2018 .

(28 publications)

W. M. Grundy, T. Bertrand, R. P. Binzel, M. W. Buie, B. J. Buratti, A. F. Cheng, J. C. Cook, D. P. Cruikshank, S. L. Devins, C. M. Dalle Ore, A. M. Earle, K. Ennico, F. Forget, P. Gao, G. R. Gladstone, C. J. A. Howett, D. E. Jennings, J. A. Kammer, T. R. Lauer, I. R. Linscott, C. M. Lisse, A. W. Lunsford, W. B. McKinnon, C. B. Olkin, A. H. Parker, S. Protopapa, E. Quirico, D. C. Reuter, B. Schmitt, K. N. Singer, J. A. Spencer, S. A. Stern, D. F. Strobel, M. E. Summers, H. A. Weaver, G. E. Weigle, M. L. Wong, E. F. Young, L. A. Young, and X. Zhang. Pluto's haze as a surface material. Icarus, 314:232-245, 2018. [ bib | DOI | ADS link ]

Pluto's atmospheric haze settles out rapidly compared with geological timescales. It needs to be accounted for as a surface material, distinct from Pluto's icy bedrock and from the volatile ices that migrate via sublimation and condensation on seasonal timescales. This paper explores how a steady supply of atmospheric haze might affect three distinct provinces on Pluto. We pose the question of why they each look so different from one another if the same haze material is settling out onto all of them. Cthulhu is a more ancient region with comparatively little present-day geological activity, where the haze appears to simply accumulate over time. Sputnik Planitia is a very active region where glacial convection, as well as sublimation and condensation rapidly refresh the surface, hiding recently deposited haze from view. Lowell Regio is a region of intermediate age featuring very distinct coloration from the rest of Pluto. Using a simple model haze particle as a colorant, we are not able to match the colors in both Lowell Regio and Cthulhu. To account for their distinct colors, we propose that after arrival at Pluto's surface, haze particles may be less inert than might be supposed from the low surface temperatures. They must either interact with local materials and environments to produce distinct products in different regions, or else the supply of haze must be non-uniform in time and/or location, such that different products are delivered to different places.

S. Lebonnois, G. Schubert, F. Forget, and A. Spiga. Planetary boundary layer and slope winds on Venus. Icarus, 314:149-158, 2018. [ bib | DOI | ADS link ]

Few constraints are available to characterize the deep atmosphere of Venus, though this region is crucial to understand the interactions between surface and atmosphere on Venus. Based on simulations performed with the IPSL Venus Global Climate Model, the possible structure and characteristics of Venus' planetary boundary layer (PBL) are investigated. The vertical profile of the potential temperature in the deepest 10 km above the surface and its diurnal variations are controlled by radiative and dynamical processes. The model predicts a diurnal cycle for the PBL activity, with a stable nocturnal PBL while convective activity develops during daytime. The diurnal convective PBL is strongly correlated with surface solar flux and is maximum around noon and in low latitude regions. It typically reaches less than 2 km above the surface, but its vertical extension is much higher over high elevations, and more precisely over the western flanks of elevated terrains. This correlation is explained by the impact of surface winds, which undergo a diurnal cycle with downward katabatic winds at night and upward anabatic winds during the day along the slopes of high-elevation terrains. The convergence of these daytime anabatic winds induces upward vertical winds, that are responsible for the correlation between height of the convective boundary layer and topography.

I. Garate-Lopez and S. Lebonnois. Latitudinal variation of clouds' structure responsible for Venus' cold collar. Icarus, 314:1-11, 2018. [ bib | DOI | ADS link ]

Global Climate Models (GCM) are very useful tools to study theoretically the general dynamics and specific phenomena in planetary atmospheres. In the case of Venus, several GCMs succeeded in reproducing the atmosphere's superrotation and the global temperature field. However, the highly variable polar temperature and the permanent cold collar present at 60o -80o latitude have not been reproduced satisfactorily yet.

Here we improve the radiative transfer scheme of the Institut Pierre Simon Laplace Venus GCM in order to numerically simulate the polar thermal features in Venus atmosphere. The main difference with the previous model is that we now take into account the latitudinal variation of the cloud structure. Both solar heating rates and infrared cooling rates have been modified to consider the cloud top's altitude decrease toward the poles and the variation in latitude of the different particle modes' abundances.

A new structure that closely resembles the observed cold collar appears in the average temperature field at 2 ×104 - 4 ×103 Pa (~ 62 - 66 km) altitude range and 60o -90o latitude band. It is not isolated from the pole as in the observation-based maps, but the obtained temperature values (220 K) are in good agreement with observed values. Temperature polar maps across this region show an inner warm region where the polar vortex is observed, but the obtained 230 K average value is colder than the observed mean value and the simulated horizontal structure does not show the fine-scale features present within the vortex.

The comparison with a simulation that does not take into account the latitudinal variation of the cloud structure in the infrared cooling computation, shows that the cloud structure is essential in the cold collar formation. Although our analysis focuses on the improvement of the radiative forcing and the variations it causes in the thermal structure, polar dynamics is definitely affected by this modified environment and a noteworthy upwelling motion is found in the cold collar area.

W. G. Read, L. K. Tamppari, N. J. Livesey, R. T. Clancy, F. Forget, P. Hartogh, S. C. R. Rafkin, and G. Chattopadhyay. Retrieval of wind, temperature, water vapor and other trace constituents in the Martian Atmosphere. Planetary and Space Science, 161:26-40, 2018. [ bib | DOI | ADS link ]

Atmospheric limb sounding is a well-established technique for measuring atmospheric temperature, composition, and wind. The theoretical capabilities of a submillimeter limb sounder placed in low Mars orbit are quantified, with a particular focus on the ability to make profile measurements of line-of-sight wind, temperature, water vapor, deuterated water vapor, several isotopes of carbon monoxide, oxygen-18 carbon dioxide, ozone, and hydrogen peroxide. We identify cases where all such measurements can be made within a single 25-70 GHz wide region of the submillimeter spectrum, enabling use of a single state-of-the-art submillimeter receiver. Six potential spectral regions, approximately centered at 335 GHz, 450 GHz, 550 GHz, 900 GHz, 1000 GHz, and 1130 GHz are found, any one of which can provide a complete measurement suite. The expected precision and vertical resolution of temperature, composition, and wind measurements from instruments in each range are quantified. This work thus follows on from that of Urban et al. (2005), Kasai et al. (2012), and earlier studies, expanding them to consider many alternative observing frequency regions. In general, performance (in terms of measurement precision and vertical resolution) is improved with increasing observation frequency. In part this is due to our choice to assume the same antenna size for each frequency, thus providing a narrower field of view for the higher frequency configurations. The general increase in emission line strengths with increasing frequency also contributes to this improved performance in some cases. However, increased instrument power needs for the higher frequency configurations may argue against their choice in some mission scenarios.

M. Golombek, M. Grott, G. Kargl, J. Andrade, J. Marshall, N. Warner, N. A. Teanby, V. Ansan, E. Hauber, J. Voigt, R. Lichtenheldt, B. Knapmeyer-Endrun, I. J. Daubar, D. Kipp, N. Muller, P. Lognonné, C. Schmelzbach, D. Banfield, A. Trebi-Ollennu, J. Maki, S. Kedar, D. Mimoun, N. Murdoch, S. Piqueux, P. Delage, W. T. Pike, C. Charalambous, R. Lorenz, L. Fayon, A. Lucas, S. Rodriguez, P. Morgan, A. Spiga, M. Panning, T. Spohn, S. Smrekar, T. Gudkova, R. Garcia, D. Giardini, U. Christensen, T. Nicollier, D. Sollberger, J. Robertsson, K. Ali, B. Kenda, and W. B. Banerdt. Geology and Physical Properties Investigations by the InSight Lander. Space Science Reviews, 214:84, 2018. [ bib | DOI | ADS link ]

Although not the prime focus of the InSight mission, the near-surface geology and physical properties investigations provide critical information for both placing the instruments (seismometer and heat flow probe with mole) on the surface and for understanding the nature of the shallow subsurface and its effect on recorded seismic waves. Two color cameras on the lander will obtain multiple stereo images of the surface and its interaction with the spacecraft. Images will be used to identify the geologic materials and features present, quantify their areal coverage, help determine the basic geologic evolution of the area, and provide ground truth for orbital remote sensing data. A radiometer will measure the hourly temperature of the surface in two spots, which will determine the thermal inertia of the surface materials present and their particle size and/or cohesion. Continuous measurements of wind speed and direction offer a unique opportunity to correlate dust devils and high winds with eolian changes imaged at the surface and to determine the threshold friction wind stress for grain motion on Mars. During the first two weeks after landing, these investigations will support the selection of instrument placement locations that are relatively smooth, flat, free of small rocks and load bearing. Soil mechanics parameters and elastic properties of near surface materials will be determined from mole penetration and thermal conductivity measurements from the surface to 3-5 m depth, the measurement of seismic waves during mole hammering, passive monitoring of seismic waves, and experiments with the arm and scoop of the lander (indentations, scraping and trenching). These investigations will determine and test the presence and mechanical properties of the expected 3-17 m thick fragmented regolith (and underlying fractured material) built up by impact and eolian processes on top of Hesperian lava flows and determine its seismic properties for the seismic investigation of Mars' interior.

T. Navarro, G. Schubert, and S. Lebonnois. Atmospheric mountain wave generation on Venus and its influence on the solid planet's rotation rate. Nature Geoscience, 11:487-491, 2018. [ bib | DOI | ADS link ]

The Akatsuki spacecraft observed a 10,000-km-long meridional structure at the top of the cloud deck of Venus that appeared stationary with respect to the surface and was interpreted as a gravity wave. Additionally, over four Venus solar days of observations, other such waves were observed to appear in the afternoon over equatorial highland regions. This indicates a direct influence of the solid planet on the whole Venusian atmosphere despite dissimilar rotation rates of 243 and 4 days, respectively. How such gravity waves might be generated on Venus is not understood. Here, we use general circulation model simulations of the Venusian atmosphere to show that the observations are consistent with stationary gravity waves over topographic highsor mountain wavesthat are generated in the afternoon in equatorial regions by the diurnal cycle of near-surface atmospheric stability. We find that these mountain waves substantially contribute to the total atmospheric torque that acts on the planet's surface. We estimate that mountain waves, along with the thermal tide and baroclinic waves, can produce a change in the rotation rate of the solid body of about 2 minutes per solar day. This interplay between the solid planet and atmosphere may explain some of the difference in rotation rates (equivalent to a change in the length of day of about 7 minutes) measured by spacecraft over the past 40 years.

J.-M. Hartmann, H. Tran, R. Armante, C. Boulet, A. Campargue, F. Forget, L. Gianfrani, I. Gordon, S. Guerlet, M. Gustafsson, J. T. Hodges, S. Kassi, D. Lisak, F. Thibault, and G. C. Toon. Recent advances in collisional effects on spectra of molecular gases and their practical consequences. Journal of Quantitative Spectroscopy and Radiative Transfer, 213:178-227, 2018. [ bib | DOI | ADS link ]

We review progress, since publication of the book “Collisional effects on molecular spectra: Laboratory experiments and models, consequences for applications“ (Elsevier, Amsterdam, 2008), on measuring, modeling and predicting the influence of pressure (ie of intermolecular collisions) on the spectra of gas molecules. We first introduce recently developed experimental techniques of high accuracy and sensitivity. We then complement the aforementioned book by presenting the theoretical approaches, results and data proposed (mostly) in the last decade on the topics of isolated line shapes, line-broadening and -shifting, line-mixing, the far wings and associated continua, and collision-induced absorption. Examples of recently demonstrated consequences of the progress in the description of spectral shapes for some practical applications (metrology, probing of gas media, climate predictions) are then given. Remaining issues and directions for future research are finally discussed.

T. Bertrand, F. Forget, O. M. Umurhan, W. M. Grundy, B. Schmitt, S. Protopapa, A. M. Zangari, O. L. White, P. M. Schenk, K. N. Singer, A. Stern, H. A. Weaver, L. A. Young, K. Ennico, and C. B. Olkin. The nitrogen cycles on Pluto over seasonal and astronomical timescales. Icarus, 309:277-296, 2018. [ bib | DOI | arXiv | ADS link ]

Pluto's landscape is shaped by the endless condensation and sublimation cycles of the volatile ices covering its surface. In particular, the Sputnik Planitia ice sheet, which is thought to be the main reservoir of nitrogen ice, displays a large diversity of terrains, with bright and dark plains, small pits and troughs, topographic depressions and evidences of recent and past glacial flows. Outside Sputnik Planitia, New Horizons also revealed numerous nitrogen ice deposits, in the eastern side of Tombaugh Regio and at mid-northern latitudes.

These observations suggest a complex history involving volatile and glacial processes occurring on different timescales. We present numerical simulations of volatile transport on Pluto performed with a model designed to simulate the nitrogen cycle over millions of years, taking into account the changes of obliquity, solar longitude of perihelion and eccentricity as experienced by Pluto. Using this model, we first explore how the volatile and glacial activity of nitrogen within Sputnik Planitia has been impacted by the diurnal, seasonal and astronomical cycles of Pluto. Results show that the obliquity dominates the N2 cycle and that over one obliquity cycle, the latitudes of Sputnik Planitia between 25degS-30degN are dominated by N2 condensation, while the northern regions between 30degN and -50degN are dominated by N2 sublimation. We find that a net amount of 1 km of ice has sublimed at the northern edge of Sputnik Planitia during the last 2 millions of years. It must have been compensated by a viscous flow of the thick ice sheet. By comparing these results with the observed geology of Sputnik Planitia, we can relate the formation of the small pits and the brightness of the ice at the center of Sputnik Planitia to the sublimation and condensation of ice occurring at the annual timescale, while the glacial flows at its eastern edge and the erosion of the water ice mountains all around the ice sheet are instead related to the astronomical timescale. We also perform simulations including a glacial flow scheme which shows that the Sputnik Planitia ice sheet is currently at its minimum extent at the northern and southern edges. We also explore the stability of N2 ice deposits outside the latitudes and longitudes of the Sputnik Planitia basin. Results show that N2 ice is not stable at the poles but rather in the equatorial regions, in particular in depressions, where thick deposits may persist over tens of millions of years, before being trapped in Sputnik Planitia. Finally, another key result is that the minimum and maximum surface pressures obtained over the simulated millions of years remain in the range of milli-Pascals and Pascals, respectively. This suggests that Pluto never encountered conditions allowing liquid nitrogen to flow directly on its surface. Instead, we suggest that the numerous geomorphological evidences of past liquid flow observed on Pluto's surface are the result of liquid nitrogen that flowed at the base of thick ancient nitrogen glaciers, which have since disappeared.

A. Spiga and I. Smith. Katabatic jumps in the Martian northern polar regions. Icarus, 308:197-208, 2018. [ bib | DOI | ADS link ]

Martian polar regions host active regional wind circulations, such as the downslope katabatic winds which develop owing to near-surface radiative cooling and sloped topography. Many observations (stratigraphy from radar profiling, frost streaks, spectral analysis of ices) concur to show that aeolian processes play a key role in glacial processes in Martian polar regions. A spectacular manifestation of this resides in elongated clouds that forms within the polar spiral troughs, a series of geological depressions in Mars' polar caps. Here we report mesoscale atmospheric modeling in Martian polar regions making use of five nested domains operating a model downscaling from horizontal resolutions of twenty kilometers to 200 m in a typical polar trough. We show that strong katabatic jumps form at the bottom of polar troughs with an horizontal morphology and location similar to trough clouds, large vertical velocity (up to +3 m/s) and temperature perturbations (up to 20 K) propitious to cloud formation. This strongly suggests that trough clouds on Mars are caused by katabatic jumps forming within polar troughs. This phenomena is analogous to the terrestrial Loewe phenomena over Antarctica's slopes and coastlines, resulting in a distinctive “wall of snow” during katabatic events. Our mesoscale modeling results thereby suggest that trough clouds might be present manifestations of the ice migration processes that yielded the internal cap structure discovered by radar observations, as part of a “cyclic step” process. This has important implications for the stability and possible migration over geological timescales of water ice surface reservoirs-and, overall, for the evolution of Mars' polar caps over geological timescales.

I. B. Smith and A. Spiga. Seasonal variability in winds in the north polar region of Mars. Icarus, 308:188-196, 2018. [ bib | DOI | ADS link ]

Surface features near Mars' polar regions are very active, suggesting that they are among the most dynamic places on the planet. Much of that activity is driven by seasonal winds that strongly influence the distribution of water ice and other particulates. Morphologic features such as the spiral troughs, Chasma Boreale, and prominent circumpolar dune fields have experienced persistent winds for several Myr. Therefore, detailing the pattern of winds throughout the year is an important step to understanding what processes affect the martian surface in contemporary and past epochs. In this study, we provide polar-focused mesoscale simulations from northern spring to summer to understand variability from the diurnal to the seasonal scales. We find that there is a strong seasonality to the diurnal surface wind speeds driven primarily by the retreat of the seasonal CO2 until about summer solstice, when the CO2 is gone. The fastest winds are found when the CO2 cap boundary is on the slopes of the north polar layered deposits, providing a strong thermal gradient that enhances the season-long katabatic effect. Mid-summer winds, while not as fast as spring winds, may play a role in dune migration for some dune fields. Late summer wind speeds pick up as the seasonal cap returns.

L. Guallini, A. P. Rossi, F. Forget, L. Marinangeli, S. E. Lauro, E. Pettinelli, R. Seu, and N. Thomas. Regional stratigraphy of the south polar layered deposits (Promethei Lingula, Mars): “Discontinuity-bounded” units in images and radargrams. Icarus, 308:76-107, 2018. [ bib | DOI | ADS link ]

The Mars South Polar Layered Deposits (SPLD) are the result of depositional and erosional events, which are marked by different stratigraphic sequences and erosional surfaces. To unambiguously define the stratigraphic units at regional scale, we mapped the SPLD on the basis of observed discontinuities (i.e., unconformities, correlative discontinuities and conformities), as commonly done in terrestrial modern stratigraphy. This methodology is defined as “Discontinuity-Bounded Units” or allostratigraphy, and is complemented by geomorphological mapping.

Our study focuses on Promethei Lingula (PL) and uses both high-resolution images (CTX, HiRISE) and radargrams (SHARAD) to combine surface and sub-surface observations and obtain a 3D geological reconstruction of the SPLD. One regional discontinuity (named AUR1) was defined within the studied stratigraphic succession and is exposed in several non-contiguous outcrops around PL as well as observed at depth within the ice sheet. This is the primary contact between two major depositional sequences, showing a different texture at CTX resolution. The lower sequence is characterized mainly by a “ridge and trough” morphology (Ridge and Trough Sequence; RTS) and the upper sequence shows mainly by a “stair-stepped” morphology (Stair-Stepped Sequence; SSS). On the basis of the observations, we defined two regional “discontinuity-bounded” units in PL, respectively coinciding with RTS and SSS sequences. Our stratigraphic reconstruction provides new hints on the major scale events that shaped this region. Oscillations in Martian axial obliquity could have controlled local climate conditions in the past, affecting the PL geological record.

I. B. Smith, S. Diniega, D. W. Beaty, T. Thorsteinsson, P. Becerra, A. M. Bramson, S. M. Clifford, C. S. Hvidberg, G. Portyankina, S. Piqueux, A. Spiga, and T. N. Titus. 6th international conference on Mars polar science and exploration: Conference summary and five top questions. Icarus, 308:2-14, 2018. [ bib | DOI | ADS link ]

We provide a historical context of the International Conference on Mars Polar Science and Exploration and summarize the proceedings from the 6th iteration of this meeting. In particular, we identify five key Mars polar science questions based primarily on presentations and discussions at the conference and discuss the overlap between some of those questions. We briefly describe the seven scientific field trips that were offered at the conference, which greatly supplemented conference discussion of Mars polar processes and landforms. We end with suggestions for measurements, modeling, and laboratory and field work that were highlighted during conference discussion as necessary steps to address key knowledge gaps.

M. W. Telfer, E. J. R. Parteli, J. Radebaugh, R. A. Beyer, T. Bertrand, F. Forget, F. Nimmo, W. M. Grundy, J. M. Moore, S. A. Stern, J. Spencer, T. R. Lauer, A. M. Earle, R. P. Binzel, H. A. Weaver, C. B. Olkin, L. A. Young, K. Ennico, K. Runyon, and aff12. Dunes on Pluto. Science, 360:992-997, 2018. [ bib | DOI | ADS link ]

The surface of Pluto is more geologically diverse and dynamic than had been expected, but the role of its tenuous atmosphere in shaping the landscape remains unclear. We describe observations from the New Horizons spacecraft of regularly spaced, linear ridges whose morphology, distribution, and orientation are consistent with being transverse dunes. These are located close to mountainous regions and are orthogonal to nearby wind streaks. We demonstrate that the wavelength of the dunes (˜0.4 to 1 kilometer) is best explained by the deposition of sand-sized (˜200 to ˜300 micrometer) particles of methane ice in moderate winds (10 meters per second). The undisturbed morphology of the dunes, and relationships with the underlying convective glacial ice, imply that the dunes have formed in the very recent geological past.

T. T. Koskinen and S. Guerlet. Atmospheric structure and helium abundance on Saturn from Cassini/UVIS and CIRS observations. Icarus, 307:161-171, 2018. [ bib | DOI | ADS link ]

We combine measurements from stellar occultations observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) and limb scans observed by the Composite Infrared Spectrometer (CIRS) to create empirical atmospheric structure models for Saturn corresponding to the locations probed by the occultations. The results cover multiple locations at low to mid-latitudes between the spring of 2005 and the fall of 2015. We connect the temperature-pressure (T-P) profiles retrieved from the CIRS limb scans in the stratosphere to the T-P profiles in the thermosphere retrieved from the UVIS occultations. We calculate the altitudes corresponding to the pressure levels in each case based on our best fit composition model that includes H2, He, CH4 and upper limits on H. We match the altitude structure to the density profile in the thermosphere that is retrieved from the occultations. Our models depend on the abundance of helium and we derive a volume mixing ratio of 11 2% for helium in the lower atmosphere based on a statistical analysis of the values derived for 32 different occultation locations. We also derive the mean temperature and methane profiles in the upper atmosphere and constrain their variability. Our results are consistent with enhanced heating at the polar auroral region and a dynamically active upper atmosphere.

S. L. Grimm, B.-O. Demory, M. Gillon, C. Dorn, E. Agol, A. Burdanov, L. Delrez, M. Sestovic, A. H. M. J. Triaud, M. Turbet, É. Bolmont, A. Caldas, J. d. Wit, E. Jehin, J. Leconte, S. N. Raymond, V. V. Grootel, A. J. Burgasser, S. Carey, D. Fabrycky, K. Heng, D. M. Hernandez, J. G. Ingalls, S. Lederer, F. Selsis, and D. Queloz. The nature of the TRAPPIST-1 exoplanets. Astronomy Astrophysics, 613:A68, 2018. [ bib | DOI | arXiv | ADS link ]

Context. The TRAPPIST-1 system hosts seven Earth-sized, temperate exoplanets orbiting an ultra-cool dwarf star. As such, it represents a remarkable setting to study the formation and evolution of terrestrial planets that formed in the same protoplanetary disk. While the sizes of the TRAPPIST-1 planets are all known to better than 5% precision, their densities have significant uncertainties (between 28% and 95%) because of poor constraints on the planet's masses. <BR /> Aims: The goal of this paper is to improve our knowledge of the TRAPPIST-1 planetary masses and densities using transit-timing variations (TTVs). The complexity of the TTV inversion problem is known to be particularly acute in multi-planetary systems (convergence issues, degeneracies and size of the parameter space), especially for resonant chain systems such as TRAPPIST-1. <BR /> Methods: To overcome these challenges, we have used a novel method that employs a genetic algorithm coupled to a full N-body integrator that we applied to a set of 284 individual transit timings. This approach enables us to efficiently explore the parameter space and to derive reliable masses and densities from TTVs for all seven planets. <BR /> Results: Our new masses result in a five- to eight-fold improvement on the planetary density uncertainties, with precisions ranging from 5% to 12%. These updated values provide new insights into the bulk structure of the TRAPPIST-1 planets. We find that TRAPPIST-1 c and e likely have largely rocky interiors, while planets b, d, f, g, and h require envelopes of volatiles in the form of thick atmospheres, oceans, or ice, in most cases with water mass fractions less than 5%.

H. Tran, M. Turbet, P. Chelin, and X. Landsheere. Measurements and modeling of absorption by CO2 + H2O mixtures in the spectral region beyond the CO2 ν3-band head. Icarus, 306:116-121, 2018. [ bib | DOI | arXiv | ADS link ]

In this work, we measured the absorption by CO2 + H2O mixtures from 2400 to 2600 cm-1 which corresponds to the spectral region beyond the ν3 band head of CO2. Transmission spectra of CO2 mixed with water vapor were recorded with a high-resolution Fourier-transform spectrometer for various pressure, temperature and concentration conditions. The continuum absorption by CO2 due to the presence of water vapor was determined by subtracting from measured spectra the contribution of local lines of both species, that of the continuum of pure CO2 as well as of the self- and CO2-continua of water vapor induced by the H2O-H2O and H2O-CO2 interactions. The obtained results are in very good agreement with the unique previous measurement (in a narrower spectral range). They confirm that the H2O-continuum of CO2 is significantly larger than that observed for pure CO2. This continuum thus must be taken into account in radiative transfer calculations for media involving CO2+ H2O mixture. An empirical model, using sub-Lorentzian line shapes based on some temperature-dependent correction factors χ is proposed which enables an accurate description of the experimental results.

L. Fernandez-Cascales, A. Lucas, S. Rodriguez, X. Gao, A. Spiga, and C. Narteau. First quantification of relationship between dune orientation and sediment availability, Olympia Undae, Mars. Earth and Planetary Science Letters, 489:241-250, 2018. [ bib | DOI | ADS link ]

Dunes provide unique information about wind regimes on planetary bodies where there is no direct meteorological data. At the eastern margin of Olympia Undae on Mars, dune orientation is measured from satellite imagery and sediment cover is estimated using the high contrast between the dune material and substrate. The analysis of these data provide the first quantification of relationship between sediment availability and dune orientation. Abrupt and smooth dune reorientations are associated with inward and outward dynamics of dunes approaching and ejecting from major sedimentary bodies, respectively. These reorientation patterns along sediment transport pathways are interpreted using a new generation dune model based on the coexistence of two dune growth mechanisms. This model also permits solving of the inverse problem of predicting the wind regime from dune orientation. For bidirectional wind regimes, solutions of this inverse problem show substantial differences in the distributions of sediment flux orientation, which can be attributed to atmospheric flow variations induced by changes in albedo at the boundaries of major dune fields. Then, we conclude that relationships between sediment cover and dune orientation can be used to constrain wind regime and dune field development on Mars and other planetary surfaces.

P. L. Read and S. Lebonnois. Superrotation on Venus, on Titan, and Elsewhere. Annual Review of Earth and Planetary Sciences, 46:175-202, 2018. [ bib | DOI | ADS link ]

The superrotation of the atmospheres of Venus and Titan has puzzled dynamicists for many years and seems to put these planets in a very different dynamical regime from most other planets. In this review, we consider how to define superrotation objectively and explore the constraints that determine its occurrence. Atmospheric superrotation also occurs elsewhere in the Solar System and beyond, and we compare Venus and Titan with Earth and other planets for which wind estimates are available. The extreme superrotation on Venus and Titan poses some difficult challenges for numerical models of atmospheric circulation, much more difficult than for more rapidly rotating planets such as Earth or Mars. We consider mechanisms for generating and maintaining a superrotating state, all of which involve a global meridional overturning circulation. The role of nonaxisymmetric eddies is crucial, however, but the detailed mechanisms may differ between Venus, Titan, and other planets.

M. Turbet, E. Bolmont, J. Leconte, F. Forget, F. Selsis, G. Tobie, A. Caldas, J. Naar, and M. Gillon. Modeling climate diversity, tidal dynamics and the fate of volatiles on TRAPPIST-1 planets. Astronomy Astrophysics, 612:A86, 2018. [ bib | DOI | arXiv | ADS link ]

TRAPPIST-1 planets are invaluable for the study of comparative planetary science outside our solar system and possibly habitability. Both transit timing variations (TTV) of the planets and the compact, resonant architecture of the system suggest that TRAPPIST-1 planets could be endowed with various volatiles today. First, we derived from N-body simulations possible planetary evolution scenarios, and show that all the planets are likely in synchronous rotation. We then used a versatile 3D global climate model (GCM) to explore the possible climates of cool planets around cool stars, with a focus on the TRAPPIST-1 system. We investigated the conditions required for cool planets to prevent possible volatile species to be lost permanently by surface condensation, irreversible burying or photochemical destruction. We also explored the resilience of the same volatiles (when in condensed phase) to a runaway greenhouse process. We find that background atmospheres made of N2, CO, or O2 are rather resistant to atmospheric collapse. However, even if TRAPPIST-1 planets were able to sustain a thick background atmosphere by surviving early X/EUV radiation and stellar wind atmospheric erosion, it is difficult for them to accumulate significant greenhouse gases like CO2, CH4, or NH3. CO2 can easily condense on the permanent nightside, forming CO2 ice glaciers that would flow toward the substellar region. A complete CO2 ice surface cover is theoretically possible on TRAPPIST-1g and h only, but CO2 ices should be gravitationally unstable and get buried beneath the water ice shell in geologically short timescales. Given TRAPPIST-1 planets large EUV irradiation (at least 103 × Titan's flux), CH4 and NH3 are photodissociated rapidly and are thus hard to accumulate in the atmosphere. Photochemical hazes could then sedimentate and form a surface layer of tholins that would progressively thicken over the age of the TRAPPIST-1 system. Regarding habitability, we confirm that few bars of CO2 would suffice to warm the surface of TRAPPIST-1f and g above the melting point of water. We also show that TRAPPIST-1e is a remarkable candidate for surface habitability. If the planet is today synchronous and abundant in water, then it should very likely sustain surface liquid water at least in the substellar region, whatever the atmosphere considered.

C. Wang, F. Forget, T. Bertrand, A. Spiga, E. Millour, and T. Navarro. Parameterization of Rocket Dust Storms on Mars in the LMD Martian GCM: Modeling Details and Validation. Journal of Geophysical Research (Planets), 123:982-1000, 2018. [ bib | DOI | ADS link ]

The origin of the detached dust layers observed by the Mars Climate Sounder aboard the Mars Reconnaissance Orbiter is still debated. Spiga et al. (2013, <A href=“https://doi.org/10.1002/jgre.20046”>https://doi.org/10.1002/jgre.20046</A>) revealed that deep mesoscale convective “rocket dust storms” are likely to play an important role in forming these dust layers. To investigate how the detached dust layers are generated by this mesoscale phenomenon and subsequently evolve at larger scales, a parameterization of rocket dust storms to represent the mesoscale dust convection is designed and included into the Laboratoire de Météorologie Dynamique (LMD) Martian Global Climate Model (GCM). The new parameterization allows dust particles in the GCM to be transported to higher altitudes than in traditional GCMs. Combined with the horizontal transport by large-scale winds, the dust particles spread out and form detached dust layers. During the Martian dusty seasons, the LMD GCM with the new parameterization is able to form detached dust layers. The formation, evolution, and decay of the simulated dust layers are largely in agreement with the Mars Climate Sounder observations. This suggests that mesoscale rocket dust storms are among the key factors to explain the observed detached dust layers on Mars. However, the detached dust layers remain absent in the GCM during the clear seasons, even with the new parameterization. This implies that other relevant atmospheric processes, operating when no dust storms are occurring, are needed to explain the Martian detached dust layers. More observations of local dust storms could improve the ad hoc aspects of this parameterization, such as the trigger and timing of dust injection.

D. Singh, M. G. Flanner, and E. Millour. Improvement of Mars Surface Snow Albedo Modeling in LMD Mars GCM With SNICAR. Journal of Geophysical Research (Planets), 123:780-791, 2018. [ bib | DOI | ADS link ]

The current version of Laboratoire de Météorologie Dynamique (LMD) Mars GCM (original-MGCM) uses annually repeating (prescribed) CO2 snow albedo values based on the Thermal Emission Spectrometer observations. We integrate the Snow, Ice, and Aerosol Radiation (SNICAR) model with MGCM (SNICAR-MGCM) to prognostically determine H2O and CO2 snow albedos interactively in the model. Using the new diagnostic capabilities of this model, we find that cryospheric surfaces (with dust) increase the global surface albedo of Mars by 0.022. Over snow-covered regions, SNICAR-MGCM simulates mean albedo that is higher by about 0.034 than prescribed values in the original-MGCM. Globally, shortwave flux into the surface decreases by 1.26 W/m2, and net CO2 snow deposition increases by about 4% with SNICAR-MGCM over one Martian annual cycle as compared to the original-MGCM simulations. SNICAR integration reduces the mean global surface temperature and the surface pressure of Mars by about 0.87% and 2.5%, respectively. Changes in albedo also show a similar distribution to dust deposition over the globe. The SNICAR-MGCM model generates albedos with higher sensitivity to surface dust content as compared to original-MGCM. For snow-covered regions, we improve the correlation between albedo and optical depth of dust from -0.91 to -0.97 with SNICAR-MGCM as compared to the original-MGCM. Dust substantially darkens Mars's cryosphere, thereby reducing its impact on the global shortwave energy budget by more than half, relative to the impact of pure snow.

J.-Y. Chaufray, R. V. Yelle, F. Gonzalez-Galindo, F. Forget, M. Lopez-Valverde, F. Leblanc, and R. Modolo. Effect of the Lateral Exospheric Transport on the Horizontal Hydrogen Distribution Near the Exobase of Mars. Journal of Geophysical Research (Space Physics), 123:2441-2454, 2018. [ bib | DOI | ADS link ]

We simulate the hydrogen density near the exobase of Mars, using the 3-D Martian Global Circulation Model of Laboratoire de Météorologie Dynamique, coupled to an exospheric ballistic model to compute the downward ballistic flux. The simulated hydrogen distribution near the exobase obtained at two different seasonsLs = 180deg and Ls = 270degis close to Zero Net Ballistic Flux equilibrium. In other words, the hydrogen density near the exobase adjusts to have a balance between the local upward ballistic and the downward ballistic flux due to a short lateral migration time in the exosphere compared to the vertical diffusion time. This equilibrium leads to a hydrogen density n near the exobase directly controlled by the exospheric temperature T by the relation nT5/2 = constant. This relation could be used to extend 1-D hydrogen exospheric model of Mars used to derive the hydrogen density and escape flux at Mars from Lyman-α observations to 3-D model based on observed or modeled exospheric temperature near the exobase, without increasing the number of free parameters.

O. Korablev, F. Montmessin, A. Trokhimovskiy, A. A. Fedorova, A. V. Shakun, A. V. Grigoriev, B. E. Moshkin, N. I. Ignatiev, F. Forget, F. Lefèvre, K. Anufreychik, I. Dzuban, Y. S. Ivanov, Y. K. Kalinnikov, T. O. Kozlova, A. Kungurov, V. Makarov, F. Martynovich, I. Maslov, D. Merzlyakov, P. P. Moiseev, Y. Nikolskiy, A. Patrakeev, D. Patsaev, A. Santos-Skripko, O. Sazonov, N. Semena, A. Semenov, V. Shashkin, A. Sidorov, A. V. Stepanov, I. Stupin, D. Timonin, A. Y. Titov, A. Viktorov, A. Zharkov, F. Altieri, G. Arnold, D. A. Belyaev, J. L. Bertaux, D. S. Betsis, N. Duxbury, T. Encrenaz, T. Fouchet, J.-C. Gérard, D. Grassi, S. Guerlet, P. Hartogh, Y. Kasaba, I. Khatuntsev, V. A. Krasnopolsky, R. O. Kuzmin, E. Lellouch, M. A. Lopez-Valverde, M. Luginin, A. Määttänen, E. Marcq, J. Martin Torres, A. S. Medvedev, E. Millour, K. S. Olsen, M. R. Patel, C. Quantin-Nataf, A. V. Rodin, V. I. Shematovich, I. Thomas, N. Thomas, L. Vazquez, M. Vincendon, V. Wilquet, C. F. Wilson, L. V. Zasova, L. M. Zelenyi, and M. P. Zorzano. The Atmospheric Chemistry Suite (ACS) of Three Spectrometers for the ExoMars 2016 Trace Gas Orbiter. Space Science Reviews, 214:7, 2018. [ bib | DOI | ADS link ]

The Atmospheric Chemistry Suite (ACS) package is an element of the Russian contribution to the ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission. ACS consists of three separate infrared spectrometers, sharing common mechanical, electrical, and thermal interfaces. This ensemble of spectrometers has been designed and developed in response to the Trace Gas Orbiter mission objectives that specifically address the requirement of high sensitivity instruments to enable the unambiguous detection of trace gases of potential geophysical or biological interest. For this reason, ACS embarks a set of instruments achieving simultaneously very high accuracy (ppt level), very high resolving power (10,000) and large spectral coverage (0.7 to 17 μmthe visible to thermal infrared range). The near-infrared (NIR) channel is a versatile spectrometer covering the 0.7-1.6 μm spectral range with a resolving power of 20,000. NIR employs the combination of an echelle grating with an AOTF (Acousto-Optical Tunable Filter) as diffraction order selector. This channel will be mainly operated in solar occultation and nadir, and can also perform limb observations. The scientific goals of NIR are the measurements of water vapor, aerosols, and dayside or night side airglows. The mid-infrared (MIR) channel is a cross-dispersion echelle instrument dedicated to solar occultation measurements in the 2.2-4.4 μm range. MIR achieves a resolving power of 50,000. It has been designed to accomplish the most sensitive measurements ever of the trace gases present in the Martian atmosphere. The thermal-infrared channel (TIRVIM) is a 2-inch double pendulum Fourier-transform spectrometer encompassing the spectral range of 1.7-17 μm with apodized resolution varying from 0.2 to 1.3 cm-1. TIRVIM is primarily dedicated to profiling temperature from the surface up to 60 km and to monitor aerosol abundance in nadir. TIRVIM also has a limb and solar occultation capability. The technical concept of the instrument, its accommodation on the spacecraft, the optical designs as well as some of the calibrations, and the expected performances for its three channels are described.

J.-Y. Chaufray, F. Gonzalez-Galindo, F. Forget, M. Lopez-Valverde, F. Leblanc, R. Modolo, and S. Hess. Reply to comment “On the hydrogen escape: Comment to variability of the hydrogen in the Martian upper atmosphere as simulated by a 3D atmosphere-exosphere coupling by J.-Y. Chaufray et al.” by V. Krasnopolsky, Icarus, 281, 262. Icarus, 301:132-135, 2018. [ bib | DOI | ADS link ]

Krasnopolsky (2017) makes a careful review of our recent results about the Martian hydrogen content of the Martian upper atmosphere (Chaufray et al., 2015). We comment here on his two major points. First, he suggests that the non-thermal escape of H2, and particularly collisions with hot oxygen, not taken into account in our general circulation model (GCM), should modify our reported H2 and H density profiles. This is an important issue; we acknowledge that future effective coupling of our GCM with comprehensive models of the Martian solar wind interaction, ideally after being validated with the latest plasma observations of H2+, would allow for better estimations of the relative importance of the H2 non-thermal and thermal escape processes. For the time being we need assumptions in the GCM, with proper and regular updates. According to a recent and detailed study of the anisotropic elastic and inelastic collision cross sections between O and H2 (Gacesa et al., 2012), the escape rates used by Krasnopolsky (2010) for this process might be overestimated. We therefore do not include non thermal escape of H2 in the model. And secondly, in response to Krasnopolsky's comment on the H escape variability with the solar cycle, we revised our calculations and found a small bug in the computation of the Jeans effusion velocity. Our revised computed H escape rates are included here. They have a small impact on our key conclusions: similar seasonal variations, a reduced variation with the solar cycle but still larger than Krasnopolsky (2017), and again a hydrogen scape systematically lower than the diffusion-limited flux. This bug does not affect the latest Mars Climate Database v5.2.

S. Erard, B. Cecconi, P. Le Sidaner, A. P. Rossi, M. T. Capria, B. Schmitt, V. Génot, N. André, A. C. Vandaele, M. Scherf, R. Hueso, A. Määttänen, W. Thuillot, B. Carry, N. Achilleos, C. Marmo, O. Santolik, K. Benson, P. Fernique, L. Beigbeder, E. Millour, B. Rousseau, F. Andrieu, C. Chauvin, M. Minin, S. Ivanoski, A. Longobardo, P. Bollard, D. Albert, M. Gangloff, N. Jourdane, M. Bouchemit, J.-M. Glorian, L. Trompet, T. Al-Ubaidi, J. Juaristi, J. Desmars, P. Guio, O. Delaa, A. Lagain, J. Soucek, and D. Pisa. VESPA: A community-driven Virtual Observatory in Planetary Science. Planetary and Space Science, 150:65-85, 2018. [ bib | DOI | arXiv | ADS link ]

The VESPA data access system focuses on applying Virtual Observatory (VO) standards and tools to Planetary Science. Building on a previous EC-funded Europlanet program, it has reached maturity during the first year of a new Europlanet 2020 program (started in 2015 for 4 years). The infrastructure has been upgraded to handle many fields of Solar System studies, with a focus both on users and data providers. This paper describes the broad lines of the current VESPA infrastructure as seen by a potential user, and provides examples of real use cases in several thematic areas. These use cases are also intended to identify hints for future developments and adaptations of VO tools to Planetary Science.

S. Guerlet, T. Fouchet, A. Spiga, F. M. Flasar, L. N. Fletcher, B. E. Hesman, and N. Gorius. Equatorial Oscillation and Planetary Wave Activity in Saturn's Stratosphere Through the Cassini Epoch. Journal of Geophysical Research (Planets), 123:246-261, 2018. [ bib | DOI | ADS link ]

Thermal infrared spectra acquired by Cassini/Composite InfraRed Spectrometer (CIRS) in limb-viewing geometry in 2015 are used to derive 2-D latitude-pressure temperature and thermal wind maps. These maps are used to study the vertical structure and evolution of Saturn's equatorial oscillation (SEO), a dynamical phenomenon presenting similarities with the Earth's quasi-biennal oscillation (QBO) and semi-annual oscillation (SAO). We report that a new local wind maximum has appeared in 2015 in the upper stratosphere and derive the descent rates of other wind extrema through time. The phase of the oscillation observed in 2015, as compared to 2005 and 2010, remains consistent with a 15 year period. The SEO does not propagate downward at a regular rate but exhibits faster descent rate in the upper stratosphere, combined with a greater vertical wind shear, compared to the lower stratosphere. Within the framework of a QBO-type oscillation, we estimate the absorbed wave momentum flux in the stratosphere to be on the order of 7 × 10-6 N m-2. On Earth, interactions between vertically propagating waves (both planetary and mesoscale) and the mean zonal flow drive the QBO and SAO. To broaden our knowledge on waves potentially driving Saturn's equatorial oscillation, we searched for thermal signatures of planetary waves in the tropical stratosphere using CIRS nadir spectra. Temperature anomalies of amplitude 1-4 K and zonal wave numbers 1 to 9 are frequently observed, and an equatorial Rossby (n = 1) wave of zonal wave number 3 is tentatively identified in November 2009.

J. M. Moore, A. D. Howard, O. M. Umurhan, O. L. White, P. M. Schenk, R. A. Beyer, W. B. McKinnon, J. R. Spencer, K. N. Singer, W. M. Grundy, A. M. Earle, B. Schmitt, S. Protopapa, F. Nimmo, D. P. Cruikshank, D. P. Hinson, L. A. Young, S. A. Stern, H. A. Weaver, C. B. Olkin, K. Ennico, G. Collins, T. Bertrand, F. Forget, F. Scipioni, and New Horizons Science Team. Bladed Terrain on Pluto: Possible origins and evolution. Icarus, 300:129-144, 2018. [ bib | DOI | ADS link ]

Bladed Terrain on Pluto consists of deposits of massive CH4, which are observed to occur within latitudes 30deg of the equator and are found almost exclusively at the highest elevations ( 2 km above the mean radius). Our analysis indicates that these deposits of CH4 preferentially precipitate at low latitudes where net annual solar energy input is lowest. CH4 and N2 will both precipitate at low elevations. However, since there is much more N2 in the atmosphere than CH4, the N2 ice will dominate at these low elevations. At high elevations the atmosphere is too warm for N2 to precipitate so only CH4 can do so. We conclude that following the time of massive CH4 emplacement; there have been sufficient excursions in Pluto's climate to partially erode these deposits via sublimation into the blades we see today. Blades composed of massive CH4 ice implies that the mechanical behavior of CH4 can support at least several hundred meters of relief at Pluto surface conditions. Bladed Terrain deposits may be widespread in the low latitudes of the poorly seen sub-Charon hemisphere, based on spectral observations. If these locations are indeed Bladed Terrain deposits, they may mark heretofore unrecognized regions of high elevation.

M. Sylvestre, N. A. Teanby, S. Vinatier, S. Lebonnois, and P. G. J. Irwin. Seasonal evolution of C2N2, C3H4, and C4H2 abundances in Titan's lower stratosphere. Astronomy Astrophysics, 609:A64, 2018. [ bib | DOI | arXiv | ADS link ]

<BR /> Aims: We study the seasonal evolution of Titan's lower stratosphere (around 15 mbar) in order to better understand the atmospheric dynamics and chemistry in this part of the atmosphere. <BR /> Methods: We analysed Cassini/CIRS far-IR observations from 2006 to 2016 in order to measure the seasonal variations of three photochemical by-products: C4H2, C3H4, and C2N2. <BR /> Results: We show that the abundances of these three gases have evolved significantly at northern and southern high latitudes since 2006. We measure a sudden and steep increase of the volume mixing ratios of C4H2, C3H4, and C2N2 at the south pole from 2012 to 2013, whereas the abundances of these gases remained approximately constant at the north pole over the same period. At northern mid-latitudes, C2N2 and C4H2 abundances decrease after 2012 while C3H4 abundances stay constant. The comparison of these volume mixing ratio variations with the predictions of photochemical and dynamical models provides constraints on the seasonal evolution of atmospheric circulation and chemical processes at play.