J.-B. Madeleine

Global modelling of the early Martian climate under a denser CO2 atmosphere: Water cycle and ice evolution

Abstract We discuss 3D global simulations of the early martian climate that we have performed assuming a faint young Sun and denser CO 2 atmosphere. We include a self-consistent representation of the water cycle, with atmosphere–surface interactions, atmospheric transport, and the radiative effects of CO 2 and H 2 O gas and clouds taken into account. We find that for atmospheric pressures greater than a fraction of a bar, the adiabatic cooling effect causes temperatures in the southern highland valley network regions to fall significantly below the global average. Long-term climate evolution simulations indicate that in these circumstances, water ice is transported to the highlands from low-lying regions for a wide range of orbital obliquities, regardless of the extent of the Tharsis bulge. In addition, an extended water ice cap forms on the southern pole, approximately corresponding to the location of the Noachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO 2 atmosphere, conditions are too cold to allow long-term surface liquid water. Limited melting occurs on warm summer days in some locations, but only for surface albedo and thermal inertia conditions that may be unrealistic for water ice. Nonetheless, meteorite impacts and volcanism could potentially cause intense episodic melting under such conditions. Because ice migration to higher altitudes is a robust mechanism for recharging highland water sources after such events, we suggest that this globally sub-zero, ‘icy highlands’ scenario for the late Noachian climate may be sufficient to explain most of the fluvial geology without the need to invoke additional long-term warming mechanisms or an early warm, wet Mars.

3D modelling of the early Martian Climate under a denser CO2 atmosphere: Temperatures and CO2 ice clouds.

On the basis of geological evidence, it is often stated that the early martian climate was warm enough for liquid water to flow on the surface thanks to the greenhouse effect of a thick atmosphere. We present 3D global climate simulations of the early martian climate performed assuming a faint young Sun and a CO2 atmosphere with surface pressure between 0.1 and 7 bars. The model includes a detailed radiative transfer model using revised CO2 gas collision induced absorption properties, and a parameterisation of the CO2 ice cloud microphysical and radiative properties. A wide range of possible climates is explored using various values of obliquities, orbital parameters, cloud microphysic parameters, atmospheric dust loading, and surface properties. Unlike on present day Mars, for pressures higher than a fraction of a bar, surface temperatures vary with altitude because of the adiabatic cooling and warming of the atmosphere when it moves vertically. In most simulations, CO2 ice clouds cover a major part of the planet. Previous studies had suggested that they could have warmed the planet thanks to their scattering greenhouse effect. However, even assuming parameters that maximize this effect, it does not exceed +15 K. Combined with the revised CO2 spectroscopy and the impact of surface CO2 ice on the planetary albedo, we find that a CO2 atmosphere could not have raised the annual mean temperature above 0 C anywhere on the planet. The collapse of the atmosphere into permanent CO2 ice caps is predicted for pressures higher than 3 bar, or conversely at pressure lower than 1 bar if the obliquity is low enough. Summertime diurnal mean surface temperatures above 0 C (a condition which could have allowed rivers and lakes to form) are predicted for obliquity larger than 40 at high latitudes but not in locations where most valley networks or layered sedimentary units are observed. In the absence of other warming mechanisms, our climate model results are thus consistent with a cold early Mars scenario in which nonclimatic mechanisms must occur to explain the evidence for liquid water. In a companion paper by Wordsworth et al. we simulate the hydrological cycle on such a planet and discuss how this could have happened in more detail.

Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone

It has been suggested that the recently discovered exoplanet GJ581d might be able to support liquid water due to its relatively low mass and orbital distance. However, GJ581d receives 35% less stellar energy than Mars and is probably locked in tidal resonance, with extremely low insolation at the poles and possibly a permanent night side. Under such conditions, it is unknown whether any habitable climate on the planet would be able to withstand global glaciation and/or atmospheric collapse. Here we present three-dimensional climate simulations which demonstrate that GJ581d will have a stable atmosphere and surface liquid water for a wide range of plausible cases, making it the first confirmed super-Earth (exoplanet of 2-10 Earth masses) in the habitable zone. We find that atmospheres with over 10 bar CO2 and varying amounts of background gas (e.g., N2) yield global mean temperatures above 0°C for both land and ocean-covered surfaces. Based on the emitted IR radiation calculated by the model, we propose observational tests that will allow these cases to be distinguished from other possible scenarios in the future.

Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling

The recently discovered exoplanet Gl 581d is extremely close to the outer edge of its system’s habitable zone, which has led to much speculation on its possible climate. We have performed a range of simulations to assess whether, given simple combinations of chemically stable greenhouse gases, the planet could sustain liquid water on its surface. For best estimates of the surface gravity, surface albedo and cloud coverage, we find that less than 10 bars of CO2 is sufficient to maintain a global mean temperature above the melting point of water. Furthermore, even with the most conservative choices of these parameters, we calculate temperatures above the water melting point for CO2 partial pressures greater than about 40 bar. However, we note that as Gl 581d is probably in a tidally resonant orbit, further simulations in 3D are required to test whether such atmospheric conditions are stable against the collapse of CO2 on the surface.

Global climate modeling of the Martian water cycle with improved microphysics and radiatively active water ice clouds

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 Meteorologie 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.

Recent Ice Ages on Mars: The role of radiatively active clouds and cloud microphysics

Global climate models (GCMs) have been successfully employed to explain the origin of many glacial deposits on Mars. However, the latitude-dependent mantle (LDM), a dust-ice mantling deposit that is thought to represent a recent “Ice Age,” remains poorly explained by GCMs. We reexamine this question by considering the effect of radiatively active water-ice clouds (RACs) and cloud microphysics. We find that when obliquity is set to 35°, as often occurred in the past 2 million years, warming of the atmosphere and polar caps by clouds modifies the water cycle and leads to the formation of a several centimeter-thick ice mantle poleward of 30° in each hemisphere during winter. This mantle can be preserved over the summer if increased atmospheric dust content obscures the surface and provides dust nuclei to low-altitude clouds. We outline a scenario for its deposition and preservation that compares favorably with the characteristics of the LDM.

Aphelion water-ice cloud mapping and property retrieval using the OMEGA imaging spectrometer onboard Mars Express

Mapping of the aphelion clouds over Tharsis and retrieval of their particle size and visible opacity are made possible by the OMEGA imaging spectrometer aboard Mars Express. Observations cover the period from MY26 Ls=330{degree sign} to MY29 Ls=180{degree sign} and are acquired at various local times, ranging from 8AM to 6PM. Cloud maps of the Tharsis region constructed using the 3.1µm ice absorption band reveal the seasonal and diurnal evolution of aphelion clouds. Four distinct types of clouds are identified: morning hazes, topographically controlled hazes, cumulus clouds and thick hazes. The location and time of occurrence of these clouds are analyzed and their respective formation process is discussed. An inverse method for retrieving cloud particle size and opacity is then developed and can only be applied to thick hazes. The relative error of these measurements is less than 30% for cloud particle size and 20% for opacity. Two groups of particles can be distinguished. The first group is found over flat plains and is composed of relatively small particles, ranging in size from 2 to 3.5µm. The second group is characterized by particle sizes of ~5µm which appear to be quite constant over Ls and local time. It is found west of Ascraeus and Pavonis Mons, and near Lunae Planum. These regions are preferentially exposed to anabatic winds, which may control the formation of these particles and explain their distinct properties. The water ice column is equal to 2.9pr.µm on average, and can reach 5.2pr.µm in the thickest clouds of Tharsis.

Correction to “Extensive MRO CRISM observations of 1.27 µm O2 airglow in Mars polar night and their comparison to MRO MCS temperature profiles and LMD GCM simulations”

The Martian polar night distribution of 1.27 μm (0-0) band emission from O 2 singlet delta [O 2 ( 1 Δ g )] is determined from an extensive set of Mars Reconnaissance Orbiter (MRO) Compact Reconnaissance Imaging Spectral Mapping (CRISM) limb scans observed over a wide range of Mars seasons, high latitudes, local times, and longitudes between 2009 and 2011. This polar nightglow reflects meridional transport and winter polar descent of atomic oxygen produced from CO 2 photodissociation. A distinct peak in 1.27 μm nightglow appears prominently over 70-90NS latitudes at 40-60 km altitudes, as retrieved for over 100 vertical profiles of O 2 ( 1 Δ g ) 1.27 μm volume emission rates (VER). We also present the first detection of much (x80±20) weaker 1.58 μm (0-1) band emission from Mars O 2 ( 1 Δ g ). Co-located polar night CRISM O 2 ( 1 Δ g ) and Mars Climate Sounder (MCS, McCleese et al, 2007) temperature profiles are compared to the same profiles as simulated by the Laboratoire de Meteorologie Dynamique (LMD) general circulation/photochemical model (e.g., Lefevre et al, 2004). Both standard and interactive aerosol LMD simulations (Madeleine et al, 2011) underproduce CRISM O 2 ( 1 Δ g ) total emission rates by 40%, due to inadequate transport of atomic oxygen to the winter polar emission regions. Incorporation of interactive cloud radiative forcing on the global circulation leads to distinct but insufficient improvements in modeled polar O 2 ( 1 Δ g ) and temperatures. The observed and modeled anti-correlations between temperatures and 1.27 μm band VER reflect the temperature dependence of the rate coefficient for O 2 ( 1 Δ g ) formation, as provided in Roble (1995).

Katabatic winds diminish precipitation contribution to the Antarctic ice mass balance

Significance Precipitation over Antarctica remains largely unknown, despite its crucial role in the surface mass balance of the Antarctic ice sheet. Using unprecedented observations covering an entire year, this work describes a previously unknown mechanism that leads to the sublimation of a large fraction of snowfall in the lower atmosphere, resulting from the interaction of precipitation and katabatic winds. Snowfall sublimation in the atmosphere, caused by katabatic winds, is in the order of 35% in the margins of East Antarctica. This process critically affects the interpretation of satellite-based remote sensing observations close to the ground and suggests that snowfall sublimation in a warming climate may counterbalance the expected increase of precipitation. Snowfall in Antarctica is a key term of the ice sheet mass budget that influences the sea level at global scale. Over the continental margins, persistent katabatic winds blow all year long and supply the lower troposphere with unsaturated air. We show that this dry air leads to significant low-level sublimation of snowfall. We found using unprecedented data collected over 1 year on the coast of Adélie Land and simulations from different atmospheric models that low-level sublimation accounts for a 17% reduction of total snowfall over the continent and up to 35% on the margins of East Antarctica, significantly affecting satellite-based estimations close to the ground. Our findings suggest that, as climate warming progresses, this process will be enhanced and will limit expected precipitation increases at the ground level.

Atmospheric hazards for entry, descent and landing of future missions to Mars: numerical simulations of fine-scale meteorological phenomena

In order to allow safe entry descent and landing, as well as mission surface operations, constraints apply to the selection of suitable landing sites. These constraints are defined in terms of general characteristics related to the specific mission profile (interplanetary transfer, waiting orbit injection and orbital waiting period and eclipse mitigation strategies) and to the EDL operations (entry conditions and environment, safe operation of the descent system with parachute deceleration, controlled rockets braking, touchdown with vented airbags). Most constraints are relative to « static » properties such as the geographical location, the altimetry, and the soil thermophysical constants. Few constraints are relative to the « dynamical » properties of the atmosphere, i.e. meteorological variations of density, temperature and winds, notwithstanding these are the most crucial characteristics to predict so as to ensure the success of the EDL phase. Martian mesoscale and microscale meteorological models are one of the relevant tools that can be used to predict the local and regional meteorological variability likely to be encountered at several proposed landing ellipses during Entry, Descent, and Landing. Most of the atmospheric hazards in the Martian lower atmosphere are not evident in current observational data and general circulation model simulations and can only be ascertained through mesoscale modeling of the region. The Laboratoire de Meteorologie Dynamique (LMD) Mesoscale Model is a versatile simulator of the Martian atmosphere and environment at horizontal scales ranging from hundreds of kilometers to tens of meters. Specific simulations with relevancy to assessment of atmospheric hazards possibly encountered in Martian landing sites can be carried out with such a tool. The need for accurate and realistic Martian mesoscale modeling remains critical for the design of upcoming missions to Mars (e.g., Mars Science Laboratory, ExoMars).