Martian upper atmosphere warming reflects the energy-materials interactions from the lower atmosphere layers. In this paper, we show a new phenomenon that enhances Mars upper atmosphere density in the equatorial region during the winter periods. First, both accelerometer-derived density and NGIMS-measured species from MAVEN show that the winter equatorial region has a secondary warming peak compared with that of the high-latitude polar warming area. Second, the major neutrals (CO2, Ar, CO, N2, and O) indicate that the phenomenon extends at least up to 240 km during both day and night sides. Furthermore, the topographic-related longitudinal structure emerged in the equatorial sector indicates that the variations are more dynamical than we expected. The warming found in this study suggest to be dust-related and influenced by the evolution of the seasonal solar insolation. Both local factors, upward small-scale gravity waves and CO2 IR-thermal effect transfer, may play key roles in shaping the warming structure.

We use the surface temperature response to Phobos transits as observed by a radiometer on board of the InSight lander to constrain the thermal properties of the uppermost layer of regolith. Modeled transit lightcurves validated by solar panel current measurements are used to modify the boundary conditions of a 1D heat conduction model. We test several model parameter sets, varying the thickness and thermal conductivity of the top layer to explore the range of parameters that match the observed temperature response within its uncertainty both during the eclipse as well as the full diurnal cycle. The measurements indicate a thermal inertia of 103+48-24 Jm-2K-1s-1/2 in the uppermost layer of 0.2 to 4 mm, significantly smaller than the thermal inertia of 200 Jm-2K-1s-1/2 derived from the diurnal temperature curve. This could be explained by larger particles, higher density, or a very small amount of cementation in the lower layers.

Observations by the Emirates eXploration Imager (EXI) on-board the Emirates Mars Mission are used to characterize the diurnal, seasonal, and spatial behavior of Aphelion Cloud Belt during Mars Year 36 L$_S$$\sim$30$^\circ$-190$^\circ$. Building from previously work with the Mars Color Imager (MARCI) onboard the Mars Reconnaissance Orbiter, we retrieve water ice extinction optical depth ($\tau_{ice}$) with an uncertainty $\pm$0.022 (excluding particle size effects). We connect EXI and MARCI using radiance and $\tau_{ice}$. Zonal and meridional diurnal trends are analyzed over 6h-18h Local True Solar Time. The retrievals show large morning-evening asymmetries about a minimum near 12h. The latitudinal distributions in early morning are extensive and particularly striking near mid-summer. Comparisons to the Mars Planetary Climate Model show reasonable agreement with the basic diurnal behavior, but noticeable departures include too much water ice in early morning, the general latitudinal extent, and behavior smaller scales like the volcanoes and other topographically distinct features.

We assimilate atmospheric temperature profiles and column dust optical depth observations from the ExoMars Trace Gas Orbiter Atmospheric Chemistry Suite thermal infrared channel (TIRVIM) into the LMD Mars Global Climate Model. The assimilation period is Mars Year 34 Ls = 182.3 - 211.4, covering the onset and peak of the 2018 global dust storm. We assimilated observations using the Local Ensemble Transform Kalman Filter with 36 ensemble members and adaptive inflation; our nominal configuration assimilated TIRVIM temperature profiles to update temperature and dust profiles, followed by dust column optical depths to update the total column dust abundance. The observation operator for temperature used the averaging kernels and prior profile from the TIRVIM retrievals. We verified our analyses against in-sample TIRVIM observations and independent Mars Climate Sounder (MCS) temperature and dust density-scaled opacity profiles. When dust observations were assimilated, the root-mean-square temperature error verified against MCS fell by 50% during the onset period of the storm, compared with assimilating temperature alone. At the peak of the storm the analysis reproduced the location and magnitude of the peak in the nighttime MCS dust distribution, along with the surface pressure diurnal cycle measured by Curiosity with a bias of less than 10 Pa. The analysis winds showed that, at the peak of the storm, the meridional circulation strengthened, a 125 m/s asymmetry developed in the midlatitude zonal jets, the diurnal tide weakened near the equator and strengthened to 10-15 K at midlatitudes, and the semi-diurnal tide strengthened almost everywhere, particularly in the equatorial lower atmosphere.

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.

HDO and the D/H ratio are essential to understand Mars past and present climate, in particular with regard to the evolution through ages of the Martian water cycle. We present here new modeling developments of the HDO cycle with the LMD Mars GCM. The present study aims at exploring the behaviour of the D/H ratio cycle and its sensitivity to the modeling of water ice clouds and the formulation of the fractionation by condensation. Our GCM simulations are compared with observations provided by the Atmospheric Chemistry Suite (ACS) on board the ESA/Roscosmos Trace Gas Orbiter, and reveal that the model quite well reproduces the temperature and water vapor fields, which offers a good basis for representing the D/H ratio cycle. The comparison also emphasizes the importance of modelling the effect of supersaturation, resulting from the microphysical processes of water ice clouds, to correctly account for the water vapor and the D/H ratio of the middle-to-upper atmosphere. This work comes jointly with a detailed comparison of the measured D/H profiles by TGO/ACS and the model outputs, conducted in the companion paper of Rossi et al. 2022 (this issue).