2019 .

(25 publications)

M. Vals, A. Spiga, F. Forget, E. Millour, L. Montabone, and F. Lott. Study of gravity waves distribution and propagation in the thermosphere of Mars based on MGS, ODY, MRO and MAVEN density measurements. Planetary and Space Science, 178:104708, 2019. [ bib | DOI | arXiv | ADS link ]

By measuring the regular oscillations of the density of CO2 in the upper atmosphere (between 120 and 190 km), the mass spectrometer MAVEN/NGIMS (Atmosphere and Volatile EvolutioN/Neutral Gas Ion Mass Spectrometer) reveals the local impact of gravity waves. This yields precious information on the activity of gravity waves and the atmospheric conditions in which they propagate and break. The intensity of gravity waves measured by MAVEN in the upper atmosphere has been shown to be dictated by saturation processes in isothermal conditions. As a result, gravity waves activity is correlated to the evolution of the inverse of the background temperature. Previous data gathered at lower altitudes (~ 95- ~ 150 km) during aerobraking by the accelerometers on board MGS (Mars Global Surveyor), ODY (Mars Odyssey) and MRO (Mars Reconnaissance Orbiter) are analyzed in the light of those recent findings with MAVEN. The anti-correlation between GW-induced density perturbations and background temperature is plausibly found in the ODY data acquired in the polar regions, but not in the MGS and MRO data. MRO data in polar regions exhibit a correlation between the density perturbations and the Brunt-Väisälä frequency (or, equivalently, static stability), obtained from Global Climate Modeling compiled in the Mars Climate Database. At lower altitude levels (between 100 and 120 km), although wave saturation might still be dominant, isothermal conditions are no longer verified. In this case, theory predicts that the intensity of gravity waves is no more correlated to background temperature, but to static stability. At other latitudes in the three aerobraking datasets, the GW-induced relative density perturbations are correlated with neither inverse temperature nor static stability; in this particular case, this means that the observed activity of gravity waves is not only controlled by saturation, but also by the effects of gravity-wave sources and wind filtering through critical levels. This result highlights the exceptional nature of MAVEN/NGIMS observations which combine both isothermal and saturated conditions contrary to aerobraking measurements.

B. Pál, Á. Kereszturi, F. Forget, and M. D. Smith. Global seasonal variations of the near-surface relative humidity levels on present-day Mars. Icarus, 333:481-495, 2019. [ bib | DOI | arXiv | ADS link ]

We investigate the global seasonal variations of near-surface relative humidity and relevant attributes, like temperature and water vapor volume mixing ratio on Mars using calculations from modelled and measurement data. We focus on 2 AM local time snapshots to eliminate daily effects related to differences in insolation, and to be able to compare calculations based on modelling data from the Laboratoire de Météorologie Dynamique Mars General Circulation Model with the observations of Mars Global Surveyor Thermal Emission Spectrometer. We study the seasonal effects by examining four specific dates in the Martian year, the northern spring equinox, summer solstice, autumn equinox, and winter solstice. We identify three specific zones, where the near-surface relative humidity levels are systematically higher than in their vicinity regardless of season. We find that these areas coincide with low thermal inertia features, which control surface temperatures on the planet, and are most likely covered with unconsolidated fine dust with grain sizes smaller than ~ 40 μm. By comparing the data of relative humidity, temperature and water vapor volume mixing ratio at three different heights (near-surface, ~ 4 m and ~ 23 m above the surface), we demonstrate that the thermal inertia could play an important role in determining near-surface humidity levels. We also notice that during the night the water vapor levels drop at ~ 4 m above the surface. This, together with the temperature and thermal inertia values, shows that water vapor likely condenses in the near-surface atmosphere and on the ground during the night at the three aforementioned regions. This condensation may be in the form of brines, wettening of the fine grains by adsorption or deliquescence. This study specifies areas of interest on the surface of present day Mars for the proposed condensation, which may be examined by in-situ measurements in the future.

J. M. Lora, T. Tokano, J. Vatant d'Ollone, S. Lebonnois, and R. D. Lorenz. A model intercomparison of Titan's climate and low-latitude environment. Icarus, 333:113-126, 2019. [ bib | DOI | ADS link ]

Cassini-Huygens provided a wealth of data with which to constrain numerical models of Titan. Such models have been employed over the last decade to investigate various aspects of Titan's atmosphere and climate, and several three-dimensional general circulation models (GCMs) now exist that simulate Titan with a high degree of fidelity. However, substantial uncertainties persist, and at the same time no dedicated intercomparisons have assessed the degree to which these models agree with each other or the observations. To address this gap, and motivated by the proposed Dragonfly Titan lander mission, we directly compare three Titan GCMs to each other and to in situ observations, and also provide multi-model expectations for the low-latitude environment during the early northern winter season. Globally, the models qualitatively agree in their representation of the atmospheric structure and circulation, though one model severely underestimates meridional temperature gradients and zonal winds. We find that, at low latitudes, simulated and observed atmospheric temperatures closely agree in all cases, while the measured winds above the boundary layer are only quantitatively matched by one model. Nevertheless, the models simulate similar near-surface winds, and all indicate these are weak. Likewise, temperatures and methane content at low latitudes are similar between models, with some differences that are largely attributable to modeling assumptions. All models predict environments that closely resemble that encountered by the Huygens probe, including little or no precipitation at low latitudes during northern winter. The most significant differences concern the methane cycle, though the models are least comparable in this area and substantial uncertainties remain. We suggest that, while the overall low-latitude environment on Titan at this season is now fairly well constrained, future in situ measurements and monitoring will transform our understanding of regional and temporal variability, atmosphere-surface coupling, Titan's methane cycle, and modeling thereof.

M. J. Wolff, R. T. Clancy, M. A. Kahre, R. M. Haberle, F. Forget, B. A. Cantor, and M. C. Malin. Mapping water ice clouds on Mars with MRO/MARCI. Icarus, 332:24-49, 2019. [ bib | DOI | ADS link ]

Observations by the Mars Color Imager (MARCI) onboard the Mars Reconnaissance Orbiter (MRO) in the ultraviolet (UV, Band 7; 320 nm) are used to characterize the spatial and temporal behavior of atmospheric water ice over a period of 6 Mars Years. Exploiting the contrast of the bright ice clouds to the low albedo surface, a radiative transfer-based retrieval algorithm is developed to derive the column-integrated optical depth of the ice (τice). Several relatively unique input products are created as part of the retrieval development process, including a zonal dust climatology based on emission phase function (EPFs) sequences from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), a spatially variable UV-reflectance model for Band 7 (as well as for Band 6, 260 nm), and a water ice scattering phase function based on a droxtal ice habit. Taking into account a radiometric precision of 7%, an error analysis estimates the uncertainty in τice to be ~0.03 (excluding particle size effects, which are discussed separately). Zonal trends are analyzed over the full temporal extent of the observations, looking at both diurnal and interannual variability. The main (zonal) features are the aphelion cloud belt (ACB) and the polar hoods. For the ACB, there can be an appreciable diurnal change in τice between the periods of 14h30-15h00 and 15h00-15h30 Local True Solar Time (LTST). The amplitude of this effect shows relatively large interannual variability, associated mainly with changes in the earlier time block. When averaged over the interval 14h00-16h00 LTST, the interannual differences in the ACB structure are appreciably smaller. When the MARCI τice are compared to those from the Thermal Emission Spectrometer (TES), there is a good correlation of features, with the most significant difference being the seasonal (LS) evolution of the ACB. For TES, the ACB zonal profile is relative symmetric about LS = 90deg. In the MARCI data, this profile is noticeably asymmetric, with the centroid shifted to later in the northern summer season (LS = 120deg). The MARCI behavior is consistent with that observed by several other instruments. The correspondence of MARCI τice zonal and meridional behaviors with that predicted by two Global Circulation Models (GCM) is good. Each model captures the general behavior seen by MARCI in the ACB, the polar hoods, and the major orographic/topographic cloud features (including Valles Mariners). However, the mismatches between GCM results and MARCI reinforce the challenging nature of water ice clouds for dynamical models. The released τice are being archived at Malin Space Science Systems at https://www.msss.com/mro_marci_iceclouds/.

T. Cavalié, V. Hue, P. Hartogh, R. Moreno, E. Lellouch, H. Feuchtgruber, C. Jarchow, T. Cassidy, L. N. Fletcher, F. Billebaud, M. Dobrijevic, L. Rezac, G. S. Orton, M. Rengel, T. Fouchet, and S. Guerlet. Herschel map of Saturn's stratospheric water, delivered by the plumes of Enceladus. Astronomy Astrophysics, 630:A87, 2019. [ bib | DOI | arXiv | ADS link ]

Context. The origin of water in the stratospheres of giant planets has been an outstanding question ever since its first detection by the Infrared Space Observatory some 20 years ago. Water can originate from interplanetary dust particles, icy rings and satellites, and large comet impacts. Analyses of Herschel Space Observatory observations have proven that the bulk of Jupiter's stratospheric water was delivered by the Shoemaker-Levy 9 impacts in 1994. In 2006, the Cassini mission detected water plumes at the South Pole of Enceladus, which made the moon a serious candidate for Saturn's stratospheric water. Further evidence was found in 2011 when Herschel demonstrated the presence of a water torus at the orbital distance of Enceladus that was fed by the moon's plumes. Finally, water falling from the rings onto Saturn's uppermost atmospheric layers at low latitudes was detected during the final orbits of Cassini's end-of-mission plunge into the atmosphere. <BR /> Aims: In this paper, we use Herschel mapping observations of water in Saturn's stratosphere to identify its source. <BR /> Methods: We tested several empirical models against the Herschel-HIFI and -PACS observations, which were collected on December 30, 2010, and January 2, 2011, respectively. <BR /> Results: We demonstrate that Saturn's stratospheric water is not uniformly mixed as a function of latitude, but peaks at the equator and decreases poleward with a Gaussian distribution. We obtain our best fit with an equatorial mole fraction 1.1 ppb and a half width at half maximum of 25deg, when accounting for a temperature increase in the two warm stratospheric vortices produced by Saturn's Great Storm of 2010-2011. <BR /> Conclusions: This work demonstrates that Enceladus is the main source of Saturn's stratospheric water.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

D. P. Cruikshank, O. M. Umurhan, R. A. Beyer, B. Schmitt, J. T. Keane, K. D. Runyon, D. Atri, O. L. White, I. Matsuyama, J. M. Moore, W. B. McKinnon, S. A. Sandford, K. N. Singer, W. M. Grundy, C. M. Dalle Ore, J. C. Cook, T. Bertrand, S. A. Stern, C. B. Olkin, H. A. Weaver, L. A. Young, J. R. Spencer, C. M. Lisse, R. P. Binzel, A. M. Earle, S. J. Robbins, G. R. Gladstone, R. J. Cartwright, and K. Ennico. Recent cryovolcanism in Virgil Fossae on Pluto. Icarus, 330:155-168, 2019. [ bib | DOI | ADS link ]

The Virgil Fossae region on Pluto exhibits three spatially coincident properties that are suggestive of recent cryovolcanic activity over an area approximately 300 by 200 km. Situated in the fossae troughs or channels and in the surrounding terrain are exposures of H2O ice in which there is entrained opaque red-colored matter of unknown composition. The H2O ice is also seen to carry spectral signatures at 1.65 and 2.2 μm of NH3 in some form, possibly as a hydrate, an ammoniated salt, or some other compound. Model calculations of NH3 destruction in H2O ice by galactic cosmic rays suggest that the maximum lifetime of NH3 in the uppermost meter of the exposed surface is 109 years, while considerations of Lyman-α ultraviolet and solar wind charged particles suggest shorter timescales by a factor of 10 or 10000. Thus, 109 y is taken as an upper limit to the age of the emplacement event, and it could be substantially younger.

The red colorant in the ammoniated H2O in Virgil Fossae and surroundings may be a macromolecular organic material (tholin) thought to give color to much of Pluto's surface, but probably different in composition and age. Owing to the limited spectral range of the New Horizons imaging spectrometer and the signal precision of the data, apart from the H2O and NH3 signatures there are no direct spectroscopic clues to the chemistry of the strongly colored deposit on Pluto. We suggest that the colored material was a component of the fluid reservoir from which the material now on the surface in this region was erupted. Although other compositions are possible, if it is indeed a complex organic material it may incorporate organics inherited from the solar nebula, further processed in a warm aqueous environment inside Pluto.

A planet-scale stress pattern in Pluto's lithosphere induced by true polar wander, freezing of a putative interior ocean, and surface loading has caused fracturing in a broad arc west of Sputnik Planitia, consistent with the structure of Virgil Fossae and similar extensional features. This faulting may have facilitated the ascent of fluid in subsurface reservoirs to reach the surface as flows and as fountains of cryoclastic materials, consistent with the appearance of colored, ammoniated H2O ice deposits in and around Virgil Fossae. Models of a cryoflow emerging from sources in Virgil Fossae indicate that the lateral extent of the flow can be several km (Umurhan et al., 2019). The deposit over the full length (200 km) of the main trough in the Virgil Fossae complex and extending through the north rim of Elliot crater and varying in elevation over a range of 2.5 km, suggests that it debouched from multiple sources, probably along the length of the strike direction of the normal faults defining the graben. The source or sources of the ammoniated H2O are one or more subsurface reservoirs that may or may not connect to the global ocean postulated for Pluto's interior. Alternatives to cryovolcanism in producing the observed characteristics of the region around Virgil Fossae are explored in the discussion section of the paper.

T. Bertrand, F. Forget, O. M. Umurhan, J. M. Moore, L. A. Young, S. Protopapa, W. M. Grundy, B. Schmitt, R. D. Dhingra, R. P. Binzel, A. M. Earle, D. P. Cruikshank, S. A. Stern, H. A. Weaver, K. Ennico, C. B. Olkin, and New Horizons Science Team. The CH4 cycles on Pluto over seasonal and astronomical timescales. Icarus, 329:148-165, 2019. [ bib | DOI | arXiv | ADS link ]

Pluto's surface is covered in numerous CH4 ice deposits, that vary in texture and brightness, as revealed by the New Horizons spacecraft as it flew by Pluto in July 2015. These observations suggest that CH4 on Pluto has a complex history, involving reservoirs of different composition, thickness and stability controlled by volatile processes occurring on different timescales. In order to interpret these observations, we use a Pluto volatile transport model able to simulate the cycles of N2 and CH4 ices over millions of years. By assuming fixed solid mixing ratios, we explore how changes in surface albedos, emissivities and thermal inertias impact volatile transport. This work is therefore a direct and natural continuation of the work by Bertrand et al. (2018), which only explored the N2 cycles. Results show that bright CH4 deposits can create cold traps for N2 ice outside Sputnik Planitia, leading to a strong coupling between the N2 and CH4 cycles. Depending on the assumed albedo for CH4 ice, the model predicts CH4 ice accumulation (1) at the same equatorial latitudes where the Bladed Terrain Deposits are observed, supporting the idea that these CH4-rich deposits are massive and perennial, or (2) at mid-latitudes (25deg- 70deg), forming a thick mantle which is consistent with New Horizons observations. In our simulations, both CH4 ice reservoirs are not in an equilibrium state and either one can dominate the other over long timescales, depending on the assumptions made for the CH4 albedo. This suggests that long-term volatile transport exists between the observed reservoirs. The model also reproduces the formation of N2 deposits at mid-latitudes and in the equatorial depressions surrounding the Bladed Terrain Deposits, as observed by New Horizons. At the poles, only seasonal CH4 and N2 deposits are obtained in Pluto's current orbital configuration. Finally, we show that Pluto's atmosphere always contained, over the last astronomical cycles, enough gaseous CH4 to absorb most of the incoming Lyman-α flux.

P. Scarica, I. Garate-Lopez, S. Lebonnois, G. Piccioni, D. Grassi, A. Migliorini, and S. Tellmann. Validation of the IPSL Venus GCM Thermal Structure with Venus Express Data. Atmosphere, 10:584, 2019. [ bib | DOI | ADS link ]

Y. J. Lee, K.-L. Jessup, S. Perez-Hoyos, D. V. Titov, S. Lebonnois, J. Peralta, T. Horinouchi, T. Imamura, S. Limaye, E. Marcq, M. Takagi, A. Yamazaki, M. Yamada, S. Watanabe, S.-y. Murakami, K. Ogohara, W. M. McClintock, G. Holsclaw, and A. Roman. Long-term Variations of Venuss 365 nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope. , 158:126, 2019. [ bib | DOI | arXiv | ADS link ]

An unknown absorber near the cloud-top level of Venus generates a broad absorption feature from the ultraviolet (UV) to visible, peaking around 360 nm, and therefore plays a critical role in the solar energy absorption. We present a quantitative study of the variability of the cloud albedo at 365 nm and its impact on Venuss solar heating rates based on an analysis of Venus Express and Akatsuki UV images and Hubble Space Telescope and MESSENGER UV spectral data; in this analysis, the calibration correction factor of the UV images of Venus Express (Venus Monitoring Camera) is updated relative to the Hubble and MESSENGER albedo measurements. Our results indicate that the 365 nm albedo varied by a factor of 2 from 2006 to 2017 over the entire planet, producing a 25%40% change in the low-latitude solar heating rate according to our radiative transfer calculations. Thus, the cloud-top level atmosphere should have experienced considerable solar heating variations over this period. Our global circulation model calculations show that this variable solar heating rate may explain the observed variations of zonal wind from 2006 to 2017. Overlaps in the timescale of the long-term UV albedo and the solar activity variations make it plausible that solar extreme UV intensity and cosmic-ray variations influenced the observed albedo trends. The albedo variations might also be linked with temporal variations of the upper cloud SO2 gas abundance, which affects the H2SO4H2O aerosol formation.

D. Cordier, D. A. Bonhommeau, S. Port, V. Chevrier, S. Lebonnois, and F. García-Sánchez. The Physical Origin of the Venus Low Atmosphere Chemical Gradient. Astrophysical Journal, 880:82, 2019. [ bib | DOI | arXiv | ADS link ]

Venus shares many similarities with the Earth, but concomitantly, some of its features are extremely original. This is especially true for its atmosphere, where high pressures and temperatures are found at the ground level. In these conditions, carbon dioxide, the main component of Venus atmosphere, is a supercritical fluid. The analysis of VeGa-2 probe data has revealed the high instability of the region located in the last few kilometers above the ground level. Recent works have suggested an explanation based on the existence of a vertical gradient of molecular nitrogen abundances, around 5 ppm per meter. Our goal was then to identify which physical processes could lead to the establishment of this intriguing nitrogen gradient, in the deep atmosphere of Venus. Using an appropriate equation of state for the binary mixture CO2N2 under supercritical conditions, and also molecular dynamics simulations, we have investigated the separation processes of N2 and CO2 in the Venusian context. Our results show that molecular diffusion is strongly inefficient, and potential phase separation is an unlikely mechanism. We have compared the quantity of CO2 required to form the proposed gradient with what could be released by a diffuse degassing from a low volcanic activity. The needed fluxes of CO2 are not so different from what can be measured over some terrestrial volcanic systems, suggesting a similar effect at work on Venus.

M. Turbet, D. Ehrenreich, C. Lovis, E. Bolmont, and T. Fauchez. The runaway greenhouse radius inflation effect. An observational diagnostic to probe water on Earth-sized planets and test the habitable zone concept. Astronomy Astrophysics, 628:A12, 2019. [ bib | DOI | arXiv | ADS link ]

Planets similar to Earth but slightly more irradiated are expected to enter into a runaway greenhouse state, where all surface water rapidly evaporates, forming an optically thick H2O-dominated atmosphere. For Earth, this extreme climate transition is thought to occur for an increase of only 6% in solar luminosity, though the exact limit at which the transition would occur is still a highly debated topic. In general, the runaway greenhouse is believed to be a fundamental process in the evolution of Earth-sized, temperate planets. Using 1D radiative-convective climate calculations accounting for thick, hot water vapor-dominated atmospheres, we evaluate the transit atmospheric thickness of a post-runaway greenhouse atmosphere, and find that it could possibly reach over a thousand kilometers (i.e., a few tens of percent of the Earth's radius). This abrupt radius inflation resulting from the runaway-greenhouse-induced transition could be detected statistically by ongoing and upcoming space missions. These include satellites such as TESS, CHEOPS, and PLATO combined with precise radial velocity mass measurements using ground-based spectrographs such as ESPRESSO, CARMENES, or SPIRou. This radius inflation could also be detected in multiplanetary systems such as TRAPPIST-1 once masses and radii are known with good enough precision. This result provides the community with an observational test of two points. The first point is the concept of runaway greenhouse, which defines the inner edge of the traditional habitable zone, and the exact limit of the runaway greenhouse transition. In particular, this could provide an empirical measurement of the irradiation at which Earth analogs transition from a temperate to a runaway greenhouse climate state. This astronomical measurement would make it possible to statistically estimate how close Earth is from the runaway greenhouse. Second, it could be used as a test for the presence (and statistical abundance) of water in temperate, Earth-sized exoplanets.

T. Encrenaz, T. K. Greathouse, S. Aoki, F. Daerden, M. Giuranna, F. Forget, F. Lefèvre, F. Montmessin, T. Fouchet, B. Bézard, S. K. Atreya, C. DeWitt, M. J. Richter, L. Neary, and S. Viscardy. Ground-based infrared mapping of H2O2 on Mars near opposition. Astronomy Astrophysics, 627:A60, 2019. [ bib | DOI | ADS link ]

We pursued our ground-based seasonal monitoring of hydrogen peroxide on Mars using thermal imaging spectroscopy, with two observations of the planet near opposition, in May 2016 (solar longitude Ls = 148.5deg, diameter = 17 arcsec) and July 2018 (Ls = 209deg, diameter = 23 arcsec). Data were recorded in the 1232-1242 cm-1 range (8.1 μm) with the Texas Echelon Cross Echelle Spectrograph (TEXES) mounted at the 3 m Infrared Telescope Facility (IRTF) at the Mauna Kea Observatories. As in the case of our previous analyses, maps of H2O2 were obtained using line depth ratios of weak transitions of H2O2 divided by a weak CO2 line. The H2O2 map of April 2016 shows a strong dichotomy between the northern and southern hemispheres, with a mean volume mixing ratio of 45 ppbv on the north side and less than 10 ppbv on the south side; this dichotomy was expected by the photochemical models developed in the LMD Mars Global Climate Model (LMD-MGCM) and with the recently developed Global Environmental Multiscale (GEM) model. The second measurement (July 2018) was taken in the middle of the MY 34 global dust storm. H2O2 was not detected with a disk-integrated 2σ upper limit of 10 ppbv, while both the LMD-MGCM and the LEM models predicted a value above 20 ppbv (also observed by TEXES in 2003) in the absence of dust storm. This depletion is probably the result of the high dust content in the atmosphere at the time of our observations, which led to a decrease in the water vapor column density, as observed by the PFS during the global dust storm. GCM simulations using the GEM model show that the H2O depletion leads to a drop in H2O2, due to the lack of HO2 radicals. Our result brings a new constraint on the photochemistry of H2O2 in the presence of a high dust content. In parallel, we reprocessed the whole TEXES dataset of H2O2 measurements using the latest version of the GEISA database (GEISA 2015). We recently found that there is a significant difference in the H2O2 line strengths between the 2003 and 2015 versions of GEISA. Therefore, all H2O2 volume mixing ratios up to 2014 from TEXES measurements must be reduced by a factor of 1.75. As a consequence, in four cases (Ls around 80deg, 100deg, 150deg, and 209deg) the H2O2 abundances show contradictory values between different Martian years. At Ls = 209deg the cause seems to be the increased dust content associated with the global dust storm. The inter-annual variability in the three other cases remains unexplained at this time.

H. Tran, M. Turbet, S. Hanoufa, X. Landsheere, P. Chelin, Q. Ma, and J.-M. Hartmann. The CO2-broadened H2O continuum in the 100-1500 cm-1 region: Measurements, predictions and empirical model. Journal of Quantitative Spectroscopy and Radiative Transfer, 230:75-80, 2019. [ bib | DOI | arXiv | ADS link ]

Transmission spectra of H2O+CO2 mixtures have been recorded, at 296, 325 and 366 K, for various pressures and mixture compositions using two experimental setups. Their analysis enables to retrieve values of the “continuum” absorption by the CO2-broadened H2O line wings between 100 and 1500 cm-1. The results are in good agreement with those, around 1300 cm-1, of the single previous experimental study available. Comparisons are also made with direct predictions based on line-shape correction factors χ calculated, almost thirty years ago, using a quasistatic approach and an input H2Osbnd CO2 intermolecular potential. They show that this model quite nicely predicts, with slightly overestimated values, the continuum over a spectral range where it varies by more than three orders of magnitude. An empirical correction is proposed, based on the experimental data, which should be useful for radiative transfer and climate studies in CO2 rich planetary atmospheres.

R. J. Lillis, M. O. Fillingim, Y. Ma, F. Gonzalez-Galindo, F. Forget, C. L. Johnson, A. Mittelholz, C. T. Russell, L. Andersson, and C. M. Fowler. Modeling Wind-Driven Ionospheric Dynamo Currents at Mars: Expectations for InSight Magnetic Field Measurements. Geophysical Research Letters, 46:5083-5091, 2019. [ bib | DOI | ADS link ]

We model expected dynamo currents above, and resulting magnetic field profiles at, InSight's landing site on Mars, including for the first time the effect of electron-ion collisions. We calculate their diurnal and seasonal variability using inputs from global models of the Martian thermosphere, ionosphere, and magnetosphere. Modeled currents primarily depend on plasma densities and the strength of the neutral wind component perpendicular to the combined crustal and draped magnetic fields that thread the ionosphere. Negligible at night, currents are the strongest in the late morning and near solstices due to stronger winds and near perihelion due to both stronger winds and higher plasma densities from solar EUV photoionization. Resulting surface magnetic fields of tens of nanotesla and occasionally 100 nT may be expected at the InSight landing site. We expect currents and surface fields to vary significantly with changes in the draped magnetic field caused by Mars' dynamic solar wind environment.

E. Meza, B. Sicardy, M. Assafin, J. L. Ortiz, T. Bertrand, E. Lellouch, J. Desmars, F. Forget, D. Bérard, A. Doressoundiram, J. Lecacheux, J. Marques Oliveira, F. Roques, T. Widemann, F. Colas, F. Vachier, S. Renner, R. Leiva, F. Braga-Ribas, G. Benedetti-Rossi, J. I. B. Camargo, A. Dias-Oliveira, B. Morgado, A. R. Gomes-Júnior, R. Vieira-Martins, R. Behrend, A. C. Tirado, R. Duffard, N. Morales, P. Santos-Sanz, M. Jelínek, R. Cunniffe, R. Querel, M. Harnisch, R. Jansen, A. Pennell, S. Todd, V. D. Ivanov, C. Opitom, M. Gillon, E. Jehin, J. Manfroid, J. Pollock, D. E. Reichart, J. B. Haislip, K. M. Ivarsen, A. P. LaCluyze, A. Maury, R. Gil-Hutton, V. Dhillon, S. Littlefair, T. Marsh, C. Veillet, K.-L. Bath, W. Beisker, H.-J. Bode, M. Kretlow, D. Herald, D. Gault, S. Kerr, H. Pavlov, O. Faragó, O. Klös, E. Frappa, M. Lavayssière, A. A. Cole, A. B. Giles, J. G. Greenhill, K. M. Hill, M. W. Buie, C. B. Olkin, E. F. Young, L. A. Young, L. H. Wasserman, M. Devogèle, R. G. French, F. B. Bianco, F. Marchis, N. Brosch, S. Kaspi, D. Polishook, I. Manulis, M. Ait Moulay Larbi, Z. Benkhaldoun, A. Daassou, Y. El Azhari, Y. Moulane, J. Broughton, J. Milner, T. Dobosz, G. Bolt, B. Lade, A. Gilmore, P. Kilmartin, W. H. Allen, P. B. Graham, B. Loader, G. McKay, J. Talbot, S. Parker, L. Abe, P. Bendjoya, J.-P. Rivet, D. Vernet, L. Di Fabrizio, V. Lorenzi, A. Magazzú, E. Molinari, K. Gazeas, L. Tzouganatos, A. Carbognani, G. Bonnoli, A. Marchini, G. Leto, R. Z. Sanchez, L. Mancini, B. Kattentidt, M. Dohrmann, K. Guhl, W. Rothe, K. Walzel, G. Wortmann, A. Eberle, D. Hampf, J. Ohlert, G. Krannich, G. Murawsky, B. Gährken, D. Gloistein, S. Alonso, A. Román, J.-E. Communal, F. Jabet, S. deVisscher, J. Sérot, T. Janik, Z. Moravec, P. Machado, A. Selva, C. Perelló, J. Rovira, M. Conti, R. Papini, F. Salvaggio, A. Noschese, V. Tsamis, K. Tigani, P. Barroy, M. Irzyk, D. Neel, J. P. Godard, D. Lanoiselée, P. Sogorb, D. Vérilhac, M. Bretton, F. Signoret, F. Ciabattari, R. Naves, M. Boutet, J. De Queiroz, P. Lindner, K. Lindner, P. Enskonatus, G. Dangl, T. Tordai, H. Eichler, J. Hattenbach, C. Peterson, L. A. Molnar, and R. R. Howell. Lower atmosphere and pressure evolution on Pluto from ground-based stellar occultations, 1988-2016. Astronomy Astrophysics, 625:A42, 2019. [ bib | DOI | arXiv | ADS link ]

Context. The tenuous nitrogen (N2) atmosphere on Pluto undergoes strong seasonal effects due to high obliquity and orbital eccentricity, and has recently (July 2015) been observed by the New Horizons spacecraft. <BR /> Aims: The main goals of this study are (i) to construct a well calibrated record of the seasonal evolution of surface pressure on Pluto and (ii) to constrain the structure of the lower atmosphere using a central flash observed in 2015. <BR /> Methods: Eleven stellar occultations by Pluto observed between 2002 and 2016 are used to retrieve atmospheric profiles (density, pressure, temperature) between altitude levels of 5 and 380 km (i.e. pressures from 10 μbar to 10 nbar). <BR /> Results: (i) Pressure has suffered a monotonic increase from 1988 to 2016, that is compared to a seasonal volatile transport model, from which tight constraints on a combination of albedo and emissivity of N2 ice are derived. (ii) A central flash observed on 2015 June 29 is consistent with New Horizons REX profiles, provided that (a) large diurnal temperature variations (not expected by current models) occur over Sputnik Planitia; and/or (b) hazes with tangential optical depth of 0.3 are present at 4-7 km altitude levels; and/or (c) the nominal REX density values are overestimated by an implausibly large factor of 20%; and/or (d) higher terrains block part of the flash in the Charon facing hemisphere.

A. C. Vandaele, O. Korablev, F. Daerden, S. Aoki, I. R. Thomas, F. Altieri, M. López-Valverde, G. Villanueva, G. Liuzzi, M. D. Smith, J. T. Erwin, L. Trompet, A. A. Fedorova, F. Montmessin, A. Trokhimovskiy, D. A. Belyaev, N. I. Ignatiev, M. Luginin, K. S. Olsen, L. Baggio, J. Alday, J.-L. Bertaux, D. Betsis, D. Bolsée, R. T. Clancy, E. Cloutis, C. Depiesse, B. Funke, M. Garcia-Comas, J.-C. Gérard, M. Giuranna, F. Gonzalez-Galindo, A. V. Grigoriev, Y. S. Ivanov, J. Kaminski, O. Karatekin, F. Lefèvre, S. Lewis, M. López-Puertas, A. Mahieux, I. Maslov, J. Mason, M. J. Mumma, L. Neary, E. Neefs, A. Patrakeev, D. Patsaev, B. Ristic, S. Robert, F. Schmidt, A. Shakun, N. A. Teanby, S. Viscardy, Y. Willame, J. Whiteway, V. Wilquet, M. J. Wolff, G. Bellucci, M. R. Patel, J.-J. López-Moreno, F. Forget, C. F. Wilson, H. Svedhem, J. L. Vago, D. Rodionov, NOMAD Science Team, A. C. Vandaele, J.-J. López-Moreno, G. Bellucci, M. R. Patel, G. Alonso-Rodrigo, S. Aoki, F. Altieri, S. Bauduin, D. Bolsée, G. Carrozzo, R. T. Clancy, E. Cloutis, M. Crismani, F. Daerden, F. da Pieve, E. D'Aversa, C. Depiesse, J. T. Erwin, G. Etiope, A. A. Fedorova, B. Funke, D. Fussen, M. Garcia-Comas, A. Geminale, J.-C. Gérard, M. Giuranna, L. Gkouvelis, F. Gonzalez-Galindo, J. Holmes, B. Hubert, N. I. Ignatiev, J. Kaminski, O. Karatekin, Y. Kasaba, D. Kass, A. Kleinböhl, O. Lanciano, F. Lefèvre, S. Lewis, G. Liuzzi, M. López-Puertas, M. López-Valverde, A. Mahieux, J. Mason, M. J. Mumma, H. Nakagawa, L. Neary, E. Neefs, R. E. Novak, F. Oliva, A. Piccialli, E. Renotte, B. Ritter, S. Robert, F. Schmidt, N. Schneider, G. Sindoni, M. D. Smith, N. A. Teanby, E. Thiemann, I. R. Thomas, A. Trokhimovskiy, L. Trompet, J. Vander Auwera, G. Villanueva, S. Viscardy, J. Whiteway, V. Wilquet, Y. Willame, M. J. Wolff, P. Wolkenberg, R. Yelle, ACS Science Team, J. Alday, F. Altieri, K. Anufreychik, G. Arnold, L. Baggio, D. A. Belyaev, J.-L. Bertaux, N. Duxbury, A. A. Fedorova, F. Forget, T. Fouchet, D. Grassi, A. V. Grigoriev, S. Guerlet, P. Hartogh, N. I. Ignatiev, Y. Kasaba, I. Khatuntsev, N. Kokonkov, O. Korablev, V. Krasnopolsky, R. Kuzmin, G. Lacombe, F. Lefèvre, E. Lellouch, M. López-Valverde, I. Maslov, M. Luginin, A. Määttänen, E. Marcq, J. Martin-Torres, A. Medvedev, E. Millour, F. Montmessin, B. Moshkin, K. S. Olsen, M. R. Patel, A. Patrakeev, D. Patsaev, C. Quantin-Nataf, D. Rodionov, A. Rodin, A. Shakun, V. Shematovich, I. R. Thomas, N. Thomas, A. Trokhimovsky, L. Vazquez, M. Vincendon, V. Wilquet, C. F. Wilson, R. Young, L. Zasova, L. Zelenyi, and M. P. Zorzano. Martian dust storm impact on atmospheric H2O and D/H observed by ExoMars Trace Gas Orbiter. Nature, 568:521-525, 2019. [ bib | DOI | ADS link ]

Global dust storms on Mars are rare1,2 but can affect the atmospheric dynamics and inflation of the atmosphere3, primarily owing to solar heating of the dust3. In turn, changes in atmospheric dynamics can affect the distribution of atmospheric water vapour, with potential implications for the atmospheric photochemistry and climate on Mars4. Recent observations of the water vapour abundance in the Martian atmosphere during dust storm conditions revealed a high-altitude increase in atmospheric water vapour that was more pronounced at high northern latitudes5,6, as well as a decrease in the water column at low latitudes7,8. Here we present concurrent, high-resolution measurements of dust, water and semiheavy water (HDO) at the onset of a global dust storm, obtained by the NOMAD and ACS instruments onboard the ExoMars Trace Gas Orbiter. We report the vertical distribution of the HDO/H2O ratio (D/H) from the planetary boundary layer up to an altitude of 80 kilometres. Our findings suggest that before the onset of the dust storm, HDO abundances were reduced to levels below detectability at altitudes above 40 kilometres. This decrease in HDO coincided with the presence of water-ice clouds. During the storm, an increase in the abundance of H2O and HDO was observed at altitudes between 40 and 80 kilometres. We propose that these increased abundances may be the result of warmer temperatures during the dust storm causing stronger atmospheric circulation and preventing ice cloud formation, which may confine water vapour to lower altitudes through gravitational fall and subsequent sublimation of ice crystals3. The observed changes in H2O and HDO abundance occurred within a few days during the development of the dust storm, suggesting a fast impact of dust storms on the Martian atmosphere.

O. Korablev, A. C. Avandaele, F. Montmessin, A. A. Fedorova, A. Trokhimovskiy, F. Forget, F. Lefèvre, F. Daerden, I. R. Thomas, L. Trompet, J. T. Erwin, S. Aoki, S. Robert, L. Neary, S. Viscardy, A. V. Grigoriev, N. I. Ignatiev, A. Shakun, A. Patrakeev, D. A. Belyaev, J.-L. Bertaux, K. S. Olsen, L. Baggio, J. Alday, Y. S. Ivanov, B. Ristic, J. Mason, Y. Willame, C. Depiesse, L. Hetey, S. Berkenbosch, R. Clairquin, C. Queirolo, B. Beeckman, E. Neefs, M. R. Patel, G. Bellucci, J.-J. López-Moreno, C. F. Wilson, G. Etiope, L. Zelenyi, H. Svedhem, J. L. Vago, ACS Science Team, NOMAD Science Team, G. Alonso-Rodrigo, F. Altieri, K. Anufreychik, G. Arnold, S. Bauduin, D. Bolsée, G. Carrozzo, R. T. Clancy, E. Cloutis, M. Crismani, F. da Pieve, E. D'Aversa, N. Duxbury, T. Encrenaz, T. Fouchet, B. Funke, D. Fussen, M. Garcia-Comas, J.-C. Gérard, M. Giuranna, L. Gkouvelis, F. Gonzalez-Galindo, D. Grassi, S. Guerlet, P. Hartogh, J. Holmes, B. Hubert, J. Kaminski, O. Karatekin, Y. Kasaba, D. Kass, I. Khatuntsev, A. Kleinböhl, N. Kokonkov, V. Krasnopolsky, R. Kuzmin, G. Lacombe, O. Lanciano, E. Lellouch, S. Lewis, M. Luginin, G. Liuzzi, M. López-Puertas, M. López-Valverde, A. Määttänen, A. Mahieux, E. Marcq, J. Martin-Torres, I. Maslov, A. Medvedev, E. Millour, B. Moshkin, M. J. Mumma, H. Nakagawa, R. E. Novak, F. Oliva, D. Patsaev, A. Piccialli, C. Quantin-Nataf, E. Renotte, B. Ritter, A. Rodin, F. Schmidt, N. Schneider, V. Shematovich, M. D. Smith, N. A. Teanby, E. Thiemann, N. Thomas, J. Vander Auwera, L. Vazquez, G. Villanueva, M. Vincendon, J. Whiteway, V. Wilquet, M. J. Wolff, P. Wolkenberg, R. Yelle, R. Young, L. Zasova, and M. P. Zorzano. No detection of methane on Mars from early ExoMars Trace Gas Orbiter observations. Nature, 568:517-520, 2019. [ bib | DOI | ADS link ]

The detection of methane on Mars has been interpreted as indicating that geochemical or biotic activities could persist on Mars today1. A number of different measurements of methane show evidence of transient, locally elevated methane concentrations and seasonal variations in background methane concentrations2-5. These measurements, however, are difficult to reconcile with our current understanding of the chemistry and physics of the Martian atmosphere6,7, whichgiven methane's lifetime of several centuriespredicts an even, well mixed distribution of methane1,6,8. Here we report highly sensitive measurements of the atmosphere of Mars in an attempt to detect methane, using the ACS and NOMAD instruments onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter from April to August 2018. We did not detect any methane over a range of latitudes in both hemispheres, obtaining an upper limit for methane of about 0.05 parts per billion by volume, which is 10 to 100 times lower than previously reported positive detections2,4. We suggest that reconciliation between the present findings and the background methane concentrations found in the Gale crater4 would require an unknown process that can rapidly remove or sequester methane from the lower atmosphere before it spreads globally.

J. Yang, J. Leconte, E. T. Wolf, T. Merlis, D. D. B. Koll, F. Forget, and D. S. Abbot. Simulations of Water Vapor and Clouds on Rapidly Rotating and Tidally Locked Planets: A 3D Model Intercomparison. Astrophysical Journal, 875:46, 2019. [ bib | DOI | ADS link ]

Robustly modeling the inner edge of the habitable zone is essential for determining the most promising potentially habitable exoplanets for atmospheric characterization. Global climate models (GCMs) have become the standard tool for calculating this boundary, but divergent results have emerged among the various GCMs. In this study, we perform an intercomparison of standard GCMs used in the field on a rapidly rotating planet receiving a G-star spectral energy distribution and on a tidally locked planet receiving an M-star spectral energy distribution. Experiments both with and without clouds are examined. We find relatively small difference (within 8 K) in global-mean surface temperature simulation among the models in the G-star case with clouds. In contrast, the global-mean surface temperature simulation in the M-star case is highly divergent (2030 K). Moreover, even differences in the simulated surface temperature when clouds are turned off are significant. These differences are caused by differences in cloud simulation and/or radiative transfer, as well as complex interactions between atmospheric dynamics and these two processes. For example we find that an increase in atmospheric absorption of shortwave radiation can lead to higher relative humidity at high altitudes globally and, therefore, a significant decrease in planetary radiation emitted to space. This study emphasizes the importance of basing conclusions about planetary climate on simulations from a variety of GCMs and motivates the eventual comparison of GCM results with terrestrial exoplanet observations to improve their performance.

M. Turbet, H. Tran, O. Pirali, F. Forget, C. Boulet, and J.-M. Hartmann. Far infrared measurements of absorptions by CH4 + CO2 and H2 + CO2 mixtures and implications for greenhouse warming on early Mars. Icarus, 321:189-199, 2019. [ bib | DOI | arXiv | ADS link ]

We present an experimental study of the absorption, between 40 and 640 cm-1, by CO2, CH4 and H2 gases as well as by H2 + CO2 and CH4 + CO2 mixtures at room temperature. A Fourier transform spectrometer associated to a multi-pass cell, whose optics were adjusted to obtain a 152 m path length, were used to record transmission spectra at total pressures up to about 0.98 bar. These measurements provide information concerning the collision-induced absorption (CIA) bands as well as about the wing of the CO2 15 μm band. Our results for the CIAs of pure gases are, within uncertainties, in agreement with previous determinations, validating our experimental and data analysis procedures. We then consider the CIAs by H2 + CO2 and CH4 + CO2 and the low frequency wing of the pure CO2 15 μm band, for which there are, to our knowledge, no previous measurements. We confirm experimentally the theoretical prediction of Wordsworth et al. (2017) that the H2 + CO2 and CH4 + CO2 CIAs are significantly stronger in the 50-550 cm-1 region than those of H2 + N2 and CH4 + N2, respectively. However, we find that the shape and the strength of these recorded CIAs differ from the aforementioned predictions. For the pure CO2 line-wings, we show that both the χ-factor deduced from measurements near 4 μm and a line-mixing model very well describe the observed strongly sub-Lorentzian behavior in the 500-600 cm-1 region. These experimental results open renewed perspectives for studies of the past climate of Mars and extrasolar analogues.

T. Encrenaz, T. K. Greathouse, E. Marcq, H. Sagawa, T. Widemann, B. Bézard, T. Fouchet, F. Lefèvre, S. Lebonnois, S. K. Atreya, Y. J. Lee, R. Giles, and S. Watanabe. HDO and SO2 thermal mapping on Venus. IV. Statistical analysis of the SO2 plumes. Astronomy Astrophysics, 623:A70, 2019. [ bib | DOI | ADS link ]

Since January 2012 we have been monitoring the behavior of sulfur dioxide and water on Venus, using the Texas Echelon Cross-Echelle Spectrograph (TEXES) imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory). We present here the observations obtained between January 2016 and September 2018. As in the case of our previous runs, data were recorded around 1345 cm-1 (7.4 μm). The molecules SO2, CO2, and HDO (used as a proxy for H2O) were observed, and the cloudtop of Venus was probed at an altitude of about 64 km. The volume mixing ratio of SO2 was estimated using the SO2/CO2 line depth ratios of weak transitions; the H2O volume mixing ratio was derived from the HDO/CO2 line depth ratio, assuming a D/H ratio of 200 times the Vienna Standard Mean Ocean Water (VSMOW). As reported in our previous analyses, the SO2 mixing ratio shows strong variations with time and also over the disk, showing evidence of the formation of SO2 plumes with a lifetime of a few hours; in contrast, the H2O abundance is remarkably uniform over the disk and shows moderate variations as a function of time. We performed a statistical analysis of the behavior of the SO2 plumes, using all TEXES data between 2012 and 2018. They appear mostly located around the equator. Their distribution as a function of local time seems to show a depletion around noon; we do not have enough data to confirm this feature definitely. The distribution of SO2 plumes as a function of longitude shows no clear feature, apart from a possible depletion around 100E-150E and around 300E-360E. There seems to be a tendency for the H2O volume mixing ratio to decrease after 2016, and for the SO2 mixing ratio to increase after 2014. However, we see no clear anti-correlation between the SO2 and H2O abundances at the cloudtop, neither on the individual maps nor over the long term. Finally, there is a good agreement between the TEXES results and those obtained in the UV range (SPICAV/Venus Express and UVI/Akatsuki) at a slightly higher altitude. This agreement shows that SO2 observations obtained in the thermal infrared can be used to extend the local time coverage of the SO2 measurements obtained in the UV range.

Y. Nishikawa, P. Lognonné, T. Kawamura, A. Spiga, E. Stutzmann, M. Schimmel, T. Bertrand, F. Forget, and K. Kurita. Mars' Background Free Oscillations. Space Science Reviews, 215:13, 2019. [ bib | DOI | ADS link ]

Observations and inversion of the eigenfrequencies of free oscillations constitute powerful tools to investigate the internal structure of a planet. On Mars, such free oscillations can be excited by atmospheric pressure and wind stresses from the Martian atmosphere, analogous to what occurs on Earth. Over long periods and on a global scale, this phenomenon may continuously excite Mars' background free oscillations (MBFs), which constitute the so-called Martian hum. However, the source exciting MBFs is related both to the global-scale atmospheric circulation on Mars and to the variations in pressure and wind at the planetary boundary layer, for which no data are available.

To overcome this drawback, we focus herein on a global-scale source and use results of simulations based on General Circular Models (GCMs). GCMs can predict and reproduce long-term, global-scale Martian pressure and wind variations and suggest that, contrary to what happens on Earth, daily correlations in the Martian hum might be generated by the solar-driven GCM. After recalling the excitation terms, we calculate MBFs by using GCM computations and estimate the contribution to the hum made by the global atmospheric circulation. Although we work at the lower limit of MBF signals, the results indicate that the signal is likely to be periodic, which would allow us to use more efficient stacking theories than can be applied to Earth's hum. We conclude by discussing the perspectives for the InSight SEIS instrument to detect the Martian hum. The amplitude of the MBF signal is on the order of nanogals and is therefore hidden by instrumental and thermal noise, which implies that, provided the predicted daily coherence in hum excitation is present, the InSight SEIS seismometer should be capable of

F. Ferri, Ö. Karatekin, S. R. Lewis, F. Forget, A. Aboudan, G. Colombatti, C. Bettanini, S. Debei, B. Van Hove, V. Dehant, A.-M. Harri, M. Leese, T. Mäkinen, E. Millour, I. Muller-Wodarg, G. G. Ori, A. Pacifici, S. Paris, M. Patel, M. Schoenenberger, J. Herath, T. Siili, A. Spiga, T. Tokano, M. Towner, P. Withers, S. Asmar, and D. Plettemeier. ExoMars Atmospheric Mars Entry and Landing Investigations and Analysis (AMELIA). Space Science Reviews, 215:8, 2019. [ bib | DOI | ADS link ]

The entry, descent and landing of Schiaparelli, the ExoMars Entry, descent and landing Demonstrator Module (EDM), offered a rare (once-per-mission) opportunity for in situ investigations of the martian environment over a wide altitude range. The aim of the ExoMars AMELIA experiment was to exploit the Entry, Descent and Landing System (EDLS) engineering measurements for scientific investigations of Mars' atmosphere and surface. Here we present the simulations, modelling and the planned investigations prior to the Entry, Descent and Landing (EDL) event that took place on 19th October 2016. Despite the unfortunate conclusion of the Schiaparelli mission, flight data recorded during the entry and the descent until the loss of signal, have been recovered. These flight data, although limited and affected by transmission interruptions and malfunctions, are essential for investigating the anomaly and validating the EDL operation, but can also contribute towards the partial achievement of AMELIA science objectives.

O. L. White, J. M. Moore, A. D. Howard, W. B. McKinnon, J. T. Keane, K. N. Singer, T. Bertrand, S. J. Robbins, P. M. Schenk, B. Schmitt, B. J. Buratti, S. A. Stern, K. Ennico, C. B. Olkin, H. A. Weaver, L. A. Young, G. New Horizons Geology, and Imaging Theme Team. Washboard and fluted terrains on Pluto as evidence for ancient glaciation. Nature Astronomy, 3:62-68, 2019. [ bib | DOI | ADS link ]

Distinctive landscapes termed `washboard' and `fluted' terrains1,2, which border the N2 ice plains of Sputnik Planitia along its northwest margin, are among the most enigmatic landforms yet seen on Pluto. These terrains consist of parallel to sub-parallel ridges that display a remarkably consistent east-northeast-west-southwest orientationa configuration that does not readily point to a simple analogous terrestrial or planetary process or landform. Here, we report on mapping and analysis of their morphometry and distribution as a means to determine their origin. Based on their occurrence in generally low-elevation, low-relief settings adjacent to Sputnik Planitia that coincide with a major tectonic system, and through comparison with fields of sublimation pits seen in southern Sputnik Planitia, we conclude that washboard and fluted terrains represent crustal debris that were buoyant in pitted glacial N2 ice that formerly covered this area, and which were deposited after the N2 ice receded via sublimation. Crater surface age estimates indicate that this N2 ice glaciation formed and disappeared early in Pluto's history, soon after formation of the Sputnik Planitia basin. These terrains constitute an entirely new category of glacial landform.

I. Ordonez-Etxeberria, R. Hueso, A. Sánchez-Lavega, E. Millour, and F. Forget. Meteorological pressure at Gale crater from a comparison of REMS/MSL data and MCD modelling: Effect of dust storms. Icarus, 317:591-609, 2019. [ bib | DOI | ADS link ]

We examine the record of atmospheric pressure in Gale crater measured in-situ by the Rover Environmental Monitoring Station (REMS) instrument (Gómez-Elvira et al., 2012) on the Mars Science Laboratory (MSL) rover over two Martian years. We compare the data with pressure predictions from the Mars Climate Database (MCD) (Forget et al., 1999; Millour et al., 2015) version 5.2, which is a climatological database derived from numerical simulations of the Martian atmosphere produced by a General Circulation Model run over several Martian years. Seasonal and daily trends in pressure data from REMS are well reproduced by the standard climatology of the MCD using its high resolution mode. This high-resolution mode extrapolates pressure values from the nominal model into the altitude of each location using a high-resolution topography model and a fine tuning of the vertical scale height that was chosen to mimic effects of slope winds not directly accounted for in the General Circulation Model on which the MCD is based. Differences between the synthetic MCD pressure data and the REMS measurements are produced by meteorological features that are identified on particular groups of sols and quantified in intensity. We show that regional dust storms outside Gale crater and dust abundance at the crater are important factors in the behaviour of the pressure exciting larger amplitudes on the daily pressure variations and causing most of the largest REMS-MCD differences. We compare the pressure signals with published data of the dust optical depth obtained by the REMS ultraviolet photodiodes and the Mastcam instrument on MSL, and with orbital images of the planet acquired by the MARCI instrument on the Mars Reconnaissance Orbiter (MRO). We show that in some cases regional dust storms induce a characteristic signature in the surface pressure measured by REMS several sols before the dust arrives to Gale crater. We explore the capability of daily pressure measurements to serve as a fast detector of the development of dust storms in the context of the MSL, Insight and Mars 2020 missions.

W. Pluriel, E. Marcq, and M. Turbet. Modeling the albedo of Earth-like magma ocean planets with H2O-CO2 atmospheres. Icarus, 317:583-590, 2019. [ bib | DOI | arXiv | ADS link ]

During accretion, the young rocky planets are so hot that they become endowed with a magma ocean. From that moment, the mantle convective thermal flux control the cooling of the planet and an atmosphere is created by outgassing. This atmosphere will then play a key role during this cooling phase. Studying this cooling phase in details is a necessary step to explain the great diversity of the observed telluric planets and especially to assess the presence of surface liquid water. We used here a radiative-convective 1D atmospheric model (H2O, CO2) to study the impact of the Bond albedo on the evolution of magma ocean planets. We derived from this model the thermal emission spectrum and the spectral reflectance of these planets, from which we calculated their Bond albedos. Compared to Marcq et al. (2017), the model now includes a new module to compute the Rayleigh scattering, and state of the art CO2 and H2O gaseous opacities data in the visible and infrared spectral ranges. We show that the Bond albedo of these planets depends on their surface temperature and results from a competition between Rayleigh scattering from the gases and Mie scattering from the clouds. The colder the surface temperature is, the thicker the clouds are, and the higher the Bond albedo is. We also evidence that the relative abundances of CO2 and H2O in the atmosphere strongly impact the Bond albedo. The Bond albedo is higher for atmospheres dominated by the CO2, better Rayleigh scatterer than H2O. Finally, we provide the community with an empirical formula for the Bond albedo that could be useful for future studies of magma ocean planets.