Temperature variability in the upper atmosphere of Mars

Abstract

Our knowledge of the temperature structure in the upper atmosphere of Mars (understood here as upper mesosphere/thermosphere, layers between ~80 and 200 km of altitude from the surface) has significantly improved in the last decade thanks to the data provided by the MAVEN/NASA, Mars Express/ESA, and ExoMars-TGO/ESA missions [e.g. 1, 2, 3]. However, aspects such as the variation of temperatures with local time or with latitude are still poorly characterized due to the sampling limitation of the different instruments, and rely mostly on the information provided by Global Climate Models (GCMs). Recent model-data comparisons [4] show that GCMs have problems in reproducing the observed local time variation of the temperatures in the mesopause, but similar comparisons are missing at other regions in the upper atmosphere.Here we analyze infrared spectra measured by the NOMAD instrument on ExoMars-TGO using the solar occultation technique [5] during 3 Mars years to derive CO2 density profiles, from which we build temperature profiles assuming hydrostatic equilibrium. These temperatures allow us to characterize, for example, the seasonal and latitudinal variability of the thermosphere and to study topics such as the effects of dust events on the thermospheric energy balance.We compare the NOMAD temperatures with predictions by the Mars Planetary Climate Model (M-PCM), a state-of-the-art ground-to-exobase GCM for Mars [6, 7]. This comparison helps to alleviate the limited coverage of the NOMAD dataset by complementing the observations with predictions at other locations and times, and is useful to interpret the observed temperature variability and to validate the model.We also compare our derived temperatures with publicly available mesospheric/thermospheric temperatures derived from other instruments and other missions, allowing a more complete local time coverage, and extending the study to a wider altitude range.Our preliminary results show that M-PCM predicts well the temperatures in the thermosphere except in the evening terminator, when observed temperatures are about 20-30 K larger than in the model. Given that the evening terminator is strongly affected by dynamical processes, this result points to deficiencies in the circulation or tidal structure predicted by the model. References[1] Jain, S.K., E. Soto, J.S. Evans, et al., Thermal structure of Mars’ middle and upper atmospheres: Understanding the impacts of dynamics and solar forcing. Icarus, 393, doi:10.1016/j.icarus.2021.114703 (2023)[2] Forget, F., F. Montmessin, J.-L. Bertaux, et al., Density and temperatures of the upper Martian atmosphere measured by stellar occultations with Mars Express SPICAM. JGR, 114, doi:10.1029/2008JE003086 (2009)[3] López-Valverde, M.A., B. Funke, A. Brines, et al., Martian atmospheric temperature and density profiles during the first year of NOMAD/TGO solar occultation measurements. JGR Planets, 128, doi:10.1029/2022JE007278 (2023)[4] Gupta, S., R.V. Yelle, N.M. Schneider, et al., Thermal structure of the Martian upper mesosphere/lower thermosphere from MAVEN/IUVS stellar occultations. JGR Planets, 127, doi:10.1029/2022JE007534 (2022)[5] Vandaele, A.-C., J.J. López-Moreno, M.R. Patel, et al., NOMAD, an integrated suite of three spectrometers for the ExoMars Trace Gas Mission: Technical description, science objectives and expected performance. Space Science Reviews, 214, doi:10.1007/s11214-018-0517-2 (2019)[6] Forget, F., F. Hourdin, R. Fournier, et al., Improved general circulation models of the Martian atmosphere from the surface to above 80 km. JGR, 104, doi:10.1029/1999JE001025 (1999)[7] González-Galindo, F., M.A. López-Valverde, F. Forget, et al., Variability of the Martian thermosphere during eight Martian years as simulated by a ground-to-exosphere global circulation model. JGR Planets, 120, doi:10.1002/2015JE004925 (2015) Acknowledgements:The IAA/CSIC team acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S and by grants PID2022-137579NB-I00, RTI2018-100920-J-I00 and PID2022-141216NB-I00 all funded by MCIN/AEI/10.13039/501100011033. A. Brines acknowledges financial support from the grant PRE2019-088355 funded by MCIN/AEI/10.13039/501100011033 and by ’ESF Investing in your future’. ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1, ST/Y000234/1 and ST/X006549/1 and Italian Space Agency through grant 2018-2-HH.0. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 101004052.