This project aims at two principal technical and scientific objectives related to the European mission Exomars Trace Gas Orbiter, in its fourth year of regular science operations from its orbit around Mars. First, an accurate analysis and inversion of very unique measurements by the instruments NOMAD and ACS, taken in a solar occultation geometry on board TGO; and secondly, an ambitious scientific exploitation plan of these data. We envision this last goal from two different angles, which comprise two major goals of the project. First the study of the vertical profiles of atmospheric temperature, density, abundance of species and aerosols, and its variability in space (latitude, longitude) and time (seasonal, daily, multiyear). This also includes their validation and comparison with independent measurements by other Mars missions. And secondly, the physical interpretation of these measurements using state-of-the-art climate models, including the goal of improving these as necessary to explain the data, and further applying them to extract conclusions on escape to espace and the long term evolution of the Martian atmosphere and volatile inventories.

The proposed research will be carried out essentially at the Instituto de Astrofísica de Andalucía (IAA), a CSIC research center with the Severo Ochoa distinction of excellence. Our team includes the co-PI of the NOMAD suite of instruments (two spectrometers in the IR, SO and LND, and one in the UV, UVIS), whose control electronic module was designed and built at the IAA. The PIs of the present proposal are co-Is of NOMAD, and one of them is also co-I of the other spectrometer on board TGO devoted to solar occulation measurements, named ACS. We are also strongly involved in the consortium developing the Mars-Planetary Climate Model. The present project, continuation of previous efforts by our team devoted to the scientific exploitation of NOMAD, is the natural culmination of the important investment of our country to the TGO mission, now in its science phase.

This project will produce significant contributions to the primary targets of the TGO mission. These include: (1) the study of trace gases, including detection, and precise derivation of species abundances and their spatial and temporal distribution; (2) retrieval and characterization with unique resolution of vertical profiles of composition (CO, H2O, HDO, aerosols) and thermal structure (temperature, density) of the Martian atmosphere; and (3) scientific interpretation of the results using state-of-the-art climate models, including improvements of the models and extraction of consequences on present and past climates on Mars. These are ambitious goals but completely feasible given the tools developed, and given the experience gained in the previous phase of the project.