We have built a new 3D Global Climate Model (GCM) to simulate Pluto as observed by New Horizons in 2015. All key processes are parametrized on the basis of theoretical equations, including ...atmospheric dynamics and transport , turbulence, radiative transfer, molecular conduction, as well as phases changes for N 2 , CH 2 and CO. Pluto's climate and ice cycles are found to be very sensitive to model parameters and initial states. Nevertheless, a reference simulation is designed by running a fast, reduced version of the GCM with simplified atmospheric transport for 40,000 Earth years to initialize the surface ice distribution and sub-surface temperatures, from which a 28-Earth-year full GCM simulation is performed. Assuming a topographic depression in a Sputnik-planum (SP)-like crater on the anti-Charon hemisphere, a realistic Pluto is obtained, with most N 2 and CO ices accumulated in the crater, methane frost covering both hemispheres except for the equatorial regions, and a surface pressure near 1.1 Pa in 2015 with an increase between 1988 and 2015, as reported from stellar occultations. Temperature profiles are in qualitative agreement with the observations. In particular, a cold atmospheric layer is obtained in the lowest kilometers above Sputnik Planum, as observed by New Horizons's REX experiment. It is shown to result from the combined effect of the topo-graphic depression and N 2 daytime sublimation. In the reference simulation with surface N 2 ice exclusively present in Sputnik Planum, the global circulation is only forced by radiative heating gradients and remains relatively weak. Surface winds are locally induced by topography slopes and by N 2 condensation and sublimation around Sputnik Planum. However, the circulation can be more intense depending on the exact distribution of surface N 2 frost. This is illustrated in an alternative simulation with N 2 condensing in the South Polar regions and N 2 frost covering latitudes between 35 • N and 48 • N. A global condensation flow is then created, inducing strong surface winds everywhere, a prograde jet in the southern high latitudes, and an equatorial superrotation likely forced by barotropic instabilities in the southern jet. Using realistic parameters, the GCM predict atmospheric concentrations of CO and CH 4 in good agreement with the observations. N 2 and CO do not condense in the atmosphere, but CH 4 ice clouds can form during daytime at low altitude near the regions covered by N 2 ice (assuming that nucleation is efficient enough). This global climate model can be used to study many aspects of the Pluto environment. For instance, organic hazes are included in the GCM and analysed in a companion paper (Bertrand and Forget, Icarus, this issue).
Water Supersaturation for Early Mars Delavois, A.; Forget, F.; Turbet, M. ...
Journal of geophysical research. Planets,
July 2023, Letnik:
128, Številka:
7
Journal Article
Recenzirano
Odprti dostop
Evidence of past liquid water flowing on the surface of Mars has been identified since the first orbital mission to the planet. However, reconstructing the climate that would allow liquid water at ...the surface is still an intense area of research. Previous studies showed that an atmosphere composed only of CO2 and H2O could not sustain surface temperatures above the freezing point of water. Different solutions have been studied, ranging from events like impacts on different atmospheric compositions, or even radiative feedback of water clouds that would create a dramatic greenhouse effect. In this context, we propose to study whether the supersaturation of water could warm the planet. Strong supersaturation is observed in the present‐day Martian atmosphere. On early Mars, supersaturation could enhance the greenhouse effect through strong absorption of the IR flux by water vapor or by modifying water clouds. While 1D modeling suggests a significant impact, our 3D model shows that warming the climate of early Mars requires a high supersaturation ratio, especially in the lower layers of the atmosphere. This configuration seems highly unrealistic since the level of supersaturation is higher than what would be expected in a dense atmosphere.
Plain Language Summary
Mars around 4 billion years ago is thought to have had a more Earth‐like climate with liquid water on its surface. Today's observations show evidence for past valleys, riverbeds, and lakes, suggesting that past conditions would have been potentially habitable on the Red Planet. This tremendous scene is, however, not well understood, since a dramatic greenhouse effect would have been needed. Previous studies were not convincing enough to predict a warm and wet climate during the Noachian/Hesperian period on Mars. Water supersaturation means that there was more water vapor in the air than could usually be held by the atmosphere, causing an eventual stronger greenhouse effect. Also, a potential modification to water clouds could have an additional warming effect. Using state‐of‐the‐art climate models, we investigate the potential role of water supersaturation on the climate of early Mars, without tackling the question of its origin. Our results show that unrealistic supersaturation conditions would be needed to reach an atmosphere with a global temperature that could host liquid water. Considering that the lower layers of the atmosphere could not host such an amount of water vapor, we conclude that the supersaturation of the atmosphere is unlikely to be the solution to the early Mars enigma.
Key Points
We simulated water supersaturation in an early Mars climate considering an abundant source of water on the surface and an arid scenario
Supersaturation can warm early Mars only with unrealistic supersaturation ratios
Supersaturation is only efficient when it occurs in the lower layers of the atmosphere in our simulations
Research highlights ao CO2 ice basal sublimation is possible under certain conditions on Mars. ao Different CO2 ice properties lead to different sublimation processes. ao Solar radiation alone can ...initiate basal sublimation in polar regions. Observations of the martian CO2 ice cap in late winter and spring have revealed exotic phenomena. Unusual dark spots, fans and blotches form as the south-polar seasonal CO2 ice cap retreats. The formation mechanisms of these features are not clearly understood. Theoretical models suggest that photons could penetrate deep into the CO2 ice down to the regolith, leading to basal sublimation and gas and dust ejection. We have developed a detailed thermal model able to simulate the temporal evolution of the regolith-CO2 ice layer-atmosphere column. It takes into account heat conduction, radiative transfer within the ice and the atmosphere, and latent heat exchange when there is a phase transition. We found that a specific algorithm, fully coupling these three components, was needed to properly predict ice sublimation below the surface. Our model allows us to determine under what conditions basal sublimation is possible and thus when and where it can occur on Mars. Our results show that basal sublimation is possible if we consider large pathlengths and very little dust content within the ice. Moreover, the model can explain how dark spots can appear very early after the end of the polar night at high latitudes. We also evaluate the importance of the different parameters in our simulations. Contrary to what was suggested by theoretical models, the role of seasonal thermal waves is found to be limited. Solar radiation alone can initiate basal sublimation, which therefore only depends on the CO2 ice properties. Three main modes were identified: one where condensation/sublimation only occurs at the surface (in the case of small grains and/or high dust content), one where basal sublimation is possible (large pathlengths and very little dust content) and an intermediate mode where sublimation within the ice may occur. We suggest that these different modes could be keys to understanding many processes that occur at the surface of Mars, like the anticryptic area behavior or the recent reported activity in gullies.
The current version of Laboratoire de Météorologie Dynamique (LMD) Mars GCM (original‐MGCM) uses annually repeating (prescribed) CO2 snow albedo values based on the Thermal Emission Spectrometer ...observations. We integrate the Snow, Ice, and Aerosol Radiation (SNICAR) model with MGCM (SNICAR‐MGCM) to prognostically determine H2O and CO2 snow albedos interactively in the model. Using the new diagnostic capabilities of this model, we find that cryospheric surfaces (with dust) increase the global surface albedo of Mars by 0.022. Over snow‐covered regions, SNICAR‐MGCM simulates mean albedo that is higher by about 0.034 than prescribed values in the original‐MGCM. Globally, shortwave flux into the surface decreases by 1.26 W/m2, and net CO2 snow deposition increases by about 4% with SNICAR‐MGCM over one Martian annual cycle as compared to the original‐MGCM simulations. SNICAR integration reduces the mean global surface temperature and the surface pressure of Mars by about 0.87% and 2.5%, respectively. Changes in albedo also show a similar distribution to dust deposition over the globe. The SNICAR‐MGCM model generates albedos with higher sensitivity to surface dust content as compared to original‐MGCM. For snow‐covered regions, we improve the correlation between albedo and optical depth of dust from −0.91 to −0.97 with SNICAR‐MGCM as compared to the original‐MGCM. Dust substantially darkens Mars's cryosphere, thereby reducing its impact on the global shortwave energy budget by more than half, relative to the impact of pure snow.
Key Points
We integrate the extended SNICAR snow model with LMD Mars GCM
Updated model prognostically determines H2O and CO2 snow albedos interactively in the model
Dust causes a substantial change in pure cryosphere albedo and the global shortwave energy budget
As the seasonal CO2 ice polar caps of Mars retreat during spring, dark spots appear on the ice in some specific regions. These features are thought to result from basal sublimation of the transparent ...CO2 ice followed by ejection of regolith‐type material, which then covers the ice. We have used Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) reflectance data, Thermal Emission Imaging System (THEMIS) visible images, and THEMIS‐derived temperature retrievals along with a thermal numerical model to constrain the physical and compositional characteristics of the seasonal cap for several areas exhibiting dark spots at both high spatial and temporal resolutions. Data analysis suggests an active period of material ejection (before solar longitude (Ls) 200), accumulation around the ejection points, and spreading of part of the ejected material over the whole area, followed by a period where no significant amount of material is ejected, followed by complete defrosting (≈ Ls 245). Dark material thickness on top of the CO2 ice is estimated to range from a few hundreds of microns to a few millimeters in the warmest spots, based on numerical modeling combined with the observed temperature evolution. The nature of the venting process and the amount of material that is moved lead to the conclusion that it could have an important impact on the surface physical properties.
Key Points
Ejection of material is monitored through THEMIS/CRISM observations
Surface temperature increases due to ejected material on top of CO2 ice
Ejected material thickness ranges from a few hundreds of microns to a few mm
Observations of the South Polar Residual Cap suggest a possible erosion of the cap, leading to an increase of the global mass of the atmosphere. We test this assumption by making the first comparison ...between Viking 1 and InSight surface pressure data, which were recorded 40 years apart. Such a comparison also allows us to determine changes in the dynamics of the seasonal ice caps between these two periods. To do so, we first had to recalibrate the InSight pressure data because of their unexpected sensitivity to the sensor temperature. Then, we had to design a procedure to compare distant pressure measurements. We propose two surface pressure interpolation methods at the local and global scale to do the comparison. The comparison of Viking and InSight seasonal surface pressure variations does not show changes larger than ±8 Pa in the CO2 cycle. Such conclusions are supported by an analysis of Mars Science Laboratory (MSL) pressure data. Further comparisons with images of the south seasonal cap taken by the Viking 2 orbiter and MARCI camera do not display significant changes in the dynamics of this cap over a 40 year period. Only a possible larger extension of the North Cap after the global storm of MY 34 is observed, but the physical mechanisms behind this anomaly are not well determined. Finally, the first comparison of MSL and InSight pressure data suggests a pressure deficit at Gale crater during southern summer, possibly resulting from a large presence of dust suspended within the crater.
Plain Language Summary
Observations of the permanent CO2 ice cap at the south pole of Mars in the 2000s suggested that the cap was eroding, possibly releasing a significant amount of CO2 into the atmosphere. To test this hypothesis, we compare surface pressures recorded by Viking in the 1970s and those recorded by InSight in 2018–2021 to confirm or refute the suspected increase of the atmospheric mass. After establishing our comparison method, we correct the influence of the sensor temperature on the InSight pressure data, which was discovered during our investigation. Comparison of the pressure data, as well as images of the seasonal caps taken by orbiters, do not reveal any change in the atmospheric mass or the dynamics of the seasonal ice caps that develop during the martian year. These conclusions are reinforced by reanalyzing the pressure data recorded by the Curiosity rover. Only small interannual changes are observed, potentially related to the effect of the dust storms that happened on Mars between 2016 and 2018. Finally, we report a possible pressure deficit at MSL's location during southern hemisphere summer, potentially explained by the unusual presence of dust in the crater air.
Key Points
We propose a recalibration of InSight pressure data to correct an unexpected sensitivity to the sensor temperature
A comparison between the InSight and Viking 1 pressure data does not show variations larger than ±8 Pa in the global atmospheric mass
This comparison also supports the absence of long‐term variability in the dynamics of seasonal cap formation and sublimation
The Thermal InfraRed channel in honor of professor Vassili Ivanovich Moroz (TIRVIM) of the Atmospheric Chemistry Suite onboard ExoMars Trace Gas Orbiter has continuously monitored the Martian ...atmosphere from 13 March 2018 until 2 December 2019, covering almost a complete Martian Year (MY). In the nadir mode of observations, infrared spectra obtained by TIRVIM in the spectral range 600–1,300 cm−1 permit retrievals of vertical temperature profiles from the surface up to 60 km of altitude, surface temperatures and column aerosol optical depths. Here we report the retrieved atmospheric thermal structure and the column dust content during the global dust storm (GDS) of MY 34 monitored from Ls = 182.2° to Ls = 211.8° (Solar Longitude), capturing the evolution of the GDS and the response of the atmospheric thermal structure to the changing dust loading. The global storm caused asymmetric atmosphere heating, predominantly in the southern hemisphere, and changed diurnal contrast of atmospheric thermal structure. We also observe a reduced diurnal contrast of surface temperatures at the peak of the GDS.
Plain Language Summary
The thermal radiation emitted by Mars in the spectral range 7.7–16.7 μm was measured by the Atmospheric Chemistry Suite Thermal InfraRed channel in honor of professor Vassili Ivanovich Moroz (ACS TIRVIM) onboard ExoMars Trace Gas Orbiter. The nadir spectra carry information about the temperature of the atmosphere at different altitudes thanks to a deep CO2 absorption present around 15 μm. Also, the dust loading can be found from the 9‐μm silicate absorption, and the surface temperature can be estimated at 7 μm where the atmosphere is mostly transparent. We follow the evolution of these parameters during the strong global dust storm of Martian Year (MY) 34 (2018). The peculiarity of the ACS TIRVIM data set is the exceptionally dense coverage providing a new look at this otherwise well‐studied dust event.
Key Points
Atmospheric thermal structure and dust content on Mars are retrieved from Atmospheric Chemistry Suite Thermal InfraRed channel nadir measurements in the spectral range 7.7–16.7 μm
The 2018 global dust storm covered the entire planet and caused an asymmetric heating of the atmosphere with a hotter southern hemisphere
We observe a reduced diurnal contrast of surface temperatures and a changed contrast of atmospheric thermal structure during the storm
In a previous work (Hernández‐Bernal et al., 2021, https://doi.org/10.1029/2020je006517) we performed an observational analysis of the Arsia Mons Elongated Cloud (AMEC), which stands out due to its ...impressive size and shape, quick dynamics, and the fact that it happens during the Martian dusty season. Observations show that its morphology can be split in a head, on the western slope of the volcano of around 120 km in diameter; and a tail, that expands to the west reaching more than 1,000 km in length, making the AMEC the longest orographic cloud observed so far in the solar system. In this work we run the Laboratoire de Météorologie Dynamique Mesoscale Model to gain insight into the physics of the AMEC. We note that it is coincident in terms of local time and seasonality with the fastest winds on the summit of Arsia Mons. A downslope windstorm on the western slope is followed by a hydraulic‐like jump triggering a strong vertical updraft that propagates upwards in the atmosphere, causing a drop in temperatures of down to 30 K at 40–50 km in altitude, spatially and temporarily coincident with the observed head of the AMEC. However the model does not reproduce the microphysics of this cloud: the optical depth is too low and the expansion of the tail does not happen in the model. The observed diurnal cycle is correctly captured by the model for the head of the cloud. This work raises new questions that will guide future observations of the AMEC.
Plain Language Summary
This is the second paper of our research on the Arsia Mons Elongated Cloud (AMEC), which is a visually impressive cloud on Mars. It appears on the western flank of the Arsia Mons volcano during a specific season right at sunrise. For 3 hr it grows, developing a thin elongated tail that has been observed to be as long as 1,800 km. In our previous work we described available observations. In this work we run a high resolution atmospheric model that captures the effect of the Arsia Mons volcano on the atmosphere. This model shows that due to the presence of the volcano and its effect on the wind, air is forced upwards next to the volcano, leading to a drop in temperatures of 30°C, which causes the formation of the cloud under extreme conditions of humidity. This is a success of the model that provides a new understanding of this outstanding cloud, however, the accurate physics behind the extreme expansion of the AMEC are not fully understood yet. This work solves some questions and raises many new ones, which will be an aid in the planning of new observations.
Key Points
We performed mesoscale model dynamic simulations of the Arsia Mons Elongated Cloud observed in the Martian southern solstice
Topography‐induced circulation causes temperatures to drop by about 30 K at the observed origin location and local time of the cloud
The cloud tail is much more elongated in the observations than in the model, which challenges our understanding of winds and microphysics
Exposed scarps images and ice‐penetrating radar measurements in the North Polar Layered Deposits (NPLD) of Mars show alternating layers that provide an archive of past climate oscillations, that are ...thought to be linked to orbital variations, akin to Milankovitch cycles on Earth. We use the Laboratoire de Météorologie Dynamique Martian Global Climate Model to study paleoclimate states to enable a better interpretation of the NPLD physical and chemical stratigraphy. When a tropical ice reservoir is present, water vapor transport from the tropics to the poles at low obliquity is modulated by the intensity of summer. At times of low and relatively constant obliquity, the flux still varies due to other orbital elements, promoting polar layer formation. Ice migrates from the tropics toward the poles in two stages. First, when surface ice is present in the tropics, and second, when the equatorial deposit is exhausted, from ice that was previously deposited in mid‐high latitudes. The polar accumulation rate is significantly higher when tropical ice is available, forming thicker layers per orbital cycle. However, the majority of the NPLD is sourced from ice that temporary resided in the mid‐high latitudes and the layers become thinner as the source location moves poleward. The migration stages imprint different D/H ratios in different sections in the PLDs. The NPLD is isotopically depleted compared to the South Polar Layered Deposits in all simulations. Thus we predict the D/H ratio of the atmosphere in contact with NPLD upper layers is biased relative to the average global ice reservoirs.
Plain Language Summary
In this work we run simulations of a Global Climate Model for Mars with a broad range of orbital elements, and ice initially placed in the tropics, we calculate the growth rate and hydrogen isotopic composition of the polar caps. These simulations help to understand the migration of ice from the tropical region to the polar caps, as believed to have occurred in the recent past. Ice transport to the poles occurs at two stages. The first is when ice is present in the tropics. The second is after the tropical reservoir has been exhausted, and the source of vapor that reaches the poles is from ice accumulated in mid‐high latitudes during the first stage. The polar caps growth rate during the first stage is larger and results in thicker layers. We find that both physical and chemical records are expected in the polar caps, controlled by the orbital elements and the surface ice distribution. The chemical record also depends on the source enrichment. These results should help to interpret ice records in order to decode past climate variations, and suggest the current hydrogen isotopic composition of the atmosphere is not representative of the total ice reservoirs on Mars.
Key Points
The North Polar Layered Deposits growth rate strongly depends on perihelion position, in late stages accumulation can be of order a meter over a precession cycle
The isotopic stratigraphic signal in the polar caps experiences secular evolution, in addition to oscillations due to orbital elements
Northern ice deposits are depleted in deuterium compared to the south, biasing the isotopic composition of the present‐day atmosphere
The IRAM Plateau de Bure Interferometer has been used to map the CO(1–0) rotational line in Mars' middle atmosphere. Absolute winds and thermal profiles were retrieved during the 1999, 2001, 2003 and ...2005 planet's oppositions. The observations sampled various seasons (
L
s
=
143
, 196, 262, 317 and 322), and different dust situations (clear, global storm, regional storm). The absolute winds were derived by measuring directly the Doppler lineshifts. The main zonal circulation near 50 km is dominated by strong retrograde winds, with typical velocities of 70–170 m/s, strongly varying seasonally, latitudinally, and longitudinally (in particular between morning and evening). Comparison of the retrieved temperature with a general circulation model indicates that the model often underestimates the temperatures in the middle (20–50 km) atmosphere, and overestimates them above 50 km.
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