that the O isotope ratios in Hsub>2O are enriched with respect to their primordial sources, which is consistent with fractionation during the escape of the Martian atmosphere into space throughout history. In the second place, more than a full Martian Year of observations are used to understand the variability of D/H and 186O/16O in water and estimate how its dissociation products are fractionated. These measurements show that while the variations of the D/H ratio in H2O are mainly driven by condensation-induced fractionation, the differential photolysis of HDO and H2O is a more important factor in determining the isotopic composition of the dissociation products (OH and H). Finally, observations encompassing strong absorption bands of carbon dioxide are used to measure the carbon and oxygen isotopic composition (13C/12C, 18O/16O and 17O/16O) of carbon dioxide at high altitudes aiming to understand how the isotope ratios in the upper atmosphere relate to those measured on the surface. This analysis shows that the C and O isotopic ratios are on average consistent with those found on Earth. The vertical distribution of the C and O isotope ratios shows a decrease above the homopause which is consistent with the expectations from diffusive separation, although some orbits also show an increase in the O isotopic ratios in the lower thermosphere which we conclude may be due to photochemical fractionation.
The measurements of the isotopic ratios presented in this thesis, as well as their interpretation in context with other atmospheric parameters, provide a significant contribution to our understanding of the isotopic composition of the Martian atmosphere, which in turn allow a better understanding of the history and evolution of the atmosphere of Mars.