Eric Chassefière

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

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

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.

Monitoring of ozone trend by stellar occultations: the GOMOS instrument

Abstract As a part of the payload of the first European Polar Platform, the GOMOS instrument has been proposed by a group of 25 scientists from six countries. It consists of a telescope feeding two spectrographs, mounted on a dedicated steerable platform. The transmittance of the atmosphere between 250 and 675 nm is measured by comparing the spectrum of a star outside the atmosphere, and through it. The ozone tangential column is determined from its UV and Chappuis band absorption. This self-calibrated method is particularly well suited for the study of ozone long term trend. The altitude of each single measurement is precisely known (± 50 m), independently of altitude uncertainties. About 25 stellar occultations per orbit, and 350 per day, spread over all latitudes can be performed from 90 km down to 15–20 km of altitude. NO 2 , NO 3 , H 2 O, T(z) and aerosols are also simultaneously determined, important parameters associated to the ozone equilibrium. The ability to measure ozone long-term trends is calculated.

Fractal aggregates 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.

Formation and growth of photochemical aerosols in Titan's atmosphere

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.

Vertical Structure and Size Distributions of Martian Aerosols from Solar Occultation Measurements

Solar occultations performed with a spectrometer on board the Soviet spacecraft Phobos 2 (Blamont et al. 1991) provided data on the vertical structure of the Martian aerosols in the equatorial region (0°–20° N latitude) near the northern spring equinox (LS = 0°–20°). All measurements were made close to the evening terminator. Five clouds were detected above 45 km altitude and their vertical structure recorded at six wavelengths between 0.28 and 3.7 μm. They have a small vertical extent (3–6 km) and a vertical optical depth less than 0.03. The thermal structure, as derived from saturated profiles of water vapor observed by our instrument in the infrared, does not allow the CO2 frost point to be reached at cloud altitude, strongly suggesting that cloud particles are formed of H2O ice. Under the assumption of spherical particles, a precise determination of their effective radius, which varies from cloud to cloud and with altitude, is obtained and ranges from 0.15 to 0.85 μm; an estimate of the effective variance of the particle size distribution is ∼ 0.2. The number density of cloud particles at the peak extinction level is ∼1 cm−3. Dust was also observed and monitored at two wavelengths, 1.9 and 3.7 μm, on nine different occasions. The top of the dust opaque layer, defined as the level above which the atmosphere becomes nearly transparent at the wavelengths of observation, is located near 25 km altitude, with variations smaller than ±3 km from place to place. The scale height of dust at this altitude is 3–4 km. The effective radius of dust particles near the top of the opaque layer is 0.95 ± 0.25 μm and increases below with a vertical gradient of ∼0.05 μm km−1. Assuming that particles are levitated by eddy mixing, the eddy diffusion coefficient, K, is found to be ∼106 cm2 sec−1 at 25 km and 105−106 cm2 sec−1 at 50 km using, respectively, dust and cloud observations. An effective variance of 0.25 (±50%) for the dust size distribution is obtained on the basis of a simple theoretical model for the observed vertical gradient of the effective radius of dust particles. Three clouds observed by Viking at midlatitude during the northern summer are reanalyzed. The analysis gives K ≈ 106 cm2 sec−1 below 50 km altitude and at least 107 cm2 sec−1 above. Since the clouds seen from Phobos 2 are observed at twilight, which coincides with the diurnal maximum of the ambient temperature, they can be assumed to be in a steady state. If their thermodynamic state were to vary quickly during the day, our optical thickness at twilight would correspond to unrealistic values in earlier hours when the temperature is lower. Clouds are well fitted by theoretical profiles obtained assuming the steady state. An atmospheric temperature of 165–170 K at ∼50 km is inferred. The negative temperature gradient above the cloud is large (1.5–2 K km−1). A parallel is established between these thin clouds and the polar mesospheric clouds observed on Earth. It is shown that upwelling in equatorial regions at equinox could be a significant factor in levitating cloud particles.

Post‐Phobos model for the altitude and size distribution of dust in the low Martian atmosphere

Four experiments flown on board Phobos 2 provided information on the characteristics of the dust particles suspended in the Martian atmosphere: Auguste (UV-visible-IR spectrometer working in solar occultation geometry), ISM (IR spectrometer measuring the light of the Sun reflected by the planet), Termoskan (scanning radiometer mapping the planetary thermal radiation), KRFM (UV-visible multiphotometer providing limb-to-limb profiles). These experiments, which sounded equatorial regions (20°S–20°N) near the northern spring equinox (LS = 0–20°), are shown to yield a reasonably consistent picture of the dust distribution over the whole altitude range from the ground level, or just above, outside the boundary layer, up to ≈25 km. The vertical profiles of particle volume mixing ratio and effective (projected area-weighted) radius deduced from Auguste measurements, performed in the 15–25 km altitude range, are extrapolated down to the ground by using a simple, physical parameterization of the altitude dependence of dust mixing ratio and radius. This parameterization, which must be understood as describing the vertical distribution of dust in the zonal average and on the mesoscale in latitude, assumes that gravitational settling and eddy diffusion are the only two processes driving vertical dust transport. The vertically averaged effective radius and optical depth of dust particles, as well as vertical profiles of related quantities, are obtained. Optical depth at 1.9-μm wavelength is found to be 0.2 on average, with a typical variation of ±0.1 with time and space. This result is similar to that obtained from ISM spectra analysis. It is also consistent with the Termoskan and KRFM measurements, which yield near-infrared optical depths of 0.12–0.26 and 0.12–0.24, respectively. The particle number density near the surface, as derived from extrapolation of solar occultation profiles, is in the range 1–3 cm-3, in good agreement with Termoskan results (1–2 cm-3). The scale height of the dust volume mixing ratio just above the surface is ≈8–9 km on average, that is, of the same order as the background atmospheric scale height. The vertically averaged effective radius of dust particles is found to lie in the range 1.7±0.2 μm, possibly ≈2 μm in the case of a large effective variance of 0.4. The most likely ISM value is 1.2 μm, with a rather large uncertainty of ±0.4 μm, mainly due to the fact that the spectral dependence of the Minnaert coefficient is not well known. Because ISM data used in the present work were obtained on the Tharsis plateau, at a mean altitude of ≈7 km, the ISM radius must be compared to the Auguste vertically averaged radius for z > 7 km, that is, ≈1.5±0.2 μm. Auguste and ISM radii are therefore consistent at the 1-σ level. Three typical vertical profiles of the dust particle radius and number density, obtained by averaging all solar occultation profiles, including their extrapolated parts below ≈15 km, are proposed as reference models, for three selected values of the effective variance of the particle size distribution (0.10, 0.25, and 0.40).

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

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.

Geophysical and Atmospheric Evolution of Habitable Planets

The evolution of Earth-like habitable planets is a complex process that depends on the geodynamical and geophysical environments. In particular, it is necessary that plate tectonics remain active over billions of years. These geophysically active environments are strongly coupled to a planets host star parameters, such as mass, luminosity and activity, orbit location of the habitable zone, and the planets initial water inventory. Depending on the host stars radiation and particle flux evolution, the composition in the thermosphere, and the availability of an active magnetic dynamo, the atmospheres of Earth-like planets within their habitable zones are differently affected due to thermal and nonthermal escape processes. For some planets, strong atmospheric escape could even effect the stability of the atmosphere.

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 Voyagers 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 Titans atmosphere. The development of simulation chambers of Titans extreme conditions is necessary for a better understanding of past and future observations. Space probes will sound Titans 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.