C. E. Newman, S. R. Lewis, P. L. Read, and F. Forget. Modeling the Martian dust cycle 2. Multiannual radiatively active dust transport simulations. Journal of Geophysical Research (Planets), 107:5124, 2002. [ bib | DOI | ADS link ]
Multiannual dust transport simulations have been performed using a Mars general circulation model containing a dust transport scheme which responds to changes in the atmospheric state. If the dust transport is “radiatively active,” the atmospheric state also responds to changes in the dust distribution. This paper examines the suspended dust distribution obtained using different lifting parameterizations, including an analysis of dust storms produced spontaneously during these simulations. The lifting mechanisms selected are lifting by (1) near-surface wind stress and (2) convective vortices known as dust devils. Each mechanism is separated into two types of parameterization: threshold-sensitive and -insensitive. The latter produce largely unrealistic annual dust cycles and storms, and no significant interannual variability. The threshold-sensitive parameterizations produce more realistic annual and interannual behavior, as well as storms with similarities to observed events, thus providing insight into how real Martian dust storms may develop. Simulations for which dust devil lifting dominates are too dusty during northern summer. This suggests either that a removal mechanism (such as dust scavenging by water ice) reduces opacities at this time or that dust devils are not the primary mechanism for storm production. Simulations for which near-surface wind stress lifting dominates produce the observed low opacities during northern spring/summer, yet appear unable to produce realistic global storms without storm decay being prevented by the occurrence of large-scale positive feedbacks on further lifting. Simulated dust levels are generally linked closely to the seasonal state of the atmosphere, and no simulation produces the observed amount of interannual variability.
C. E. Newman, S. R. Lewis, P. L. Read, and F. Forget. Modeling the Martian dust cycle, 1. Representations of dust transport processes. Journal of Geophysical Research (Planets), 107:5123, 2002. [ bib | DOI | ADS link ]
A dust transport scheme has been developed for a general circulation model of the Martian atmosphere. This enables radiatively active dust transport, with the atmospheric state responding to changes in the dust distribution via atmospheric heating, as well as dust transport being determined by atmospheric conditions. The scheme includes dust lifting, advection by model winds, atmospheric mixing, and gravitational sedimentation. Parameterizations of lifting initiated by (1) near-surface wind stress and (2) convective vortices known as dust devils are considered. Two parameterizations are defined for each mechanism and are first investigated offline using data previously output from the non-dust-transporting model. The threshold-insensitive parameterizations predict some lifting over most regions, varying smoothly in space and time. The threshold-sensitive parameterizations predict lifting only during extreme atmospheric conditions (such as exceptionally strong winds), so lifting is rarer and more confined to specific regions and times. Wind stress lifting is predicted to peak during southern summer, largely between latitudes 15deg and 35degS, with maxima also in regions of strong slope winds or thermal contrast flows. These areas are consistent with observed storm onset regions and dark streak surface features. Dust devil lifting is also predicted to peak during southern summer, with a moderate peak during northern summer. The greatest dust devil lifting occurs in early afternoon, particularly in the Noachis, Arcadia/Amazonis, Sirenum, and Thaumasia regions. Radiatively active dust transport experiments reveal strong positive feedbacks on lifting by near-surface wind stress and negative feedbacks on lifting by dust devils.
S. Lebonnois, E. L. O. Bakes, and C. P. McKay. Transition from Gaseous Compounds to Aerosols in Titan's Atmosphere. Icarus, 159:505-517, 2002. [ bib | DOI | ADS link ]
We investigate the chemical transition of simple molecules like C 2H 2 and HCN into aerosol particles in the context of Titan's atmosphere. Experiments that synthesize analogs (tholins) for these aerosols can help illuminate and constrain these polymerization mechanisms. Using information available from these experiments, we suggest chemical pathways that can link simple molecules to macromolecules, which will be the precursors to aerosol particles: polymers of acetylene and cyanoacetylene, polycyclic aromatics, polymers of HCN and other nitriles, and polyynes. Although our goal here is not to build a detailed kinetic model for this transition, we propose parameterizations to estimate the production rates of these macromolecules, their C/N and C/H ratios, and the loss of parent molecules (C 2H 2, HCN, HC 3N and other nitriles, and C 6H 6) from the gas phase to the haze. We use a one-dimensional photochemical model of Titan's atmosphere to estimate the formation rate of precursor macromolecules. We find a production zone slightly lower than 200 km altitude with a total production rate of 4×10 -14 g cm -2 s -1 and a C/N4. These results are compared with experimental data, and to microphysical model requirements. The Cassini/Huygens mission will bring a detailed picture of the haze distribution and properties, which will be a great challenge for our understanding of these chemical processes.
E. Van den Acker, T. Van Hoolst, O. de Viron, P. Defraigne, F. Forget, F. Hourdin, and V. Dehant. Influence of the seasonal winds and the CO2 mass exchange between atmosphere and polar caps on Mars' rotation. Journal of Geophysical Research (Planets), 107:5055, 2002. [ bib | DOI | ADS link ]
The Martian atmosphere and the CO2 polar ice caps exchange mass. This exchange, together with the atmospheric response to solar heating, induces variations of the rotation of Mars. Using the angular momentum budget equation of the system solid-Mars-atmosphere-polar ice caps, the variations of Mars' rotation can be deduced from the variations of the angular momentum of the superficial layer; this later is associated with the winds, that is, the motion term, and with the mass redistribution, that is, the matter term. For the “mean” Martian atmosphere, without global dust storms, total amplitudes of 10 cm on the surface are obtained for both the annual and semiannual polar motion excited by the atmosphere and ice caps. The atmospheric pressure variations are the dominant contribution to these amplitudes. Length-of-day (lod) variations have amplitudes of 0.253 ms for the annual signal and of 0.246 ms for the semiannual signal. The lod variations are mainly associated with changes in the atmospheric contribution to the mass term, partly compensated by the polar ice cap contribution. We computed lod variations and polar motion for three scenarios having different atmospheric dust contents. The differences between the three sets of results for lod variations are about one order of magnitude larger than the expected accuracy of the NEtlander Ionosphere and Geodesy Experiment (NEIGE) for lod. It will thus be possible to constrain the global atmospheric circulation models from the NEIGE measurements.
F. Costard, F. Forget, N. Mangold, and J. P. Peulvast. Formation of Recent Martian Debris Flows by Melting of Near-Surface Ground Ice at High Obliquity. Science, 295:110-113, 2002. [ bib | DOI | ADS link ]
The observation of small gullies associated with recent surface runoff on Mars has renewed the question of liquid water stability at the surface of Mars. The gullies could be formed by groundwater seepage from underground aquifers; however, observations of gullies originating from isolated peaks and dune crests question this scenario. We show that these landforms may result from the melting of water ice in the top few meters of the martian subsurface at high obliquity. Our conclusions are based on the analogy between the martian gullies and terrestrial debris flows observed in Greenland and numerical simulations that show that above-freezing temperatures can occur at high obliquities in the near surface of Mars, and that such temperatures are only predicted at latitudes and for slope orientations corresponding to where the gullies have been observed on Mars.
F. Forget, R. M. Haberle, E. Marcq, A. Colaprete, J. Schaeffer, and Y. Wanherdrick. Simulation of The Early Mars Climate With A General Circulation Model. EG Quaternary Science Journal, 27, 2002. [ bib | ADS link ]
In spite of a solar luminosity significantly lower than it is today, geological evidence shows that, early in its history during the Noachian erra, Mars was warm enough to support flowing water. The atmospheric conditions required to produce and sustain a warm, wet climate on early Mars remains uncertain. Several models have been used in the past to study this enigma, but all of them were simple 1D model. To improve our understanding of the early Mars Climate, we have developped a 3D general circula- tion model similar to the one used on current Earth or Mars to study the details of the climate today. To simulate what could have been the early Mars conditions, we have coupled the Martian General Circulation model developped at LMD with a sophis- ticated correlated k distribution model developped at NASA Ames Research Center. As a first step, we assumed that the early Mars atmosphere was composed of 2 bars of pure CO2, with CO2 ice clouds possibly condensing in the atmosphere. Using the correlated k distribution model allowed us to take into account the complex radiative transfer processes which occur in such an atmosphere (gaseous and scaterring green- house effect). Preliminary results suggest that temperature near or above the freezing point of water could have easily been obtained seasonally in the summer hemisphere even if the scaterring greenhouse effect of the clouds is negelected, whereas warming CO2 ice clouds would have especially formed in the winter hemisphere.
F. Jegou, E. Chassefiere, E. Lellouch, F. Forget, T. Encrenaz, and R. Moreno. Ground-based Millimeter Observations Of The Middle Atmosphere Of Mars. EG Quaternary Science Journal, 27, 2002. [ bib | ADS link ]
Measurements of Doppler shift winds and thermal structure of Mars middle atmo- sphere (40-70 km) have been obtained for ten years from ground-based millimeter- wave observations. Recently, new measurements were obtained with I.R.A.M. 30m single dish antenna in april 1999 and in june 2001. This set of data was completed in april 1999 and in july 2001 by interferometric observations (IRAM Plateau de Bure), with a 5-fold improvement in spatial resolution.
The Doppler shift wind measurements are of particular interest. The winds of the middle atmosphere infered from the observations are almost always retrograde and stronger than expected, compared to the winds deduced from T.E.S. temperature ob- servations and GCM simulations. This discrepancy may result from our poor under- standing of the actual dynamics of the Martian middle atmosphere, but may also be due to the fact that the measured signal includes some contributions from a large range of altitude and locations in a way difficult to interpret directly.
To address this issue we have developed a geometric-radiative transfer model able to synthesize the CO rotational lines emitted by a 3D varying temperature wind fields on Mars as it would be observed from Earth. The synthetic lines can then be compared with the ground-based millimeter-wave observations.
The model can be applied to GCM simulations, providing a good diagnostic for the martian GCM in the middle atmosphere, especially with regard to the interactions between the atmospheric tides and the mean flow.
Indeed, one possible explanation of the strong retrograde jet could be the effect of the zonal acceleration induced by dissipation of tides in the mesosphere. A retrograde acceleration due to momentum deposit is well modeled at low latitudes. Zonal accel- eration results in the formation of a dynamical meridional cell, from 0 to 30 degrees of latitude.
Unfortunatly the amplitude and the spread of the jet in the GCM simulations are too weak compared to the observations. A best understanding of the dissipation of the tides could bring nearer the observations and the theory.
F. Forget, G. Beaudin, A. D. P. Encrenaz, M. Gheudin, B. Thomas, M. Capderou, K. Dassas, P. Ricaud, J. Urban, M. F. S. Gulkis, M. Janssen, L. Riley, T. Encrenaz, E. Lellouch, and P. H. T. Clancy. Microwave Sounding Of The Martian Atmosphere With Mambo. EG Quaternary Science Journal, 27, 2002. [ bib | ADS link ]
Aboard the CNES orbiter currently planned for 2007, the Mars Atmosphere Mi- crowave Brightness Observer (MAMBO) aims to characterize the water cycle, the dynamics, and the photo-chemistry of the Martian atmosphere, with an unprecedented sensitivity and coverage. For this purpose, MAMBO will analyse the thermal emission of the atmosphere using heterodyne spectroscopy, for the first time from orbit around another planet. In practice, MAMBO will be a 20 kg instrument including a 20-25 cm rotating antenna to perform measurements at the atmospheric limb and at nadir, with a receiver dedicated to the monitoring of selected lines of key molecule around 320-350 GHz: H2O, CO, 13CO, HDO, O3, H2O2.
The inversion of CO and 13CO will allow to directly map 3D winds for the first time on Mars (by Doppler shift, with an accuracy of about 10m/s), and map the temperature from the surface up to the homopause around 120 km. Combined with data assimila- tion technique, these measurements will allow us to determine the 3D atmospheric circulation day after day and perform comparative meteorology. Water vapor profiles will be measured with an accuracy and a sensitivity much better than any previous ex- periments. In addition, we will learn about possible fractionation processes in the wa- ter cycle by monitoring the variations of the D/H ratio by simultaneous spectroscopy of H2O and HDO. Last, a global view of the martian atmosphere photochemistry will be obtained by profiling key species CO, H2O, O3, and H2O2. This last species has never been observed on Mars. However, several models have shown its key impor- tance for the chemical equilibrium of the atmosphere and for its role in oxydizing the martian soil, a problem of key interest for exobiology.
E. Chassefiere, J.-L. Bertaux, J.-J. Berthelier, J.-C. Cerisier, F. Forget, M. Menvielle, G. Hulot, M. Mandea, H. Reme, J.-P. Barriot, M. Blanc, M. Parrot, J.-G. Trotignon, F. Barlier, P. Touboul, J. Lilensten, A. Nagy, S. Bougher, H. Waite, D. Yung, J. Clarke, R. Lin, J. Luhmann, S. Smrekar, M. Purucker, D. Breuer, and K. Szego. Dynamo : An Instrumental Suite To Characterize Atmospheric Escape and To Complete Magnetic Field Mapping of Mars From The 07 Cnes Orbiter. EG Quaternary Science Journal, 27, 2002. [ bib | ADS link ]
DYNAMO is a suite of instruments to be implemented in the frame of the NASA/CNES Mars program and planned to be flown onboard the 2007 CNES or- biter. It is primarily aimed at characterizing current atmospheric escape, which is still poorly constrained, and at providing a detailed and complete mapping of the crustal magnetic field. These objectives are achieved from a low periapsis orbit (150-170 km x 1000 km), during an extended phase of the mission. They are reached by using a nominal payload composed of : a low-energy ion and neutral mass spectrometer measuring the thermal and hot populations, and wind velocities, complemented by specific sensors of the neutral atmosphere, an energetic particle spectrometer mea- suring all kinds of particles resulting from solar wind interaction, according to their nature, flux and velocity distribution, a Langmuir probe for adequate restitution of the ionospheric environment, associated with more specific plasma instruments, a UV airglow spectrometer allowing to probe high exospheric levels, as well as lower parts of the atmosphere, a 3-axis magnetometer to measure the static part of the magnetic field vector. A radio science experiment is under study, in order to measure electron profiles in the ionosphere, and to improve the resolution of the gravity field.
F. Montmessin, P. Rannou, F. Forget, and M. Cabane. Study of Martian Water Ice Clouds Using A General Circulation Model. EG Quaternary Science Journal, 27, 2002. [ bib | ADS link ]
By means of the LMD/MGCM, we investigated the spatial and temporal variations of water ice clouds in the atmosphere of Mars. We employed a simplified cloud scheme, where ice particles are submitted to condensational growth, sublimation, sedimenta- tion and transported by 3D dynamics. Radiative effects of clouds are also accounted for. We present the results of our simulation, emphasizing the radiative aspect of clouds and their effects on the global water cycle.
A. I. M. Coll, F. Forget, F. Hourdin, J. Wanherdrick, M. A. Lopez-Valverde, S. R. Lewis, and P. L. Read. First Results With The Mars Thermospheric Lmd General Circulation Model. EG Quaternary Science Journal, 27, 2002. [ bib | ADS link ]
The LMD-AOPP Mars General Circulation Model developed jointly by LMD (Paris), AOPP (Oxford) and IAA (Granada) has been extended into the thermosphere. The model reaches a height of approximately 240 km and includes the effects of thermal conduction, molecular viscosity and EUV absorption which are relevant processes at high altitudes. The model also includes non-LTE effects in the upper atmosphere. This is the first model to be able to simulate the planet's atmosphere from the ground up to thermospheric heights. This full coupling of the various atmospheric regions yields an excellent tool to study the effects of lower atmospheric processes, such as dust storms or upward propagating waves, on the upper atmosphere. First results of the improved Mars LMD TGCM will be shown.
This research was funded under CNES and ESTEC Contract 11369/95/NL/JG
S. Lebonnois, F. Hourdin, P. Rannou, and D. Luz. Impact of The Seasonal Variations of Ethane and Acetylene Distributions On The Temperature Field of Titan's Stratosphere. EG Quaternary Science Journal, 27, 2002. [ bib | ADS link ]
An enrichment has been observed for several compounds in Titan's stratosphere by IRIS/Voyager 1, for high latitudes coming out of winter (Coustenis and Bézard, Icarus 115, 1995). Bézard et al. (Icarus 113, 1995) showed that this enrichment sould be capable of affecting the temperature field in Titan's stratosphere by inducing a relative cooling at those latitudes.
The impact of meridional dynamics on the composition of Titan's atmosphere has been studied recently with a 2-dimensional photochemical model (Lebonnois et al., Icarus 152, 2001).To go further into the study of the coupling between photochemistry and dynamics, a photochemical module has been directly introduced into the general cir- culation model of the Laboratoire de Météorologie Dynamique (described in Hourdin et al., Icarus 117, 1995). A simplified module has also been implemented, that uses for a limited number of species a simple linear relaxation toward a prescribed vertical profile, instead of the complex photochemical calculations. This technique, presented in Lebonnois et al. (2001), allows to reproduce the latitudinal variations observed in the stratosphere of Titan for some species. We consider here the case of C2H2 and C2H6, because the distributions of these compounds are taken into account in the ra- diative calculations of the GCM. The contributions of other trace compounds have been neglected until now, based on their equatorial mole fractions.
Using this simplified coupled model, we have studied the impact on the temperature field of the variations of the distributions of C2H2 and C2H6 induced by meridional dynamics. The enrichment obtained in the descending branch of the Hadley cell is associated with a relative cooling for pressures under approximately 1 mbar (altitudes above 200 km). For lower altitudes, the effect is a slight heating.
We will present these effects and their interpretation, and will discuss their signifi- cance, especially when compared to the thermal effects of the coupling between dy- namics and the aerosols' distribution.
N. Mangold, F. Forget, and F. C. J.-P. Peulvast. Narrow Gullies Over High Sand Dunes On Mars: Evidence For Recent Liquid Flows. EG Quaternary Science Journal, 27, 2002. [ bib | ADS link ]
The discovery of recent gullies on Mars is one of the more unexpected observation of Mars Global Surveyor. The characteristics of these landforms suggest the local occurrence of a fluid emanating from alcoves located mostly in the upper part of poleward-facing slopes at mid and high latitudes (Malin and Edgett, Science, June 2000). However, observations of gullies originating from the top of peaks and sand dunes question this scenario. According to observations related to terrestrial debris flows analogous to martian gullies, we have shown that most gullies are more likely to result from the melting of liquid water in the first meters of the Martian sub- surface (Costard et al., Science, January 2002). Numerical simulations show that above-freezing temperatures can occur at high obliquities (? =40-45deg) in the near- surface of Mars. Here, we focus on gullies observed on the flanks of high sand dunes. Narrow gullies interpreted as the result of liquid flows on frozen dunes are observed on 6 MOC images in the latitudes of 40 to 60degS. Among these dunes, a large scale sand dunes reaching an elevation of 600 m above surrounding plains covers the floor of Russel crater. This dune is striped by narrow channels less than 20 m wide and 1 to 2 km long. The sinuousity of several channels shows that the flow is not very fast (less than 10 m/s) and inconsistent with CO2 driven flows. Indeed, flows triggered by CO2 would give high velocities strictly larger than 20 m/s like pyroclastic flows on Earth. Lateral ridges, also called “levées”, are observed on every channels like on terrestrial debris flows. This demonstrates that the liquid of the flow is a viscous slurry with a given shear strength. These gullies over dunes are therefore due to flows of liquid water mixed with sand and dust. This favors an external cause for the occurrence of liquid water by melting of water ice inside the near-surface porosity of dunes during a recent period of high obliquity in agreement with temperature variations found by simulations. The fact that gullies over dunes are still observable at the present time also shows that dunes may be frozen and inactive for thousand years explaining the conservation of these landforms.
W. J. Markiewicz, H. U. Keller, N. Thomas, D. Titov, and F. Forget. Optical properties of the Martian aerosols in the visible spectral range. Advances in Space Research, 29:175-181, 2002. [ bib | DOI | ADS link ]
Imager for Mars Pathfinder (IMP) obtained data of sky brightness as a function of the scattering angle, wavelength, time of day and Sol. This data set is fitted with model calculations to extract the size distribution, shape and the refractive index of the aerosols suspended in the atmosphere. The inferred optical parameters are discussed in context of diurnal variations and compared to those derived from Viking Landers cameras and Phobos KRFM radiometer data. The effects of the scattering and absorption of the solar radiation by the atmospheric aerosols are discussed in terms of their influence on the spectrophotometry of the Martian surface.