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.

Figure 1. MEX Nadir over TGO Sun Occultation point

The authors acknowledge the contributions of the European Space Agency, Roscomos, all National Agencies, research institutions and teams involved in the success of the Mars Express and ExoMars 2016 missions. ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos (Russia).

References:

[1] Cardesin Moinelo, A. et al: First year of coordinated science observations by Mars Express and ExoMars 2016 Trace Gas Orbiter, ICARUS 353, 2021

[2] Geiger, B et al: Long Term Planning for the ExoMars Trace Gas Orbiter Mission: Opportunity Analysis and Observation Scheduling, Proceedings of SpaceOps Conference, 2018

[3] Vandaele, A. C. et al: NOMAD, an Integrated Suite of Three Spectrometers for the ExoMars Trace Gas Mission: Technical Description, Science Objectives and Expected Performance, Space Sci Rev 214: 80. 2018

[4] Korablev, O. et al: The Atmospheric Chemistry Suite (ACS) of Three Spectrometers for the ExoMars 2016 Trace Gas Orbiter Space Sci Rev 214: 7. 2018

[5] Montmessin et al, SPICAM on Mars Express: A 10 year in-depth survey of the Martian atmosphere. Icarus, Volume 297, p. 195-216. 2017

[6] Bibring, J.-P., et al., OMEGA: Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité, ESA SP-1240, pp. 3–16. 2004

[7] Formisano V. et al: The Planetary Fourier Spectrometer (PFS) onboard the European Mars Express mission, Planetary and Space Science, Volume 53, Issue 10, p. 963-974. 2005