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.
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.