The inner edge of the classical habitable zone is often defined by the critical flux needed to trigger the runaway greenhouse instability. This 1D notion of a critical flux, however, may not be all ...that relevant for inhomogeneously irradiated planets, or when the water content is limited (land planets). Based on results from our 3D global climate model, we present general features of the climate and large-scale circulation on close-in terrestrial planets. We find that the circulation pattern can shift from super-rotation to stellar/anti stellar circulation when the equatorial Rossby deformation radius significantly exceeds the planetary radius, changing the redistribution properties of the atmosphere. Using analytical and numerical arguments, we also demonstrate the presence of systematic biases among mean surface temperatures and among temperature profiles predicted from either 1D or 3D simulations. After including a complete modeling of the water cycle, we further demonstrate that two stable climate regimes can exist for land planets closer than the inner edge of the classical habitable zone. One is the classical runaway state where all the water is vaporized, and the other is a collapsed state where water is captured in permanent cold traps. We identify this “moist” bistability as the result of a competition between the greenhouse effect of water vapor and its condensation on the night side or near the poles, highlighting the dynamical nature of the runaway greenhouse effect. We also present synthetic spectra showing the observable signature of these two states. Taking the example of two prototype planets in this regime, namely Gl 581 c and HD 85512 b, we argue that depending on the rate of water delivery and atmospheric escape during the life of these planets, they could accumulate a significant amount of water ice at their surface. If such a thick ice cap is present, various physical mechanisms observed on Earth (e.g., gravity driven ice flows, geothermal flux) should come into play to produce long-lived liquid water at the edge and/or bottom of the ice cap. Consequently, the habitability of planets at smaller orbital distance than the inner edge of the classical habitable zone cannot be ruled out. Transiting planets in this regime represent promising targets for upcoming exoplanet characterization observatories, such as EChO and JWST.
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•We have produced a multiannual climatology of the dust distribution on Mars.•We grid retrievals of column dust optical depth from 3 heterogeneous instruments.•Biases among different ...instruments appear when carrying out detailed validation.•We show the interannual and interseasonal variability of dust over 8 martian years.•The years without global-scale storms display four phases in the dust distribution.
We have produced a multiannual climatology of airborne dust from martian year 24–31 using multiple datasets of retrieved or estimated column optical depths. The datasets are based on observations of the martian atmosphere from April 1999 to July 2013 made by different orbiting instruments: the Thermal Emission Spectrometer (TES) aboard Mars Global Surveyor, the Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey, and the Mars Climate Sounder (MCS) aboard Mars Reconnaissance Orbiter (MRO). The procedure we have adopted consists of gridding the available retrievals of column dust optical depth (CDOD) from TES and THEMIS nadir observations, as well as the estimates of this quantity from MCS limb observations. Our gridding method calculates averages and uncertainties on a regularly spaced spatio-temporal grid, using an iterative procedure that is weighted in space, time, and retrieval quality. The lack of observations at certain times and locations introduces missing grid points in the maps, which therefore may result in irregularly gridded (i.e. incomplete) fields. In order to evaluate the strengths and weaknesses of the resulting gridded maps, we compare with independent observations of CDOD by PanCam cameras and Mini-TES spectrometers aboard the Mars Exploration Rovers “Spirit” and “Opportunity”, by the Surface Stereo Imager aboard the Phoenix lander, and by the Compact Reconnaissance Imaging Spectrometer for Mars aboard MRO. We have statistically analyzed the irregularly gridded maps to provide an overview of the dust climatology on Mars over eight years, specifically in relation to its interseasonal and interannual variability, in addition to provide a basis for instrument intercomparison. Finally, we have produced regularly gridded maps of CDOD by spatially interpolating the irregularly gridded maps using a kriging method. These complete maps are used as dust scenarios in the Mars Climate Database (MCD) version 5, and are useful in many modeling applications. The two datasets for the eight available martian years are publicly available and distributed with open access on the MCD website.
The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) spacecraft landed successfully on Mars and imaged the surface to characterize the surficial geology. Here ...we report on the geology and subsurface structure of the landing site to aid in situ geophysical investigations. InSight landed in a degraded impact crater in Elysium Planitia on a smooth sandy, granule- and pebble-rich surface with few rocks. Superposed impact craters are common and eolian bedforms are sparse. During landing, pulsed retrorockets modified the surface to reveal a near surface stratigraphy of surficial dust, over thin unconsolidated sand, underlain by a variable thickness duricrust, with poorly sorted, unconsolidated sand with rocks beneath. Impact, eolian, and mass wasting processes have dominantly modified the surface. Surface observations are consistent with expectations made from remote sensing data prior to landing indicating a surface composed of an impact-fragmented regolith overlying basaltic lava flows.
Recent geological observations in the northern mid-latitudes of Mars show evidence for past glacial activity during the late Amazonian, similar to the integrated glacial landsystems in the Dry ...Valleys of Antarctica. The large accumulation of ice (many hundreds of meters) required to create the observed glacial deposits points to significant atmospheric precipitation, snow and ice accumulation, and glacial flow. In order to understand the climate scenario required for these conditions, we used the LMD (Laboratoire de Météorologie Dynamique) Mars GCM (General Circulation Model), which is able to reproduce the present-day water cycle, and to predict past deposition of ice consistent with geological observations in many cases. Prior to this analysis, however, significant mid-latitude glaciation had not been simulated by the model, run under a range of parameters.
In this analysis, we studied the response of the GCM to a wider range of orbital configurations and water ice reservoirs, and show that during periods of moderate obliquity (
ϵ
=
25–35°) and high dust opacity (
τ
dust
=
1.5–2.5), broad-scale glaciation in the northern mid-latitudes occurs if water ice deposited on the flanks of the Tharsis volcanoes at higher obliquity is available for sublimation. We find that high dust contents of the atmosphere increase its water vapor holding capacity, thereby moving the saturation region to the northern mid-latitudes. Precipitation events are then controlled by topographic forcing of stationary planetary waves and transient weather systems, producing surface ice distribution and amounts that are consistent with the geological record. Ice accumulation rates of ∼10
mm
yr
−1 lead to the formation of a 500–1000
m thick regional ice sheet that will produce glacial flow patterns consistent with the geological observations.
Radiatively active water ice clouds (RAC) play a key role in shaping the thermal structure of the Martian atmosphere. In this paper, RAC are implemented in the LMD Mars Global Climate Model (GCM) and ...the simulated temperatures are compared to Thermal Emission Spectrometer observations over a full year. RAC change the temperature gradients and global dynamics of the atmosphere and this change in dynamics in turn implies large‐scale adiabatic temperature changes. Therefore, clouds have both a direct and indirect effect on atmospheric temperatures. RAC successfully reduce major GCM temperature biases, especially in the regions of formation of the aphelion cloud belt where a cold bias of more than 10 K is corrected. Departures from the observations are however seen in the polar regions, and highlight the need for better modeling of cloud formation and evolution.
Key Points
Radiatively active clouds (RAC) are implemented in the LMD global climate model
Whatever the season, including RAC is required to fit the observed temperatures
Renewed attention on the polar regions, where cold biases remain, is needed
Hydrogen chloride was discovered in the atmosphere of Mars for the first time during the global dust storm in Mars year (MY) 34 (July 2018) using the Atmospheric Chemistry Suite mid-infrared channel ...(ACS MIR) on the ExoMars Trace Gas Orbiter. The simultaneity of variations in dust and HCl, and a correlation between water vapour and HCl, led to the proposal of a novel surface-atmosphere coupling analogous to terrestrial HCl production in the troposphere from salt aerosols. After seasonal dust activity restarted in MY 35 (August 2020), we have been monitoring HCl activity to determine whether such a coupling was validated. Here we present a new technique for analysing the absorption features of trace gases close to the ACS MIR noise level and report that HCl mixing ratios are observed to rapidly increase in both hemispheres coincidentally with the onset of the MY 35 perihelion dust season. We present the temporal evolution of the vertical distribution of HCl (0.1–6 ppbv) and of dust activity in both hemispheres. We also report two observations of >2 ppbv HCl below 10 km in the northern hemisphere during the aphelion period.
Airborne dust is the main driver of Martian atmospheric temperature, and accurately accounting for its radiative effect in Global Climate Models (GCMs) is essential. This requires the modeling of the ...dust distribution and radiative properties, and when trying to simulate the true climate variability, the use of the observed dust column opacity to guide the model. A recurrent problem has been the inability of Mars GCMs to predict realistic temperatures while using both the observed dust radiative properties and column opacity. One would have to drive the model with a tuned opacity to reach an agreement with the observations, thereby losing its self‐consistency. In this paper, we show that using the most recently derived dust radiative properties in the LMD (Laboratoire de Météorologie Dynamique) GCM solves this problem, which was mainly due to the underestimation of the dust single scattering albedo in the solar domain. However, an overall warm temperature bias remains above the 1 hPa pressure level. We therefore refine the model by implementing a “semi‐interactive” dust transport scheme which is coupled to the radiative transfer calculations. This scheme allows a better representation of the dust layer depth in the model and thereby removes the remaining warm bias. The LMD/GCM is now able to predict accurate temperatures without any tuning of the dust opacity used to guide the model. Remaining discrepancies are discussed, and seem to be primarily due to the neglect of the radiative effect of water‐ice clouds, and secondarily to persisting uncertainties in the dust spatial distribution.
Key Points
Most recent dust optical properties significantly improve Mars GCM predictions
A new strategy for transporting dust is coupled to the radiative transfer model
Simulating changes in dust layer depth is key to predict realistic temperatures
The Martian planetary boundary layer (PBL) is a crucial component of the Martian climate system. Global climate models (GCMs) and mesoscale models (MMs) lack the resolution to predict PBL mixing ...which is therefore parameterized. Here we propose to adapt the “thermal plume” model, recently developed for Earth climate modeling, to Martian GCMs, MMs, and single‐column models. The aim of this physically based parameterization is to represent the effect of organized turbulent structures (updrafts and downdrafts) on the daytime PBL transport, as it is resolved in large‐eddy simulations (LESs). We find that the terrestrial thermal plume model needs to be modified to satisfyingly account for deep turbulent plumes found in the Martian convective PBL. Our Martian thermal plume model qualitatively and quantitatively reproduces the thermal structure of the daytime PBL on Mars: superadiabatic near‐surface layer, mixing layer, and overshoot region at PBL top. This model is coupled to surface layer parameterizations taking into account stability and turbulent gustiness to calculate surface‐atmosphere fluxes. Those new parameterizations for the surface and mixed layers are validated against near‐surface lander measurements. Using a thermal plume model moreover enables a first‐order estimation of key turbulent quantities (e.g., PBL height and convective plume velocity) in Martian GCMs and MMs without having to run costly LESs.
Key Points
A terrestrial thermal plume model is adapted to Mars’ boundary layer convection
This new model reproduces caracteristics resolved through Large Eddy Simulations
Lander measurements can now be consistently reproduced in climate models
Water ice clouds play a key role in the radiative transfer of the Martian atmosphere, impacting its thermal structure, its circulation, and, in turn, the water cycle. Recent studies including the ...radiative effects of clouds in global climate models (GCMs) have found that the corresponding feedbacks amplify the model defaults. In particular, it prevents models with simple microphysics from reproducing even the basic characteristics of the water cycle. Within that context, we propose a new implementation of the water cycle in GCMs, including a detailed cloud microphysics taking into account nucleation on dust particles, ice particle growth, and scavenging of dust particles due to the condensation of ice. We implement these new methods in the Laboratoire de Météorologie Dynamique GCM and find satisfying agreement with the Thermal Emission Spectrometer observations of both water vapor and cloud opacities, with a significant improvement when compared to GCMs taking into account radiative effects of water ice clouds without this implementation. However, a lack of water vapor in the tropics after Ls = 180° is persistent in simulations compared to observations, as a consequence of aphelion cloud radiative effects strengthening the Hadley cell. Our improvements also allow us to explore questions raised by recent observations of the Martian atmosphere. Supersaturation above the hygropause is predicted in line with Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars observations. The model also suggests for the first time that the scavenging of dust by water ice clouds alone fails to fully account for the detached dust layers observed by the Mars Climate Sounder.
Key Points
Radiatively active clouds impact atmospheric water vapor and ice in a GCMCloud microphysics with dynamic nuclei is needed in GCMs
Massive reservoirs of subsurface water ice in equilibrium with atmospheric water vapor are found poleward of 45° latitude on Mars. The absence of CO2 frost on steep pole‐facing slopes and simulations ...of atmospheric‐soil water exchanges suggested that water ice could be stable underneath these slopes down to 25° latitude. We revisit these arguments with a new slope microclimate model. Our model shows that below 30° latitude, slopes are warmer than previously estimated as the air above is heated by warm surrounding plains. This additional heat prevents the formation of surface CO2 frost and subsurface water ice for most slopes. Our model suggests the presence of subsurface water ice beneath pole‐facing slopes down to 30° latitude, and possibly 25° latitude on sparse steep dusty slopes. While unstable ice deposits might be present, our results suggest that water ice is rarer than previously thought in the ±30° latitude range considered for human exploration.
Plain Language Summary
The presence of water ice near the equator is a key issue for future human exploration of Mars. In the current climate, this ice cannot exist near the equator but could be stable at accessible depths below pole‐facing slopes down to latitudes of 25°, that is, close enough to the equator for a crewed mission. Here, we study the possible presence of this subsurface ice with a new model that simulates the microclimates associated with slopes on Mars. Our results show that, contrary to the arguments put forward in the literature, the slopes close to the equator (20°–30°) may in fact be too warm to allow subsurface water ice to be stable, and that the observations that suggested the presence of ice under these slopes can be explained otherwise by our model. Thus, the widespread presence of water ice under these slopes at subtropical latitudes is not demonstrated. However, our model cannot rule out the presence of ancient ice reservoirs, that would be slowly sublimating today.
Key Points
We use a new model of steep slope microclimates to explore the stability of subsurface water ice on Mars at latitudes lower than 30°
Our model shows that warm plains and large‐scale atmospheric dynamics heat these slopes, preventing ice from being stable
Subsurface ice is predicted to be present down to 30° of latitude, possibly down to 25° but for sparse slopes with favorable conditions
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