2012 .

(25 publications)

S. Lebonnois, C. Covey, A. Grossman, H. Parish, G. Schubert, R. Walterscheid, P. Lauritzen, and C. Jablonowski. Angular momentum budget in General Circulation Models of superrotating atmospheres: A critical diagnostic. Journal of Geophysical Research (Planets), 117:E12004, 2012. [ bib | DOI | ADS link ]

To help understand the large disparity in the results of circulation modeling for the atmospheres of Titan and Venus, where the whole atmosphere rotates faster than the surface (superrotation), the atmospheric angular momentum budget is detailed for two General Circulation Models (GCMs). The LMD GCM is tested for both Venus (with simplified and with more realistic physical forcings) and Titan (realistic physical forcings). The Community Atmosphere Model is tested for both Earth and Venus with simplified physical forcings. These analyses demonstrate that errors related to atmospheric angular momentum conservation are significant, especially for Venus when the physical forcings are simplified. Unphysical residuals that have to be balanced by surface friction and mountain torques therefore affect the overall circulation. The presence of topography increases exchanges of angular momentum between surface and atmosphere, reducing the impact of these numerical errors. The behavior of GCM dynamical cores with regard to angular momentum conservation under Venus conditions provides an explanation of why recent GCMs predict dissimilar results despite identical thermal forcing. The present study illustrates the need for careful and detailed analysis of the angular momentum budget for any GCM used to simulate superrotating atmospheres.

J.-B. Madeleine, F. Forget, E. Millour, T. Navarro, and A. Spiga. The influence of radiatively active water ice clouds on the Martian climate. Geophysical Research Letters, 39:L23202, 2012. [ bib | DOI | ADS link ]

Radiatively active water ice clouds (RAC) play a key role in shaping the thermal structure of the Martian atmosphere. In this paper, RAC are implemented in the LMD Mars Global Climate Model (GCM) and the simulated temperatures are compared to Thermal Emission Spectrometer observations over a full year. RAC change the temperature gradients and global dynamics of the atmosphere and this change in dynamics in turn implies large-scale adiabatic temperature changes. Therefore, clouds have both a direct and indirect effect on atmospheric temperatures. RAC successfully reduce major GCM temperature biases, especially in the regions of formation of the aphelion cloud belt where a cold bias of more than 10 K is corrected. Departures from the observations are however seen in the polar regions, and highlight the need for better modeling of cloud formation and evolution.

M. R. Balme, A. Pathare, S. M. Metzger, M. C. Towner, S. R. Lewis, A. Spiga, L. K. Fenton, N. O. Renno, H. M. Elliott, F. A. Saca, T. I. Michaels, P. Russell, and J. Verdasca. Field measurements of horizontal forward motion velocities of terrestrial dust devils: Towards a proxy for ambient winds on Mars and Earth. Icarus, 221:632-645, 2012. [ bib | DOI | ADS link ]

Dust devils - convective vortices made visible by the dust and debris they entrain - are common in arid environments and have been observed on Earth and Mars. Martian dust devils have been identified both in images taken at the surface and in remote sensing observations from orbiting spacecraft. Observations from landing craft and orbiting instruments have allowed the dust devil translational forward motion (ground velocity) to be calculated, but it is unclear how these velocities relate to the local ambient wind conditions, for (i) only model wind speeds are generally available for Mars, and (ii) on Earth only anecdotal evidence exists that compares dust devil ground velocity with ambient wind velocity. If dust devil ground velocity can be reliably correlated to the ambient wind regime, observations of dust devils could provide a proxy for wind speed and direction measurements on Mars. Hence, dust devil ground velocities could be used to probe the circulation of the martian boundary layer and help constrain climate models or assess the safety of future landing sites.

We present results from a field study of terrestrial dust devils performed in the southwest USA in which we measured dust devil horizontal velocity as a function of ambient wind velocity. We acquired stereo images of more than a 100 active dust devils and recorded multiple size and position measurements for each dust devil. We used these data to calculate dust devil translational velocity. The dust devils were within a study area bounded by 10 m high meteorology towers such that dust devil speed and direction could be correlated with the local ambient wind speed and direction measurements.

Daily (10:00-16:00 local time) and 2-h averaged dust devil ground speeds correlate well with ambient wind speeds averaged over the same period. Unsurprisingly, individual measurements of dust devil ground speed match instantaneous measurements of ambient wind speed more poorly; a 20-min smoothing window applied to the ambient wind speed data improves the correlation. In general, dust devils travel 10-20% faster than ambient wind speed measured at 10 m height, suggesting that their ground speeds are representative of the boundary layer winds a few tens of meters above ground level. Dust devil ground motion direction closely matches the measured ambient wind direction.

The link between ambient winds and dust devil ground velocity demonstrated here suggests that a similar one should apply on Mars. Determining the details of the martian relationship between dust devil ground velocity and ambient wind velocity might require new in situ or modelling studies but, if completed successfully, would provide a quantitative means of measuring wind velocities on Mars that would otherwise be impossible to obtain.

A. Spiga. Comment on ”Observing desert dust devils with a pressure logger“ by Lorenz (2012) - insights on measured pressure fluctuations from large-eddy simulations. Geoscientific Instrumentation, Methods and Data Systems, 1:151-154, 2012. [ bib | DOI | ADS link ]

Lorenz et al. (2012) proposes to use pressure loggers for long-term field measurements in terrestrial deserts. The dataset obtained through this method features both pressure drops (reminiscent of dust devils) and periodic convective signatures. Here we use large-eddy simulations to provide an explanation for those periodic convective signatures and to argue that pressure measurements in deserts have broader applications than monitoring dust devils.

G. Tinetti, J. P. Beaulieu, T. Henning, M. Meyer, G. Micela, I. Ribas, D. Stam, M. Swain, O. Krause, M. Ollivier, E. Pace, B. Swinyard, A. Aylward, R. van Boekel, A. Coradini, T. Encrenaz, I. Snellen, M. R. Zapatero-Osorio, J. Bouwman, J. Y.-K. Cho, V. Coudé de Foresto, T. Guillot, M. Lopez-Morales, I. Mueller-Wodarg, E. Palle, F. Selsis, A. Sozzetti, P. A. R. Ade, N. Achilleos, A. Adriani, C. B. Agnor, C. Afonso, C. Allende Prieto, G. Bakos, R. J. Barber, M. Barlow, V. Batista, P. Bernath, B. Bézard, P. Bordé, L. R. Brown, A. Cassan, C. Cavarroc, A. Ciaravella, C. Cockell, A. Coustenis, C. Danielski, L. Decin, R. De Kok, O. Demangeon, P. Deroo, P. Doel, P. Drossart, L. N. Fletcher, M. Focardi, F. Forget, S. Fossey, P. Fouqué, J. Frith, M. Galand, P. Gaulme, J. I. G. Hernández, O. Grasset, D. Grassi, J. L. Grenfell, M. J. Griffin, C. A. Griffith, U. Grözinger, M. Guedel, P. Guio, O. Hainaut, R. Hargreaves, P. H. Hauschildt, K. Heng, D. Heyrovsky, R. Hueso, P. Irwin, L. Kaltenegger, P. Kervella, D. Kipping, T. T. Koskinen, G. Kovács, A. La Barbera, H. Lammer, E. Lellouch, G. Leto, M. Lopez Morales, M. A. Lopez Valverde, M. Lopez-Puertas, C. Lovis, A. Maggio, J. P. Maillard, J. Maldonado Prado, J. B. Marquette, F. J. Martin-Torres, P. Maxted, S. Miller, S. Molinari, D. Montes, A. Moro-Martin, J. I. Moses, O. Mousis, N. Nguyen Tuong, R. Nelson, G. S. Orton, E. Pantin, E. Pascale, S. Pezzuto, D. Pinfield, E. Poretti, R. Prinja, L. Prisinzano, J. M. Rees, A. Reiners, B. Samuel, A. Sánchez-Lavega, J. S. Forcada, D. Sasselov, G. Savini, B. Sicardy, A. Smith, L. Stixrude, G. Strazzulla, J. Tennyson, M. Tessenyi, G. Vasisht, S. Vinatier, S. Viti, I. Waldmann, G. J. White, T. Widemann, R. Wordsworth, R. Yelle, Y. Yung, and S. N. Yurchenko. EChO. Exoplanet characterisation observatory. Experimental Astronomy, 34:311-353, 2012. [ bib | DOI | arXiv | ADS link ]

A dedicated mission to investigate exoplanetary atmospheres represents a major milestone in our quest to understand our place in the universe by placing our Solar System in context and by addressing the suitability of planets for the presence of life. EChOthe Exoplanet Characterisation Observatoryis a mission concept specifically geared for this purpose. EChO will provide simultaneous, multi-wavelength spectroscopic observations on a stable platform that will allow very long exposures. The use of passive cooling, few moving parts and well established technology gives a low-risk and potentially long-lived mission. EChO will build on observations by Hubble, Spitzer and ground-based telescopes, which discovered the first molecules and atoms in exoplanetary atmospheres. However, EChO's configuration and specifications are designed to study a number of systems in a consistent manner that will eliminate the ambiguities affecting prior observations. EChO will simultaneously observe a broad enough spectral regionfrom the visible to the mid-infraredto constrain from one single spectrum the temperature structure of the atmosphere, the abundances of the major carbon and oxygen bearing species, the expected photochemically-produced species and magnetospheric signatures. The spectral range and resolution are tailored to separate bands belonging to up to 30 molecules and retrieve the composition and temperature structure of planetary atmospheres. The target list for EChO includes planets ranging from Jupiter-sized with equilibrium temperatures T eq up to 2,000 K, to those of a few Earth masses, with T eq u223c 300 K. The list will include planets with no Solar System analog, such as the recently discovered planets GJ1214b, whose density lies between that of terrestrial and gaseous planets, or the rocky-iron planet 55 Cnc e, with day-side temperature close to 3,000 K. As the number of detected exoplanets is growing rapidly each year, and the mass and radius of those detected steadily decreases, the target list will be constantly adjusted to include the most interesting systems. We have baselined a dispersive spectrograph design covering continuously the 0.4-16 μm spectral range in 6 channels (1 in the visible, 5 in the InfraRed), which allows the spectral resolution to be adapted from several tens to several hundreds, depending on the target brightness. The instrument will be mounted behind a 1.5 m class telescope, passively cooled to 50 K, with the instrument structure and optics passively cooled to u223c45 K. EChO will be placed in a grand halo orbit around L2. This orbit, in combination with an optimised thermal shield design, provides a highly stable thermal environment and a high degree of visibility of the sky to observe repeatedly several tens of targets over the year. Both the baseline and alternative designs have been evaluated and no critical items with Technology Readiness Level (TRL) less than 4-5 have been identified. We have also undertaken a first-order cost and development plan analysis and find that EChO is easily compatible with the ESA M-class mission framework.

R. D. Lorenz, C. E. Newman, T. Tokano, J. L. Mitchell, B. Charnay, S. Lebonnois, and R. K. Achterberg. Formulation of a wind specification for Titan late polar summer exploration. Planetary and Space Science, 70:73-83, 2012. [ bib | DOI | ADS link ]

Titan's polar regions, and its hydrocarbon lakes in particular, are of interest for future exploration. The polar conditions have considerable seasonal variation and are distinct from the equatorial environment experienced by Huygens. Thus specific environmental models are required for these regions. This paper, informed by Cassini and groundbased observations and four independent Global Circulation Models (GCMs), summarizes northern summer polar conditions (specifically, regions north of 65degN, during the 2023-2024 period, or solar longitude Ls150o-170deg) and presents a simple analytical formulation of expected, minimum and maximum winds as a function of altitude to aid spacecraft and instrument design for future exploration, with particular reference to the descent dispersions of the Titan Mare Explorer (TiME) mission concept presently under development. We also consider winds on the surface, noting that these (of relevance for impact conditions, for waves, and for wind-driven drift of a floating capsule) are weaker than those in the lowest cell in most GCMs: some previously-reported estimates of 'surface' wind speeds (actually at 90-500 m altitude) should be reduced by 20-35% to refer to the standard 10 m 'anemometer height' applicable for surface phenomena. A Weibull distribution with scale speed C=0.4 m/s and shape parameter k=2.0 embraces the GCM-predicted surface wind speeds.

V. Dehant, B. Banerdt, P. Lognonné, M. Grott, S. Asmar, J. Biele, D. Breuer, F. Forget, R. Jaumann, C. Johnson, M. Knapmeyer, B. Langlais, M. Le Feuvre, D. Mimoun, A. Mocquet, P. Read, A. Rivoldini, O. Romberg, G. Schubert, S. Smrekar, T. Spohn, P. Tortora, S. Ulamec, and S. Vennerstrøm. Future Mars geophysical observatories for understanding its internal structure, rotation, and evolution. Planetary and Space Science, 68:123-145, 2012. [ bib | DOI | ADS link ]

Our fundamental understanding of the interior of the Earth comes from seismology, geodesy, geochemistry, geomagnetism, geothermal studies, and petrology. For the Earth, measurements in those disciplines of geophysics have revealed the basic internal layering of the Earth, its dynamical regime, its thermal structure, its gross compositional stratification, as well as significant lateral variations in these quantities. Planetary interiors not only record evidence of conditions of planetary accretion and differentiation, they exert significant control on surface environments. We present recent advances in possible in-situ investigations of the interior of Mars, experiments and strategies that can provide unique and critical information about the fundamental processes of terrestrial planet formation and evolution. Such investigations applied on Mars have been ranked as a high priority in virtually every set of European, US and international high-level planetary science recommendations for the past 30 years. New seismological methods and approaches based on the cross-correlation of seismic noise by two seismic stations/landers on the surface of Mars and on joint seismic/orbiter detection of meteorite impacts, as well as the improvement of the performance of Very Broad-Band (VBB) seismometers have made it possible to secure a rich scientific return with only two simultaneously recording stations. In parallel, use of interferometric methods based on two Earth-Mars radio links simultaneously from landers tracked from Earth has increased the precision of radio science experiments by one order of magnitude. Magnetometer and heat flow measurements will complement seismic and geodetic data in order to obtain the best information on the interior of Mars. In addition to studying the present structure and dynamics of Mars, these measurements will provide important constraints for the astrobiology of Mars by helping to understand why Mars failed to sustain a magnetic field, by helping to understand the planet's climate evolution, and by providing a limit for the energy available to the chemoautotrophic biosphere through a measurement of the surface heat flow. The landers of the mission will also provide meteorological stations to monitor the climate and obtain new measurements in the atmospheric boundary layer.

R. T. Clancy, B. J. Sandor, M. J. Wolff, M. D. Smith, F. Lefèvre, J.-B. Madeleine, F. Forget, S. L. Murchie, F. P. Seelos, K. D. Seelos, H. A. Nair, A. D. Toigo, D. Humm, D. M. Kass, A. Kleinböhl, and N. Heavens. 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. Journal of Geophysical Research (Planets), 117:E00J10, 2012. [ bib | DOI | ADS link ]

The Martian polar night distribution of 1.27 μm (0-0) band emission from O2 singlet delta [O2(1Δg)] 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 CO2 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 O2(1Δg) 1.27 μm volume emission rates (VER). We also present the first detection of much (×80 20) weaker 1.58 μm (0-1) band emission from Mars O2(1Δg). Co-located polar night CRISM O2(1Δg) and Mars Climate Sounder (MCS) (McCleese et al., 2008) temperature profiles are compared to the same profiles as simulated by the Laboratoire de Météorologie Dynamique (LMD) general circulation/photochemical model (e.g., Lefèvre et al., 2004). Both standard and interactive aerosol LMD simulations (Madeleine et al., 2011a) underproduce CRISM O2(1Δg) 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 O2(1Δg) 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 O2(1Δg) formation, as provided in Roble (1995).

N. Schorghofer and F. Forget. History and anatomy of subsurface ice on Mars. Icarus, 220:1112-1120, 2012. [ bib | DOI | ADS link ]

Ice buried beneath a thin layer of soil has been revealed by neutron spectroscopy and explored by the Phoenix Mars Lander. It has also been exposed by recent impacts. This subsurface ice is thought to lose and gain volume in response to orbital variations (Milankovitch cycles). We use a powerful numerical model to follow the growth and retreat of near-surface ice as a result of regolith-atmosphere exchange continuously over millions of years. If a thick layer of almost pure ice has been deposited recently, it has not yet reached equilibrium with the atmospheric water vapor and may still remain as far equatorward as 43degN, where ice has been revealed by recent impacts. A potentially observable consequence is present-day humidity output from the still retreating ice. We also demonstrate that in a sublimation environment, subsurface pore ice can accumulate in two ways. The first mode, widely known, is the progressive filling of pores by ice over a range of depths. The second mode occurs on top of an already impermeable ice layer; subsequent ice accumulates in the form of pasted on horizontal layers such that beneath the ice table, the pores are completely full with ice. Most or all of the pore ice on Mars today may be of the second type. At the Phoenix landing site, where such a layer is also expected to exist above an underlying ice sheet, it may be extremely thin, due to exceptionally small variations in ice stability over time.

F. Altieri, A. Spiga, L. Zasova, G. Bellucci, and J.-P. Bibring. Gravity waves mapped by the OMEGA/MEX instrument through O2 dayglow at 1.27 μm: Data analysis and atmospheric modeling. Journal of Geophysical Research (Planets), 117:E00J08, 2012. [ bib | DOI | ADS link ]

We present the occurrence of waves patterns on the southern polar region of Mars as traced by the O2 dayglow emission at λ = 1.27 μm during late winter/early spring of MY 28. The observations were carried out by the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) imaging spectrometer on board Mars Express (MEX). Waves are found preferentially at high incidence angles and latitudes between 55deg and 75degS. The dayglow intensity fluctuations are of the order of 3% at incidence angle 88.5deg and they can be explained by the propagation of gravity waves in the Martian atmosphere. Mesoscale meteorological modeling predicts gravity wave activity in the same range of latitude as the observed O2(a1Δg) wave patterns with temperature oscillations consistent with existing measurements. Moreover, gravity waves simulated through mesoscale modeling can induce dayglow fluctuations of the same order-of-magnitude as observed in the OMEGA maps. This study confirms that airglow imagery is a powerful method to detect and study the bi-dimensional propagation of gravity waves, as foreseen in previous studies coupling photochemical and dynamical models.

S. Oshchepkov, A. Bril, T. Yokota, I. Morino, Y. Yoshida, T. Matsunaga, D. Belikov, D. Wunch, P. Wennberg, G. Toon, C. O'Dell, A. Butz, S. Guerlet, A. Cogan, H. Boesch, N. Eguchi, N. Deutscher, D. Griffith, R. Macatangay, J. Notholt, R. Sussmann, M. Rettinger, V. Sherlock, J. Robinson, E. Kyrö, P. Heikkinen, D. G. Feist, T. Nagahama, N. Kadygrov, S. Maksyutov, O. Uchino, and H. Watanabe. Effects of atmospheric light scattering on spectroscopic observations of greenhouse gases from space: Validation of PPDF-based CO2 retrievals from GOSAT. Journal of Geophysical Research (Atmospheres), 117:D12305, 2012. [ bib | DOI | ADS link ]

This report describes a validation study of Greenhouse gases Observing Satellite (GOSAT) data processing using ground-based measurements of the Total Carbon Column Observing Network (TCCON) as reference data for column-averaged dry air mole fractions of atmospheric carbon dioxide (XCO2). We applied the photon path length probability density function method to validate XCO2retrievals from GOSAT data direct evaluation of optical path modifications due to atmospheric light scattering that would have a negligible impact on ground-based TCCON measurements but could significantly affect gas retrievals when observing reflected sunlight from space. Our results reveal effects of optical path lengthening over Northern Hemispheric stations, essentially from May-September of each year, and of optical path shortening for sun-glint observations in tropical regions. These effects are supported by seasonal trends in aerosol optical depth derived from an offline three-dimensional aerosol transport model and by cirrus optical depth derived from space-based measurements of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument. Removal of observations that were highly contaminated by aerosol and cloud from the GOSAT data set resulted in acceptable agreement in the seasonal variability of XCO2 over each station as compared with TCCON measurements. Statistical comparisons between GOSAT and TCCON coincident measurements of CO2column abundance show a correlation coefficient of 0.85, standard deviation of 1.80 ppm, and a sub-ppm negative bias of retrieved XCO2 with a spatial resolution of 2.5deg latitude × 2.5deg longitude show agreement within 2.5 ppm with those predicted by the atmospheric tracer transport model.

M. Reuter, M. Buchwitz, O. Schneising, F. Hase, J. Heymann, S. Guerlet, A. J. Cogan, H. Bovensmann, and J. P. Burrows. A simple empirical model estimating atmospheric CO2 background concentrations. Atmospheric Measurement Techniques, 5:1349-1357, 2012. [ bib | DOI | ADS link ]

A simple empirical CO2 model (SECM) is presented to estimate column-average dry-air mole fractions of atmospheric CO2 (XCO2) as well as mixing ratio profiles. SECM is based on a simple equation depending on 17 empirical parameters, latitude, and date. The empirical parameters have been determined by least squares fitting to NOAA's (National Oceanic and Atmospheric Administration) assimilation system CarbonTracker version 2010 (CT2010). Comparisons with TCCON (total carbon column observing network) FTS (Fourier transform spectrometer) measurements show that SECM XCO2 agrees quite well with reality. The synthetic XCO2 values have a standard error of 1.39 ppm and systematic station-to-station biases of 0.46 ppm. Typical column averaging kernels of the TCCON FTS, a SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY), and two GOSAT (Greenhouse gases Observing SATellite) XCO2 retrieval algorithms have been used to assess the smoothing error introduced by using SECM profiles instead of CT2010 profiles as a priori. The additional smoothing error amounts to 0.17 ppm for a typical SCIAMACHY averaging kernel and is most times much smaller for the other instruments (e.g. 0.05 ppm for a typical TCCON FTS averaging kernel). Therefore, SECM is well suited to provide a priori information for state-of-the-art ground-based (FTS) and satellite-based (GOSAT, SCIAMACHY) XCO2 retrievals. Other potential applications are: (i) near real-time processing systems (that cannot make use of models like CT2010 operated in delayed mode), (ii) “CO2 proxy” methods for XCH4 retrievals (as correction for the XCO2 background), and (iii) observing system simulation experiments especially for future satellite missions.

J.-B. Madeleine, F. Forget, A. Spiga, M. J. Wolff, F. Montmessin, M. Vincendon, D. Jouglet, B. Gondet, J.-P. Bibring, Y. Langevin, and B. Schmitt. Aphelion water-ice cloud mapping and property retrieval using the OMEGA imaging spectrometer onboard Mars Express. Journal of Geophysical Research (Planets), 117:E00J07, 2012. [ bib | DOI | ADS link ]

Mapping of the aphelion clouds over the Tharsis plateau 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 = 330deg to MY29 Ls = 180deg and are acquired at various local times, ranging from 8 AM to 6 PM. Cloud maps of the Tharsis region constructed using the 3.1 μm 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 3.5 μm. The second group is characterized by particle sizes of 5 μm 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 2.9 pr.μm on average, and can reach 5.2 pr.μm in the thickest clouds of Tharsis.

D. Schepers, S. Guerlet, A. Butz, J. Landgraf, C. Frankenberg, O. Hasekamp, J.-F. Blavier, N. M. Deutscher, D. W. T. Griffith, F. Hase, E. Kyro, I. Morino, V. Sherlock, R. Sussmann, and I. Aben. Methane retrievals from Greenhouse Gases Observing Satellite (GOSAT) shortwave infrared measurements: Performance comparison of proxy and physics retrieval algorithms. Journal of Geophysical Research (Atmospheres), 117:D10307, 2012. [ bib | DOI | ADS link ]

We compare two conceptually different methods for determining methane column-averaged mixing ratios ? from Greenhouse Gases Observing Satellite (GOSAT) shortwave infrared (SWIR) measurements. These methods account differently for light scattering by aerosol and cirrus. The proxy method retrieves a CO2 column which, in conjunction with prior knowledge on CO2acts as a proxy for scattering effects. The physics-based method accounts for scattering by retrieving three effective parameters of a scattering layer. Both retrievals are the Total Carbon Column Observing Network (TCCON), showing comparable performance: for the proxy retrieval we find station-dependent retrieval biases from -0.312% to 0.421% of ? a standard deviation of 0.22% and a typical precision of 17 ppb. The physics method shows biases between -0.836% and -0.081% with a standard deviation of 0.24% and a precision similar to the proxy method. Complementing this validation we compared both retrievals with simulated methane fields from a global chemistry-transport model. This identified shortcomings of both retrievals causing biases of up to 1ings and provide a satisfying validation of any methane retrieval from space-borne SWIR measurements, in our opinion it is essential to further expand the network of TCCON stations.

L. Kerber, J. W. Head, J.-B. Madeleine, F. Forget, and L. Wilson. The dispersal of pyroclasts from ancient explosive volcanoes on Mars: Implications for the friable layered deposits. Icarus, 219:358-381, 2012. [ bib | DOI | ADS link ]

A number of voluminous, fine-grained, friable deposits have been mapped on Mars. The modes of origin for these deposits are debated. The feasibility for an origin by volcanic airfall for the friable deposits is tested using a global circulation model to simulate the dispersal of pyroclasts from candidate source volcanoes near each deposit. It is concluded that the Medusae Fossae Formation and Electris deposits are easily formed through volcanic processes, and that the Hellas deposits and south polar pitted deposits could have some contribution from volcanic sources in specific atmospheric regimes. The Arabia and Argyre deposits are not well replicated by modeled pyroclast dispersal, suggesting that these deposits were most likely emplaced by other means.

J. L. Fastook, J. W. Head, D. R. Marchant, F. Forget, and J.-B. Madeleine. Early Mars climate near the Noachian-Hesperian boundary: Independent evidence for cold conditions from basal melting of the south polar ice sheet (Dorsa Argentea Formation) and implications for valley network formation. Icarus, 219:25-40, 2012. [ bib | DOI | ADS link ]

Currently, and throughout much of the Amazonian, the mean annual surface temperatures of Mars are so cold that basal melting does not occur in ice sheets and glaciers and they are cold-based. The documented evidence for extensive and well-developed eskers (sediment-filled former sub-glacial meltwater channels) in the south circumpolar Dorsa Argentea Formation is an indication that basal melting and wet-based glaciation occurred at the South Pole near the Noachian-Hesperian boundary. We employ glacial accumulation and ice-flow models to distinguish between basal melting from bottom-up heat sources (elevated geothermal fluxes) and top-down induced basal melting (elevated atmospheric temperatures warming the ice). We show that under mean annual south polar atmospheric temperatures (-100 degC) simulated in typical Amazonian climate experiments and typical Noachian-Hesperian geothermal heat fluxes (45-65 mW/m2), south polar ice accumulations remain cold-based. In order to produce significant basal melting with these typical geothermal heat fluxes, the mean annual south polar atmospheric temperatures must be raised from today's temperature at the surface (-100 degC) to the range of -50 to -75 degC. This mean annual polar surface atmospheric temperature range implies lower latitude mean annual temperatures that are likely to be below the melting point of water, and thus does not favor a “warm and wet” early Mars. Seasonal temperatures at lower latitudes, however, could range above the melting point of water, perhaps explaining the concurrent development of valley networks and open basin lakes in these areas. This treatment provides an independent estimate of the polar (and non-polar) surface temperatures near the Noachian-Hesperian boundary of Mars history and implies a cold and relatively dry Mars climate, similar to the Antarctic Dry Valleys, where seasonal melting forms transient streams and permanent ice-covered lakes in an otherwise hyperarid, hypothermal climate.

E. Hébrard, C. Listowski, P. Coll, B. Marticorena, G. Bergametti, A. Määttänen, F. Montmessin, and F. Forget. An aerodynamic roughness length map derived from extended Martian rock abundance data. Journal of Geophysical Research (Planets), 117:E04008, 2012. [ bib | DOI | ADS link ]

Many boundary layer processes simulated within a Mars General Circulation Model (MGCM), including the description of the processes controlling dust rising from the Martian surface, are highly sensitive to the aerodynamic roughness length z0. On the basis of rock-size frequency distributions inferred from different Martian landing sites and Earth analog sites, we have first established that lognormal-modeled rock-size frequency distributions are able to reproduce correctly the observed Martian rock populations. We have validated the hypothesis that the rock abundance ζ of a given area could be estimated at a first order from its thermophysical properties, namely its thermal inertia I and its albedo α. We have demonstrated the possibility of using rock abundance ζ to estimate the roughness density λ on Mars and to retrieve subsequently the aerodynamic roughness length by using semi-empirical relationships based on terrestrial wind-tunnel and field measurements. By combining our methodology with remote sensing measurements of the Thermal Emission Spectrometer aboard Mars Global Surveyor, we have derived a global map of the aeolian aerodynamic roughness length with a 1/8deg × 1/8deg resolution over the entire Martian surface. Contrary to what is often assumed, the Martian aeolian aerodynamic roughness length is spatially highly heterogeneous. At the fullest resolution, the Martian aerodynamic roughness length varies from 10-3 cm to 2.33 cm. About 84% of the Martian surface seems to be characterized by an aeolian aerodynamic roughness length value lower than 1 cm, the spatially uniform value that most of the MGCMs simulations have assumed recently. Since the aerodynamic roughness length z0 is a key parameter in deriving the erosion threshold wind velocities, we anticipate a significant impact of our findings on the efficiencies for lifting dust in future MGCMs.

A. L. Sprague, W. V. Boynton, F. Forget, Y. Lian, M. Richardson, R. Starr, A. E. Metzger, D. Hamara, and T. Economou. Interannual similarity and variation in seasonal circulation of Mars' atmospheric Ar as seen by the Gamma Ray Spectrometer on Mars Odyssey. Journal of Geophysical Research (Planets), 117:E04005, 2012. [ bib | DOI | ADS link ]

More than 3 Mars' years (MY) of atmospheric argon (Ar) measurements are used to study annual and seasonal variations in atmospheric transport and mixing. Data are obtained over the period 20 May 2002 to 4 May 2008 by the Gamma Subsystem (GS) of the Gamma Ray Spectrometer (GRS) on the Mars Odyssey spacecraft in orbit around Mars. Here we augment previous studies of Mars' Ar in which strong seasonal variations were observed and horizontal meridional mixing coefficients for the southern hemisphere were computed. Comparison of year-to-year seasonal abundance shows strong similarity but also some short-period (15deg-30deg Ls) and interannual variations. Evidence for short periods of strong eddy transport is exhibited during autumn and winter. The seasonal change in Ar concentration for southern latitudes is relatively gradual and well defined, but seasonal changes at high northern latitudes are chaotic and indicate that atmospheric disturbance is ubiquitous. Major topographic landforms (Elysium, Tharsis, Noachis Terra, Hellas) apparently have little control over seasonal Ar concentration at the spatial resolution of the GRS data set. Some indication of local enhanced Ar concentration is present from 30degN to 60degN for the Hellas and Tharsis sectors in late winter and early spring. The data show some significant (3σ) differences between MY 26 and MY 27 in geographical sectors that are likely produced by local weather. The GS data do not show seasonal variation of Ar at equatorial and low-latitude zones, in contrast to those from the Alpha Particle X-ray Spectrometer (APXS) measurements from the Mars Exploration Rovers.

C. F. Wilson, E. Chassefière, E. Hinglais, K. H. Baines, T. S. Balint, J.-J. Berthelier, J. Blamont, G. Durry, C. S. Ferencz, R. E. Grimm, T. Imamura, J.-L. Josset, F. Leblanc, S. Lebonnois, J. J. Leitner, S. S. Limaye, B. Marty, E. Palomba, S. V. Pogrebenko, S. C. R. Rafkin, D. L. Talboys, R. Wieler, L. V. Zasova, and C. Szopa. The 2010 European Venus Explorer (EVE) mission proposal. Experimental Astronomy, 33:305-335, 2012. [ bib | DOI | ADS link ]

The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an `M-class' mission under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective cloud layer of Venus at an altitude of 55 km, where temperatures and pressures are benign (25degC and 0.5 bar). The balloon float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet. This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets in our solar system and beyond.

S. Lebonnois, J. Burgalat, P. Rannou, and B. Charnay. Titan global climate model: A new 3-dimensional version of the IPSL Titan GCM. Icarus, 218:707-722, 2012. [ bib | DOI | ADS link ]

We have developed a new 3-dimensional climate model for Titans atmosphere, using the physics of the IPSL Titan 2-dimensional climate model with the current version of the LMDZ General Circulation Model dynamical core. Microphysics and photochemistry are still computed as zonal averages. This GCM covers altitudes from surface to 500 km altitude, with barotropic waves now being resolved and the diurnal cycle included. The boundary layer scheme has been changed, yielding a strong improvement in the tropospheric zonal wind profile modeled at Huygens descent position and season. The potential temperature profile is fairly consistent with Huygens observations in the lowest 10 km. The latitudinal profile of the near-surface temperature is close to observed values. The minimum of zonal wind observed by the Huygens probe just above the tropopause is also present in these simulations, and its origin is discussed by comparing solar heating and dynamical transport of energy. The stratospheric temperature and wind fields are consistent with our previous works. Compared to observations, the zonal wind peak is too weak (around 120 m/s) and too low (around 200 km). The temperature structures appear to be compressed in altitude, and depart strongly from observations in the upper stratosphere. These discrepancies are correlated, and most probably related to the altitude of the haze production. The model produces a detached haze layer located more than 150 km lower than observed by the Cassini instruments. This low production altitude is due to the current position of the GCM upper boundary. However, the temporal behaviour of the detached haze layer in the model may explain the seasonal differences observed between Cassini and Voyager 1. The waves present in the GCM are analyzed, together with their respective roles in the angular momentum budget. Though the role of the mean meridional circulation in momentum transport is similar to previous work, and the transport by barotropic waves is clearly seen in the stratosphere, a significant part of the transport at high latitudes is done all year long through low-frequency tropospheric waves that may be baroclinic waves.

D. Cordier, O. Mousis, J. I. Lunine, S. Lebonnois, P. Rannou, P. Lavvas, L. Q. Lobo, and A. G. M. Ferreira. Titan's lakes chemical composition: Sources of uncertainties and variability. Planetary and Space Science, 61:99-107, 2012. [ bib | DOI | arXiv | ADS link ]

Between 2004 and 2007 the instruments of the Cassini spacecraft, orbiting within the Saturn system, discovered dark patches in the polar regions of Titan. These features are interpreted as hydrocarbon lakes and seas with ethane and methane identified as the main compounds. In this context, we have developed a lake-atmosphere equilibrium model allowing the determination of the chemical composition of these liquid areas present on Titan. The model is based on uncertain thermodynamic data and precipitation rates of organic species predicted to be present in the lakes and seas that are subject to spatial and temporal variations. Here we explore and discuss the influence of these uncertainties and variations. The errors and uncertainties relevant to thermodynamic data are simulated via Monte Carlo simulations. Global circulation models (GCM) are also employed in order to investigate the possibility of chemical asymmetry between the south and the north poles, due to differences in precipitation rates. We find that mole fractions of compounds in the liquid phase have a high sensitivity to thermodynamic data used as inputs, in particular molar volumes and enthalpies of vaporization. When we combine all considered uncertainties, the ranges of obtained mole fractions are rather large (up to ~8500%) but the distributions of values are narrow. The relative standard deviations remain between 10% and ~300% depending on the compound considered. Compared to other sources of uncertainties and variability, deviation caused by surface pressure variations are clearly negligible, remaining of the order of a few percent up to ~20%. Moreover, no significant difference is found between the composition of lakes located in north and south poles. Because the theory of regular solutions employed here is sensitive to thermodynamic data and is not suitable for polar molecules such as HCN and CH3CN, our work strongly underlines the need for experimental simulations and the improvement of Titan's atmospheric models.

B. Charnay and S. Lebonnois. Two boundary layers in Titan's lower troposphere inferred from a climate model. Nature Geoscience, 5:106-109, 2012. [ bib | DOI | ADS link ]

Saturn's moon Titan has a dense atmosphere, but its thermal structure is poorly known. Conflicting information has been gathered on the nature, extent and evolution of Titan's planetary boundary layer-the layer of the atmosphere that is influenced by the surface-from radio-occultation observations by the Voyager 1 spacecraft and the Cassini orbiter, measurements by the Huygens probe and by dune-spacing analyses. Specifically, initial analyses of the Huygens data suggested a boundary layer of 300m depth with no diurnal evolution, incompatible with alternative estimates of 2-3km (refs , , ). Here we use a three-dimensional general circulation model, albeit not explicitly simulating the methane cycle, to analyse the dynamics leading to the thermal profile of Titan's lowermost atmosphere. In our simulations, a convective boundary layer develops in the course of the day, rising to an altitude of 800m. In addition, a seasonal boundary of 2km depth is produced by the reversal of the Hadley cell at the equinox, with a dramatic impact on atmospheric circulation. We interpret fog that had been discovered at Titan's south pole earlier as boundary layer clouds. We conclude that Titan's troposphere is well structured, featuring two boundary layers that control wind patterns, dune spacing and cloud formation at low altitudes.

A. Migliorini, D. Grassi, L. Montabone, S. Lebonnois, P. Drossart, and G. Piccioni. Investigation of air temperature on the nightside of Venus derived from VIRTIS-H on board Venus-Express. Icarus, 217:640-647, 2012. [ bib | DOI | ADS link ]

We present the spatial distribution of air temperature on Venus' night side, as observed by the high spectral resolution channel of VIRTIS (Visible and Infrared Thermal Imaging Spectrometer), or VIRTIS-H, on board the ESA mission Venus Express. The present work extends the investigation of the average thermal fields in the northern hemisphere of Venus, by including the VIRTIS-H data. We show results in the pressure range of 100-4 mbar, which corresponds to the altitude range of 65-80 km. With these new retrievals, we are able to compare the thermal structure of the Venus' mesosphere in both hemispheres. The major thermal features reported in previous investigations, i.e. the cold collar at about 65-70degS latitude, 100 mbar pressure level, and the asymmetry between the evening and morning sides, are confirmed here. By comparing the temperatures retrieved by the VIRTIS spectrometer in the North and South we find that similarities exist between the two hemispheres. Solar thermal tides are clearly visible in the average temperature fields. To interpret the thermal tide signals (otherwise impossible without day site observations), we apply model simulations using the Venus global circulation model Venus GCM (Lebonnois, S., Hourdin, F., Forget, F., Eymet, V., Fournier, R. [2010b]. International Venus Conference, Aussois, 20-26 June 2010) of the Laboratoire de Météorologie Dynamique (LMD). We suggest that the signal detected at about 60-70deg latitude and pressure of 100 mbar is a diurnal component, while those located at equatorial latitudes are semi-diurnal. Other tide-related features are clearly identified in the upper levels of the atmosphere.

M. Massé, O. Bourgeois, S. Le Mouélic, C. Verpoorter, A. Spiga, and L. Le Deit. Wide distribution and glacial origin of polar gypsum on Mars. Earth and Planetary Science Letters, 317:44-55, 2012. [ bib | DOI | ADS link ]

The North Polar Cap of Mars is associated with different kinds of superficial sediments, including the Circumpolar Dune Field, interior dune fields and sedimentary veneers scattered over the ice cap. In order to resolve the mineralogical composition and the regional distribution of these sediments, we processed OMEGA and CRISM hyperspectral data with an original method based on spectral derivation. We find that gypsum is present in all areas where undefined hydrated minerals had been previously detected, including superficial sedimentary veneers covering the North Polar Cap, interior dune fields and the whole Circumpolar Dune Field. Morphological and structural analyses reveal that these gypsum crystals derive directly from the interior of the ice cap. The source of superficial sedimentary veneers is the dust that was previously contained in the upper part of the ice cap, the ice-rich North Polar Layered Deposits (NPLD). This gypsum-bearing dust was released, on south-facing slopes of spiral troughs and arcuate scarps, by ice ablation controlled by katabatic winds. By the analysis of all associations of erosional scarps and dune fields over the North Polar Cap, we also demonstrate that the polar dunes are composed of sand-sized particles that were previously contained in the sediment-rich Basal Unit (BU), corresponding to the lower part of the ice cap. These particles contain gypsum and were released from the BU, by regressive ablation of ice at marginal scarps that border the North Polar Cap and by vertical ablation of ice on Olympia Planum. From a reconstruction of wind streamlines over and around the ice cap, we infer that katabatic winds descending from the polar high and rotating around the North Polar Cap control the release of these gypsum-bearing particles by ice ablation and the redistribution of these particles in the Circumpolar Dune Field.

A. Spiga, F. González-Galindo, M.-Á. López-Valverde, and F. Forget. Gravity waves, cold pockets and CO2 clouds in the Martian mesosphere. Geophysical Research Letters, 39:L02201, 2012. [ bib | DOI | ADS link ]

Many independent measurements have shown that extremely cold temperatures are found in the Martian mesosphere. These mesospheric cold pockets may result from the propagation of atmospheric waves. Recent observational achievements also hint at such cold pockets by revealing mesospheric clouds formed through the condensation of CO2, the major component of the Martian atmosphere. Thus far, modeling studies addressing the presence of cold pockets in the Martian mesosphere have explored the influence of large-scale circulations. Mesoscale phenomena, such as gravity waves, have received less attention. Here we show through multiscale meteorological modeling that mesoscale gravity waves could play a key role in the formation of mesospheric cold pockets propitious to CO2 condensation.