Spacecraft-to-spacecraft radio occultations experiments are being conducted at Mars between the Mars Express (MEX) and Trace Gas Orbiter (TGO) spacecraft, the first ever periodic inter-spacecraft occultations at a planet other than Earth. Here we present results from the first 83 such occultations, conducted between 2\textsuperscript{nd} Nov 2020 and 5\textsuperscript{th} of July 2023. Of these, 44 observations have to-date resulted in the extraction of vertical electron density profiles. These observations are the successful results of a major feasibility study conducted by the European Space Agency to use pre-existing relay communication equipment for radio science purposes. Mutual radio occultations have numerous advantages over traditional spacecraft-to-ground station occultations. In this work, we demonstrate how raw data are transformed into electron density values and validated with models and other instruments.

The mid-infrared channel of the Atmospheric Chemistry Suite (ACS MIR) onboard the ExoMars Trace Gas Orbiter is capable of observing the infrared absorption of ozone (O3) in the atmosphere of Mars. During solar occulations, the 003-000 band (3000-3060 cm-1) is observed with spectral sampling of ˜0.045 cm-1. Around the equinoxes in both hemispheres and over the southern winters, we regularly observe around 200-500 ppbv of O3 below 30 km. The warm southern summers, near perihelion, produce enough atmospheric moisture that O3 is not detectable at all, and observations are rare even at high northern latitudes. During the northern summers, water vapour is restricted to below 10 km, and an O3 layer (100-300 ppbv) is visible between 20-30 km. At this same time, the aphelion cloud belt forms, condensing water vapour and allowing O3 to build up between 30-40 km. A comparison to vertical profiles of water vapour and temperature in each season reveals that water vapour abundance is controlled by atmospheric temperature, and H2O and O3 are anti-correlated as expected. When the atmosphere cools, over time or over altitude, water vapour condenses (observed as a reduction in its mixing ratio) and the production of odd hydrogen species is reduced, which allows O3 to build up. Conversely, warmer temperatures lead to water vapour enhancements and ozone loss. The LMD Mars Global Climate Model is able to reproduce vertical structure and seasonal changes of temperature, H2O, and O3 that we observe. However, the observed O3 abundance is larger by a factor of 2-6, indicating important differences in the rate of odd-hydrogen photochemistry.