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.
Thermal tides are atmospheric planetary‐scale waves with periods that are harmonics of the solar day. In the Martian atmosphere thermal tides are known to be especially significant compared to any ...other known planet. Based on the data set of pressure timeseries produced by the InSight lander, which is unprecedented in terms of accuracy and temporal coverage, we investigate thermal tides on Mars and we find harmonics even beyond the number 24, which exceeds significantly the number of harmonics previously reported by other works. We explore comparatively the characteristics and seasonal evolution of tidal harmonics and find that even and odd harmonics exhibit some clearly differentiated trends that evolve seasonally and respond to dust events. High‐order tidal harmonics with small amplitudes could transiently interfere constructively to produce meteorologically relevant patterns.
Plain Language Summary
In analogy to the string of a guitar, which can oscillate in integer harmonics, planetary atmospheres exhibit oscillations that are harmonics of the solar day (Harmonic 1 with a period of 24 hr; harmonic 2, 12 hr; harmonic 3, 8 hr; etc.). These oscillations are part of the so‐called “atmospheric thermal tides”, which retain a complex global structure. They are conceptually related to ocean gravitational tides, and they have been observed in atmospheres of the solar system whose main source of energy is the light from the sun: Earth, Mars, Venus, and Titan. On Mars, thermal tides are particularly strong and they play a key role in atmospheric dynamics, presenting interactions with meteorological phenomena such as dust storms. Most studies on thermal tides focus on low‐order harmonics (1, 2, 3, and sometimes 4). In this study, we use a particularly sensitive pressure sensor that landed on Mars with the InSight mission to explore the existence of high‐order harmonics, and we find clear harmonics with very small amplitudes even beyond harmonic 24, which corresponds to 24 oscillations per solar day. We compare the characteristics of those harmonics and analyze their seasonal behavior, and we find that even and odd harmonics exhibit clearly different behaviors.
Key Points
Analysis of an unprecedented data set of pressure obtained by InSight suggests that tidal harmonics beyond 24 are present on Mars
Even and odd modes exhibit distinct patterns with a seasonal dependency centered on equinoxes and solstices, and response to dust events
The InSight mission, featuring continuous high‐frequency high‐sensitivity pressure measurements, is in ideal position to study the active atmospheric turbulence of Mars. Data acquired during 1.25 ...Martian year allows us to study the seasonal evolution of turbulence and its diurnal cycle. We investigate vortices (abrupt pressure drops), local turbulence (frequency range 0.01−2 Hz) and non‐local turbulence often caused by convection cells and plumes (frequency range 0.002−0.01 Hz). Contrary to non‐local turbulence, local turbulence is strongly sensitive at all local times and seasons to the ambient wind. We report many remarkable events with the arrival of northern autumn at the InSight landing site: a spectacular burst of daytime vortices, the appearance of nighttime vortices, and the development of nighttime local turbulence as intense as its daytime counterpart. Nighttime turbulence at this dusty season appears as a result of the combination of a stronger low‐level jet, producing shear‐driven turbulence, and a weaker stability.
Plain Language Summary
The weather station on board the InSight lander on Mars includes a very sensitive barometer to measure atmospheric pressure all the time. We use pressure records by InSight during more than a Martian year to study how the fast (from seconds to minutes) changes in the atmosphere (named turbulence) varies with seasons on Mars. Because of the heating of Mars surface by sunlight, turbulence during the day is strong and takes the form of convection plumes and whirlwinds that are detected by InSight. At night, turbulence is usually not expected because the colder surface prevents convection to occur. We discovered that turbulence at InSight landing site was unusual in autumn/winter: daytime whirlwinds are much more numerous, whirlwinds form even during the night, and there is nearly as much turbulence during the night than during the day. We explain the increase of nocturnal turbulence at this dusty season on Mars by warmer and windier nights.
Key Points
InSight's pressure sensor unveils seasonal variability of daytime and nighttime atmospheric turbulence on Mars
Local turbulence (>0.01 Hz) is sensitive to wind at all local times and seasons contrary to non‐local turbulence (<0.01 Hz, i.e., plumes/cells)
Northern autumn/winter hosts a remarkable burst of daytime vortices, and nighttime turbulence (including vortices) triggered by strong wind
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
Terrestrial and Martian atmospheres are both characterised by a large variety of mesoscale meteorological events, occurring at horizontal scales of hundreds of kilometres and below. Available ...measurements from space exploration and recently developed high-resolution numerical tools have given insights into Martian mesoscale phenomena, as well as similarities and differences with their terrestrial counterparts. The remarkable intensity of Martian mesoscale events compared to terrestrial phenomena mainly results from low density and strong radiative control. This is exemplified in the present paper by discussing two mesoscale phenomena encountered in the lowest atmospheric levels of both planets with notable differences: nighttime katabatic winds (drainage flow down sloping terrains) and daytime boundary layer convection (vertical growth of mixed layer over heated surfaces). While observations of katabatic events are difficult on Earth, except over vast ice sheets, intense clear-cut downslope circulations are expected to be widespread on Mars. Convective motions in the daytime Martian boundary layer are primarily driven by radiative contributions, usually negligible on Earth where sensible heat flux dominates, and exhibit turbulent variances one order of magnitude larger. Martian maximum heat fluxes are not attained close to the surface as on Earth but a few hundreds of metres above, which implies generalised definitions for mixing layer scales such as vertical velocity
w
⁎. Measurements on Mars of winds in uneven topographical areas and of heat fluxes over flat terrains could be useful to assess general principles of mesoscale meteorology applicable to both terrestrial and Martian environments.
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
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
Context.
Hot Jupiters are tidally locked gaseous exoplanets that exhibit large day-night temperature contrasts. Their cooler nightsides are thought to host clouds, as has been suggested by numerous ...observations. However, the exact nature of these clouds, their spatial distribution, and their impact on atmospheric dynamics, thermal structure, and spectra is still unclear.
Aims.
We investigate the atmosphere of WASP-43 b, a short period hot Jupiter recently observed with
James Webb
Space Telescope (JWST), to understand the radiative and dynamical impact of clouds on the atmospheric circulation and thermal structure. We aim to understand the impact of different kinds of condensates potentially forming in WASP-43 b, with various sizes and atmospheric metallicities.
Methods.
We used a 3D global climate model (GCM) with a new temperature-dependent cloud model that includes radiative feedbacks coupled with hydrodynamical integrations to study the atmospheric properties of WASP-43 b. We produced observables from our GCM simulations and compared them to spectral phase curves from various observations to derive constraints on the atmospheric properties.
Results.
We show that clouds have a net warming effect, meaning that the greenhouse effect caused by clouds is stronger than the albedo cooling effect. We show that the radiative effect of clouds has various impacts on the dynamical and thermal structure of WASP-43 b. Depending on the type of condensates and their sizes, the radiative-dynamical feedback will modify the horizontal and vertical temperature gradient and reduce the wind speed. For super-solar metallicity atmospheres, fewer clouds form in the atmosphere, leading to a weaker feedback. Comparisons with spectral phase curves observed with HST,
Spitzer
, and JWST indicate that WASP-43 b's nightside is cloudy and rule out sub-micron Mg
2
SiO
4
cloud particles as the main opacity source. Distinguishing between cloudy solarand cloudy super-solar-metallicity atmospheres is not straightforward, and further observations of both reflected light and thermal emission are needed.
•New method to derive motion of active features on Mars with CRISM and CTX/HiRISE data.•We derived the horizontal motion (speed and direction) of dust devils on Mars.•Comparison with Mars Climate ...Database (MCD) wind speeds and directions.•Broad agreement with ambient wind speeds and directions within the Planetary Boundary Layer (PBL).•Dust devils on Mars move faster than near-surface winds.
We derived the horizontal motion (speed and direction) of dust devils from time-delayed Mars Reconnaissance Orbiter (MRO) coordinated image data sets of the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) to the Context Camera (CTX) and/or the High Resolution Imaging Science Experiment (HiRISE) acquired between 2008 and 2011. In total, 47 dust devils were observed in 15 regions with diameters ranging from 15 to 280m with an average diameter of 100m and heights from 40 to 4400m. Horizontal speeds of 44 dust devils range from 4 to 25ms−1 with average speeds of 12ms−1. The majority of dust devils were observed in the northern hemisphere (79%), mainly in Amazonis Planitia (67.5% from the northern hemisphere dust devils). Seasonal occurrence of dust devils in the northern hemisphere is predominant in early and mid spring (76%). We compared our measured dust devil horizontal speeds and directions of motion to the monthly climatologies (wind speed and direction) released in the Mars Climate Database (MCD) derived from General Circulation Model (GCM) predictions. There is a broad agreement between dust devil horizontal speeds and MCD wind speed predictions within the Planetary Boundary Layer (PBL) as well as dust devil directions of motion and MCD predicted wind directions occurring within the PBL. Comparisons between dust devil horizontal speeds and MCD near-surface wind speed predictions at 10m height above the surface do not correlate well: dust devils move about a factor of 2 faster than MCD near-surface wind predictions. The largest offsets between dust devil motion and MCD predictions were related to three dust devils occurring near the Phoenix landing site when the lander was still active. The offsets could be explained by a regional weather front passing over the Phoenix landing site. In general, the good agreement between dust devil horizontal speeds and directions of motion, and ambient wind speeds and directions predicted within the PBL through GCM, show that dust devils on Mars move with ambient winds in the PBL, hence faster than near surface winds.
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