Using a ground‐to‐exosphere general circulation model for Mars we have simulated the variability of the dayside temperatures at the exobase during eight Martian years (MY, from MY24 to MY31, ...approximately from 1998 to 2013), taking into account the observed day‐to‐day solar and dust load variability. We show that the simulated temperatures are in good agreement with the exospheric temperatures derived from Precise Orbit Determination of Mars Global Surveyor. We then study the effects of the solar variability and of two planetary‐encircling dust storms on the simulated temperatures. The seasonal effect produced by the large eccentricity of the Martian orbit translates in an aphelion‐to‐perihelion temperature contrast in every simulated year. However, the magnitude of this seasonal temperature variation is strongly affected by the solar conditions, ranging from 50 K for years corresponding to solar minimum conditions to almost 140 K during the last solar maximum. The 27 day solar rotation cycle is observed on the simulated temperatures at the exobase, with average amplitude of the temperature oscillation of 2.6 K but with a significant interannual variability. These two results highlight the importance of taking into account the solar variability when simulating the Martian upper atmosphere and likely have important implications concerning the atmospheric escape rate. We also show that the global dust storms in MY25 and MY28 have a significant effect on the simulated temperatures. In general, they increase the exospheric temperatures over the low latitude and midlatitude regions and decrease them in the polar regions.
Key Points
Eight Martian years simulated with global model including day‐to‐day variable solar flux and dust
Important effect of solar cycle and solar rotation on simulated exobase temperatures
Significant effects of planetary‐encircling dust storms on simulated exobase temperatures
The impact of gravity waves (GW) on diurnal tides and the global circulation in the middle/upper atmosphere of Mars is investigated using a general circulation model (GCM). We have implemented a ...stochastic parameterization of non‐orographic GW into the Laboratoire de Météorologie Dynamique (LMD) Mars GCM (LMD‐MGCM) following an innovative approach. The source is assumed to be located above typical convective cells (
∼250 Pa), and the effect of GW on the circulation and predicted thermal structure above 1 Pa (
∼50 km) is analyzed. We focus on the comparison between model simulations and observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter during Martian Year 29. MCS data provide the only systematic measurements of the Martian mesosphere up to 80 km to date. The primary effect of GW is to damp the thermal tides by reducing the diurnal oscillation of the meridional and zonal winds. The GW drag reaches magnitudes of the order of 1 m/s/sol above 10
−2 Pa in the northern hemisphere winter solstice and produces major changes in the zonal wind field (from tens to hundreds of m/s), while the impact on the temperature field is relatively moderate (10–20 K). It suggests that GW‐induced alteration of the meridional flow is the main responsible for the simulated temperature variation. The results also show that with the GW scheme included, the maximum day‐night temperature difference due to the diurnal tide is around 10 K, and the peak of the tide is shifted toward lower altitudes, in better agreement with MCS observations.
Key Points
A stochastic non‐orographic gravity wave (GW) scheme is implemented into the LMD‐MGCM
Non‐orographic GW generated above typical convective layers control diurnal tides
The implemented GW scheme improves the accuracy of the LMD‐MGCM between 1 and 0.01 Pa in comparison with MCS
Studying the atmospheric planetary boundary layer (PBL) is crucial to understand the climate of a planet. The meteorological measurements by the instruments onboard InSight at a latitude of 4.5°N ...make a unique rich data set to study the active turbulent dynamics of the daytime PBL on Mars. Here we use the high‐sensitivity continuous pressure, wind, and temperature measurements in the first 400 sols of InSight operations (from northern late winter to midsummer) to analyze wind gusts, convective cells, and vortices in Mars’ daytime PBL. We compare InSight measurements to turbulence‐resolving large‐eddy simulations (LES). The daytime PBL turbulence at the InSight landing site is very active, with clearly identified signatures of convective cells and a vast population of 6,000 recorded vortex encounters, adequately represented by a power law with a 3.4 exponent. While the daily variability of vortex encounters at InSight can be explained by the statistical nature of turbulence, the seasonal variability is positively correlated with ambient wind speed, which is supported by LES. However, wind gustiness is positively correlated to surface temperature rather than ambient wind speed and sensible heat flux, confirming the radiative control of the daytime Martian PBL; and fewer convective vortices are forming in LES when the background wind is doubled. Thus, the long‐term seasonal variability of vortex encounters at the InSight landing site is mainly controlled by the advection of convective vortices by ambient wind speed. Typical tracks followed by vortices forming in the LES show a similar distribution in direction and length as orbital imagery.
Plain Language Summary
InSight is a lander sent to the surface of Mars with a weather station capable, like never before, to measure pressure, temperature, and winds continuously and at high cadence. We use this InSight atmospheric data set acquired over half a Martian year, along with computer simulations, to study the intense turbulence that develops in the daytime hours on Mars. InSight detects periodic variations in the measurements of the weather station, corresponding to air motions driven by convection. We also detect a large population of 6,000 whirlwinds passing close to the InSight lander and causing the pressure at the weather station to suddenly drop. The number of those whirlwind encounters varies from day to day, because of the random turbulence, and, on a seasonal basis, because of the varying ambient wind that transports the whirlwinds toward InSight. Unlike the population of whirlwinds, the strength of wind gusts follows the ground temperature varying with season. Whirlwinds also leave graffiti‐like dark tracks at the surface of Mars that can be imaged by satellites in the InSight region and reproduced by our numerical simulations.
Key Points
We study daytime turbulence with InSight atmospheric measurements and large‐eddy simulations
We identify convective cells, as well as numerous convective vortices following a power law of exponent 3.4
Seasonal variability of convective vortices controlled by wind advection and conversely turbulent gustiness correlated with surface temperature
Context. The detection of sulphur species in the Martian atmosphere would be a strong indicator of volcanic outgassing from the surface of Mars. Aims. We wish to establish the presence of SO2, H2S, ...or OCS in the Martian atmosphere or determine upper limits on their concentration in the absence of a detection. Methods. We perform a comprehensive analysis of solar occultation data from the mid-infrared channel of the Atmospheric Chemistry Suite instrument, on board the ExoMars Trace Gas Orbiter, obtained during Martian years 34 and 35. Results. For the most optimal sensitivity conditions, we determine 1σ upper limits of SO2 at 20 ppbv, H2S at 15 ppbv, and OCS at 0.4 ppbv; the last value is lower than any previous upper limits imposed on OCS in the literature. We find no evidence of any of these species above a 3σ confidence threshold. We therefore infer that passive volcanic outgassing of SO2 must be below 2 ktons day−1.
Different solutions have been proposed to solve the “faint young Sun problem,” defined by the fact that the Earth was not fully frozen during the Archean despite the fainter Sun. Most previous ...studies were performed with simple 1‐D radiative convective models and did not account well for the clouds and ice‐albedo feedback or the atmospheric and oceanic transport of energy. We apply a global climate model (GCM) to test the different solutions to the faint young Sun problem. We explore the effect of greenhouse gases (CO2 and CH4), atmospheric pressure, cloud droplet size, land distribution, and Earth's rotation rate. We show that neglecting organic haze, 100 mbar of CO2 with 2 mbar of CH4 at 3.8 Ga and 10 mbar of CO2 with 2 mbar of CH4 at 2.5 Ga allow a temperate climate (mean surface temperature between 10°C and 20°C). Such amounts of greenhouse gases remain consistent with the geological data. Removing continents produces a warming lower than +4°C. The effect of rotation rate is even more limited. Larger droplets (radii of 17 μm versus 12 μm) and a doubling of the atmospheric pressure produce a similar warming of around +7°C. In our model, ice‐free water belts can be maintained up to 25°N/S with less than 1 mbar of CO2 and no methane. An interesting cloud feedback appears above cold oceans, stopping the glaciation. Such a resistance against full glaciation tends to strongly mitigate the faint young Sun problem.
Key PointsNew constraints on the amount of greenhouse gases to get a temperate climateWaterbelt can be maintained with less than 1 mbar of CO2 and no methaneThe effects of larger cloud droplets and a higher pressure are quantified
Mars harbors ice deposits in several forms, on the surface and in the subsurface, which exchange with each other on various timescales. We seek to study the pore ice evolution over millennial time ...scales and how it contributes to and affects the Polar cap's evolution. We calculate the evolution of SubSurface Ice (SSI) pore filling by coupling two models, the Mars LMD Global Climate Model, which calculates the atmospheric and surface evolution on an annual timescale, and the dynamical version of the Mars Subsurface Ice Model, which calculates the evolution of the SSI on a millennial timescale. The SSI latitudinal boundary fluctuates over more than 25° in one obliquity cycle, overall extending equatorward of latitude ±35° at high obliquity, and receding to about ±60° at low obliquity. In locations where the SSI is stable continuously over orbital cycles, the simulations predict layering caused by a sublimation front at the SSI top boundary. Between 5 and 2.5 Myr ago, the subsurface lost at least ∼95 m of polar equivalent layer ice. The SSI flux routinely reaches ∼1 mm/Mars year. In addition to the direct contribution to the growth of the North Polar Layered Deposits (NPLD), the SSI causes variations in the NPLD accumulation rate due to the changes in the SSI distribution that affect the seasonal energy budget. These variations are comparable to the change in rate due to variations in orbital elements. When running paleo‐climate simulations, particularly to reconstruct the NPLD profile, changes in the SSI distribution should be considered.
Plain Language Summary
Ice on Mars is abundant and can be found on the surface and in the subsurface. In this work, we model the evolution of subsurface ice by diffusion. We examine how the evolution of the subsurface affects the North Polar cap stratigraphy. We use humidity calculated by a Global Climate Model and a thermal diffusion model to calculate the growth and retreat of ice in the subsurface. We calculate the depth and the fraction of pore‐filling ice at present. The subsurface can contribute ∼95 m (of the overall ∼2,000 m) of the North Polar Cap column. Subsurface ice indirectly affects the accumulation rate and isotopic ratio of the North Polar Cap growth by changing the seasonal energy budget. This occurs because when ice extends in the subsurface, it changes the thermal properties of the ground. These results emphasize the importance of the subsurface ice in the Martian water cycle, and it needs to be considered when reconstructing the North Polar Cap profile.
Key Points
Subsurface‐atmosphere vapor (ice) exchange is modeled over Milankovitch cycles using Mars climate and subsurface models
In regions where subsurface ice periodically forms and disappears due to vapor diffusion, ice concentrations are expected to be small
The pore‐ice volume lost between 5 and 2.5 Myr ago is equivalent to a layer of max‐imum thickness 95 m distributed over the North Polar Layered Deposits
The Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter monitors the Martian atmosphere through different spectral intervals in the infrared light. We present a retrieval ...algorithm tailored to the analysis of spectra acquired in nadir geometry by Thermal InfraRed channel in honor of professor Vassilii Ivanovich Moroz (TIRVIM), the thermal infrared channel of ACS. Our algorithm simultaneously retrieves vertical profile of atmospheric temperature up to 50 km, surface temperature, and integrated optical depth of dust and water ice clouds. The specificity of the TIRVIM data set lies in its capacity to resolve the diurnal cycle over a 54 sol period. However, it is uncertain to what extent can the desired atmospheric quantities be accurately estimated at different times of day. Here we first present an Observing System Simulation Experiment (OSSE). We produce synthetic observations at various latitudes, seasons and local times and run our retrieval algorithm on these synthetic data, to evaluate its robustness. Different sources of biases are documented, in particular regarding aerosol retrievals. Atmospheric temperature retrievals are found robust even when dust and/or water ice cloud opacities are not well estimated in our OSSE. We then apply our algorithm to TIRVIM observations in April–May 2018 and perform a cross‐validation of retrieved atmospheric temperature and dust integrated opacity by comparisons with thousands of colocated Mars Climate Sounder (MCS) retrievals. Most differences between TIRVIM and MCS atmospheric temperatures can be attributed to differences in vertical sensitivity. Daytime dust opacities agree well with each other, while biases are found in nighttime dust opacity retrieved from TIRVIM at this season.
Plain Language Summary
The Martian surface and atmosphere undergo strong variations in temperature and amount of aerosols (dust or water ice cloud particles). Our knowledge on their variations at diurnal scale is however limited, due to lack of appropriate observations. We present a method to analyze thermal emission spectra of Mars' surface and atmosphere recorded by Thermal InfraRed channel in honor of professor Vassilii Ivanovich Moroz (TIRVIM), a spectrometer onboard the ExoMars Trace Gas Orbiter. We have developed a program to derive surface and atmospheric temperatures from these spectra, along with an estimation of the amount of aerosols. The specificity of the TIRVIM data set is its capacity to resolve the diurnal cycle over a 54 sol period. However, atmospheric quantities cannot be accurately estimated at all times of day. One of the goals of our paper is to assess the robustness of our algorithm with the help of simulated observations. The retrieval of aerosol opacity is assessed to be challenging at some times of day, but atmospheric temperature is well determined. We have then applied our algorithm to tens of thousands of TIRVIM observations obtained in April–May 2018 and showed that our derived atmospheric temperatures compare very well with independent measurements obtained from the Mars Climate Sounder, reinforcing our confidence in our method.
Key Points
We exploit Thermal InfraRed channel in honor of professor Vassilii Ivanovich Moroz (TIRVIM) spectra to determine Martian atmospheric, surface temperature, as well as integrated opacity of dust and water ice clouds
Different sources of biases are investigated with the help of simulated observations at different local times, latitudes and seasons
Atmospheric temperatures retrieved from TIRVIM in April–May 2018 are in excellent agreement with colocated Mars Climate Sounder observations
Abstract
Migrating tides dominate the tropical climate on Mars and are known to reach high amplitudes during global dust events (GDE). In this study, we characterize the amplitude, phase and vertical ...wavelength of the diurnal and semidiurnal migrating tides in Mars' lower atmosphere (up to 50 km) by exploiting temperature vertical profiles retrieved from TIRVIM, an infrared spectrometer onboard the ExoMars Trace Gas Orbiter covering multiple local times. Observations from the Mars Climate Sounder onboard the Mars Reconnaissance Orbiter are used to complement the local time coverage when needed, and to estimate a seasonal trend to subtract from TIRVIM observations. We focus on two time periods in Martian Year 34, near Ls = 150° and near Ls = 200° (during the 2018 GDE). The characteristics of the migrating tides at Ls = 150° agree very well with tidal theory: a downward propagation, amplitudes of typically 2–5 K, and a larger vertical wavelength for the semidiurnal compared to the diurnal mode. Comparisons with model predictions from the Mars Planetary Climate Model reveal an excellent agreement, except for a slightly different phase of the diurnal tide. During the GDE, the tide pattern changes spectacularly: the diurnal tide amplitude reaches 35 K at 65°S and 17 K at 50°N, being vertically trapped up to 10 Pa. The semidiurnal tide is maximum near 20–30°S with an amplitude of 8–12 K. The phase of this mode is tilted with latitude, which was not the case before the storm. This indicates a significant contribution of the asymmetric Hough modes due to hemispheric asymmetry in the dust distribution.
Plain Language Summary
Important temperature variations are observed in the Martian atmosphere at diurnal scale. Part of these variations are due to a phenomenon called migrating thermal tides, which take the form of global oscillations with periods that are diurnal, or fractions of a day (half a day period = semidiurnal, a third of a day = terdiurnal, etc). Using satellite observations sampling different local times on Mars, we separated these different components and derived their amplitude and phase (the local time at which these oscillations peak) in particular during the global dust event that occurred in 2018. During this major storm, diurnal temperature variations reached 65 K at high southern latitudes near 25 km altitude between the late morning (minimum of temperature) and late evening (maximum of temperature). We also report on the characteristics of the semidiurnal tide and even the terdiurnal tide, which have been less studied during dust storms. The phase of the semidiurnal tide is not the same at all latitudes, which we interpret as a consequence of the asymmetric dust distribution (more dust in the southern hemisphere). Our results agree with predictions from a Global Climate Model, although some small disagreements are found, calling for future improvements in the model.
Key Points
The characteristics of the migrating tides derived from TIRVIM at Ls = 150° agree with tidal theory and model predictions
During the 2018 global dust event (GDE), the diurnal tide reached an amplitude of 35 K at 65°S and we tentatively detect the terdiurnal tide
During the 2018 GDE, the semidiurnal tide amplitude reached 10 K at 20°S and exhibited a phase tilt with latitude
A large number of surface features (e.g., frost, gullies, slope streaks, recurring slope lineae) are observed on Martian slopes. Their activity is often associated with the specific microclimates on ...these slopes, which have been mostly studied with one‐dimensional radiative balance models to date. We develop here a parameterization to simulate these microclimates in 3D Global Climate Models. We first demonstrate that any Martian slope can be thermally represented by a poleward or equatorward slope, that is, the daily average, minimum, and maximum surface temperatures depend on the North‐South component of the slope. Based on this observation, we implement here a subgrid‐scale parameterization to represent slope microclimates (radiative fluxes, volatile condensation, ignoring slope winds for now) in the Mars Planetary Climate Model and validate it through comparisons with surface temperature measurements and frost detections on sloped terrains. With this new model, we show that slope microclimates do not have a significant impact on the seasonal CO2 and H2O cycles on a global scale. Furthermore, short‐scale slopes (i.e., less than ∼1 km in length) do not significantly impact the thermal state of the atmosphere. Ninety‐one percent of the active gullies are found where our model predicts CO2 frost, suggesting that their activity is related to processes involving CO2 ice. However, the low thicknesses (≤tens of cm) predicted at mid‐latitudes rule out mechanisms involving large amounts (∼meters) of ice. This model opens the way to new studies on surface‐atmosphere interactions in present and past climates.
Plain Language Summary
Martian slopes present traces of present and past activities (e.g., dust avalanches, moraines created by past glaciers). Such activities are linked to the microclimate of these slopes: a slope oriented toward the pole will be more in the shadow and therefore colder than a slope oriented toward the equator. For now, mostly 1D models have been used to determine the microclimates present on these slopes, neglecting potential 3D atmospheric effects. Here we propose a parameterization allowing us to model such microclimates in 3D Global Climate Models. We have implemented and validated this parameterization in the Mars Planetary Climate Model by comparing our model with observations and measurements of these microclimates on the slopes. Our model shows that these slope microclimates do not significantly influence the global climate of Mars. We also provide a new CO2 ice thickness map which constrains the possible mechanisms that form gullies on Mars.
Key Points
We develop a sub‐grid slope parameterization to simulate slope microclimates in the Mars Planetary Climate Model
We test and validate this parameterization against observations; and reappraise frost thicknesses on Martian slopes
This novel model opens the way to new studies on surface‐atmosphere interactions for the present and past climates of Mars
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 (3,000‐3,060 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 vapor is restricted to below 10 km, and an O3 layer (100–300 ppbv) is visible between 20 and 30 km. At this same time, the aphelion cloud belt forms, condensing water vapor and allowing O3 to build up between 30 and 40 km. A comparison to vertical profiles of water vapor and temperature in each season reveals that water vapor 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 vapor 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 vapor 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 factors between 2 and 6, indicating important differences in the rate of odd‐hydrogen photochemistry.
Plain Language Summary
Ozone on Mars is part of the so‐called odd‐oxygen family of reactive, oxidizing gases. It is part of many chemical cycles that help convert one type of gas into another, facilitating the transfer of carbon or hydrogen. Odd‐oxygen is crucial to linking the cycles of water vapor and carbon dioxide, or the destruction of trace gases, such as methane. With the Atmospheric Chemistry Suite onboard the ExoMars Trace Gas Orbiter, we are able to study the vertical structure of ozone in the Martian atmosphere and make direct comparisons between it and water vapor and temperature. We have observed ozone abundances several times larger than predicted, suggesting that the oxidizing power of the Martian atmosphere is stronger or faster than expected. We have also observed and measured the relationship between these products: temperature controls the abundance of water vapor, and when the atmosphere cools and water condenses, ozone is able to build up. It is the by‐products of when water vapor breaks down in sunlight that remove odd‐oxygen from the atmosphere.
Key Points
Observations of the vertical distribution of ozone on Mars over 3 years
Direct comparison of water vapor, ozone, and temperature, revealing trends and correlations
Ozone is observed in higher abundances than photochemical models predict
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