F. González-Galindo, S. W. Bougher, M. A. López-Valverde, F. Forget, and J. Murphy. Thermal and wind structure of the Martian thermosphere as given by two General Circulation Models. Planetary and Space Science, 58:1832-1849, 2010. [ bib | DOI | ADS link ]
We have used two different General Circulation Models to study the thermal and wind structure of the Martian upper atmosphere (mesosphere and thermosphere). Both models take into account the effects of waves propagating from the lower atmosphere, although they use different methods for this purpose. We present the results of three simulations that allow us to take into account the seasonal variability of the Martian atmosphere. Simplified dust scenarios and a common set of input parameters are used. The temperatures and winds predicted by both models show an overall good agreement. However, some differences have been identified, generally of a local nature. The magnitude of these differences tends to increase with the amount of dust in the lower atmosphere. The different heating terms of the upper atmosphere predicted by both models are in good agreement, which suggests that the differences between the models have their origin in the propagation of waves from the lower atmosphere. This study has allowed us to confirm the important role of the redistribution of the energy by the winds in producing the longitude-latitude structure of the temperatures. Both models predict also a thermospheric polar warming during the Southern summer solstice, although its intensity seems to be model-dependent and connected to lower atmosphere dust loading.
R. Wordsworth, F. Forget, and V. Eymet. Infrared collision-induced and far-line absorption in dense CO 2 atmospheres. Icarus, 210:992-997, 2010. [ bib | DOI | ADS link ]
Collision-induced absorption is of great importance to the overall radiative budget in dense CO 2-rich atmospheres, but its representation in climate models remains uncertain, mainly due to a lack of accurate experimental and theoretical data. Here we compare several parameterisations of the effect, including a new one that makes use of previously unused measurements in the 1200-1800 cm -1 spectral range. We find that a widely used parameterisation strongly overestimates absorption in pure CO 2 atmospheres compared to later results, and propose a new approach that we believe is the most accurate possible given currently available data.
R. D. Wordsworth, F. Forget, F. Selsis, J.-B. Madeleine, E. Millour, and V. Eymet. Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling. Astronomy Astrophysics, 522:A22, 2010. [ bib | DOI | arXiv | ADS link ]
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.
A. Spiga, F. Forget, S. R. Lewis, and D. P. Hinson. Correction to: `Structure and dynamics of the convective boundary layer on Mars as inferred from large-eddy simulations and remote-sensing measurements'. Quarterly Journal of the Royal Meteorological Society, 136:2205-2206, 2010. [ bib | DOI | ADS link ]
M. Vincendon, F. Forget, and J. Mustard. Water ice at low to midlatitudes on Mars. Journal of Geophysical Research (Planets), 115:E10001, 2010. [ bib | DOI | arXiv | ADS link ]
In this paper, we analyze water ice occurrences at the surface of Mars using near-infrared observations, and we study their distribution with a climate model. Latitudes between 45degS and 50degN are considered. Data from the Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Actitité and the Compact Reconnaissance Imaging Spectrometer for Mars are used to assess the presence of surface water ice as a function of location and season. A modeling approach combining the 1-D and 3-D versions of the General Circulation Model of the Laboratoire de Météorologie Dynamique de Jussieu is developed and successfully compared to observations. Ice deposits 2-200 μm thick are observed during the day on pole facing slopes in local fall, winter, and early spring. Ice extends down to 13deg latitude in the Southern Hemisphere but is restricted to latitudes higher than 32deg in the north. On a given slope, the pattern of ice observations at the surface is mainly controlled by the global variability of atmospheric water (precipitation and vapor), with local ground properties playing a lower role. Only seasonal surface ice is observed: no exposed patches of perennial ground ice have been detected. Surface seasonal ice is however sensitive to subsurface properties: the results presented in this study are consistent with the recent discovery of low-latitude subsurface ice obtained through the analysis of CO2 frost.
A. V. Pathare, M. R. Balme, S. M. Metzger, A. Spiga, M. C. Towner, N. O. Renno, and F. Saca. Assessing the power law hypothesis for the size-frequency distribution of terrestrial and martian dust devils. Icarus, 209:851-853, 2010. [ bib | DOI | ADS link ]
Competing hypotheses for the diameter dependence of terrestrial and martian dust devil frequency are assessed using new field observations from two sites in the southwestern United States. We show that at diameters less than 12 m, our observed dust devil size-frequency distributions are better fit by an exponential function than by a power law formulation, and discuss the implications for larger dust devils on Earth and Mars.
S. Guerlet, T. Fouchet, B. Bézard, J. I. Moses, L. N. Fletcher, A. A. Simon-Miller, and F. Michael Flasar. Meridional distribution of CH3C2H and C4H2 in Saturns stratosphere from CIRS/Cassini limb and nadir observations. Icarus, 209:682-695, 2010. [ bib | DOI | ADS link ]
Limb and nadir spectra acquired by Cassini/CIRS (Composite InfraRed Spectrometer) are analyzed in order to derive, for the first time, the meridional variations of diacetylene (C 4H 2) and methylacetylene (CH 3C 2H) mixing ratios in Saturn's stratosphere, from 5 hPa up to 0.05 hPa and 80degS to 45degN. We find that the C 4H 2 and CH 3C 2H meridional distributions mimic that of acetylene (C 2H 2), exhibiting small-scale variations that are not present in photochemical model predictions. The most striking feature of the meridional distribution of both molecules is an asymmetry between mid-southern and mid-northern latitudes. The mid-southern latitudes are found depleted in hydrocarbons relative to their northern counterparts. In contrast, photochemical models predict similar abundances at north and south mid-latitudes. We favor a dynamical explanation for this asymmetry, with upwelling in the south and downwelling in the north, the latter coinciding with the region undergoing ring shadowing. The depletion in hydrocarbons at mid-southern latitudes could also result from chemical reactions with oxygen-bearing molecules. Poleward of 60degS, at 0.1 and 0.05 hPa, we find that the CH 3C 2H and C 4H 2 abundances increase dramatically. This behavior is in sharp contradiction with photochemical model predictions, which exhibit a strong decrease towards the south pole. Several processes could explain our observations, such as subsidence, a large vertical eddy diffusion coefficient at high altitudes, auroral chemistry that enhances CH 3C 2H and C 4H 2 production, or shielding from photolysis by aerosols or molecules produced from auroral chemistry. However, problems remain with all these hypotheses, including the lack of similar behavior at lower altitudes. Our derived mean mixing ratios at 0.5 hPa of (2.4 0.3) × 10 -10 for C 4H 2 and of (1.1 0.3) × 10 -9 for CH 3C 2H are compatible with the analysis of global-average ISO observations performed by Moses et al. (Moses, J.I., Bézard, B., Lellouch, E., Gladstone, G.R., Feuchtgruber, H., Allen, M. [2000a]. Icarus 143, 244-298). Finally, we provide values for the ratios [CH 3C 2H]/[C 2H 2] and [C 4H 2]/[C 2H 2] that can constrain the coupled chemistry of these hydrocarbons.
A. Määttänen, F. Montmessin, B. Gondet, F. Scholten, H. Hoffmann, F. González-Galindo, A. Spiga, F. Forget, E. Hauber, G. Neukum, J.-P. Bibring, and J.-L. Bertaux. Mapping the mesospheric CO 2 clouds on Mars: MEx/OMEGA and MEx/HRSC observations and challenges for atmospheric models. Icarus, 209:452-469, 2010. [ bib | DOI | ADS link ]
This study presents the latest results on the mesospheric CO 2 clouds in the martian atmosphere based on observations by OMEGA and HRSC onboard Mars Express. We have mapped the mesospheric CO 2 clouds during nearly three martian years of OMEGA data yielding a cloud dataset of 60 occurrences. The global mapping shows that the equatorial clouds are mainly observed in a distinct longitudinal corridor, at seasons Ls = 0-60deg and again at and after Ls = 90deg. A recent observation shows that the equatorial CO 2 cloud season may start as early as at Ls = 330deg. Three cases of mesospheric midlatitude autumn clouds have been observed. Two cloud shadow observations enabled the mapping of the cloud optical depth ( τ = 0.01-0.6 with median values of 0.13-0.2 at λ = 1 μm) and the effective radii (mainly 1-3 μm with median values of 2.0-2.3 μm) of the cloud crystals. The HRSC dataset of 28 high-altitude cloud observations shows that the observed clouds reside mainly in the altitude range 60-85 km and their east-west speeds range from 15 to 107 m/s. Two clouds at southern midlatitudes were observed at an altitude range of 53-62 km. The speed of one of these southern midlatitude clouds was measured, and it exhibited west-east oriented speeds between 5 and 42 m/s. The seasonal and geographical distribution as well as the observed altitudes are mostly in line with previous work. The LMD Mars Global Climate Model shows that at the cloud altitude range (65-85 km) the temperatures exhibit significant daily variability (caused by the thermal tides) with the coldest temperatures towards the end of the afternoon. The GCM predicts the coldest temperatures of this altitude range and the season Ls = 0-30deg in the longitudinal corridor where most of the cloud observations have been made. However, the model does not predict supersaturation, but the GCM-predicted winds are in fair agreement with the HRSC-measured cloud speeds. The clouds exhibit variable morphologies, but mainly cirrus-type, filamented clouds are observed (nearly all HRSC observations and most of OMEGA observations). In 15% of OMEGA observations, clumpy, round cloud structures are observed, but very few clouds in the HRSC dataset show similar morphology. These observations of clumpy, cumuliform-type clouds raise questions on the possibility of mesospheric convection on Mars, and we discuss this hypothesis based on Convective Available Potential Energy calculations.
D. Cordier, O. Mousis, J. I. Lunine, S. Lebonnois, P. Lavvas, L. Q. Lobo, and A. G. M. Ferreira. About the Possible Role of Hydrocarbon Lakes in the Origin of Titan's Noble Gas Atmospheric Depletion. Astrophysical Journal, 721:L117-L120, 2010. [ bib | DOI | arXiv | ADS link ]
An unexpected feature of Titan's atmosphere is the strong depletion in primordial noble gases revealed by the Gas Chromatograph Mass Spectrometer aboard the Huygens probe during its descent on 2005 January 14. Although several plausible explanations have already been formulated, no definitive response to this issue has yet been found. Here, we investigate the possible sequestration of these noble gases in the liquid contained in lakes and wet terrains on Titan and the consequences for their atmospheric abundances. Considering the atmosphere and the liquid existing on the soil as a whole system, we compute the abundance of each noble gas relative to nitrogen. To do so, we make the assumption of thermodynamic equilibrium between the liquid and the atmosphere, the abundances of the different constituents being determined via regular solution theory. We find that xenon's atmospheric depletion can be explained by its dissolution at ambient temperature in the liquid presumably present on Titan's soil. In the cases of argon and krypton, we find that the fractions incorporated in the liquid are negligible, implying that an alternative mechanism must be invoked to explain their atmospheric depletion.
D. Grassi, A. Migliorini, L. Montabone, S. Lebonnois, A. Cardesìn-Moinelo, G. Piccioni, P. Drossart, and L. V. Zasova. Thermal structure of Venusian nighttime mesosphere as observed by VIRTIS-Venus Express. Journal of Geophysical Research (Planets), 115:E09007, 2010. [ bib | DOI | ADS link ]
The mapping IR channel of the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS-M) on board the Venus Express spacecraft observes the CO2 band at 4.3 μm at a spectral resolution adequate to retrieve the atmospheric temperature profiles in the 65-96 km altitude range. Observations acquired in the period June 2006 to July 2008 were used to derive average temperature fields as a function of latitude, subsolar longitude (i.e., local time, LT), and pressure. Coverage presented here is limited to the nighttime because of the adverse effects of daytime non-LTE emission on the retrieval procedure and to southernmost latitudes because of the orientation of the Venus-Express orbit. Maps of air temperature variability are also presented as the standard deviation of the population included in each averaging bin. At the 100 mbar level (about 65 km above the reference surface), temperatures tend to decrease from the evening to the morning side despite a local maximum observed around 20-21LT. The cold collar is evident around 65S, with a minimum temperature at 3LT. Moving to higher altitudes, local time trends become less evident at 12.6 mbar (about 75 km) where the temperature monotonically increases from middle latitudes to the southern pole. Nonetheless, at this pressure level, two weaker local time temperature minima are observed at 23LT and 2LT equatorward of 60S. Local time trends in temperature reverse about 85 km, where the morning side is the warmer. The variability at the 100 mbar level is maximum around 80S and stronger toward the morning side. Moving to higher altitudes, the morning side always shows the stronger variability. Southward of 60S, standard deviation presents minimum values around 12.6 mbar for all the local times.
T. Encrenaz, T. K. Greathouse, B. Bézard, T. Fouchet, F. Lefèvre, F. Montmessin, M. Bitner, A. Kruger, M. J. Richter, J. H. Lacy, F. Forget, and S. K. Atreya. Water vapor map of Mars near summer solstice using ground-based infrared spectroscopy. Astronomy Astrophysics, 520:A33, 2010. [ bib | DOI | ADS link ]
Ground-based spatial mapping of Mars provides a unique way to retrieve the global distribution of minor atmospheric species and to study transient phenomena or possible variations with the local hour. We have obtained an instantaneous map of water vapor on Mars near summer solstice (Ls = 80deg) using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF) at Mauna Kea Observatory. Data have been obtained in the 1230-1245 cm-1 range (λ = 8.1 μm), with a spatial resolution of 1.1 arcsec (after convolution) and a spectral resolution of 0.012 cm-1 (R = 105). The map has been retrieved from the line depth of a weak HDO transition, compared with the line depth of a weak CO2 nearby transition. The TEXES map exhibits a strong maximum around the northern pole, as expected from previous observations and from climate model predictions. More interestingly, it shows longitudinal variations, both at high northern latitudes and at mid-latitudes, in close agreement with the predictions of the Global Climate Model developed at the Laboratoire de Meteorologie Dynamique (LMD GCM). The inferred water vapor mixing ratio is also in good agreement with the model predictions. The longitudinal variations at mid latitudes show a general enhancement toward the east. They do not seem to be due to the effect of local hour, but can be explained by dynamical effects generated by the topography. The map of surface temperatures, inferred from the continuum flux, is surprisingly different from the map expected from the climate models; the source of this discrepancy is still unclear.
F. Schmidt, B. Schmitt, S. Douté, F. Forget, J.-J. Jian, P. Martin, Y. Langevin, J.-P. Bibring, and OMEGA Team. Sublimation of the Martian CO 2 Seasonal South Polar Cap. Planetary and Space Science, 58:1129-1138, 2010. [ bib | DOI | arXiv | ADS link ]
The polar condensation/sublimation of CO 2, that involve about one fourth of the atmosphere mass, is the major Martian climatic cycle. Early observations in visible and thermal infrared have shown that the sublimation of the Seasonal South Polar Cap (SSPC) is not symmetric around the geographic South Pole. Here we use observations by OMEGA/Mars Express in the near-infrared to detect unambiguously the presence of CO 2 at the surface, and to estimate albedo. Second, we estimate the sublimation of CO 2 released in the atmosphere and show that there is a two-step process. From Ls=180deg to 220deg, the sublimation is nearly symmetric with a slight advantage for the cryptic region. After Ls=220deg the anti-cryptic region sublimation is stronger. Those two phases are not balanced such that there is 22% 9 more mass the anti-cryptic region, arguing for more snow precipitation. We compare those results with the MOLA height measurements. Finally we discuss implications for the Martian atmosphere about general circulation and gas tracers, e.g. Ar.
G. A. Morgan, J. W. Head, F. Forget, J.-B. Madeleine, and A. Spiga. Gully formation on Mars: Two recent phases of formation suggested by links between morphology, slope orientation and insolation history. Icarus, 208:658-666, 2010. [ bib | DOI | ADS link ]
The unusual 80 km diameter Noachian-aged Asimov crater in Noachis Terra (46degS, 5degE) is characterized by extensive Noachian-Hesperian crater fill and a younger superposed annulus of valleys encircling the margins of the crater floor. These valleys provide an opportunity to study the relationships of gully geomorphology as a function of changing slope orientation relative to solar insolation. We found that the level of development of gullies was highly correlated with slope orientation and solar insolation. The largest and most complex gully systems, with the most well-developed fluvial landforms, are restricted to pole-facing slopes. In contrast, gullies on equator-facing slopes are smaller, more poorly developed and integrated, more highly degraded, and contain more impact craters. We used a 1D version of the Laboratoire de Météorologie Dynamique GCM, and slope geometries (orientation and angle), driven by predicted spin-axis/orbital parameter history, to assess the distribution and history of surface temperatures in these valleys during recent geological history. Surface temperatures on pole-facing slopes preferential for water ice accumulation and subsequent melting are predicted to occur as recently as 0.5-2.1 Ma, which is consistent with age estimates of gully activity elsewhere on Mars. In contrast, the 1D model predicts that water ice cannot accumulate on equator-facing slopes until obliquities exceed 45deg, suggesting they are unlikely to have been active over the last 5 Ma. The correlation of the temperature predictions and the geological evidence for age differences suggests that there were two phases of gully formation in the last few million years: an older phase in which top-down melting occurred on equator-facing slopes and a younger more robust phase on pole-facing slopes. The similarities of small-scale fluvial erosion features seen in the gullies on Mars and those observed in gullies cut by seasonal and perennial snowmelt in the Antarctic Dry Valleys supports a top-down melting origin for these gullies on Mars.
R. J. Lillis, S. W. Bougher, F. González-Galindo, F. Forget, M. D. Smith, and P. C. Chamberlin. Four Martian years of nightside upper thermospheric mass densities derived from electron reflectometry: Method extension and comparison with GCM simulations. Journal of Geophysical Research (Planets), 115:E07014, 2010. [ bib | DOI | ADS link ]
The long-term dynamics of the Martian upper thermosphere near the exobase (160-200 km) are still relatively poorly constrained by data. Electron reflectometry (ER) provides a way to derive, from electron loss cones, neutral mass densities at these altitudes in the night hemisphere. Because the Mars Global Surveyor Electron Reflectometer was not designed for this purpose, uncertainties in individual measurements are large and thus upper thermospheric variability can be characterized only on time scales of weeks or longer. Density measurements are presented at 2 A.M. local time and 185 km altitude, from April 1999 until November 2006, spanning 4 Martian years. We observe a weaker correlation with lower atmospheric dust activity than is seen in the lower thermosphere and a weaker correlation with solar EUV flux than is observed in the dayside exosphere. Seasonally repeating features are (1) overall expansion/contraction of the nighttime thermosphere with heliocentric distance, (2) much lower densities at the aphelion winter pole compared to the perihelion winter pole, and (3) a short-lived local density maximum at aphelion in the southern hemisphere. Interannual differences are also observed; in particular, the interval of low densities in the southern winter occurs progressively later as solar EUV flux decreases from solar maximum to solar minimum. Results are compared with predictions from the Mars Thermosphere General Circulation Model and LMD Mars Global Circulation atmospheric model frameworks for Ls = 90deg-180deg, which generally underestimate and overestimate neutral densities, respectively. This disagreement reflects the difficulty in simulating nightside dynamical and cooling processes.
L. N. Fletcher, R. K. Achterberg, T. K. Greathouse, G. S. Orton, B. J. Conrath, A. A. Simon-Miller, N. Teanby, S. Guerlet, P. G. J. Irwin, and F. M. Flasar. Seasonal change on Saturn from Cassini/CIRS observations, 2004-2009. Icarus, 208:337-352, 2010. [ bib | DOI | ADS link ]
Five years of thermal infrared spectra from the Cassini Composite Infrared Spectrometer (CIRS) are analyzed to determine the response of Saturn's atmosphere to seasonal changes in insolation. Hemispheric mapping sequences at 15.0 cm -1 spectral resolution are used to retrieve the variation in the zonal mean temperatures in the stratosphere (0.5-5.0 mbar) and upper troposphere (75-800 mbar) between October 2004 (shortly after the summer solstice in the southern hemisphere) and July 2009 (shortly before the autumnal equinox). Saturn's northern mid-latitudes show signs of dramatic warming in the stratosphere (by 6-10 K) as they emerge from ring-shadow into springtime conditions, whereas southern mid-latitudes show evidence for cooling (4-6 K). The 40-K asymmetry in stratospheric temperatures between northern and southern hemispheres (at 1 mbar) slowly decreased during the timespan of the observations. Tropospheric temperatures also show temporal variations but with a smaller range, consistent with the increasing radiative time constant of the atmospheric response with increasing pressure. The tropospheric response to the insolation changes shows the largest magnitude at the locations of the broad retrograde jets. Saturn's warm south-polar stratospheric hood has cooled over the course of the mission, but remains present. Stratospheric temperatures are compared to a radiative climate model which accounts for the spatial distribution of the stratospheric coolants. The model successfully predicts the magnitude and morphology of the observed changes at most latitudes. However, the model fails at locations where strong dynamical perturbations dominate the temporal changes in the thermal field, such as the hot polar vortices and the equatorial semi-annual oscillation (Orton, G., and 27 colleagues . Nature 453, 196-198). Furthermore, observed temperatures in Saturn's ring-shadowed regions are larger than predicted by all radiative-climate models to date due to the incomplete characterization of the dynamical response to the shadow. Finally, far-infrared CIRS spectra are used to demonstrate variability of the para-hydrogen distribution over the 5-year span of the dataset, which may be related to observed changes in Saturn's tropospheric haze in the spring hemisphere.
S. Lebonnois, F. Hourdin, V. Eymet, A. Crespin, R. Fournier, and F. Forget. Superrotation of Venus' atmosphere analyzed with a full general circulation model. Journal of Geophysical Research (Planets), 115:E06006, 2010. [ bib | DOI | ADS link ]
A general circulation model (GCM) has been developed for the Venus atmosphere, from the surface up to 100 km altitude, based on the GCM developed for Earth at our laboratory. Key features of this new GCM include topography, diurnal cycle, dependence of the specific heat on temperature, and a consistent radiative transfer module based on net exchange rate matrices. This allows a consistent computation of the temperature field, in contrast to previous GCMs of Venus atmosphere that used simplified temperature forcing. The circulation is analyzed after 350 Venus days (111 Earth years). Superrotation is obtained above roughly 40 km altitude. Below, the zonal wind remains very small compared to observed values, which is a major pending question. The meridional circulation consists of equator-to-pole cells, the dominant one being located within the cloud layers. The modeled temperature structure is globally consistent with observations, though discrepancies persist in the stability of the lowest layers and equator-pole temperature contrast within the clouds (10 K in the model compared to the observed 40 K). In agreement with observational data, a convective layer is found between the base of the clouds (around 47 km) and the middle of the clouds (55-60 km altitude). The transport of angular momentum is analyzed, and comparison between the reference simulation and a simulation without diurnal cycle illustrates the role played by thermal tides in the equatorial region. Without diurnal cycle, the Gierasch-Rossow-Williams mechanism controls angular momentum transport. The diurnal tides add a significant downward transport of momentum in the equatorial region, causing low latitude momentum accumulation.
T. L. McDunn, S. W. Bougher, J. Murphy, M. D. Smith, F. Forget, J.-L. Bertaux, and F. Montmessin. Simulating the density and thermal structure of the middle atmosphere (80-130 km) of Mars using the MGCM-MTGCM: A comparison with MEX/SPICAM observations. Icarus, 206:5-17, 2010. [ bib | DOI | ADS link ]
The objective of this work is to advance the understanding of the structure and dynamics of the middle altitude region of the martian atmosphere. While numerous advancements have been made in the level of scientific understanding of Mars's upper and lower atmospheres over the past several decades, insight into the mechanisms of the middle atmosphere has come at a significantly slower pace due to the small number of datasets available for this region. Over the past decade this has begun to change, with renewed interest by NASA and ESA to send spacecraft to Mars. The result of these recent efforts is a growing database for Mars's middle atmosphere, enabling long-awaited and necessary studies characterizing the middle altitude region. Various numerical models of the martian atmosphere can now be validated and constrained using this database. We utilize the Mars Express/Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (MEX/SPICAM) density and temperature datasets to characterize the middle atmosphere as well as validate and constrain the coupled multi-dimensional Mars General Circulation Model-Mars Thermosphere General Circulation Model (MGCM-MTGCM) at middle altitudes in order to explore the underlying physics controlling the structure and dynamics at these levels. The results of this study stress the importance of proper dust prescription within the MGCM-MTGCM for accurately reproducing the density and thermal structure of the middle and upper atmosphere regions on Mars. Simulations conducted with horizontal dust opacities that are consistent with the SPICAM observation period (i.e. Mars Odyssey/THEMIS opacities) yield modeled densities and temperatures that are closer to the observed values than simulations conducted with “typical” dust conditions (i.e. Mars Global Surveyor/TES opacities). We show that the MGCM-MTGCM closely reproduces the observed densities during low-dust and high-dust scenarios but displays difficulty during the pre-dust-season “ramp-up” period ( L s 120-200deg) during MY27. In addition, we show that the MGCM-MTGCM accurately reproduces the temperature profiles below the mesopause, but, the mesopause altitude is too low and its temperature warmer (5-10 K) than observations. This may be related to nightside dynamical heating processes that require further refinement. In addition, CO 2 15-μm cooling rates may be too small, which would be consistent with underestimated atomic O abundances.
A. Spiga, F. Forget, S. R. Lewis, and D. P. Hinson. Structure and dynamics of the convective boundary layer on Mars as inferred from large-eddy simulations and remote-sensing measurements. Quarterly Journal of the Royal Meteorological Society, 136:414-428, 2010. [ bib | ADS link ]
The structure of the Martian convective boundary layer (BL) is decribed by means of a novel approach involving both modelling and data analysis. Mars Express radio-occultation (RO) temperature profiles are compared to large-eddy simulations (LESs) performed with the Martian mesoscale model. The model combines the Martian radiative transfer, soil and surface layer schemes designed at Laboratoire de MÃtÃorologie Dynamique (LMD) with the most recent version of the Weather Research and Forecast (WRF) fully compressible non-hydrostatic dynamical core. The key roles of the vertical resolution and, to lesser extent, of the domain horizontal extent have been investigated to ensure the robustness of the LES results. The dramatic regional variations of the BL depth are quantitatively reproduced by the Martian LES. Intense BL dynamics are found to underlie the measured depths (up to 9 km): vertical speed up to 20 m s-1, heat flux up to 2.7 K m s-1 and turbulent kinetic energy up to 26 m2 s-2. Under specific conditions, both the model and the measurements show a distinctive positive correlation between surface topography and BL depth. Our interpretation is that, in the tenuous CO2 Martian near-surface environment, the daytime BL is to first order controlled by the infrared radiative heating, fairly independent of elevation, which implies a simple correlation between the BL potential temperature and the inverse pressure (“pressure effect”). No prominent “pressure effect” is in action on Earth where sensible heat flux dominates the BL energy budget. Both RO observations and numerical simulations confirm the terrain-following behaviour of near-surface temperature on Mars induced by the dominant radiative influence. The contribution of the Martian sensible heat flux is not negligible and results in a given isotherm in the BL being comparatively closer to the ground at higher surface elevation. The strong radiative control of the Martian convective BL implies a generalised formulation for the BL dimensionless quantities. Based on this formulation and the variety of simulated BL depths by the LES, new similarity relationships for the Martian convective BL in quasi-steady midday conditions are derived. Rigorous comparisons between the Martian and terrestrial BL and fast computations of the mean Martian BL turbulent statistics are now made possible by such similarity laws.
M. Vincendon, J. Mustard, F. Forget, M. Kreslavsky, A. Spiga, S. Murchie, and J.-P. Bibring. Near-tropical subsurface ice on Mars. Geophysical Research Letters, 37:L01202, 2010. [ bib | DOI | arXiv | ADS link ]
Near-surface perennial water ice on Mars has been previously inferred down to latitudes of about 45deg and could result from either water vapor diffusion through the regolith under current conditions or previous ice ages precipitations. In this paper we show that at latitudes as low as 25deg in the southern hemisphere buried water ice in the shallow (1 m) subsurface is required to explain the observed surface distribution of seasonal CO2 frost on pole facing slopes. This result shows that possible remnants of the last ice age, as well as water that will be needed for the future exploration of Mars, are accessible significantly closer to the equator than previously thought, where mild conditions for both robotic and human exploration lie.
A. Spiga and S. R. Lewis. Martian mesoscale and microscale wind variability of relevance for dust lifting. International Journal of Mars Science and Exploration, 5:146-158, 2010. [ bib | DOI | ADS link ]
Background: Mars is both a windy and dusty environment. Ariborne dust is a crucial climate component on Mars. It impacts atmospheric circulations at large-, meso- and micro-scales, which in turn control dust lifting from the surface and transport in the atmosphere. Dust lifting processes and feedbacks on atmospheric circulations are currently not well understood. Method: Our purpose is to show how mesoscale models and large-eddy simulations help to explore small-scale circulation patterns which are potentially important for lifting dust into the atmosphere but which are unresolved by global climate models. We focus on variations of friction velocity, u*, relevant for dust lifting, in particular investigating maximum values and the spatial and temporal variability of u*. Conclusion: Meteorological scales between 100 km and 10 km can be studied by high-resolution global circulation and limited-area mesoscale models, which both show strong topographic control of the daytime and nighttime near-surface winds. Scales below 10 km and 1 km are dominated by turbulent gusts and dust devils, two distinct convective boundary layer processes likely to lift dust from the surface. In low-latitude regions, boundary layer depth and friction velocity u* are correlated with surface altimetry. Further studies will be carried out to parameterize lifting by boundary layer processes and dust radiative effects once transported in the atmosphere.