Atmospheric observations performed simultaneously in 2020-2021 by ESA’s Mars Express and Trace Gas Orbiter

In this contribution we provide an update of the observations performed simultaneously (or quasi-simultaneously) by the two European satellites around Mars: Mars Express (MEX) and ExoMars 2016 Trace Gas Orbiter (TGO).

The scientific objectives of both MEX and TGO missions are very complementary and there is a lot of synergy between the science teams, especially for the study of the atmosphere. Since the start of the TGO science operations in April 2018 we have executed hundreds of coordinated observations of the Martian atmosphere, first reported in [1] and now extended here. The science data obtained during these coordinated observations, both in sun occultation and nadir geometry, provide useful input for cross-calibration of the instruments, comparison of atmospheric vertical profiles and potential for the study of the temperature, composition, meteorology and climate of Mars.

These observations are routinely coordinated by the instrument teams, in particular the spectrometers NOMAD and ACS onboard TGO [3,4] and SPICAM, OMEGA and PFS onboard MEX [5,6,7] and the Science Operations Centers (SOCs) of both missions [1, 2]:

• MEX-TGO Sun Occultations (within 15min and distance <1000km)
• MEX-TGO Nadir crossings  (simultaneous, within <5deg seen from Mars center)
• MEX Nadir over TGO Sun Occultation point (simultaneous within distance <500km)
• MEX-TGO Radio Occultation points (preliminary testing during 2021)

Vertical profiles of the atmosphere are observed during solar occultations by the spectrometers SPICAM (MEX) and ACS-NOMAD (TGO) for both ingress and egress points (i.e. dusk and/or dawn terminator). We report here the quasi-simultaneous observations (<15minutes difference) in the same region of the planet (<1000km distance). In nadir geometry, we observe regularly with all spectrometers and have so far performed hundreds of simultaneous observations of the crossing points (where both spacecraft are <5deg as seen from Mars) and quasi-simultaneous observations of the same location within a few minutes difference, at various distances and illumination conditions. With Mars Express we have also observed directly in nadir geometry the solar occultation point seen by Trace Gas orbiter (Fig.1), allowing to retrieve nadir temperature measurements at the same time of the solar occultation vertical profile. Finally, we have also started the preliminary testing for spacecraft-to-spacecraft radio occultations, that may in the future provide useful information of the ionosphere.

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