Michel Cabane

Volatile, Isotope, and Organic Analysis of Martian Fines with the Mars Curiosity Rover

Samples from the Rocknest aeolian deposit were heated to ~835°C under helium flow and evolved gases analyzed by Curiosity’s Sample Analysis at Mars instrument suite. H2O, SO2, CO2, and O2 were the major gases released. Water abundance (1.5 to 3 weight percent) and release temperature suggest that H2O is bound within an amorphous component of the sample. Decomposition of fine-grained Fe or Mg carbonate is the likely source of much of the evolved CO2. Evolved O2 is coincident with the release of Cl, suggesting that oxygen is produced from thermal decomposition of an oxychloride compound. Elevated δD values are consistent with recent atmospheric exchange. Carbon isotopes indicate multiple carbon sources in the fines. Several simple organic compounds were detected, but they are not definitively martian in origin.

Origin and role of water ice clouds in the Martian water cycle as inferred from a general circulation model

In this paper, we present the results obtained by the general circulation model developed at the Laboratoire de Meteorologie 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 (

Complex organic matter in Titan's atmospheric aerosols from in situ pyrolysis and analysis.

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.

Physical properties of the organic aerosols and clouds on Titan

Aerosols in Titans atmosphere play an important role in determining its thermal structure. They also serve as sinks for organic vapours and can act as condensation nuclei for the formation of clouds, where the condensation efficiency will depend on the chemical composition of the aerosols. So far, however, no direct information has been available on the chemical composition of these particles. Here we report an in situ chemical analysis of Titans aerosols by pyrolysis at 600 °C. Ammonia (NH3) and hydrogen cyanide (HCN) have been identified as the main pyrolysis products. This clearly shows that the aerosol particles include a solid organic refractory core. NH3 and HCN are gaseous chemical fingerprints of the complex organics that constitute this core, and their presence demonstrates that carbon and nitrogen are in the aerosols.

A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO

Abstract Titans haze is optically thick in the visible, with an optical depth at 0.5 μm of about three. The haze varies with latitude in a seasonal cycle and has a detached upper layer. Microphysical models, photochemical models, and laboratory simulations all imply that the production rate of the haze is in the range of 0.5– 2×10 −14 g cm −2 s −1 . Given the rate of sedimentation, the total mass loading is about 250 mg m −2 . The transparency of the haze is high for wavelengths above 1 μm because the haze material becomes almost purely scattering and the optical depth decreases with increasing wavelength. The particles in the main haze deck are probably fractal in structure with an equivalent volume radius of 0.2 μm . The haze material is organic and, if similar to laboratory tholin, has a C/N ratio in the range of 2–4 and a C/H ratio of about unity. The haze significantly affects the thermal balance of Titan, causing an antigreenhouse effect that cools the surface by 9 K. Titans faintly banded appearance suggests strong zonal winds in the lower stratosphere. Condensate clouds of ethane or methane, if present, are thin, patchy, or transient. Stratospheric clouds of condensed nitriles and (possibly) hydrocarbons appear to be associated with, though not contained entirely in, the polar shadow, suggesting abundances may vary with the season. Precipitating condensate particles from the stratosphere probably act as nucleating centers for the formation and rapid growth of methane ice particles in the troposphere, where the gas phase appears to be highly supersaturated. Once formed, fallout times for these hailstones are ∼2 h or less. Melting, and possible subsequent fragmentation of methane raindrops should occur at ∼12 km and below. Almost complete evaporation should occur just above the surface. A thin residue of ethane-enriched fog particles would then slowly settle to the surface, steadily modifying an existing surface or subsurface residue of liquid hydrocarbons. The optical properties of the haze in the 1 to 3 μm spectral region and the implications for the visibility of the surface are probably the most pressing current research questions. Other key questions include the nature of the high altitude detached haze layer, altitude and seasonal changes in composition of the haze, the role of haze particles as condensation nuclei for clouds, and the nature of any condensate clouds.

Fractal aggregates in Titan's atmosphere

Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60–100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar–antisolar circulation in the thermosphere (above 100 km), 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 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90–120 km 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 100 km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40 years 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 80–90 km for which we have no explanation.

Formation and growth of photochemical aerosols in Titan's atmosphere

Abstract It has been suggested that the haze aerosols in Titans atmosphere might present an irregular structure, rather similar to the morphology of aggregates experimentally synthesized by Bar-Nun et al. ( J. geophys. Res. 93 , 8383, 1988). The theoretical approach of West ( Appl. Opt. 30 ,5316, 1991) and West and Smith ( Icarus 90 , 330, 1991). which uses a fractal concept to numerically generate aggregates, allowed us to support this idea and to provide constraints on their size and shape by comparing the observed and modelled polarization properties of such particles. The building mechanism of these aerosols, when analysed using microphysical modelling (Cabane et al., Icarus 96 , 176. 1992). leads naturally to aggregates. They are formed of spherical compact monomers, which build up in the region of photochemical synthesis, and whose radius depends mainly on the atmospheric pressure at the formation level. The subsequent growth of aggregates in the settling phase is treated here by introducing the fractal dimension as a parameter of the model ( D f ≈ 2 in the case of cluster-cluster aggregation). Using this fractal model, a vertical distribution of size and number density of the aggregates is obtained down to ≈ 80 km for different production altitudes. The previous estimate of the formation altitude of photochemical aerosols (≈ 350–400 km) is confirmed when comparing the number of monomers per aggregate deduced from the present study with the value proposed by West and Smith. The vertical profile of the effective radius of aggregates is calculated as a function of the visible optical depth derived from Voyager imaging. A good fit with the radius derived from Voyager forward-scattering measurements is obtained (≈ 0.3–0.5 μm), still using a low formation altitude. Finally, it must be emphasized that, for the first time, observational and theoretical results about the size and the structure of particles are reconciled.

A new interpretation of scattered light measurements at Titan's limb

Recent developments in our understanding of the morphology (shape and size) of haze aerosols in Titans atmosphere, aggregate particles and their associated optical properties (West 1990, West and Smith 1991), have been considered in light of a microphysical modeling of aerosols. Using an Eulerian model rather similar to the one of Toon et al. (1980), it is shown that the growth of particles may be divided in two stages. The first one corresponds to the initial growth near the formation altitude by accretion of very small elementary particles (synthetized oligomers). Due to the large number of accreted small particles, this stage leads to the formation of nearly spherical particles (called “monomers” by West and Smith). In the second stage, these particles (“monomers”) settle in the atmosphere and stick together forming aggregates, of the same kind as those obtained experimentally by Bar-Nun et al. (1988). The classical assumption of spherical particles, which is made in all existing models, is shown to be valid during the first growth stage. The mean ratio between the masses of two particles colliding together is quantified as a function of altitude using a specific index (mass sharing index, MSI). From this criterion, the two previously defined regions (the formation region inside which monomers are generated, the settling region where aggregates build up) may be clearly separated and the boundary between them precisely defined and located. Identifying the monomer radius with the mass-averaged radius at the altitude of the boundary, it is shown that this radius is extremely sensitive to the altitude where aerosols are created. The reason is that the residence time of particles near the formation altitude, which determines to a large extent the size of monomers, is an increasing function of the pressure. The strong dependence of the monomer radius on the pressure (thus the altitude) is calculated using different sets of parameters (eddy diffusion coefficient, electrical charge, mass production rate, …) in order to cover the widest range of possibilities. Using parameters favored by West in his analysis of the polarizing properties of Titans haze, the formation altitude of aerosols is found to lie in the range from 350 to 400 km.

Mean-field approximation of Mie scattering by fractal aggregates of identical spheres

Images of Titan, taken by Voyager 2 at phase angles Φ=140° and Φ=155° have provided radial intensity profiles at the bright and dark limbs, which provide information on the vertical and latitudinal distribution of organic hazes. In previous work, the deduced extinction coefficient, using ad hoc particle sizes, was obtained without help of microphysics, and it appeared difficult to compare it with coefficients computed from theoretical models. We use here our fractal approach of microphysical modeling and optics of agregates to compute intensity profiles of the main haze at the bright limb, and compare to the Voyager observations. Fractal aerosol distributions are obtained using different production altitudes and rates. Scattering and absorption of light are described by an improved model, based on the use of fractal aggregates made of spherical (Mie) particles. We show that the fractal dimension of aggregates has to be Df≈2, as predicted by microphysical arguments. Only a production altitude z0≈385±60 km, corresponding to a monomer radius rm≈0.066 μm, is fully consistent with both phase angle data. We also point out that the production rate of the aerosols decreases by a factor ≈2 between 30°S and the midnorthern latitude and further, increases up to 80°N. The average value of the production rate is Q≈1.4×10−13 kg/m2/s; we give arguments in favor of dynamical processes rather than of a purely microphysical mechanisms to explain such latitudinal variations.

The study of the martian atmosphere from top to bottom with SPICAM light on mars express

We apply the recent exact theory of multiple electromagnetic scattering by sphere aggregates to statistically isotropic finite fractal clusters of identical spheres. In the mean-field approximation the usual Mie expansion of the scattered wave is shown to be still valid, with renormalized Mie coefficients as the multipolar terms. We give an efficient method of computing these coefficients, and we compare this mean-field approach with exact results for silica aggregates of fractal dimension 2.