F. Hourdin, S. Lebonnois, D. Luz, and P. Rannou. Titan's stratospheric composition driven by condensation and dynamics. Journal of Geophysical Research (Planets), 109:E12005, 2004. [ bib | DOI | ADS link ]
Atmospheric transport of chemical compounds and organic haze in the stratosphere of Titan is investigated with an axisymmetric general circulation model. It has been shown previously that the meridional circulation, dominated by global Hadley cells, is responsible both for the creation of an intense stratospheric zonal flow and for the accumulation of chemical compounds and haze in high latitudes. The modified composition in turn intensifies the meridional circulation and equator-to-pole thermal contrasts. This paper analyzes in detail the transport processes responsible for the observed vertical and latitudinal variations of atmospheric composition. It is shown that the competition between rapid sinking of air from the upper stratosphere in the winter polar vortex and latitudinal mixing by barotropic planetary waves (parameterized in the model) controls the vertical gradient of chemical compounds. The magnitude of polar enrichment (of a factor 1.4 to 20 depending on the particular species) with respect to low latitudes is mostly controlled by the way the meridional advection increases the concentrations of chemical compounds in the clean air which is rising from the troposphere, where most of the chemical compounds are removed by condensation (the temperature at the tropopause being close to 70 K). The agreement between the observed and simulated contrasts provides an indirect but strong validation of the simulated dynamics, thus confirming the explanation put forward for atmospheric superrotation. It is shown also that by measuring the atmospheric composition, the Cassini-Huygens mission will provide a strong constraint about Titan's atmospheric circulation.
H. M. Böttger, S. R. Lewis, P. L. Read, and F. Forget. The effect of a global dust storm on simulations of the Martian water cycle. Geophysical Research Letters, 31:L22702, 2004. [ bib | DOI | ADS link ]
Recent Mars Global Surveyor Thermal Emission Spectrometer (TES) observations have shown the effects of a global dust storm on the water cycle on Mars. Simulations using a Mars General Circulation Model were conducted to assess the influence of an arbitrary global dust storm on the modelled water cycle. Further, the effects of an adsorbing regolith during the dust storm were examined. Both with an active and a passive regolith the water cycle is substantially affected during the course of the dust storm, but returns to ambient conditions soon after the storm has abated. Differences between the simulations do exist, especially in the southern hemisphere during summer. When comparing the simulations with observations both the active and passive regolith simulations fail to fully replicate the general trends observed by TES. However, the actions of an adsorbing regolith appear to result in a water cycle more closely resembling the observations.
B. Levrard, F. Forget, F. Montmessin, and J. Laskar. Recent ice-rich deposits formed at high latitudes on Mars by sublimation of unstable equatorial ice during low obliquity. Nature, 431:1072-1075, 2004. [ bib | DOI | ADS link ]
Observations from the gamma-ray spectrometer instrument suite on the Mars Odyssey spacecraft have been interpreted as indicating the presence of vast reservoirs of near-surface ice in high latitudes of both martian hemispheres. Ice concentrations are estimated to range from 70 per cent at 60deg latitude to 100 per cent near the poles, possibly overlain by a few centimetres of ice-free material in most places. This result is supported by morphological evidence of metres-thick layered deposits that are rich in water-ice and periglacial-like features found only at high latitudes. Diffusive exchange of water between the pore space of the regolith and the atmosphere has been proposed to explain this distribution, but such a degree of concentration is difficult to accommodate with such processes. Alternatively, there are suggestions that ice-rich deposits form by transport of ice from polar reservoirs and direct redeposition in high latitudes during periods of higher obliquity, but these results have been difficult to reproduce with other models. Here we propose instead that, during periods of low obliquity (less than 25deg), high-latitude ice deposits form in both hemispheres by direct deposition of ice, as a result of sublimation from an equatorial ice reservoir that formed earlier, during a prolonged high-obliquity excursion. Using the ice accumulation rates estimated from global climate model simulations we show that, over the past ten million years, large variations of Mars' obliquity have allowed the formation of such metres-thick, sedimentary layered deposits in high latitude and polar regions.
F. Montmessin, F. Forget, P. Rannou, M. Cabane, and R. M. Haberle. Origin and role of water ice clouds in the Martian water cycle as inferred from a general circulation model. Journal of Geophysical Research (Planets), 109:E10004, 2004. [ bib | DOI | ADS link ]
In this paper, we present the results obtained by the general circulation model developed at the Laboratoire de Météorologie Dynamique which has been used to simulate the Martian hydrological cycle. Our model, which employs a simplified cloud scheme, reproduces the observed Martian water cycle with unprecedented agreement. The modeled seasonal evolution of cloudiness, which also compares well with data, is described in terms of the meteorological phenomena that control the Martian cloud distribution. Whereas cloud formation in the tropical region results from seasonal changes in the overturning circulation, Polar Hood clouds are mostly driven by variations of atmospheric wave activity. A sensitivity study allows us to quantify the effects of the transport of water ice clouds on the seasonal evolution of the water cycle. The residence time of cloud particles is long enough to allow cloud advection over great distances (typically thousands of kilometers). Despite the relatively low proportion of clouds (˜10%) in the total atmospheric inventory of water, their ability to be transported over large distances generally acts at the expense of the north polar cap and generates a water cycle globally wetter by a factor of 2 than a cycle produced by a model neglecting cloud transport. Around aphelion season, clouds modulate the north to south migration of water in a significant fashion and participate just as much as vapor in the cross-equatorial transport of total water. Most of the year, atmospheric waves generate an equatorward motion of water ice clouds near the polar vortex boundaries, partially balancing the opposite poleward flux of water vapor. The combination of both effects delays the return of water to the north polar cap and allows water to build up in the Martian tropics.
M. Capderou and F. Forget. Optimal orbits for Mars atmosphere remote sensing. Planetary and Space Science, 52:789-798, 2004. [ bib | DOI | ADS link ]
Most of the spacecrafts currently around Mars (or planned to reach Mars in the near future) use Sun-synchronous or near-polar orbits. Such orbits offer a very poor sampling of the diurnal cycle. Yet, sampling the diurnal cycle is of key importance to study Mars meteorology and climate. A comprehensive remote sensing data set should have been obtained by the end of the MRO mission, launched in 2005. For later windows, time-varying phenomena should be given the highest priority for remote sensing investigations. We present possible orbits for such missions which provide a rich spatial and temporal sampling with a relatively short repeat cycle (50 sols). After computation and determination of these orbits, said “optimal orbits”, we illustrate our results by tables of sampling and comparison with other orbits.
N. Mangold, S. Maurice, W. C. Feldman, F. Costard, and F. Forget. Spatial relationships between patterned ground and ground ice detected by the Neutron Spectrometer on Mars. Journal of Geophysical Research (Planets), 109:E08001, 2004. [ bib | DOI | ADS link ]
Patterned grounds, like polygonal features, are the signature of climatic effects in periglacial regions on Earth. Identifying similar features on Mars is important for an understanding of the past Martian climate. In this study we mapped fresh patterned landforms from the systematic analysis of Mars Orbiter Camera high-resolution images. We show that most of them are distributed at latitudes poleward of +/-55deg, making a climatic control likely. This distribution correlates to the distribution of ground ice detected by the Neutron Spectrometer aboard Mars Odyssey. This correlation is likely the consequence of the Neutron Spectrometer detecting ice no deeper than about 1 m. Patterned ground formation requires ice in this range of depth because these features are triggered by the propagation of a thermal wave that is driven by seasonal or diurnal changes in insolation, which affect the temperature in the uppermost ground layers. Sublimation seems to play a role in the shaping of many of the small patterns observed at latitudes between 55deg and 70deg. No widespread polygonal features are correlated to the equatorial regions where hydrogen is detected by the Neutron Spectrometer.
T. Encrenaz, B. Bézard, T. K. Greathouse, M. J. Richter, J. H. Lacy, S. K. Atreya, A. S. Wong, S. Lebonnois, F. Lefèvre, and F. Forget. Hydrogen peroxide on Mars: evidence for spatial and seasonal variations. Icarus, 170:424-429, 2004. [ bib | DOI | ADS link ]
Hydrogen peroxide (H 2O 2) has been suggested as a possible oxidizer of the martian surface. Photochemical models predict a mean column density in the range of 10 15-10 16 cm -2. However, a stringent upper limit of the H 2O 2 abundance on Mars (9×10 14 cm -2) was derived in February 2001 from ground-based infrared spectroscopy, at a time corresponding to a maximum water vapor abundance in the northern summer (30 pr. μm, Ls=112deg). Here we report the detection of H 2O 2 on Mars in June 2003, and its mapping over the martian disk using the same technique, during the southern spring ( Ls=206deg) when the global water vapor abundance was 10 pr. μm. The spatial distribution of H 2O 2 shows a maximum in the morning around the sub-solar latitude. The mean H 2O 2 column density (6×10 15 cm -2) is significantly greater than our previous upper limit, pointing to seasonal variations. Our new result is globally consistent with the predictions of photochemical models, and also with submillimeter ground-based measurements obtained in September 2003 ( Ls=254deg), averaged over the martian disk (Clancy et al., 2004, Icarus 168, 116-121).
F. Lefèvre, S. Lebonnois, F. Montmessin, and F. Forget. Three-dimensional modeling of ozone on Mars. Journal of Geophysical Research (Planets), 109:E07004, 2004. [ bib | DOI | ADS link ]
We present the first three-dimensional model simulations of ozone on Mars. The model couples a state-of-the-art gas-phase photochemical package to the general circulation model developed at Laboratoire de Météorologie Dynamique (LMD). The results do not contradict the classical picture of a global anticorrelation between the ozone (O3) and water vapor columns. However, the quantitative approach shows significant departures from this relationship, related to substantial orbital variations in the O3 vertical distribution. Over the period Ls = 180deg-330deg, low-latitude to midlatitude O3 is essentially confined below 20 km, has a weak diurnal cycle, and is largely modulated by topography. During the rest of the year (Ls = 330deg-180deg) the model predicts the formation of an O3 layer at 25-70 km altitude, characterized by nighttime densities about one order of magnitude larger than during the day. Throughout the year, high-latitude O3 peaks near the surface and reaches maximum integrated amounts (˜40 μm-atm) in the winter polar vortex, with considerable (30 to 50%) dynamically induced day-to-day variations. The most stringent comparison to date with O3 observational data reveals contrasted results. A good quantitative agreement is found in the postperihelion period (Ls = 290deg-10deg), but the model fails to reproduce O3 columns as large as those measured near aphelion (Ls = 61deg-67deg). Current uncertainties in absorption cross sections and gas-phase kinetics data do not seem to provide credible explanations to explain this discrepancy, which may suggest the existence of heterogeneous processes.
J.-P. Bibring, Y. Langevin, F. Poulet, A. Gendrin, B. Gondet, M. Berthé, A. Soufflot, P. Drossart, M. Combes, G. Bellucci, V. Moroz, N. Mangold, B. Schmitt, OMEGA Team, S. Erard, O. Forni, N. Manaud, G. Poulleau, T. Encrenaz, T. Fouchet, R. Melchiorri, F. Altieri, V. Formisano, G. Bonello, S. Fonti, F. Capaccioni, P. Cerroni, A. Coradini, V. Kottsov, N. Ignatiev, D. Titov, L. Zasova, P. Pinet, C. Sotin, E. Hauber, H. Hoffman, R. Jaumann, U. Keller, R. Arvidson, J. Mustard, T. Duxbury, and F. Forget. Perennial water ice identified in the south polar cap of Mars. Nature, 428:627-630, 2004. [ bib | DOI | ADS link ]
The inventory of water and carbon dioxide reservoirs on Mars are important clues for understanding the geological, climatic and potentially exobiological evolution of the planet. From the early mapping observation of the permanent ice caps on the martian poles, the northern cap was believed to be mainly composed of water ice, whereas the southern cap was thought to be constituted of carbon dioxide ice. However, recent missions (NASA missions Mars Global Surveyor and Odyssey) have revealed surface structures, altimetry profiles, underlying buried hydrogen, and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide. Here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes, at a resolution of 2km, during the local summer, when the extent of the polar ice is at its minimum. We observe that this south polar cap contains perennial water ice in extended areas: as a small admixture to carbon dioxide in the bright regions; associated with dust, without carbon dioxide, at the edges of this bright cap; and, unexpectedly, in large areas tens of kilometres away from the bright cap.
M. Angelats i Coll, F. Forget, M. A. López-Valverde, P. L. Read, and S. R. Lewis. Upper atmosphere of Mars up to 120 km: Mars Global Surveyor accelerometer data analysis with the LMD general circulation model. Journal of Geophysical Research (Planets), 109:E01011, 2004. [ bib | DOI | ADS link ]
Mars Global Surveyor (MGS) aerobraking accelerometer density measurements are analyzed with the use of the general circulation model (GCM) at the Laboratoire de Météorologie Dynamique (LMD). MGS constant altitude density data are used, obtaining longitudinal wavelike structures at fixed local times which appear to be dominated by low zonal wave number harmonics. Comparisons with simulated data for different seasons and latitudinal bands at constant altitude are performed. Excellent agreement is obtained between the simulated and observational data for low latitudes, with accuracy in both mean and zonal structure. Higher latitudes show a reduction in agreement between GCM results and MGS data. Comparisons that result in good agreement with the observational data allow for the study of wave composition in the MGS data. In particular, the excellent agreement between the simulations and the data obtained at 115 km during areocentric longitude Ls ˜ 65deg allows the extraction of the major contributors to the signature, with the eastward propagating diurnal waves of wave numbers one to three being the major players. Significant contributions are also obtained for eastward propagating semidiurnal waves of wave numbers two, three, and five and diurnal wave number five. A sensitivity study is performed to delineate the effects of the near-IR tidal forcing of the upper atmosphere on the wave content at those heights. Simulations without this forcing yield reduced amplitudes for diurnal eastward propagating waves two and three along with a more latitudinally symmetric response for these two components as well as for diurnal eastward propagating wave number one.
E. Chassefière, J.-L. Bertaux, J.-J. Berthelier, M. Cabane, V. Ciarletti, G. Durry, F. Forget, M. Hamelin, F. Leblanc, M. Menvielle, M. Gerasimov, O. Korablev, S. Linkin, G. Managadze, A. Jambon, G. Manhès, P. Lognonné, P. Agrinier, P. Cartigny, D. Giardini, T. Pike, W. Kofman, A. Herique, P. Coll, A. Person, F. Costard, P. Sarda, P. Paillou, M. Chaussidon, B. Marty, F. Robert, S. Maurice, M. Blanc, C. d'Uston, J.-C. Sabroux, J.-F. Pineau, and P. Rochette. MEP (Mars Environment Package): toward a package for studying environmental conditions at the surface of Mars from future lander/rover missions. Advances in Space Research, 34:1702-1709, 2004. [ bib | DOI | ADS link ]
In view to prepare Mars human exploration, it is necessary to promote and lead, at the international level, a highly interdisciplinary program, involving specialists of geochemistry, geophysics, atmospheric science, space weather, and biology. The goal of this program will be to elaborate concepts of individual instruments, then of integrated instrumental packages, able to collect exhaustive data sets of environmental parameters from future landers and rovers of Mars, and to favour the conditions of their implementation. Such a program is one of the most urgent need for preparing human exploration, in order to develop mitigation strategies aimed at ensuring the safety of human explorers, and minimizing risk for surface operations. A few main areas of investigation may be listed: particle and radiation environment, chemical composition of atmosphere, meteorology, chemical composition of dust, surface and subsurface material, water in the subsurface, physical properties of the soil, search for an hypothesized microbial activity, characterization of radio-electric properties of the Martian ionosphere. Scientists at the origin of the present paper, already involved at a high degree of responsibility in several Mars missions, and actively preparing in situ instrumentation for future landed platforms (Netlandernow cancelled, MSL-09), express their readiness to participate in both ESA/AURORA and NASA programs of Mars human exploration. They think that the formation of a Mars Environment working group at ESA, in the course of the AURORA definition phase, could act positively in favour of the program, by increasing its scientific cross-section and making it still more focused on human exploration.
E. Chassefière, A. Nagy, M. Mandea, F. Primdahl, H. Rème, J.-A. Sauvaud, R. Lin, S. Barabash, D. Mitchell, T. Zurbuchen, F. Leblanc, J.-J. Berthelier, H. Waite, D. T. Young, J. Clarke, M. Parrot, J.-G. Trotignon, J.-L. Bertaux, E. Quèmerais, F. Barlier, K. Szegö, S. Szalaï, S. Bougher, F. Forget, J. Lilensten, J.-P. Barriot, G. Chanteur, J. Luhmann, G. Hulot, M. Purucker, D. Breuer, S. Smrekar, B. Jakosky, M. Menvielle, S. Sasaki, M. Acuna, G. Keating, P. Touboul, J.-C. Gérard, P. Rochus, S. Orsini, G. Cerutti-Maori, J. Porteneuve, M. Meftah, and C. Malique. DYNAMO: a Mars upper atmosphere package for investigating solar wind interaction and escape processes, and mapping Martian fields. Advances in Space Research, 33:2228-2235, 2004. [ bib | DOI | ADS link ]
DYNAMO is a small multi-instrument payload aimed at characterizing current atmospheric escape, which is still poorly constrained, and improving gravity and magnetic field representations, in order to better understand the magnetic, geologic and thermal history of Mars. The internal structure and evolution of Mars is thought to have influenced climate evolution. The collapse of the primitive magnetosphere early in Mars history could have enhanced atmospheric escape and favored transition to the present arid climate. These objectives are achieved by using a low periapsis orbit. DYNAMO has been proposed in response to the AO released in February 2002 for instruments to be flown as a complementary payload onboard the CNES Orbiter to Mars (MO-07), foreseen to be launched in 2007 in the framework of the French PREMIER Mars exploration program. MO-07 orbital phase 2b (with an elliptical orbit of periapsis 170 km), and in a lesser extent 2a, offers an unprecedented opportunity to investigate by in situ probing the chemical and dynamical properties of the deep ionosphere, thermosphere, and the interaction between the atmosphere and the solar wind, and therefore the present atmospheric escape rate. Ultraviolet remote sensing is an essential complement to characterize high, tenuous, layers of the atmosphere. One Martian year of operation, with about 5,000 low passes, should allow DYNAMO to map in great detail the residual magnetic field, together with the gravity field. Additional data on the internal structure will be obtained by mapping the electric conductivity, sinergistically with the NETLANDER magnetic data. Three options have been recommended by the International Science and Technical Review Board (ISTRB), who met on July 1st and 2nd, 2002. One of them is centered on DYNAMO. The final choice, which should be made before the end of 2002, will depend on available funding resources at CNES.