2011 .

(22 publications)

J.-B. Madeleine, F. Forget, E. Millour, L. Montabone, and M. J. Wolff. Revisiting the radiative impact of dust on Mars using the LMD Global Climate Model. Journal of Geophysical Research (Planets), 116:E11010, 2011. [ bib | DOI | ADS link ]

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.

L. Kerber, J. W. Head, J.-B. Madeleine, F. Forget, and L. Wilson. The dispersal of pyroclasts from Apollinaris Patera, Mars: Implications for the origin of the Medusae Fossae Formation. Icarus, 216:212-220, 2011. [ bib | DOI | ADS link ]

The Medusae Fossae Formation (MFF) has long been thought to be of Amazonian age, but recent studies propose that a significant part of its emplacement occurred in the Hesperian and that many of the Amazonian ages represent modification (erosional and redepositional) ages. On the basis of the new formational age, we assess the hypothesis that explosive eruptions from Apollinaris Patera might have been the source of the Medusae Fossae Formation. In order to assess the likelihood of this hypothesis, we examine stratigraphic relationships between Apollinaris Patera and the MFF and analyze the relief of the MFF using topographic data. We predict the areal distribution of tephra erupted from Apollinaris Patera using a Mars Global Circulation Model (GCM) combined with a semi-analytical explosive eruption model for Mars, and compare this with the distribution of the MFF. We conclude that Apollinaris Patera could have been responsible for the emplacement of the Medusae Fossae Formation.

J. L. Fastook, J. W. Head, F. Forget, J.-B. Madeleine, and D. R. Marchant. Evidence for Amazonian northern mid-latitude regional glacial landsystems on Mars: Glacial flow models using GCM-driven climate results and comparisons to geological observations. Icarus, 216:23-39, 2011. [ bib | DOI | ADS link ]

A fretted valley system on Mars located at the northern mid-latitude dichotomy boundary contains lineated valley fill (LVF) with extensive flow-like features interpreted to be glacial in origin. We have modeled this deposit using glacial flow models linked to atmospheric general circulation models (GCM) for conditions consistent with the deposition of snow and ice in amounts sufficient to explain the interpreted glaciation. In the first glacial flow model simulation, sources were modeled in the alcoves only and were found to be consistent with the alpine valley glaciation interpretation for various environments of flow in the system. These results supported the interpretation of the observed LVF deposits as resulting from initial ice accumulation in the alcoves, accompanied by debris cover that led to advancing alpine glacial landsystems to the extent observed today, with preservation of their flow texture and the underlying ice during downwasting in the waning stages of glaciation. In the second glacial flow model simulation, the regional accumulation patterns predicted by a GCM linked to simulation of a glacial period were used. This glacial flow model simulation produced a much wider region of thick ice accumulation, and significant glaciation on the plateaus and in the regional plains surrounding the dichotomy boundary. Deglaciation produced decreasing ice thicknesses, with flow centered on the fretted valleys. As plateaus lost ice, scarps and cliffs of the valley and dichotomy boundary walls were exposed, providing considerable potential for the production of a rock debris cover that could preserve the underlying ice and the surface flow patterns seen today. In this model, the lineated valley fill and lobate debris aprons were the product of final retreat and downwasting of a much larger, regional glacial landsystem, rather than representing the maximum extent of an alpine valley glacial landsystem. These results favor the interpretation that periods of mid-latitude glaciation were characterized by extensive plateau and plains ice cover, rather than being restricted to alcoves and adjacent valleys, and that the observed lineated valley fill and lobate debris aprons represent debris-covered residual remnants of a once more extensive glaciation.

F. González-Galindo, A. Määttänen, F. Forget, and A. Spiga. The martian mesosphere as revealed by CO 2 cloud observations and General Circulation Modeling. Icarus, 216:10-22, 2011. [ bib | DOI | ADS link ]

Different missions have observed mesospheric clouds on Mars in the last years. The presence of these clouds implies, among other conditions, mesospheric temperatures below CO 2 condensation temperature. We use a General Circulation Model to study the mesospheric temperatures and compare the observed distribution of the mesospheric clouds and the predicted climatology of mesospheric temperatures. Although the model does not usually predict temperatures below condensation for daytime conditions, in some regions the predicted temperatures are close enough to condensation that perturbations caused by small scale processes could produce local excursions below condensation. The location and time of the lowest temperatures predicted by the GCM correspond to a first order with the two observed populations of mesospheric clouds: equatorial clouds observed before and after the Northern summer solstice, and mid-latitude clouds observed around the Northern winter solstice. For the equatorial clouds season, the model predicts temperatures close to condensation at the longitude, latitude, altitude and local time where they have been observed. We find that the diurnal migrating thermal tide and non-migrating tides are at the root of the spatial confinement of the equatorial clouds. For the mid-latitude clouds season, the temperatures predicted by the model at the location of the observed clouds is too high. Stereo observations by two different instruments allow for the determination of the zonal speed of these clouds producing a rare dataset of mesospheric winds. We compare the mesospheric zonal winds predicted by the model with these observations, finding a good agreement, although in some cases the observed variability exceeds that predicted by the model.

J.-Y. Chaufray, K. D. Retherford, D. G. Horvath, J.-L. Bertaux, F. Forget, and F. Leblanc. The density of the upper martian atmosphere measured by Lyman-α absorption with Mars Express SPICAM. Icarus, 215:522-525, 2011. [ bib | DOI | ADS link ]

Absorption of interplanetary Lyman-α emission by Mars' nightside lower thermosphere was observed by Mars Express Spectrometer for Investigation of Characteristics of the Atmosphere of Mars (SPICAM), and is analyzed to derive the CO 2 density at 110 km during a martian year. The observed density seasonal variability is consistent with recent observations obtained by stellar occultations, proving that this method, though not as accurate as stellar occultations could be used complementary to them to characterize large variations of thermospheric density on Mars and provide a better spatial coverage by Lyman-α imagery.

L. Maltagliati, F. Montmessin, A. Fedorova, O. Korablev, F. Forget, and J.-L. Bertaux. Evidence of Water Vapor in Excess of Saturation in the Atmosphere of Mars. Science, 333:1868, 2011. [ bib | DOI | ADS link ]

The vertical distribution of water vapor is key to the study of Mars hydrological cycle. To date, it has been explored mainly through global climate models because of a lack of direct measurements. However, these models assume the absence of supersaturation in the atmosphere of Mars. Here, we report observations made using the SPICAM (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) instrument onboard Mars Express that provide evidence of the frequent presence of water vapor in excess of saturation, by an amount far surpassing that encountered in Earths atmosphere. This result contradicts the widespread assumption that atmospheric water on Mars cannot exist in a supersaturated state, directly affecting our long-term representation of water transport, accumulation, escape, and chemistry on a global scale.

A. Petrosyan, B. Galperin, S. E. Larsen, S. R. Lewis, A. Määttänen, P. L. Read, N. Renno, L. P. H. T. Rogberg, H. Savijärvi, T. Siili, A. Spiga, A. Toigo, and L. Vázquez. The Martian Atmospheric Boundary Layer. Reviews of Geophysics, 49:RG3005, 2011. [ bib | DOI | ADS link ]

The planetary boundary layer (PBL) represents the part of the atmosphere that is strongly influenced by the presence of the underlying surface and mediates the key interactions between the atmosphere and the surface. On Mars, this represents the lowest 10 km of the atmosphere during the daytime. This portion of the atmosphere is extremely important, both scientifically and operationally, because it is the region within which surface lander spacecraft must operate and also determines exchanges of heat, momentum, dust, water, and other tracers between surface and subsurface reservoirs and the free atmosphere. To date, this region of the atmosphere has been studied directly, by instrumented lander spacecraft, and from orbital remote sensing, though not to the extent that is necessary to fully constrain its character and behavior. Current data strongly suggest that as for the Earth's PBL, classical Monin-Obukhov similarity theory applies reasonably well to the Martian PBL under most conditions, though with some intriguing differences relating to the lower atmospheric density at the Martian surface and the likely greater role of direct radiative heating of the atmosphere within the PBL itself. Most of the modeling techniques used for the PBL on Earth are also being applied to the Martian PBL, including novel uses of very high resolution large eddy simulation methods. We conclude with those aspects of the PBL that require new measurements in order to constrain models and discuss the extent to which anticipated missions to Mars in the near future will fulfill these requirements.

A. Spiga. Elements of comparison between Martian and terrestrial mesoscale meteorological phenomena: Katabatic winds and boundary layer convection. Planetary and Space Science, 59:915-922, 2011. [ bib | DOI | ADS link ]

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.

F. Selsis, R. D. Wordsworth, and F. Forget. Thermal phase curves of nontransiting terrestrial exoplanets. I. Characterizing atmospheres. Astronomy Astrophysics, 532:A1, 2011. [ bib | DOI | arXiv | ADS link ]

Context. Although transit spectroscopy is a very powerful method for studying the composition, thermal properties, and dynamics of exoplanet atmospheres, only a few transiting terrestrial exoplanets will be close enough to allow significant transit spectroscopy with the current and forthcoming generations of instruments. Thermal phase curves (variations in the apparent infrared emission of the planet with its orbital phase) have been observed for hot Jupiters in both transiting and nontransiting configurations, and have been used to put constraints on the temperature distribution and atmospheric circulation. This method could be applied to hot terrestrial exoplanets. <BR /> Aims: We study the wavelength and phase changes of the thermal emission of a tidally-locked terrestrial planet as atmospheric pressure increases. We address the observability of these multiband phase curves and the ability to use them to detect atmospheric constituents. <BR /> Methods: We used a 3D climate model (GCM) to simulate the CO2 atmosphere of a terrestrial planet on an 8-day orbit around an M 3 dwarf and its apparent infrared emission as a function of its orbital phase. We estimated the signal to photon-noise ratio in narrow bands between 2.5 and 20 μm for a 10 pc target observed with a 6 m and a 1.5 m telescope (respectively the sizes of JWST and EChO). <BR /> Results: Atmospheric absorption bands produce associated signatures in what we call the variation spectrum. Atmospheric windows probing the near surface atmospheric layers are needed to produce large, observable phase-curve amplitudes. The number and transparency of these windows, hence the observability of the phase curves and the molecular signatures, decreases with increasing pressure. Planets with no atmosphere produce large variations and can be easily distinguished from dense absorbing atmospheres. <BR /> Conclusions: Photon-noise limited spectro-photometry of nearby systems could allow us to detect and characterize the atmosphere of nontransiting terrestrial planets known from radial velocity surveys. Two obvious impediments to these types of observations are the required photometric sensitivity (10-5) over the duration of at least one orbit (8-days in the studied case) and the intrinsic stellar variability. However, overcoming these obstacles would give access to one order of magnitude more targets than does transit spectroscopy.

A. Butz, S. Guerlet, O. Hasekamp, D. Schepers, A. Galli, I. Aben, C. Frankenberg, J.-M. Hartmann, H. Tran, A. Kuze, G. Keppel-Aleks, G. Toon, D. Wunch, P. Wennberg, N. Deutscher, D. Griffith, R. Macatangay, J. Messerschmidt, J. Notholt, and T. Warneke. Toward accurate CO2 and CH4 observations from GOSAT. Geophysical Research Letters, 38:L14812, 2011. [ bib | DOI | ADS link ]

The column-average dry air mole fractions of atmospheric carbon dioxide and methane (XCO<SUB>2</SUB> and XCH<SUB>4</SUB>) are inferred from observations of backscattered sunlight conducted by the Greenhouse gases Observing SATellite (GOSAT). Comparing the first year of GOSAT retrievals over land with colocated ground-based observations of the Total Carbon Column Observing Network (TCCON), we find an average difference (bias) of -0.05% and -0.30% for XCO<SUB>2</SUB> and XCH<SUB>4</SUB> with a station-to-station variability (standard deviation of the bias) of 0.37% and 0.26% among the 6 considered TCCON sites. The root-mean square deviation of the bias-corrected satellite retrievals from colocated TCCON observations amounts to 2.8 ppm for XCO<SUB>2</SUB> and 0.015 ppm for XCH<SUB>4</SUB>. Without any data averaging, the GOSAT records reproduce general source/sink patterns such as the seasonal cycle of XCO<SUB>2</SUB> suggesting the use of the satellite retrievals for constraining surface fluxes.

L. Cottereau, N. Rambaux, S. Lebonnois, and J. Souchay. The various contributions in Venus rotation rate and LOD. Astronomy Astrophysics, 531:A45, 2011. [ bib | DOI | arXiv | ADS link ]

Context. Thanks to the Venus Express Mission, new data on the properties of Venus could be obtained, in particular concerning its rotation. <BR /> Aims: In view of these upcoming results, the purpose of this paper is to determine and compare the major physical processes influencing the rotation of Venus and, more particularly, the angular rotation rate. <BR /> Methods: Applying models already used for Earth, the effect of the triaxiality of a rigid Venus on its period of rotation are computed. Then the variations of Venus rotation caused by the elasticity, the atmosphere, and the core of the planet are evaluated. <BR /> Results: Although the largest irregularities in the rotation rate of the Earth on short time scales are caused by its atmosphere and elastic deformations, we show that the irregularities for Venus are dominated by the tidal torque exerted by the Sun on its solid body. Indeed, as Venus has a slow rotation, these effects have a large amplitude of two minutes of time (mn). These variations in the rotation rate are greater than the one induced by atmospheric wind variations that can reach 25-50 s of time (s), depending on the simulation used. The variations due to the core effects that vary with its size between 3 and 20 s are smaller. Compared to these effects, the influence of the elastic deformation caused by the zonal tidal potential is negligible. <BR /> Conclusions: As the variations in the rotation of Venus reported here are close to 3 mn peak to peak, they should influence past, present, and future observations, thereby providing further constraints on the planet's internal structure and atmosphere.

T. Toyota, K. Kurita, and A. Spiga. Distribution and time-variation of spire streaks at Pavonis Mons on Mars. Planetary and Space Science, 59:672-682, 2011. [ bib | DOI | ADS link ]

We documented the distribution and the time-variation of the specific dark wind streaks at Pavonis Mons. We focused on the streaks we named Spire Streaks, which are overlapping spindle shaped dark streaks at the upper boundary of the coalesced dark streaks on Tharsis volcanoes. We investigated both visible and infrared images obtained by Viking orbiter camera, Mars Orbiter Camera (MOC), THEMIS, CTX and HiRISE of the spire streaks at Pavonis Mons. We also used topographic data obtained by Mars Orbiter Laser Altimeter (MOLA) to see the relationship between the topography and the distribution of the spire streaks. The spire streaks at Pavonis Mons provide us high-resolution information about the direction of the nighttime slope wind, and could be indirect clues for the time-variation of the nighttime environment. We conclude that the spire streaks are erosional features. However, some features of the spire streaks reported in this paper are outside the scope of previous modeling for erosional process, and we need a new category of model for the formation.

L. Maltagliati, D. V. Titov, T. Encrenaz, R. Melchiorri, F. Forget, H. U. Keller, and J.-P. Bibring. Annual survey of water vapor behavior from the OMEGA mapping spectrometer onboard Mars Express. Icarus, 213:480-495, 2011. [ bib | DOI | ADS link ]

We present here the annual behavior of atmospheric water vapor on Mars, as observed by the OMEGA spectrometer on board Mars Express during its first martian year. We consider all the different features of the cycle of water vapor: temporal evolution, both at a seasonal and at a diurnal scale; longitudinal distribution; and the vertical profile, through the variations in the saturation height. We put our results into the context of the current knowledge on the water cycle through a systematic comparison with the already published datasets. The seasonal behavior is in very good agreement with past and simultaneous retrievals both qualitatively and quantitatively, within the uncertainties. The average water vapor abundance during the year is 10 pr. μm, with an imbalance between northern and southern hemisphere, in favor of the first. The maximum of activity, up to 60 pr. μm, occurs at high northern latitudes during local summer and shows the dominance of the northern polar cap within the driving processes of the water cycle. A corresponding maximum at southern polar latitudes during the local summer is present, but less structured and intense. It reaches 25 pr. μm at its peak. Global circulation has some influence in shaping the water cycle, but it is less prominent than the results from previous instruments suggest. No significant correlation between water vapor column density and local hour is detected. We can constrain the amount of water vapor exchanged between the surface and the atmosphere to few pr. μm. This is consistent with recent results by OMEGA and PFS-LW. The action of the regolith layer on the global water cycle seems to be minor, but it cannot be precisely constrained. The distribution of water vapor on the planet, after removing the topography, shows the already known two-maxima system, over Tharsis and Arabia Terra. However, the Arabia Terra increase is quite fragmented compared with previous observations. A deep zone of minimum separates the two regions. The saturation height of water vapor is mainly governed by the variations of insolation during the year. It is confined within 5-15 km from the surface at aphelion, while in the perihelion season it stretches up to 55 km of altitude.

R. D. Wordsworth, F. Forget, F. Selsis, E. Millour, B. Charnay, and J.-B. Madeleine. Gliese 581d is the First Discovered Terrestrial-mass Exoplanet in the Habitable Zone. Astrophysical Journal, 733:L48, 2011. [ bib | DOI | arXiv | ADS link ]

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 0degC 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.

T. Encrenaz, T. K. Greathouse, M. J. Richter, J. H. Lacy, T. Fouchet, B. Bézard, F. Lefèvre, F. Forget, and S. K. Atreya. A stringent upper limit to SO2 in the Martian atmosphere. Astronomy Astrophysics, 530:A37, 2011. [ bib | DOI | ADS link ]

Surfur-bearing molecules have been found at the surface of Mars by the Viking lander, the Spirit and Opportunity rovers, and the OMEGA infrared spectrometer aboard Mars Express. However, no gaseous sulfur-bearing species have ever been detected in the Martian atmosphere. We search for SO2 signatures in the thermal spectrum of Mars at 7.4 μm using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). Data were obtained on Oct. 12, 2009 (Ls = 353deg), in the 1350-1360 cm-1 range, with a spatial resolution of 1 arcsec (after convolution over three pixels along the N-S axis and two steps along the E-W axis) and a resolving power of 80 000. To improve the signal-to-noise ratio (S/N), we co-added the Martian spectrum around the positions of nine selected SO2 transitions with a high S/N and no telluric contamination. From a mean spectrum, averaged over 35 pixels in the region of maximum continuum, we infer a 2σ upper limit of 0.3 ppb to the SO2 mixing ratio, assuming that our instrumental errors are combined according to Gaussian statistics. Our upper limit is three times lower than the upper limit derived by Krasnopolsky (2005, Icarus, 178, 487), who used the same technique on previous TEXES data. In addition, we derive an upper limit of 2 ppb at each spatial pixel of the region observed by TEXES, which covers the longitude ranges 50 E-170 E for latitudes above 30 N, 100 E-170 E for latitudes between 0 and 30 N, and 110 E-170 E for latitudes between 15 S and 0. The non-detection of localized SO2 sources in the observed area is consistent with a homogeneous distribution being expected around equinox for non-condensible species with a lifetime longer than the global mixing time. In view of the typically large SO2/CH4 ratio observed in terrestrial volcanoes, and assuming a comparable volcanic composition for Mars and the Earth, our result reaffirms that a volcanic origin is unlikely for any methane in the Martian atmosphere.

T. Appéré, B. Schmitt, Y. Langevin, S. Douté, A. Pommerol, F. Forget, A. Spiga, B. Gondet, and J.-P. Bibring. Winter and spring evolution of northern seasonal deposits on Mars from OMEGA on Mars Express. Journal of Geophysical Research (Planets), 116:E05001, 2011. [ bib | DOI | ADS link ]

The OMEGA visible/near-infrared imaging spectrometer on Mars Express has observed the retreat of the northern seasonal deposits during Martian year 27-28 from the period of maximum extension, close to the northern winter solstice, to the end of the retreat at Ls 95deg. We present the temporal and spatial distributions of both CO2 and H2O ices and propose a scenario that describes the winter and spring evolution of the northern seasonal deposits. During winter, the CO2-rich condensates are initially transparent and could be in slab form. A water ice annulus surrounds the sublimating CO2 ice, extending over 6deg of latitude at Ls 320deg, decreasing to 2deg at Ls 350deg, and gradually increasing to 4.5deg at Ls 50deg. This annulus first consists of thin frost as observed by the Viking Lander 2 and is then overlaid by H2O grains trapped in the CO2-rich ice layer and released during CO2 sublimation. By Ls 50deg, H2O ice spectrally dominates most of the deposits. In order to hide the still several tens of centimeters thick CO2 ice layer in central areas of the cap we propose the buildup of an optically thick top layer of H2O ice from ice grains previously embedded in the CO2 ice and by cold trapping of water vapor from the sublimating water ice annulus. The CO2 ice signature locally reappears between Ls 50deg and 70deg. What emerges from our observations is a very active surface-atmosphere water cycle. These data provide additional constraints to the general circulation models simulating the Martian climate.

C. Pilorget, F. Forget, E. Millour, M. Vincendon, and J. B. Madeleine. Dark spots and cold jets in the polar regions of Mars: New clues from a thermal model of surface CO 2 ice. Icarus, 213:131-149, 2011. [ bib | DOI | ADS link ]

Observations of the martian CO 2 ice cap in late winter and spring have revealed exotic phenomena. Unusual dark spots, fans and blotches form as the south-polar seasonal CO 2 ice cap retreats. The formation mechanisms of these features are not clearly understood. Theoretical models suggest that photons could penetrate deep into the CO 2 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-CO 2 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 CO 2 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.

S. Guerlet, T. Fouchet, B. Bézard, F. M. Flasar, and A. A. Simon-Miller. Evolution of the equatorial oscillation in Saturn's stratosphere between 2005 and 2010 from Cassini/CIRS limb data analysis. Geophysical Research Letters, 38:L09201, 2011. [ bib | DOI | ADS link ]

We present an analysis of thermal infrared spectra acquired in limb viewing geometry by Cassini/CIRS in February 2010. We retrieve vertical profiles of Saturn's stratospheric temperature from 20 hPa to 10-2 hPa, at 9 latitudes between 20degN and 20degS. Using the gradient thermal wind equation, we derive a map of the zonal wind field. Both the temperature and the zonal wind vertical profiles exhibit an oscillation in the equatorial region. These results are compared to the temperature and zonal wind maps obtained from 2005-2006 CIRS limb data, when this oscillation was first reported. In both epochs, strong temperature anomalies at the equator (up to 20K) are consistent with adiabatic heating (cooling) due to a sinking (rising) motion at a speed of 0.1-0.2 mm/s. Finally, we show that the altitude of the maximum eastward wind has moved downwards by 1.3 scale heights in 4.2 years, hence with a phase speed of 0.5 mm/s. This rate is consistent with the estimated period of 14.7 years for the equatorial oscillation, and requires a local zonal acceleration of 1.1 × 10-6 m.s-2 at the 2.5 hPa pressure level. This downward propagation of the oscillation is consistent with it being driven by absorption of upwardly propagating waves.

A. Spiga, F. Forget, J.-B. Madeleine, L. Montabone, S. R. Lewis, and E. Millour. The impact of martian mesoscale winds on surface temperature and on the determination of thermal inertia. Icarus, 212:504-519, 2011. [ bib | DOI | ADS link ]

Radiative control of surface temperature is a key characteristic of the martian environment and its low-density atmosphere. Here we show through meteorological modeling that surface temperature can be far from radiative equilibrium over numerous sloping terrains on Mars, where nighttime mesoscale katabatic winds impact the surface energy budget. Katabatic circulations induce both adiabatic atmospheric heating and enhancement of downward sensible heat flux, which then becomes comparable to radiative flux and acts to warm the ground. Through this mechanism, surface temperature can increase up to 20 K. One consequence is that warm signatures of surface temperature over slopes, observed through infrared spectrometry, cannot be systematically associated with contrasts of intrinsic soil thermal inertia. Apparent thermal inertia maps retrieved thus far possibly contain wind-induced structures. Another consequence is that surface temperature observations close to sloping terrains could allow the validation of model predictions for martian katabatic winds, provided contrasts in intrinsic thermal inertia can be ruled out. The thermal impact of winds is mostly discussed in this paper in the particular cases of Olympus Mons/Lycus Sulci and Terra Meridiani but is generally significant over any sloped terrains in low thermal inertia areas. It is even general enough to apply under daytime conditions, thereby providing a possible explanation for observed afternoon surface cooling, and to ice-covered terrains, thereby providing new insights on how winds could have shaped the present surface of Mars.

H. F. Parish, G. Schubert, C. Covey, R. L. Walterscheid, A. Grossman, and S. Lebonnois. Decadal variations in a Venus general circulation model. Icarus, 212:42-65, 2011. [ bib | DOI | ADS link ]

The Community Atmosphere Model (CAM), a 3-dimensional Earth-based climate model, has been modified to simulate the dynamics of the Venus atmosphere. The most current finite volume version of CAM is used with Earth-related processes removed, parameters appropriate for Venus introduced, and some basic physics approximations adopted. A simplified Newtonian cooling approximation has been used for the radiation scheme. We use a high resolution (1deg by 1deg in latitude and longitude) to take account of small-scale dynamical processes that might be important on Venus. A Rayleigh friction approach is used at the lower boundary to represent surface drag, and a similar approach is implemented in the uppermost few model levels providing a sponge layer to prevent wave reflection from the upper boundary. The simulations generate superrotation with wind velocities comparable to those measured in the Venus atmosphere by probes and around 50-60% of those measured by cloud tracking. At cloud heights and above the atmosphere is always superrotating with mid-latitude zonal jets that wax and wane on an approximate 10 year cycle. However, below the clouds, the zonal winds vary periodically on a decadal timescale between superrotation and subrotation. Both subrotating and superrotating mid-latitude jets are found in the approximate 40-60 km altitude range. The growth and decay of the sub-cloud level jets also occur on the decadal timescale. Though subrotating zonal winds are found below the clouds, the total angular momentum of the atmosphere is always in the sense of superrotation. The global relative angular momentum of the atmosphere oscillates with an amplitude of about 5% on the approximate 10 year timescale. Symmetric instability in the near surface equatorial atmosphere might be the source of the decadal oscillation in the atmospheric state. Analyses of angular momentum transport show that all the jets are built up by poleward transport by a meridional circulation while angular momentum is redistributed to lower latitudes primarily by transient eddies. Possible changes in the structure of Venus cloud level mid-latitude jets measured by Mariner 10, Pioneer Venus, and Venus Express suggest that a cyclic variation similar to that found in the model might occur in the real Venus atmosphere, although no subrotating winds below the cloud level have been observed to date. Venus atmosphere must be observed over multi-year timescales and below the clouds if we are to understand its dynamics.

P.-Y. Meslin, R. Gough, F. Lefèvre, and F. Forget. Little variability of methane on Mars induced by adsorption in the regolith. Planetary and Space Science, 59:247-258, 2011. [ bib | DOI | ADS link ]

The mechanisms that can induce short term variations of methane in the Martian atmosphere, and thus explain the observations currently available, are yet to be discovered. Seasonal exchange with the regolith, caused by reversible adsorption, is expected to induce both spatial and time variabilities without the need for additional sources and sinks, thus avoiding difficulties raised by other scenarios. However, a comprehensive view of the role of reversible exchanges with the subsurface was still lacking. We have investigated the efficiency of such a process by implementing a coupled subsurface-atmosphere transport module in a Global Climate Model, taking into account both the thermodynamics and the kinetics of the adsorption process. It is based on recent experimental data on the adsorption of methane. We show that even with an optimistic set of parameters, and although the regolith can potentially take up a large fraction of the atmospheric reservoir, the seasonal variability induced by an exchange with the subsurface is very limited. If a local plume is detected, however, the apparent decay rate of methane in the atmosphere can be affected by the regolith uptake. This study could be extended to any trace gas reacting with the regolith, to help interpret future in situ or orbital measurements.

S. K. Atreya, O. Witasse, V. F. Chevrier, F. Forget, P. R. Mahaffy, P. Buford Price, C. R. Webster, and R. W. Zurek. Methane on Mars: Current observations, interpretation, and future plans. Planetary and Space Science, 59:133-136, 2011. [ bib | DOI | ADS link ]