J.-L. Bertaux, A.-C. Vandaele, O. Korablev, E. Villard, A. Fedorova, D. Fussen, E. Quémerais, D. Belyaev, A. Mahieux, F. Montmessin, C. Muller, E. Neefs, D. Nevejans, V. Wilquet, J. P. Dubois, A. Hauchecorne, A. Stepanov, I. Vinogradov, A. Rodin, J.-L. Bertaux, D. Nevejans, O. Korablev, F. Montmessin, A.-C. Vandaele, A. Fedorova, M. Cabane, E. Chassefière, J. Y. Chaufray, E. Dimarellis, J. P. Dubois, A. Hauchecorne, F. Leblanc, F. Lefèvre, P. Rannou, E. Quémerais, E. Villard, D. Fussen, C. Muller, E. Neefs, E. van Ransbeeck, V. Wilquet, A. Rodin, A. Stepanov, I. Vinogradov, L. Zasova, F. Forget, S. Lebonnois, D. Titov, S. Rafkin, G. Durry, J. C. Gérard, and B. Sandel. A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO. Nature, 450:646-649, 2007. [ bib | DOI | ADS link ]
Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60km. The 60-100km region (the mesosphere) is a transition region between the 4day retrograde superrotation at the top of the thick clouds and the solar-antisolar circulation in the thermosphere (above 100km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60km. Here we report the detection of an extensive layer of warm air at altitudes 90-120km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the `cryosphere' above 100km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40years ago. HDO/H2O is enhanced by a factor of ˜2.5 with respect to the lower atmosphere, and there is a general depletion of H2O around 80-90km for which we have no explanation.
P. Drossart, G. Piccioni, J. C. Gérard, M. A. Lopez-Valverde, A. Sanchez-Lavega, L. Zasova, R. Hueso, F. W. Taylor, B. Bézard, A. Adriani, F. Angrilli, G. Arnold, K. H. Baines, G. Bellucci, J. Benkhoff, J. P. Bibring, A. Blanco, M. I. Blecka, R. W. Carlson, A. Coradini, A. di Lellis, T. Encrenaz, S. Erard, S. Fonti, V. Formisano, T. Fouchet, R. Garcia, R. Haus, J. Helbert, N. I. Ignatiev, P. Irwin, Y. Langevin, S. Lebonnois, D. Luz, L. Marinangeli, V. Orofino, A. V. Rodin, M. C. Roos-Serote, B. Saggin, D. M. Stam, D. Titov, G. Visconti, M. Zambelli, C. Tsang, E. Ammannito, A. Barbis, R. Berlin, C. Bettanini, A. Boccaccini, G. Bonnello, M. Bouyé, F. Capaccioni, A. Cardesin, F. Carraro, G. Cherubini, M. Cosi, M. Dami, M. de Nino, D. Del Vento, M. di Giampietro, A. Donati, O. Dupuis, S. Espinasse, A. Fabbri, A. Fave, I. Ficai Veltroni, G. Filacchione, K. Garceran, Y. Ghomchi, M. Giustizi, B. Gondet, Y. Hello, F. Henry, S. Hofer, G. Huntzinger, J. Kachlicki, R. Knoll, D. Kouach, A. Mazzoni, R. Melchiorri, G. Mondello, F. Monti, C. Neumann, F. Nuccilli, J. Parisot, C. Pasqui, S. Perferi, G. Peter, A. Piacentino, C. Pompei, J.-M. Réess, J.-P. Rivet, A. Romano, N. Russ, M. Santoni, A. Scarpelli, A. Sémery, A. Soufflot, D. Stefanovitch, E. Suetta, F. Tarchi, N. Tonetti, F. Tosi, and B. Ulmer. A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express. Nature, 450:641-645, 2007. [ bib | DOI | ADS link ]
The upper atmosphere of a planet is a transition region in which energy is transferred between the deeper atmosphere and outer space. Molecular emissions from the upper atmosphere (90-120km altitude) of Venus can be used to investigate the energetics and to trace the circulation of this hitherto little-studied region. Previous spacecraft and ground-based observations of infrared emission from CO2, O2 and NO have established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus. These data, however, have left unresolved the precise altitude of the emission owing to a lack of data and of an adequate observing geometry. Here we report measurements of day-side CO2 non-local thermodynamic equilibrium emission at 4.3m, extending from 90 to 120km altitude, and of night-side O2 emission extending from 95 to 100km. The CO2 emission peak occurs at ˜115km and varies with solar zenith angle over a range of ˜10km. This confirms previous modelling, and permits the beginning of a systematic study of the variability of the emission. The O2 peak emission happens at 96km+/-1km, which is consistent with three-body recombination of oxygen atoms transported from the day side by a global thermospheric sub-solar to anti-solar circulation, as previously predicted.
G. Piccioni, P. Drossart, A. Sanchez-Lavega, R. Hueso, F. W. Taylor, C. F. Wilson, D. Grassi, L. Zasova, M. Moriconi, A. Adriani, S. Lebonnois, A. Coradini, B. Bézard, F. Angrilli, G. Arnold, K. H. Baines, G. Bellucci, J. Benkhoff, J. P. Bibring, A. Blanco, M. I. Blecka, R. W. Carlson, A. di Lellis, T. Encrenaz, S. Erard, S. Fonti, V. Formisano, T. Fouchet, R. Garcia, R. Haus, J. Helbert, N. I. Ignatiev, P. G. J. Irwin, Y. Langevin, M. A. Lopez-Valverde, D. Luz, L. Marinangeli, V. Orofino, A. V. Rodin, M. C. Roos-Serote, B. Saggin, D. M. Stam, D. Titov, G. Visconti, M. Zambelli, E. Ammannito, A. Barbis, R. Berlin, C. Bettanini, A. Boccaccini, G. Bonnello, M. Bouye, F. Capaccioni, A. Cardesin Moinelo, F. Carraro, G. Cherubini, M. Cosi, M. Dami, M. de Nino, D. Del Vento, M. di Giampietro, A. Donati, O. Dupuis, S. Espinasse, A. Fabbri, A. Fave, I. F. Veltroni, G. Filacchione, K. Garceran, Y. Ghomchi, M. Giustini, B. Gondet, Y. Hello, F. Henry, S. Hofer, G. Huntzinger, J. Kachlicki, R. Knoll, K. Driss, A. Mazzoni, R. Melchiorri, G. Mondello, F. Monti, C. Neumann, F. Nuccilli, J. Parisot, C. Pasqui, S. Perferi, G. Peter, A. Piacentino, C. Pompei, J.-M. Reess, J.-P. Rivet, A. Romano, N. Russ, M. Santoni, A. Scarpelli, A. Semery, A. Soufflot, D. Stefanovitch, E. Suetta, F. Tarchi, N. Tonetti, F. Tosi, and B. Ulmer. South-polar features on Venus similar to those near the north pole. Nature, 450:637-640, 2007. [ bib | DOI | ADS link ]
Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright `dipole' feature surrounded by a cold `collar' at its north pole. The polar dipole is a `double-eye' feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus' south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.
F. Montmessin, B. Gondet, J.-P. Bibring, Y. Langevin, P. Drossart, F. Forget, and T. Fouchet. Hyperspectral imaging of convective CO2 ice clouds in the equatorial mesosphere of Mars. Journal of Geophysical Research (Planets), 112:E11S90, 2007. [ bib | DOI | ADS link ]
A unique feature of the Martian climate is the possibility for carbon dioxide, the main atmospheric constituent, to condense as ice. CO2 ice is usually detected as frost but is also known to exist as clouds. This paper presents the first unambiguous observation of CO2 ice clouds on Mars. These images were obtained by the visible and near-infrared imaging spectrometer OMEGA on board Mars Express. The data set encompasses 19 different occurrences. Compositional identification is based on the detection of a diagnostic spectral feature around 4.26 μm which is produced by resonant scattering of solar photons by mesospheric CO2 ice particles in a spectral interval otherwise dominated by saturated gaseous absorption. Observed clouds exhibit a strong seasonal and geographic dependence, concentrating in the near-equatorial regions during two periods before and after northern summer solstice (Ls 45deg and 135deg). Radiative transfer modeling indicates that the 4.26 μm feature is very sensitive to cloud altitude, opacity, and particle size, thereby explaining the variety of spectra associated with the cloud images. On two orbits, the simultaneous detection of clouds with their shadow provides straightforward and robust estimates of cloud properties. These images confirm the conclusions established from modeling: clouds are thick, with normal opacities greater than 0.2 in the near infrared, and are lofted in the mesosphere above 80 km. The mean radius of CO2 ice crystals is found to exceed 1 μm, an unexpected value considering this altitude range. This finding implies the existence of high-altitude atmospheric updrafts which are strong enough to counteract the rapid gravitational fall of particles. This statement is consistent with the cumuliform morphology of the clouds which may be linked to a moist convective origin generated by the latent heat released during CO2 condensation.
V. De La Haye, J. H. Waite, T. E. Cravens, A. F. Nagy, R. E. Johnson, S. Lebonnois, and I. P. Robertson. Titan's corona: The contribution of exothermic chemistry. Icarus, 191:236-250, 2007. [ bib | DOI | ADS link ]
The contribution of exothermic ion and neutral chemistry to Titan's corona is studied. The production rates for fast neutrals N 2, CH 4, H, H 2, 3CH 2, CH 3, C 2H 4, C 2H 5, C 2H 6, N( 4S), NH, and HCN are determined using a coupled ion and neutral model of Titan's upper atmosphere. After production, the formation of the suprathermal particles is modeled using a two-stream simulation, as they travel simultaneously through a thermal mixture of N 2, CH 4, and H 2. The resulting suprathermal fluxes, hot density profiles, and energy distributions are compared to the N 2 and CH 4 INMS exospheric data presented in [De La Haye, V., Waite Jr., J.H., Johnson, R.E., Yelle, R.V., Cravens, T.E., Luhmann, J.G., Kasprzak, W.T., Gell, D.A., Magee, B., Leblanc, F., Michael, M., Jurac, S., Robertson, I.P., 2007. J. Geophys. Res., doi:10.1029/2006JA012222, in press], and are found insufficient for producing the suprathermal populations measured. Global losses of nitrogen atoms and carbon atoms in all forms due to exothermic chemistry are estimated to be 8.3×10 Ns and 7.2×10 Cs.
J.-L. Bertaux, D. Nevejans, O. Korablev, E. Villard, E. Quémerais, E. Neefs, F. Montmessin, F. Leblanc, J. P. Dubois, E. Dimarellis, A. Hauchecorne, F. Lefèvre, P. Rannou, J. Y. Chaufray, M. Cabane, G. Cernogora, G. Souchon, F. Semelin, A. Reberac, E. Van Ransbeek, S. Berkenbosch, R. Clairquin, C. Muller, F. Forget, F. Hourdin, O. Talagrand, A. Rodin, A. Fedorova, A. Stepanov, I. Vinogradov, A. Kiselev, Y. Kalinnikov, G. Durry, B. Sandel, A. Stern, and J. C. Gérard. SPICAV on Venus Express: Three spectrometers to study the global structure and composition of the Venus atmosphere. Planetary and Space Science, 55:1673-1700, 2007. [ bib | DOI | ADS link ]
Spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAV) is a suite of three spectrometers in the UV and IR range with a total mass of 13.9 kg flying on the Venus Express (VEX) orbiter, dedicated to the study of the atmosphere of Venus from ground level to the outermost hydrogen corona at more than 40,000 km. It is derived from the SPICAM instrument already flying on board Mars Express (MEX) with great success, with the addition of a new IR high-resolution spectrometer, solar occultation IR (SOIR), working in the solar occultation mode. The instrument consists of three spectrometers and a simple data processing unit providing the interface of these channels with the spacecraft. A UV spectrometer (118-320 nm, resolution 1.5 nm) is identical to the MEX version. It is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation. In nadir orientation, SPICAV UV will analyse the albedo spectrum (solar light scattered back from the clouds) to retrieve SO 2, and the distribution of the UV-blue absorber (of still unknown origin) on the dayside with implications for cloud structure and atmospheric dynamics. On the nightside, γ and δ bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode the UV sensor will measure the vertical profiles of CO 2, temperature, SO 2, SO, clouds and aerosols. The density/temperature profiles obtained with SPICAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (60 km) to 160 km in the atmosphere. This is essential for future missions that would rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO +, and CO 2+, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere. The SPICAV VIS-IR sensor (0.7-1.7 μm, resolution 0.5-1.2 nm) employs a pioneering technology: an acousto-optical tunable filter (AOTF). On the nightside, it will study the thermal emission peeping through the clouds, complementing the observations of both VIRTIS and Planetary Fourier Spectrometer (PFS) on VEX. In solar occultation mode this channel will study the vertical structure of H 2O, CO 2, and aerosols. The SOIR spectrometer is a new solar occultation IR spectrometer in the range λ=2.2-4.3 μm, with a spectral resolution λ/Δ λ15,000, the highest on board VEX. This new concept includes a combination of an echelle grating and an AOTF crystal to sort out one order at a time. The main objective is to measure HDO and H 2O in solar occultation, in order to characterize the escape of D atoms from the upper atmosphere and give more insight about the evolution of water on Venus. It will also study isotopes of CO 2 and minor species, and provides a sensitive search for new species in the upper atmosphere of Venus. It will attempt to measure also the nightside emission, which would allow a sensitive measurement of HDO in the lower atmosphere, to be compared to the ratio in the upper atmosphere, and possibly discover new minor atmospheric constituents.
P. Drossart, G. Piccioni, A. Adriani, F. Angrilli, G. Arnold, K. H. Baines, G. Bellucci, J. Benkhoff, B. Bézard, J.-P. Bibring, A. Blanco, M. I. Blecka, R. W. Carlson, A. Coradini, A. Di Lellis, T. Encrenaz, S. Erard, S. Fonti, V. Formisano, T. Fouchet, R. Garcia, R. Haus, J. Helbert, N. I. Ignatiev, P. G. J. Irwin, Y. Langevin, S. Lebonnois, M. A. Lopez-Valverde, D. Luz, L. Marinangeli, V. Orofino, A. V. Rodin, M. C. Roos-Serote, B. Saggin, A. Sanchez-Lavega, D. M. Stam, F. W. Taylor, D. Titov, G. Visconti, M. Zambelli, R. Hueso, C. C. C. Tsang, C. F. Wilson, and T. Z. Afanasenko. Scientific goals for the observation of Venus by VIRTIS on ESA/Venus express mission. Planetary and Space Science, 55:1653-1672, 2007. [ bib | DOI | ADS link ]
The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the ESA/Venus Express mission has technical specifications well suited for many science objectives of Venus exploration. VIRTIS will both comprehensively explore a plethora of atmospheric properties and processes and map optical properties of the surface through its three channels, VIRTIS-M-vis (imaging spectrometer in the 0.3-1 μm range), VIRTIS-M-IR (imaging spectrometer in the 1-5 μm range) and VIRTIS-H (aperture high-resolution spectrometer in the 2-5 μm range). The atmospheric composition below the clouds will be repeatedly measured in the night side infrared windows over a wide range of latitudes and longitudes, thereby providing information on Venus's chemical cycles. In particular, CO, H 2O, OCS and SO 2 can be studied. The cloud structure will be repeatedly mapped from the brightness contrasts in the near-infrared night side windows, providing new insights into Venusian meteorology. The global circulation and local dynamics of Venus will be extensively studied from infrared and visible spectral images. The thermal structure above the clouds will be retrieved in the night side using the 4.3 μm fundamental band of CO 2. The surface of Venus is detectable in the short-wave infrared windows on the night side at 1.01, 1.10 and 1.18 μm, providing constraints on surface properties and the extent of active volcanism. Many more tentative studies are also possible, such as lightning detection, the composition of volcanic emissions, and mesospheric wave propagation.
T. Fouchet, E. Lellouch, N. I. Ignatiev, F. Forget, D. V. Titov, M. Tschimmel, F. Montmessin, V. Formisano, M. Giuranna, A. Maturilli, and T. Encrenaz. Martian water vapor: Mars Express PFS/LW observations. Icarus, 190:32-49, 2007. [ bib | DOI | ADS link ]
We present the seasonal and geographical variations of the martian water vapor monitored from the Planetary Fourier Spectrometer Long Wavelength Channel aboard the Mars Express spacecraft. Our dataset covers one martian year (end of Mars Year 26, Mars Year 27), but the seasonal coverage is far from complete. The seasonal and latitudinal behavior of the water vapor is globally consistent with previous datasets, Viking Orbiter Mars Atmospheric Water Detectors (MAWD) and Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES), and with simultaneous results obtained from other Mars Express instruments, OMEGA and SPICAM. However, our absolute water columns are lower and higher by a factor of 1.5 than the values obtained by TES and SPICAM, respectively. In particular, we retrieve a Northern midsummer maximum of 60 pr-μm, lower than the 100-pr-μm observed by TES. The geographical distribution of water exhibits two local maxima at low latitudes, located over Tharsis and Arabia. Global Climate Model (GCM) simulations suggest that these local enhancements are controlled by atmospheric dynamics. During Northern spring, we observe a bulge of water vapor over the seasonal polar cap edge, consistent with the northward transport of water from the retreating seasonal cap to the permanent polar cap. In terms of vertical distribution, we find that the water volume mixing ratio over the large volcanos remains constant with the surface altitude within a factor of two. However, on the whole dataset we find that the water column, normalized to a fixed pressure, is anti-correlated with the surface pressure, indicating a vertical distribution intermediate between control by atmospheric saturation and confinement to a surface layer. This anti-correlation is not reproduced by GCM simulations of the water cycle, which do not include exchange between atmospheric and subsurface water. This situation suggests a possible role for regolith-atmosphere exchange in the martian water cycle.
F. Montmessin, R. M. Haberle, F. Forget, Y. Langevin, R. T. Clancy, and J.-P. Bibring. On the origin of perennial water ice at the south pole of Mars: A precession-controlled mechanism? Journal of Geophysical Research (Planets), 112:E08S17, 2007. [ bib | DOI | ADS link ]
The poles of Mars are known to have recorded recent (107 years) climatic changes. While the south polar region appears to have preserved its million-year-old environment from major resurfacing events, except for the small portion containing the CO2 residual cap, the discovery of residual water ice units in areas adjacent to the cap provides compelling evidence for recent glaciological activity. The mapping and characterization of these H2O-rich terrains by Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) on board Mars Express, which have supplemented earlier findings by Mars Odyssey and Mars Global Surveyor, have raised a number of questions related to their origin. We propose that these water ice deposits are the relics of Mars' orbit precession cycle and that they were laid down when perihelion was synchronized with northern summer, i.e., more than 10,000 years ago. We favor precession over other possible explanations because (1) as shown by our General Circulation Model (GCM) and previous studies, current climate is not conducive to the accumulation of water at the south pole due to an unfavorable volatile transport and insolation configuration, (2) the residual CO2 ice cap, which is known to cold trap water molecules on its surface and which probably controls the current extent of the water ice units, is geologically younger, (3) our GCM shows that 21,500 years ago, when perihelion occurred during northern spring, water ice at the north pole was no longer stable and accumulated instead near the south pole with rates as high as 1 mm yr-1. This could have led to the formation of a meters-thick circumpolar water ice mantle. As perihelion slowly shifted back to the current value, southern summer insolation intensified and the water ice layer became unstable. The layer recessed poleward until the residual CO2 ice cover eventually formed on top of it and protected water ice from further sublimation. In this polar accumulation process, water ice clouds play a critical role since they regulate the exchange of water between hemispheres. The so-called “Clancy effect,” which sequesters water in the spring/summer hemisphere coinciding with aphelion due to cloud sedimentation, is demonstrated to be comparable in magnitude to the circulation bias forced by the north-to-south topographic dichotomy. However, we predict that the response of Mars' water cycle to the precession cycle should be asymmetric between hemispheres not only because of the topographic bias in circulation but also because of an asymmetry in the dust cycle. We predict that under a “reversed perihelion” climate, dust activity during northern summer is less pronounced than during southern summer in the opposite perihelion configuration (i.e., today's regime). When averaged over a precession cycle, this reduced potential for dust lifting will force a significantly colder summer in the north and, by virtue of the Clancy effect, will curtail the ability of the northern hemisphere to transfer volatiles to the south. This process may have helped create the observed morphological differences in the layered deposits between the poles and could help explain the large disparity in their resurfacing ages.
A. Spiga, F. Forget, B. Dolla, S. Vinatier, R. Melchiorri, P. Drossart, A. Gendrin, J.-P. Bibring, Y. Langevin, and B. Gondet. Remote sensing of surface pressure on Mars with the Mars Express/OMEGA spectrometer: 2. Meteorological maps. Journal of Geophysical Research (Planets), 112:E08S16, 2007. [ bib | DOI | ADS link ]
Surface pressure measurements help to achieve a better understanding of the main dynamical phenomena occurring in the atmosphere of a planet. The use of the Mars Express OMEGA visible and near-IR imaging spectrometer allows us to tentatively perform an unprecedented remote sensing measurement of Martian surface pressure. OMEGA reflectances in the CO2 absorption band at 2 μm are used to retrieve a hydrostatic estimation of surface pressure (see companion paper by Forget et al. (2007)) with a precision sufficient to draw maps of this field and thus analyze meteorological events in the Martian atmosphere. Prior to any meteorological analysis, OMEGA observations have to pass quality controls on insolation and albedo conditions, atmosphere dust opacity, and occurrence of water ice clouds and frosts. For the selected observations, registration shifts with the MOLA reference are corrected. “Sea-level” surface pressure reduction is then carried out in order to remove the topographical component of the surface pressure field. Three main phenomena are observed in the resulting OMEGA surface pressure maps: horizontal pressure gradients, atmospheric oscillations, and pressure perturbations in the vicinity of topographical obstacles. The observed pressure oscillations are identified as possible signatures of phenomena such as inertia-gravity waves or convective rolls. The pressure perturbations detected around the Martian hills and craters may be the signatures of complex interactions between an incoming flow and topographical obstacles. Highly idealized mesoscale simulations using the WRF model enable a preliminary study of these complex interactions, but more realistic mesoscale simulations are necessary. The maps provide valuable insights for future synoptic and mesoscale modeling, which will in turn help in the interpretation of observations.
F. Forget, A. Spiga, B. Dolla, S. Vinatier, R. Melchiorri, P. Drossart, A. Gendrin, J.-P. Bibring, Y. Langevin, and B. Gondet. Remote sensing of surface pressure on Mars with the Mars Express/OMEGA spectrometer: 1. Retrieval method. Journal of Geophysical Research (Planets), 112:E08S15, 2007. [ bib | DOI | ADS link ]
Observing and analyzing the variations of pressure on the surface of a planet is essential to understand the dynamics of its atmosphere. On Mars the absorption by atmospheric CO2 of the solar light reflected on the surface allows us to measure the surface pressure by remote sensing. We use the imaging spectrometer OMEGA aboard Mars Express, which provides an excellent signal to noise ratio and the ability to produce maps of surface pressure with a resolution ranging from 400 m to a few kilometers. Surface pressure is measured by fitting spectra of the CO2 absorption band centered at 2 μm. To process the hundreds of thousands of pixels present in each OMEGA image, we have developed a fast and accurate algorithm based on a line-by-line radiative transfer model which includes scattering and absorption by dust aerosols. In each pixel the temperature profile, the dust opacity, and the surface spectrum are carefully determined from the OMEGA data set or from other sources to maximize the accuracy of the retrieval. We estimate the 1-σ relative error to be around 7 Pa in bright regions and about 10 Pa in darker regions, with a possible systematic bias on the absolute pressure lower than 30 Pa (4%). The method is first tested by comparing an OMEGA pressure retrieval obtained over the Viking Lander 1 (VL1) landing site with in situ measurements recorded 30 years ago by the VL1 barometer. The retrievals are further validated using a surface pressure predictor which combines the VL1 pressure records with the MOLA topography and meteorological pressure gradients simulated with a General Circulation Model. A good agreement is obtained. In particular, OMEGA is able to monitor the seasonal variations of the surface pressure in Isidis Planitia. Such a tool can be applied to detect meteorological phenomena, as described by Spiga et al. (2007).
Y. Langevin, J.-P. Bibring, F. Montmessin, F. Forget, M. Vincendon, S. Douté, F. Poulet, and B. Gondet. Observations of the south seasonal cap of Mars during recession in 2004-2006 by the OMEGA visible/near-infrared imaging spectrometer on board Mars Express. Journal of Geophysical Research (Planets), 112:E08S12, 2007. [ bib | DOI | ADS link ]
The OMEGA visible/near-infrared imaging spectrometer on board Mars Express has observed the southern seasonal cap in late 2004 and 2005 and then in the summer of 2006. These observations extended from the period of maximum extension, close to the southern winter solstice, to the end of the recession at Ls 325deg. The spectral range and spectral resolution of OMEGA make it possible to monitor the extent and effective grain size of CO2 ice and H2O ice on the ground, the level of contamination of CO2 ice and H2O ice by dust, and the column density of μm-sized ice grains in the atmosphere. The CO2 seasonal cap is very clean and clear in early southern winter. Contamination by H2O ice spreads eastward from the Hellas basin until the southern spring equinox. During southern spring and summer, there is a very complex evolution in terms of effective grain size of CO2 ice and contamination by dust or H2O ice. H2O ice does not play a significant role close to the southern summer solstice. Contamination of CO2 ice by H2O ice is only observed close to the end of the recession, as well as the few H2O ice patches already reported by Bibring et al. (2004a). These observations have been compared to the results of a general circulation model, with good qualitative agreement on the distribution of H2O ice on the surface and in the atmosphere. Resolving the remaining discrepancies will improve our understanding of the water cycle on Mars.
D. Grassi, V. Formisano, F. Forget, C. Fiorenza, N. I. Ignatiev, A. Maturilli, and L. V. Zasova. The martian atmosphere in the region of Hellas basin as observed by the planetary Fourier spectrometer (PFS-MEX). Planetary and Space Science, 55:1346-1357, 2007. [ bib | DOI | ADS link ]
This work presents a review of the observations acquired by the planetary Fourier spectrometer (PFS) in the region of the Hellas basin. Taking advantage of the high spectral resolution of PFS, the vertical air temperature profile can be investigated with a previously unexperienced vertical resolution. Extensive comparisons with the expectations of EMCD 4.0 database highlight moderate discrepancies, strongly dependant on season. Namely, the morning observations acquired around Ls=45deg show a series of temperature deficiencies with recurrent spatial patterns in different observations, correlated with the topography profile. Trends of integrated dust loads as a function of the field of view (FOV) elevation are also described. Values are consistent with the retrieval hypothesis of a dust scale height equal to the gas one, even far from the season of main dust storms.
B. Levrard, F. Forget, F. Montmessin, and J. Laskar. Recent formation and evolution of northern Martian polar layered deposits as inferred from a Global Climate Model. Journal of Geophysical Research (Planets), 112:E06012, 2007. [ bib | DOI | ADS link ]
We present a time-marching model which simulates the exchange of water ice between the Martian northern cap, the tropics, and a high-latitude surface reservoir. Net annual exchange rates of water and their sensitivity to variations in orbital/rotational parameters are examined using the Martian water cycle modeled by the LMD three-dimensional Global Climate Model. These rates are propagated over the last 10 Myr to follow the thickness of the reservoirs. The effect of a sublimation dust lag is taken account to test simple models of layer formation. Periods of high mean polar summer insolation (˜5-10 Ma ago) lead to a rapid exhaustion of a northern polar cap and a prolonged formation of tropical glaciers. The formation of a northern cap and of a high-latitude icy mantle may have started 4 Ma ago with the average decrease of polar insolation. Tropical ice may have disappeared around 2.7 Ma ago, but small glaciers could have formed during the last peaks of polar summer insolation. Over the last 4 Myr, most of the present cap may have formed at the expense of tropical and high-latitude reservoirs forming distinct layers at almost each ˜51-kyr/120-kyr insolation cycle. Layers thickness ranges from 10 to 80 m, variations being produced by the modulation of the obliquity with ˜2.4 and 1.3 Myr periods. Because only ˜30 insolation cycles have occurred since 4 Ma ago, we found an inconsistency between the recent astronomical forcing, the observed number of layers, and simple astronomically based scenarios of layers formation.