2015 .

(19 publications)

J. Leconte, F. Forget, and H. Lammer. On the (anticipated) diversity of terrestrial planet atmospheres. Experimental Astronomy, 40:449-467, 2015. [ bib | DOI | ADS link ]

On our way toward the characterization of smaller and more temperate planets, missions dedicated to the spectroscopic observation of exoplanets will teach us about the wide diversity of classes of planetary atmospheres, many of them probably having no equivalent in the Solar System. But what kind of atmospheres can we expect? To start answering this question, many theoretical studies have tried to understand and model the various processes controlling the formation and evolution of planetary atmospheres, with some success in the Solar System. Here, we shortly review these processes and we try to give an idea of the various type of atmospheres that these processes can create. As will be made clear, current atmosphere evolution models have many shortcomings yet, and need heavy calibrations. With that in mind, we will thus discuss how observations with a mission similar to EChO would help us unravel the link between a planet's environment and its atmosphere.

G. Tinetti, P. Drossart, P. Eccleston, P. Hartogh, K. Isaak, M. Linder, C. Lovis, G. Micela, M. Ollivier, L. Puig, I. Ribas, I. Snellen, B. Swinyard, F. Allard, J. Barstow, J. Cho, A. Coustenis, C. Cockell, A. Correia, L. Decin, R. de Kok, P. Deroo, T. Encrenaz, F. Forget, A. Glasse, C. Griffith, T. Guillot, T. Koskinen, H. Lammer, J. Leconte, P. Maxted, I. Mueller-Wodarg, R. Nelson, C. North, E. Pallé, I. Pagano, G. Piccioni, D. Pinfield, F. Selsis, A. Sozzetti, L. Stixrude, J. Tennyson, D. Turrini, M. Zapatero-Osorio, J.-P. Beaulieu, D. Grodent, M. Guedel, D. Luz, H. U. Nørgaard-Nielsen, T. Ray, H. Rickman, A. Selig, M. Swain, M. Banaszkiewicz, M. Barlow, N. Bowles, G. Branduardi-Raymont, V. C. du Foresto, J.-C. Gerard, L. Gizon, A. Hornstrup, C. Jarchow, F. Kerschbaum, G. Kovacs, P.-O. Lagage, T. Lim, M. Lopez-Morales, G. Malaguti, E. Pace, E. Pascale, B. Vandenbussche, G. Wright, G. Ramos Zapata, A. Adriani, R. Azzollini, A. Balado, I. Bryson, R. Burston, J. Colomé, M. Crook, A. Di Giorgio, M. Griffin, R. Hoogeveen, R. Ottensamer, R. Irshad, K. Middleton, G. Morgante, F. Pinsard, M. Rataj, J.-M. Reess, G. Savini, J.-R. Schrader, R. Stamper, B. Winter, L. Abe, M. Abreu, N. Achilleos, P. Ade, V. Adybekian, L. Affer, C. Agnor, M. Agundez, C. Alard, J. Alcala, C. Allende Prieto, F. J. Alonso Floriano, F. Altieri, C. A. Alvarez Iglesias, P. Amado, A. Andersen, A. Aylward, C. Baffa, G. Bakos, P. Ballerini, M. Banaszkiewicz, R. J. Barber, D. Barrado, E. J. Barton, V. Batista, G. Bellucci, J. A. Belmonte Avilés, D. Berry, B. Bézard, D. Biondi, M. Blecka, I. Boisse, B. Bonfond, P. Bordé, P. Börner, H. Bouy, L. Brown, L. Buchhave, J. Budaj, A. Bulgarelli, M. Burleigh, A. Cabral, M. T. Capria, A. Cassan, C. Cavarroc, C. Cecchi-Pestellini, R. Cerulli, J. Chadney, S. Chamberlain, S. Charnoz, N. Christian Jessen, A. Ciaravella, A. Claret, R. Claudi, A. Coates, R. Cole, A. Collura, D. Cordier, E. Covino, C. Danielski, M. Damasso, H. J. Deeg, E. Delgado-Mena, C. Del Vecchio, O. Demangeon, A. De Sio, J. De Wit, M. Dobrijévic, P. Doel, C. Dominic, E. Dorfi, S. Eales, C. Eiroa, M. Espinoza Contreras, M. Esposito, V. Eymet, N. Fabrizio, M. Fernández, B. Femenía Castella, P. Figueira, G. Filacchione, L. Fletcher, M. Focardi, S. Fossey, P. Fouqué, J. Frith, M. Galand, L. Gambicorti, P. Gaulme, R. J. García López, A. Garcia-Piquer, W. Gear, J.-C. Gerard, L. Gesa, E. Giani, F. Gianotti, M. Gillon, E. Giro, M. Giuranna, H. Gomez, I. Gomez-Leal, J. Gonzalez Hernandez, B. González Merino, R. Graczyk, D. Grassi, J. Guardia, P. Guio, J. Gustin, P. Hargrave, J. Haigh, E. Hébrard, U. Heiter, R. L. Heredero, E. Herrero, F. Hersant, D. Heyrovsky, M. Hollis, B. Hubert, R. Hueso, G. Israelian, N. Iro, P. Irwin, S. Jacquemoud, G. Jones, H. Jones, K. Justtanont, T. Kehoe, F. Kerschbaum, E. Kerins, P. Kervella, D. Kipping, T. Koskinen, N. Krupp, O. Lahav, B. Laken, N. Lanza, E. Lellouch, G. Leto, J. Licandro Goldaracena, C. Lithgow-Bertelloni, S. J. Liu, U. Lo Cicero, N. Lodieu, P. Lognonné, M. Lopez-Puertas, M. A. Lopez-Valverde, I. Lundgaard Rasmussen, A. Luntzer, P. Machado, C. MacTavish, A. Maggio, J.-P. Maillard, W. Magnes, J. Maldonado, U. Mall, J.-B. Marquette, P. Mauskopf, F. Massi, A.-S. Maurin, A. Medvedev, C. Michaut, P. Miles-Paez, M. Montalto, P. Montañés Rodríguez, M. Monteiro, D. Montes, H. Morais, J. C. Morales, M. Morales-Calderón, G. Morello, A. Moro Martín, J. Moses, A. Moya Bedon, F. Murgas Alcaino, E. Oliva, G. Orton, F. Palla, M. Pancrazzi, E. Pantin, V. Parmentier, H. Parviainen, K. Y. Peña Ramírez, J. Peralta, S. Perez-Hoyos, R. Petrov, S. Pezzuto, R. Pietrzak, E. Pilat-Lohinger, N. Piskunov, R. Prinja, L. Prisinzano, I. Polichtchouk, E. Poretti, A. Radioti, A. A. Ramos, T. Rank-Lüftinger, P. Read, K. Readorn, R. Rebolo López, J. Rebordão, M. Rengel, L. Rezac, M. Rocchetto, F. Rodler, V. J. Sánchez Béjar, A. Sanchez Lavega, E. Sanromá, N. Santos, J. Sanz Forcada, G. Scandariato, F.-X. Schmider, A. Scholz, S. Scuderi, J. Sethenadh, S. Shore, A. Showman, B. Sicardy, P. Sitek, A. Smith, L. Soret, S. Sousa, A. Stiepen, M. Stolarski, G. Strazzulla, H. M. Tabernero, P. Tanga, M. Tecsa, J. Temple, L. Terenzi, M. Tessenyi, L. Testi, S. Thompson, H. Thrastarson, B. W. Tingley, M. The EChO science case. Experimental Astronomy, 40:329-391, 2015. [ bib | DOI | arXiv | ADS link ]

The discovery of almost two thousand exoplanets has revealed an unexpectedly diverse planet population. We see gas giants in few-day orbits, whole multi-planet systems within the orbit of Mercury, and new populations of planets with masses between that of the Earth and Neptuneall unknown in the Solar System. Observations to date have shown that our Solar System is certainly not representative of the general population of planets in our Milky Way. The key science questions that urgently need addressing are therefore: What are exoplanets made of? Why are planets as they are? How do planetary systems work and what causes the exceptional diversity observed as compared to the Solar System? The EChO (Exoplanet Characterisation Observatory) space mission was conceived to take up the challenge to explain this diversity in terms of formation, evolution, internal structure and planet and atmospheric composition. This requires in-depth spectroscopic knowledge of the atmospheres of a large and well-defined planet sample for which precise physical, chemical and dynamical information can be obtained. In order to fulfil this ambitious scientific program, EChO was designed as a dedicated survey mission for transit and eclipse spectroscopy capable of observing a large, diverse and well-defined planet sample within its 4-year mission lifetime. The transit and eclipse spectroscopy method, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allows us to measure atmospheric signals from the planet at levels of at least 10-4 relative to the star. This can only be achieved in conjunction with a carefully designed stable payload and satellite platform. It is also necessary to provide broad instantaneous wavelength coverage to detect as many molecular species as possible, to probe the thermal structure of the planetary atmospheres and to correct for the contaminating effects of the stellar photosphere. This requires wavelength coverage of at least 0.55 to 11 μm with a goal of covering from 0.4 to 16 μm. Only modest spectral resolving power is needed, with R ˜ 300 for wavelengths less than 5 μm and R ˜ 30 for wavelengths greater than this. The transit spectroscopy technique means that no spatial resolution is required. A telescope collecting area of about 1 m2 is sufficiently large to achieve the necessary spectro-photometric precision: for the Phase A study a 1.13 m2 telescope, diffraction limited at 3 μm has been adopted. Placing the satellite at L2 provides a cold and stable thermal environment as well as a large field of regard to allow efficient time-critical observation of targets randomly distributed over the sky. EChO has been conceived to achieve a single goal: exoplanet spectroscopy. The spectral coverage and signal-to-noise to be achieved by EChO, thanks to its high stability and dedicated design, would be a game changer by allowing atmospheric composition to be measured with unparalleled exactness: at least a factor 10 more precise and a factor 10 to 1000 more accurate than current observations. This would enable the detection of molecular abundances three orders of magnitude lower than currently possible and a fourfold increase from the handful of molecules detected to date. Combining these data with estimates of planetary bulk compositions from accurate measurements of their radii and masses would allow degeneracies associated with planetary interior modelling to be broken, giving unique insight into the interior structure and elemental abundances of these alien worlds. EChO would allow scientists to study exoplanets both as a population and as individuals. The mission can target super-Earths, Neptune-like, and Jupiter-like planets, in the very hot to temperate zones (planet temperatures of 300-3000 K) of F to M-type host stars. The EChO core science would be delivered by a three-tier survey. The EChO Chemical Census: This is a broad survey of a few-hundred exoplanets, which allows us to explore the spectroscopic and chemical diversity of the exoplanet population as a whole. The EChO Origin: This is a deep survey of a subsample of tens of exoplanets for which significantly higher signal to noise and spectral resolution spectra can be obtained to explain the origin of the exoplanet diversity (such as formation mechanisms, chemical processes, atmospheric escape). The EChO Rosetta Stones: This is an ultra-high accuracy survey targeting a subsample of select exoplanets. These will be the bright “benchmark” cases for which a large number of measurements would be taken to explore temporal variations, and to obtain two and three dimensional spatial information on the atmospheric conditions through eclipse-mapping techniques. If EChO were launched today, the exoplanets currently observed are sufficient to provide a large and diverse sample. The Chemical Census survey would consist of 160 exoplanets with a range of planetary sizes, temperatures, orbital parameters and stellar host properties. Additionally, over the next 10 years, several new ground- and space-based transit photometric surveys and missions will come on-line (e.g. NGTS, CHEOPS, TESS, PLATO), which will specifically focus on finding bright, nearby systems. The current rapid rate of discovery would allow the target list to be further optimised in the years prior to EChO's launch and enable the atmospheric characterisation of hundreds of planets.

F. González-Galindo, M. A. López-Valverde, F. Forget, M. García-Comas, E. Millour, and L. Montabone. Variability of the Martian thermosphere during eight Martian years as simulated by a ground-to-exosphere global circulation model. Journal of Geophysical Research (Planets), 120:2020-2035, 2015. [ bib | DOI | ADS link ]

Using a ground-to-exosphere general circulation model for Mars we have simulated the variability of the dayside temperatures at the exobase during eight Martian years (MY, from MY24 to MY31, approximately from 1998 to 2013), taking into account the observed day-to-day solar and dust load variability. We show that the simulated temperatures are in good agreement with the exospheric temperatures derived from Precise Orbit Determination of Mars Global Surveyor. We then study the effects of the solar variability and of two planetary-encircling dust storms on the simulated temperatures. The seasonal effect produced by the large eccentricity of the Martian orbit translates in an aphelion-to-perihelion temperature contrast in every simulated year. However, the magnitude of this seasonal temperature variation is strongly affected by the solar conditions, ranging from 50 K for years corresponding to solar minimum conditions to almost 140 K during the last solar maximum. The 27 day solar rotation cycle is observed on the simulated temperatures at the exobase, with average amplitude of the temperature oscillation of 2.6 K but with a significant interannual variability. These two results highlight the importance of taking into account the solar variability when simulating the Martian upper atmosphere and likely have important implications concerning the atmospheric escape rate. We also show that the global dust storms in MY25 and MY28 have a significant effect on the simulated temperatures. In general, they increase the exospheric temperatures over the low latitude and midlatitude regions and decrease them in the polar regions.

L. Kerber, F. Forget, and R. Wordsworth. Sulfur in the early martian atmosphere revisited: Experiments with a 3-D Global Climate Model. Icarus, 261:133-148, 2015. [ bib | DOI | ADS link ]

Volcanic SO2 in the martian atmosphere has been invoked as a way to create a sustained or transient greenhouse during early martian history. Many modeling studies have been performed to test the feasibility of this hypothesis, resulting in a range of conclusions, from highly feasible to highly improbable. In this study we perform a wide range of simulations using the 3-D Laboratoire de Météorologie Dynamique Generic Global Climate Model (GCM) in order to place earlier results into context and to explore the sensitivity of model outcomes to parameters such as SO2 mixing ratio, atmospheric H2O content, background atmospheric pressure, and aerosol size, abundance, and composition. We conclude that SO2 is incapable of creating a sustained greenhouse on early Mars, and that even in the absence of aerosols, local and daily temperatures rise above 273 K for only for limited periods with favorable background CO2 pressures. In the presence of even small amounts of aerosols, the surface is dramatically cooled for realistic aerosol sizes. Brief, mildly warm conditions require the co-occurrence of many improbable factors, while cooling is achieved for a wide range of model parameters. Instead of causing warming, sulfur in the martian atmosphere may have caused substantial cooling, leading to the end of clement climate conditions on early Mars.

M. Sylvestre, S. Guerlet, T. Fouchet, A. Spiga, F. M. Flasar, B. Hesman, and G. L. Bjoraker. Seasonal changes in Saturn's stratosphere inferred from Cassini/CIRS limb observations. Icarus, 258:224-238, 2015. [ bib | DOI | ADS link ]

We present temperature and hydrocarbons abundances (C2H6, C2H2, C3H8) retrieved from Cassini/CIRS limb spectra, acquired during northern spring in 2010 (LS = 12 deg) and 2012 (LS = 31 deg). We compare them to the previous limb measurements performed by Guerlet et al. (Guerlet, S. et al. [2009]. Icarus 203, 214-232) during northern winter. The latitudinal coverage (from 79degN to 70degS) and the sensitivity of our observations to a broad range of pressure levels (from 20 hPa to 0.003 hPa) allow us to probe the meridional and vertical structure of Saturn's stratosphere during northern spring. Our results show that in the northern hemisphere, the lower stratosphere (1 hPa) has experienced the strongest warming from northern winter to spring (11 0.91.1 K), while the southern hemisphere exhibits weak variations of temperature at the same pressure level. We investigate the radiative contribution in the thermal seasonal evolution by comparing these results to the radiative-convective model of Guerlet et al. (Guerlet, S. et al. [2014]. Icarus 238, 110-124). We show that radiative heating and cooling by atmospheric minor constituents is not always sufficient to reproduce the measured variations of temperature (depending on the pressure level). The measurements of the hydrocarbons abundances and their comparison with the predictions of the 1D photochemical model of Moses and Greathouse (Moses, J.I., Greathouse, T.K. [2005]. J. Geophys. Res. (Planets) 110, 9007) give insights into large scale atmospheric dynamics. At 1 hPa, C2H6, C2H2, and C3H8 abundances are remarkably constant from northern winter to spring. At the same pressure level, C2H6 and C3H8 exhibit homogeneous meridional distributions unpredicted by this photochemical model, unlike C2H2. This is consistent with the existence of a meridional circulation at 1 hPa, as suggested by previous studies.

S. Guerlet, T. Fouchet, S. Vinatier, A. A. Simon, E. Dartois, and A. Spiga. Stratospheric benzene and hydrocarbon aerosols detected in Saturn's auroral regions. Astronomy Astrophysics, 580:A89, 2015. [ bib | DOI | ADS link ]

Context. Saturn's polar upper atmosphere exhibits significant auroral activity; however, its impact on stratospheric chemistry (i.e. the production of benzene and heavier hydrocarbons) and thermal structure remains poorly documented. <BR /> Aims: We aim to bring new constraints on the benzene distribution in Saturn's stratosphere, to characterize polar aerosols (their vertical distribution, composition, thermal infrared optical properties), and to quantify the aerosols' radiative impact on the thermal structure. <BR /> Methods: Infrared spectra acquired by the Composite Infrared Spectrometer (CIRS) on board Cassini in limb viewing geometry are analysed to derive benzene column abundances and aerosol opacity profiles over the 3 to 0.1 mbar pressure range. The spectral dependency of the haze opacity is assessed in the ranges 680-900 and 1360-1440 cm-1. Then, a radiative climate model is used to compute equilibrium temperature profiles, with and without haze, given the haze properties derived from CIRS measurements. <BR /> Results: On Saturn's auroral region (80degS), benzene is found to be slightly enhanced compared to its equatorial and mid-latitude values. This contrasts with the Moses Greathouse (2005, J. Geophys. Res., 110, 9007) photochemical model, which predicts a benzene abundance 50 times lower at 80degS than at the equator. This advocates for the inclusion of ion-related reactions in Saturn's chemical models. The polar stratosphere is also enriched in aerosols, with spectral signatures consistent with vibration modes assigned to aromatic and aliphatic hydrocarbons, and presenting similarities with the signatures observed in Titan's stratosphere. The aerosol mass loading at 80degS is estimated to be 1-4 × 10-5 g cm-2, an order of magnitude less than on Jupiter, which is consistent with the order of magnitude weaker auroral power at Saturn. We estimate that this polar haze warms the middle stratosphere by 6 K in summer and cools the upper stratosphere by 5 K in winter. Hence, aerosols linked with auroral activity can partly account for the warm polar hood observed in Saturn's summer stratosphere.

T. C. Brothers, J. W. Holt, and A. Spiga. Planum Boreum basal unit topography, Mars: Irregularities and insights from SHARAD. Journal of Geophysical Research (Planets), 120:1357-1375, 2015. [ bib | DOI | ADS link ]

Shallow Radar investigations of Planum Boreum, Mars' “basal unit” (BU) deposit have revealed multiple reentrants, morphologic irregularities, and thickness trends that differ from those of the overlying north polar layered deposits. We present detailed subsurface maps for these features and offer explanation for genesis of the deposit's morphologic asymmetry, expressed in different erosional characteristics between 0degE-180degE and 180degE-360degE. Additionally, this work revealed a depression in the basal unit that may have provided a site for spiral trough initiation. Interpretations of the findings suggest that antecedent BU topography has a marked impact on modern morphology and that aeolian forces have been the dominant driver of polar deposit accumulation since at least the end of rupes unit emplacement. We find no results requiring explanation beyond common Martian surface processes, including aeolian erosion and impact armoring. To add to the detailed morphologic study of the BU, we mapped the variability of the BU radar reflection character. Combining generalized katabatic wind flow with the radar mapping results suggests that rupes unit material sourced the younger cavi. We present clear evidence that, while compositionally distinct from the overlying layered deposits, the BU and its morphology are intimately linked to the morphology of the north polar layered deposits. Combining geologic evidence with paleoclimate modeling, the deposits contain evidence for a long history of aeolian emplacement and modification.

I. B. Smith, A. Spiga, and J. W. Holt. Aeolian processes as drivers of landform evolution at the South Pole of Mars. Geomorphology, 240:54-69, 2015. [ bib | DOI | ADS link ]

We combine observations of surface morphology, topography, subsurface stratigraphy, and near surface clouds with mesoscale simulations of south polar winds and temperature to investigate processes governing the evolution of spiral troughs on the South Pole of Mars. In general we find that the south polar troughs are cyclic steps that all formed during an erosional period, contrary to the troughs at the North Pole, which are constructional features. The Shallow Radar instrument (SHARAD) onboard Mars Reconnaissance Orbiter detects subsurface stratigraphy indicating relatively recent accumulation that occurred post trough formation in many locations. Using optical instruments, especially the Thermal Emission Imaging System (THEMIS), we find low altitude trough clouds in over 500 images spanning 6 Mars years. The locations of detected clouds correspond to where recent accumulation is detected by SHARAD, and offers clues about surface evolution. The clouds migrate by season, moving poleward from 71deg S at ˜ Ls 200deg until Ls 318deg, when the last cloud is detected. Our atmospheric simulations find that the fastest winds on the pole are found roughly near the external boundary of the seasonal CO2 ice cap. Thus, we find that the migration of clouds (and katabatic jumps) corresponds spatially to the retreat of the CO2 seasonal ice as detected by Titus (2005) and that trough morphology, through recent accumulation, is integrally related to this seasonal retreat.

R. D. Wordsworth, L. Kerber, R. T. Pierrehumbert, F. Forget, and J. W. Head. Comparison of “warm and wet” and “cold and icy” scenarios for early Mars in a 3-D climate model. Journal of Geophysical Research (Planets), 120:1201-1219, 2015. [ bib | DOI | arXiv | ADS link ]

We use a 3-D general circulation model to compare the primitive Martian hydrological cycle in “warm and wet” and “cold and icy” scenarios. In the warm and wet scenario, an anomalously high solar flux or intense greenhouse warming artificially added to the climate model are required to maintain warm conditions and an ice-free northern ocean. Precipitation shows strong surface variations, with high rates around Hellas basin and west of Tharsis but low rates around Margaritifer Sinus (where the observed valley network drainage density is nonetheless high). In the cold and icy scenario, snow migration is a function of both obliquity and surface pressure, and limited episodic melting is possible through combinations of seasonal, volcanic, and impact forcing. At surface pressures above those required to avoid atmospheric collapse (0.5 bar) and moderate to high obliquity, snow is transported to the equatorial highland regions where the concentration of valley networks is highest. Snow accumulation in the Aeolis quadrangle is high, indicating an ice-free northern ocean is not required to supply water to Gale crater. At lower surface pressures and obliquities, both H2O and CO2 are trapped as ice at the poles and the equatorial regions become extremely dry. The valley network distribution is positively correlated with snow accumulation produced by the cold and icy simulation at 41.8 obliquity but uncorrelated with precipitation produced by the warm and wet simulation. Because our simulations make specific predictions for precipitation patterns under different climate scenarios, they motivate future targeted geological studies.

S. Lebonnois, V. Eymet, C. Lee, and J. Vatant d'Ollone. Analysis of the radiative budget of the Venusian atmosphere based on infrared Net Exchange Rate formalism. Journal of Geophysical Research (Planets), 120:1186-1200, 2015. [ bib | DOI | ADS link ]

A detailed one-dimensional analysis of the energy balance in Venus atmosphere is proposed in this work, based on the Net Exchange Rate formalism that allows the identification in each altitude region of the dominant energy exchanges controlling the temperature. Well-known parameters that control the temperature profile are the solar flux deposition and the cloud particle distribution. Balance between solar heating and infrared energy exchanges is analyzed for each region: upper atmosphere (from cloud top to 100 km), upper cloud, middle cloud, cloud base, and deep atmosphere (cloud base to surface). The energy accumulated below the clouds is transferred to the cloud base through infrared windows, mostly at 3-4 μm and 5-7 μm. The continuum opacity in these spectral regions is not well known for the hot temperatures and large pressures of Venus's deep atmosphere but strongly affects the temperature profile from cloud base to surface. From cloud base, upward transport of energy goes through convection and short-range radiative exchanges up to the middle cloud where the atmosphere is thin enough in the 20-30 μm window to cool directly to space. Total opacity in this spectral window between the 15 μm CO2 band and the CO2 collision-induced absorption has a strong impact on the temperature in the cloud convective layer. Improving our knowledge of the gas opacities in these different windows through new laboratory measurements or ab initio computations, as well as improving the constraints on cloud opacities would help to separate gas and cloud contributions and secure a better understanding of Venus's atmosphere energy balance.

T. Encrenaz, T. K. Greathouse, F. Lefèvre, F. Montmessin, F. Forget, T. Fouchet, C. DeWitt, M. J. Richter, J. H. Lacy, B. Bézard, and S. K. Atreya. Seasonal variations of hydrogen peroxide and water vapor on Mars: Further indications of heterogeneous chemistry. Astronomy Astrophysics, 578:A127, 2015. [ bib | DOI | ADS link ]

We have completed our seasonal monitoring of hydrogen peroxide and water vapor on Mars using ground-based thermal imaging spectroscopy, by observing the planet in March 2014, when water vapor is maximum, and July 2014, when, according to photochemical models, hydrogen peroxide is expected to be maximum. Data have been obtained with the Texas Echelon Cross Echelle Spectrograph (TEXES) mounted at the 3 m-Infrared Telescope Facility (IRTF) at Maunakea Observatory. Maps of HDO and H2O2 have been obtained using line depth ratios of weak transitions of HDO and H2O2 divided by CO2. The retrieved maps of H2O2 are in good agreement with predictions including a chemical transport model, for both the March data (maximum water vapor) and the July data (maximum hydrogen peroxide). The retrieved maps of HDO are compared with simulations by Montmessin et al. (2005, J. Geophys. Res., 110, 03006) and H2O maps are inferred assuming a mean martian D/H ratio of 5 times the terrestrial value. For regions of maximum values of H2O and H2O2, we derive, for March 1 2014 (Ls = 96deg), H2O2 = 20+/-7 ppbv, HDO = 450 +/-75 ppbv (45 +/-8 pr-nm), and for July 3, 2014 (Ls = 156deg), H2O2 = 30+/-7 ppbv, HDO = 375+/-70 ppbv (22+/-3 pr-nm). In addition, the new observations are compared with LMD global climate model results and we favor simulations of H2O2 including heterogeneous reactions on water-ice clouds.

B. Charnay, E. Barth, S. Rafkin, C. Narteau, S. Lebonnois, S. Rodriguez, S. Courrech Du Pont, and A. Lucas. Methane storms as a driver of Titan's dune orientation. Nature Geoscience, 8:362-366, 2015. [ bib | DOI | arXiv | ADS link ]

The equatorial regions of Saturn's moon Titan are covered by linear dunes that propagate eastwards. Global climate models (GCMs), however, predict westward mean surface winds at low latitudes on Titan, similar to the trade winds on Earth. This apparent contradiction has been attributed to Saturn's gravitational tides, large-scale topography and wind statistics, but none of these hypotheses fully explains the global eastward propagation of dunes in Titan's equatorial band. However, above altitudes of about 5 km, Titan's atmosphere is in eastward super-rotation, suggesting that this momentum may be delivered to the surface. Here we assess the influence of equatorial tropical methane storms-which develop at high altitudes during the equinox-on Titan's dune orientation, using mesoscale simulations of convective methane clouds with a GCM wind profile that includes super-rotation. We find that these storms produce fast eastward gust fronts above the surface that exceed the normal westward surface winds. These episodic gusts generated by tropical storms are expected to dominate aeolian transport, leading to eastward propagation of dunes. We therefore suggest a coupling between super-rotation, tropical methane storms and dune formation on Titan. This framework, applied to GCM predictions and analogies to some terrestrial dune fields, explains the linear shape, eastward propagation and poleward divergence of Titan's dunes, and implies an equatorial origin of dune sand.

A. S. Medvedev, F. González-Galindo, E. Yigit, A. G. Feofilov, F. Forget, and P. Hartogh. Cooling of the Martian thermosphere by CO2 radiation and gravity waves: An intercomparison study with two general circulation models. Journal of Geophysical Research (Planets), 120:913-927, 2015. [ bib | DOI | arXiv | ADS link ]

Observations show that the lower thermosphere of Mars (100-140 km) is up to 40 K colder than the current general circulation models (GCMs) can reproduce. Possible candidates for physical processes missing in the models are larger abundances of atomic oxygen facilitating stronger CO2 radiative cooling and thermal effects of gravity waves. Using two state-of-the-art Martian GCMs, the Laboratoire de Météorologie Dynamique and Max Planck Institute models that self-consistently cover the atmosphere from the surface to the thermosphere, these physical mechanisms are investigated. Simulations demonstrate that the CO2 radiative cooling with a sufficiently large atomic oxygen abundance and the gravity wave-induced cooling can alone result in up to 40 K colder temperature in the lower thermosphere. Accounting for both mechanisms produce stronger cooling at high latitudes. However, radiative cooling effects peak above the mesopause, while gravity wave cooling rates continuously increase with height. Although both mechanisms act simultaneously, these peculiarities could help to further quantify their relative contributions from future observations.

D. P. Mulholland, A. Spiga, C. Listowski, and P. L. Read. An assessment of the impact of local processes on dust lifting in martian climate models. Icarus, 252:212-227, 2015. [ bib | DOI | ADS link ]

Simulation of the lifting of dust from the planetary surface is of substantially greater importance on Mars than on Earth, due to the fundamental role that atmospheric dust plays in the former's climate, yet the dust emission parameterisations used to date in martian global climate models (MGCMs) lag, understandably, behind their terrestrial counterparts in terms of sophistication. Recent developments in estimating surface roughness length over all martian terrains and in modelling atmospheric circulations at regional to local scales (less than O(100 km)) presents an opportunity to formulate an improved wind stress lifting parameterisation. We have upgraded the conventional scheme by including the spatially varying roughness length in the lifting parameterisation in a fully consistent manner (thereby correcting a possible underestimation of the true threshold level for wind stress lifting), and used a modification to account for deviations from neutral stability in the surface layer. Following these improvements, it is found that wind speeds at typical MGCM resolution never reach the lifting threshold at most gridpoints: winds fall particularly short in the southern midlatitudes, where mean roughness is large. Sub-grid scale variability, manifested in both the near-surface wind field and the surface roughness, is then considered, and is found to be a crucial means of bridging the gap between model winds and thresholds. Both forms of small-scale variability contribute to the formation of dust emission 'hotspots': areas within the model gridbox with particularly favourable conditions for lifting, namely a smooth surface combined with strong near-surface gusts. Such small-scale emission could in fact be particularly influential on Mars, due both to the intense positive radiative feedbacks that can drive storm growth and a strong hysteresis effect on saltation. By modelling this variability, dust lifting is predicted at the locations at which dust storms are frequently observed, including the flushing storm sources of Chryse and Utopia, and southern midlatitude areas from which larger storms tend to initiate, such as Hellas and Solis Planum. The seasonal cycle of emission, which includes a double-peaked structure in northern autumn and winter, also appears realistic. Significant increases to lifting rates are produced for any sensible choices of parameters controlling the sub-grid distributions used, but results are sensitive to the smallest scale of variability considered, which high-resolution modelling suggests should be O(1 km) or less. Use of such models in future will permit the use of a diagnosed (rather than prescribed) variable gustiness intensity, which should further enhance dust lifting in the southern hemisphere in particular.

L. Montabone, F. Forget, E. Millour, R. J. Wilson, S. R. Lewis, B. Cantor, D. Kass, A. Kleinböhl, M. T. Lemmon, M. D. Smith, and M. J. Wolff. Eight-year climatology of dust optical depth on Mars. Icarus, 251:65-95, 2015. [ bib | DOI | arXiv | ADS link ]

We have produced a multiannual climatology of airborne dust from martian year 24-31 using multiple datasets of retrieved or estimated column optical depths. The datasets are based on observations of the martian atmosphere from April 1999 to July 2013 made by different orbiting instruments: the Thermal Emission Spectrometer (TES) aboard Mars Global Surveyor, the Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey, and the Mars Climate Sounder (MCS) aboard Mars Reconnaissance Orbiter (MRO). The procedure we have adopted consists of gridding the available retrievals of column dust optical depth (CDOD) from TES and THEMIS nadir observations, as well as the estimates of this quantity from MCS limb observations. Our gridding method calculates averages and uncertainties on a regularly spaced spatio-temporal grid, using an iterative procedure that is weighted in space, time, and retrieval quality. The lack of observations at certain times and locations introduces missing grid points in the maps, which therefore may result in irregularly gridded (i.e. incomplete) fields. In order to evaluate the strengths and weaknesses of the resulting gridded maps, we compare with independent observations of CDOD by PanCam cameras and Mini-TES spectrometers aboard the Mars Exploration Rovers “Spirit” and “Opportunity”, by the Surface Stereo Imager aboard the Phoenix lander, and by the Compact Reconnaissance Imaging Spectrometer for Mars aboard MRO. We have statistically analyzed the irregularly gridded maps to provide an overview of the dust climatology on Mars over eight years, specifically in relation to its interseasonal and interannual variability, in addition to provide a basis for instrument intercomparison. Finally, we have produced regularly gridded maps of CDOD by spatially interpolating the irregularly gridded maps using a kriging method. These complete maps are used as dust scenarios in the Mars Climate Database (MCD) version 5, and are useful in many modeling applications. The two datasets for the eight available martian years are publicly available and distributed with open access on the MCD website.

S. Vinatier, B. Bézard, S. Lebonnois, N. A. Teanby, R. K. Achterberg, N. Gorius, A. Mamoutkine, E. Guandique, A. Jolly, D. E. Jennings, and F. M. Flasar. Seasonal variations in Titan's middle atmosphere during the northern spring derived from Cassini/CIRS observations. Icarus, 250:95-115, 2015. [ bib | DOI | ADS link ]

We analyzed spectra acquired at the limb of Titan in the 2006-2013 period by the Cassini/Composite Infrared Spectrometer (CIRS) in order to monitor the seasonal evolution of the thermal, gas composition and aerosol spatial distributions. We are primarily interested here in the seasonal changes after the northern spring equinox and interpret our results in term of global circulation seasonal changes. Data cover the 600-1500 cm-1 spectral range at a resolution of 0.5 or 15.5 cm-1 and probe the 150-500 km vertical range with a vertical resolution of about 30 km. Retrievals of the limb spectra acquired at 15.5 cm-1 resolution allowed us to derive eight global maps of temperature, aerosols and C2H2, C2H6 and HCN molecular mixing ratios between July 2009 and May 2013. In order to have a better understanding of the global changes taking place after the northern spring equinox, we analyzed 0.5 cm-1 resolution limb spectra to infer the mixing ratio profiles of 10 molecules for some latitudes. These profiles are compared with CIRS observations performed during the northern winter. Our observations are compatible with the coexistence of two circulation cells upwelling at mid-latitudes and downwelling at both poles from at last January 2010 to at least June 2010. One year later, in June 2011, there are indications that the global circulation had reversed compared to the winter situation, with a single pole-to-pole cell upwelling at the north pole and downwelling at the south pole. Our observations show that in December 2011, this new pole-to-pole cell has settled with a downward velocity of 4.4 mm/s at 450 km above the south pole. Therefore, in about two years after the equinox, the global circulation observed during the northern winter has totally reversed, which is in agreement with the predictions of general circulation models. We observe a sudden unexpected temperature decrease above the south pole in February 2012, which is probably related to the strong enhancement of molecular gas in this region, acting as radiative coolers. In July and November 2012, we observe a detached haze layer located around 320-330 km, which is comparable to the altitude of the detached haze layer observed by the Cassini Imaging Science Subsystem (ISS) in the UV.

E. Lellouch, C. de Bergh, B. Sicardy, F. Forget, M. Vangvichith, and H.-U. Käufl. Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere. Icarus, 246:268-278, 2015. [ bib | DOI | arXiv | ADS link ]

High-resolution spectra of Pluto in the 1.66 μm region, recorded with the VLT/CRIRES instrument in 2008 (2 spectra) and 2012 (5 spectra), are analyzed to constrain the spatial and vertical distribution of methane in Pluto's atmosphere and to search for mid-term (4 year) variability. A sensitivity study to model assumptions (temperature structure, surface pressure, Pluto's radius) is performed. Results indicate that (i) no variation of the CH4 atmospheric content (column-density or mixing ratio) with Pluto rotational phase is present in excess of 20%, (ii) CH4 column densities show at most marginal variations between 2008 and 2012, with a best guess estimate of a ~20% decrease over this time frame. As stellar occultations indicate that Pluto's surface pressure has continued to increase over this period, this implies a concomitant decrease of the methane mixing ratio (iii) the data do not show evidence for an altitude-varying methane distribution; in particular, they imply a roughly uniform mixing ratio in at least the first 22-27 km of the atmosphere, and high concentrations of low-temperature methane near the surface can be ruled out. Our results are also best consistent with a relatively large (1180 km) Pluto radius. Comparison with predictions from a recently developed global climate model indicates that these features are best explained if the source of methane occurs in regional-scale CH4 ice deposits, including both low latitudes and high Northern latitudes, evidence for which is present from the rotational and secular evolution of the near-IR features due to CH4 ice. Our “best guess” predictions for the New Horizons encounter in 2015 are: a 1184 km radius, a 17 μbar surface pressure, and a 0.44% CH4 mixing ratio with negligible longitudinal variations.

J.-Y. Chaufray, F. Gonzalez-Galindo, F. Forget, M. A. Lopez-Valverde, F. Leblanc, R. Modolo, and S. Hess. Variability of the hydrogen in the martian upper atmosphere as simulated by a 3D atmosphere-exosphere coupling. Icarus, 245:282-294, 2015. [ bib | DOI | ADS link ]

We present the temporal variability of the atomic and molecular hydrogen density derived from a 3D General Circulation Model describing the martian atmosphere from the surface to the exobase. A kinetic exospheric model is used to compute the hydrogen density above the exobase. We use these models to study the diurnal and seasonal variations of the hydrogen density and the Jeans escape rate as well as their variations with solar activity, assuming a classic dust scenario. We find that the diurnal variations of the hydrogen density are important with a peak in the dawn region during equinoxes and a peak on the nightside during solstices. These features result from the dynamics of the martian upper atmosphere. The variations of the atomic hydrogen Jeans escape with seasons and solar activity are in the range 1.3 × 1025 s-1-4.4 × 1026 s-1. A factor ~8 is due to the seasonal variations with a maximum during the winter solstice in the northern hemisphere and a minimum during the summer solstice in the northern hemisphere that we attribute to the variation of the Mars-Sun distance. A factor ~5 is due to the solar cycle with a maximum escape rate at high solar activity. The variations of the molecular hydrogen Jeans escape with seasons and solar activity are in the range 3 × 1022 s-1-6 × 1024 s-1. A factor ~10 is due to the seasonal variations with a maximum during the winter solstice in the northern hemisphere and a minimum during the summer solstice in the northern hemisphere. A factor ~20 is due to the solar cycle with a maximum escape rate at high solar activity. If Jeans escape is the major escape channel for hydrogen, the hydrogen escape is never limited by diffusion. The hydrogen density above 10,000 km presents seasonal and solar cycle variations similar to the Jeans escape rate at all latitudes and local times. This 3D temporal model of the hydrogen thermosphere/exosphere will be useful to interpret future MAVEN observations and the consequences of the hydrogen corona variability on the martian plasma environment.

M. Alexe, P. Bergamaschi, A. Segers, R. Detmers, A. Butz, O. Hasekamp, S. Guerlet, R. Parker, H. Boesch, C. Frankenberg, R. A. Scheepmaker, E. Dlugokencky, C. Sweeney, S. C. Wofsy, and E. A. Kort. Inverse modelling of CH4 emissions for 2010-2011 using different satellite retrieval products from GOSAT and SCIAMACHY. Atmospheric Chemistry & Physics, 15:113-133, 2015. [ bib | DOI | ADS link ]

At the beginning of 2009 new space-borne observations of dry-air column-averaged mole fractions of atmospheric methane (XCH4) became available from the Thermal And Near infrared Sensor for carbon Observations-Fourier Transform Spectrometer (TANSO-FTS) instrument on board the Greenhouse Gases Observing SATellite (GOSAT). Until April 2012 concurrent {methane (CH4) retrievals} were provided by the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) instrument on board the ENVironmental SATellite (ENVISAT). The GOSAT and SCIAMACHY XCH4 retrievals can be compared emissions between January 2010 and December 2011, using the TM5-4DVAR inverse modelling system. In addition to satellite data, high-accuracy measurements from the Cooperative Air Sampling Network of the National Oceanic and Atmospheric Administration Earth System Research Laboratory (NOAA ESRL) are used, providing strong constraints on the remote surface atmosphere. We discuss five inversion scenarios that make use of different GOSAT and SCIAMACHY XCH4 retrieval products, including two sets of GOSAT proxy retrievals processed independently by the Netherlands Institute for Space Research (SRON)/Karlsruhe Institute of Technology (KIT), and the University of Leicester (UL), and the RemoTeC “Full-Physics” (FP) XCH4 retrievals available from SRON/KIT. The GOSAT-based inversions show significant reductions in the root mean square (rms) difference between retrieved and modelled XCH4, and require much smaller bias corrections compared to the inversion using SCIAMACHY retrievals, reflecting the higher precision and relative accuracy of the GOSAT XCH4. Despite the large differences between the GOSAT and SCIAMACHY retrievals, 2-year average emission maps show overall good agreement among all satellite-based inversions, with consistent flux adjustment patterns, particularly across equatorial Africa and North America. Over North America, the satellite inversions result in a significant redistribution of CH4 emissions from North-East to South-Central United States. This result is consistent with recent independent studies suggesting a systematic underestimation of CH4 emissions from North American fossil fuel sources in bottom-up inventories, likely related to natural gas production facilities. Furthermore, all four satellite inversions yield lower CH4 fluxes across the Congo basin compared to the NOAA-only scenario, but higher emissions across tropical East Africa. The GOSAT and SCIAMACHY inversions show similar performance when validated against independent shipboard and aircraft observations, and XCH4 retrievals available from the Total Carbon Column Observing Network (TCCON).