Laboratory simulations of Titan's atmosphere: organic gases and aerosols

Titan, the main satellite of Saturn, has been observed by remote sensing for many years, both from interplanetary probes (Pioneer and Voyager's flybys) and from the Earth. Its N2 atmosphere, containing a small fraction of CH4 (approximately 2%), with T approximately 90 K and P approximately 1.5 bar at the ground level, is irradiated by solar UV photons and deeply bombarded by energetic particles, i.e. Saturn mangetospheric electrons and protons, interplanetary electrons and cosmic rays. The resulting energy deposition, which takes place mainly below 1000 km, initiates chemical reactions which yield gaseous hydrocarbons and nitriles and, through polymerisation processes, solid aerosol particles which grow by coagulation and settle down to the ground. At the present time, photochemical models strongly require the results of specific laboratory studies. Chemical rate constants are not well known at low temperatures, charged-particle-induced reactions are difficult to model and laboratory simulations of atmospheric processes are therefore of great interest. Moreover, the synthesis of organic compounds which have not been detected to date provides valuable information for future observations. The origin and chemical composition of aerosols depend on the nature of chemical and energy sources. Their production from gaseous species may be monitored in laboratory chambers and their optical or microphysical properties compared to those deduced from the observations of Titan's atmosphere. The development of simulation chambers of Titan's extreme conditions is necessary for a better understanding of past and future observations. Space probes will sound Titan's atmosphere by remote sensing and in situ analysis in the near future (Cassini-Huygens mission). It appears necessary, as a preliminary step to test on-board experiments in such chambers, and as a final step, when new space data have been acquired, to use them for more general scientific purposes.     

Experimental investigation of the kinetics of Ostwald ripening of quartz in silicic melts

To characterise the kinetics of Ostwald ripening of quartz, we conducted four series of experiments in systems consisting of quartz, with an initial grain size of ƹ.3 to 6 µm, in equilibrium with hydrous silicic liquids. Two series were performed with a haplogranitic liquid containing 6.5 wt% H₂O at 900 °C and 1 GPa. The third series was made in the quartz-anorthite system at water saturation, 900 °C, and 1 GPa. The last series was made in the quartz-albite system at water saturation, 800 °C, and 0.2 GPa. In all series, we observed a relatively small but systematic increase of the mean grain size of quartz, % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8YjY-vipgYlH8Gipec8Eeeu0xXdbba9frFj % 0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9 % vr0-vr0-vqpWqaaeaabiGaciaacaqabeaabaqaamaaaOqaaiqbdsga % Kzaaraaaaa!29AA! [`(d)] \bar d , with increasing run duration, t. The largest increase was in the quartz-anorthite system: % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8YjY-vipgYlH8Gipec8Eeeu0xXdbba9frFj % 0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9 % vr0-vr0-vqpWqaaeaabiGaciaacaqabeaabaqaamaaaOqaaiqbdsga % Kzaaraaaaa!29AA! [`(d)] \bar d increased by a factor of 4.3 after 326 h; the smallest increase was measured in the quartz-albite system: % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8YjY-vipgYlH8Gipec8Eeeu0xXdbba9frFj % 0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9 % vr0-vr0-vqpWqaaeaabiGaciaacaqabeaabaqaamaaaOqaaiqbdsga % Kzaaraaaaa!29AA! [`(d)] \bar d increased by a factor of only ƹ.6 after 1,173 h. The experimental data yield very good linear fits in both ln t vs ln% MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8YjY-vipgYlH8Gipec8Eeeu0xXdbba9frFj % 0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9 % vr0-vr0-vqpWqaaeaabiGaciaacaqabeaabaqaamaaaOqaaiqbdsga % Kzaaraaaaa!29AA! [`(d)] \bar d and ln t vs % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8YjY-vipgYlH8Gipec8Eeeu0xXdbba9frFj % 0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9 % vr0-vr0-vqpWqaaeaabiGaciaacaqabeaabaqaamaaaOqaaiqbdsga % Kzaaraaaaa!29AA! [`(d)] \bar d diagrams. The slopes in the ln t vs ln% MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8YjY-vipgYlH8Gipec8Eeeu0xXdbba9frFj % 0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9 % vr0-vr0-vqpWqaaeaabiGaciaacaqabeaabaqaamaaaOqaaiqbdsga % Kzaaraaaaa!29AA! [`(d)] \bar d diagrams, between 1/5 and 1/7, are, however, much smaller than the value of 1/3 predicted by the Lifshitz-Slyozov-Wagner theory for diffusion-controlled Ostwald ripening. A possible explanation for this discrepancy is that the diffusion-controlled regime in our experiments is only attained after a transient regime lasting from 20 h in series III to 100 h in series IV. A more straightforward explanation of the experimental results is that the rate-limiting mechanism for Ostwald ripening is quartz growth by surface nucleation not diffusion in the silicic liquid. Finally, we extrapolated our data to geological time scales to evaluate the importance of Ostwald ripening in natural quartz-bearing magmatic systems: (1) quartz cannot coarsen measurably by Ostwald ripening over reasonable time scales if the initial grain size is ƹ mm or more; and (2) Ostwald ripening may be very active at the end of nucleation events and result in the consumption of a significant proportion of crystalline nuclei.     

An experimental study of Ostwald ripening of olivine and plagioclase in silicate melts: implications for the growth and size of crystals in magmas

We carried out an experimental study to characterize the kinetics of Ostwald ripening in the forsterite-basalt system and in the plagioclase (An₆₅)-andesite system. Eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution (CSD) with time t; the experiments were performed in a 1-atmosphere quench furnace, at 1,250°C for plagioclase and 1,300°C for olivine. Very contrasted coarsening kinetics were observed in the two series. In the plagioclase series, the mean grain size increased as log(t), from ≈3 μm to only 8.7 μm in 336 h. The kinetic law in log(t) means that Ostwald ripening was rate-limited by surface nucleation at plagioclase-liquid interfaces. In the olivine series, the mean grain size increased as t ^1/3, from ≈3 μm to 23.2 μm in 496 h. A kinetic law in t ^1/3 is expected when Ostwald ripening is rate-limited either by diffusion in the liquid or by grain growth/dissolution controlled by a screw dislocation mechanism. The shape of olivine CSDs, in particular their positive skewness, indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism, not by diffusion. As the degrees of undercooling ΔT (or supersaturation) involved in Ostwald ripening are essentially <1°C, the mechanisms of crystal growth identified in our experiments are expected to be those prevailing during the slow crystallisation of large magma chambers. We extrapolated our experimental data to geological time scales to estimate the effect of Ostwald ripening on the size of crystals in magmas. In the case of plagioclase, Ostwald ripening is only efficient for mean grain sizes of a few microns to 20 μm, even for a time scale of 10⁵ years. It can, however, result in a significant decrease of the number of small crystals per unit volume, and contribute to the development of convex upwards CSDs. For olivine, the mean grain size increases from 2–3 μm to ≈70 μm in 1 year and 700 μm in 10³ years; a mean grain size of 3 mm is reached in 10⁵ years. Accordingly, the rate of grain size-dependent processes, such as compaction of olivine-rich cumulates or melt extraction from partially molten peridotites, may significantly be enhanced by textural coarsening.     

Two formation regions for Titan's hazes: indirect clues and possible synthesis mechanisms

It is suggested that aerosol particles forming the detached and main haze layers of Titan's atmosphere do not originate in the same atmospheric levels. Particles present above approximately 350 km could be formed of polyacetylenes synthetized in the 500-800 km altitude range through successive insertion reactions involving the C2H radical under the action of solar ultraviolet photons (Yung et al., Astrophys. J. Suppl. 55, 465, 1984). They might contain C-N oligomers in comparable amounts, as well as C-H-N oligomers synthetized at high altitude (900-1000 km) by the action of suprathermal Saturn plasma electrons. Physically, they are expected to consist of fluffy aggregates of density approximately 0.01-0.1 g cm-3. Their mass production rate is small (10(-15)-10(-14) kg m-2 s-1), that is typically 10% or less of the main haze production rate. Due to their low fall velocity, they are very sensitive to large scale horizontal motions and one substantial part of them may be swept away by meridional circulation at the detached haze level. The altitude range where these aerosols are created is well above the range proposed by Cabane et al. (Planet. Space Sci. 41, 257, 1993) for aerosols of the main haze layer, on the basis of a new fractal microphysical modeling of Titan's aggregates, that is approximately 350-400 km. A natural outcome of this apparent discrepancy is to suppose that there is a second formation region, below approximately 400 km altitude, giving rise to the main haze layer. The aim of the present paper is to review the different possible formation mechanisms of this main haze layer and assess their ability to account for the observed characteristics of the haze. Several conditions are established. The first one, called "condition A", concerns the formation altitude range imposed by fractal modeling. Possible chemical and energy sources are examined. Two additional constraints, relative to the minimum gas mass ("condition B") and input energy ("condition C") required for efficient conversion of gas into aerosols, are defined. By comparing the production rates of the haze, as derived from microphysical models, and of gaseous chemical species, as derived from photochemical models, five possible source constituents are identified: N2, CH4, C2H2, C2H6 and HCN. Polymerization of C2H2 into (C2H2)n through action of solar ultraviolet photons is shown to be rather improbable (condition A is hardly satisfied). From both our current knowledge of the gaseous phase photochemistry, through modeling and laboratory experiments, and existing models of the interaction between Saturn magnetosphere and Titan atmosphere, the formation of C-H-N polymers through action of Saturn magnetospheric energetic particles (E approximately 100 keV), is proposed as the basic polymerization mechanism in the lower formation region (conditions A,B and C are jointly satisfied).     

Light Scattering by Dust Under Microgravity Conditions

Most of our knowledge on heterogeneous media in the Universe comes from the light they scatter. This light is mainly linearly polarized, and the polarization phase curves contain information about the properties of the scattering dust. In the solar system, the dust seems to be made of irregular aggregates with a size greater than a few microns and a fractal structure. Many constraints appear in the scattering computations, due to the trickiness of the mathematical calculations, and to our ignorance of the precise structure of the dust. This leads to the necessity to perform light scattering measurements on characteristic aggregates, built under low velocity ballistic collisions. Microgravity is a sensible way to achieve such measurements on a cloud of levitating and aggregating dust particles. A first step has been the PROGRA² experiment, which operates during parabolic flights on an aircraft. The instrument is a polar nephelometer measuring successively the light scattered by a dust sample at various angles; it is fully operational, and will provide a data base of polarization phase curves. A second step is the CODAG-SR experiment, which uses the duration of a rocket flight to build up dust aggregates. The instrument measures simultaneously the light scattered at numerous phase angles; it is now space qualified, and should provide in a near future a monitoring of the intensity and polarization phase curves while the aggregation processes are taking place. This revised version was published online in July 2006 with corrections to the Cover Date.     

A new extraction technique for in situ analyses of amino and carboxylic acids on Mars by gas chromatography mass spectrometry

In order to target key organic compounds in the Martian regolith using gas chromatography mass spectrometry (GC-MS), we have developed a new extraction procedure coupled with chemical derivatization. This new technique was tested on a Mars analog soil sample collected from the Atacama Desert in Chile. We found that amino and carboxylic acids can be extracted from the Atacama soil in a 1:1 mixture of isopropanol and water after ultrasonic treatment for 30 min. The extracted organic compounds were then derivatized in a single-step reaction using N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide (MTBSTFA) as the silylating agent in order to transform these compounds into volatile species that can then be detected by GC-MS. We are currently developing a miniaturized reaction cell suited for spaceflight, where both organic extraction and chemical derivatization processes can take place in a single step.