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International audience; The Mars Organic Molecule Analyzer (MOMA) and Sample Analysis at Mars (SAM) instruments onboard the Exomars 2022 and Mars Science Laboratory rovers, respectively, are capable of organic matter detection and differentiating potentially biogenic from abiotic organics in martian samples. To identify organics, both these instruments utilize pyrolysis–gas chromatography coupled to mass spectrometry, and the thermochemolysis agent tetramethylammonium hydroxide (TMAH) is also used to increase organic volatility. However, the reactivity and efficiency of TMAH thermochemolysis are affected by the presence of calcium perchlorate on the martian surface. In this study, we determined the products of TMAH pyrolysis in the presence and absence of calcium perchlorate at different heating rates (flash pyrolysis and SAM-like ramp pyrolysis with a 35°C·min−1 heating rate). The decomposition mechanism of TMAH pyrolysis in the presence of calcium perchlorate was studied by using stepped pyrolysis. Moreover, the effect of calcium perchlorate (at Mars-relevant concentrations) on the recovery rate of fatty acids with TMAH thermochemolysis was studied. Results demonstrate that flash pyrolysis yields more diversity and greater abundances of TMAH thermochemolysis products than does the SAM-like ramp pyrolysis method. There is no obvious effect of calcium perchlorate on TMAH degradation when the [ClO4−] is lower than 10 weight percent (wt %). Most importantly, the presence of calcium perchlorate does not significantly impact the recovery rate of fatty acids with TMAH thermochemolysis under laboratory conditions, which is promising for the detection of fatty acids via TMAH thermochemolysis with the SAM and MOMA instruments on Mars.

"Multi-column gas chromatography analysis of chlorohydrocarbons 1with the SAM experiment onboard NASA’s Mars Curiosity rover" est le titre provisoire de la publication.; International audience; The Sample Analysis at Mars (SAM) instrument is a gas chromatograph-mass spectrometer onboard the NASA Curiosity rover, currently operating on the surface of Mars. Organic compounds are of major importance with regard to questions of habitability and the potential presence of life on Mars, and one of the mission’s main objectives is to analyze the organic content of soil and rock samples. In SAM’s first chromatographic measurements, however, unexpected chlorine-bearing organic molecules were detected. These molecules have different origins but the presence of perchlorates and chlorates detected at the surface of Mars suggests that reactivity between organic molecules and thermal decomposition products from oxychlorines is one of the major sources of the chlorinated organic molecules. Here we perform a comprehensive and systematic study of the separation of volatile chlorohydrocarbons with the chromatographic columns used in the SAM instrument. Despite the constrained operating conditions of the flight instrument, we demonstrate that SAM’s capillary chromatographic columns allow for effective separation and identification of a wide range of chlorine-bearing species. We also show that instrumental limitations prevent the detection of certain molecules, obscuring our ability to make definitive conclusions about the origin of these organic materials.

Molecular hydrogen (H2) from volcanic emissions is suggested to warm the Martian surface when carbon dioxide (CO2) levels dropped from the Noachian (4100 to 3700 Myr) to the Hesperian (3700 to 3000 Myr). Its presence is expected to shift the conversion of molecular nitrogen (N2) into different forms of fixed nitrogen (N). Here we present experimental data and theoretical calculations that investigate the efficiency of nitrogen fixation by bolide impacts in CO2‐N2 atmospheres with or without H2. Surprisingly, nitric oxide (NO) was produced more efficiently in 20% H2 in spite of being a reducing agent and not likely to increase the rate of nitrogen oxidation. Nevertheless, its presence led to a faster cooling of the shock wave raising the freeze‐out temperature of NO resulting in an enhanced yield. We estimate that the nitrogen fixation rate by bolide impacts varied from 7 × 10−4 to 2 × 10−3 g N·Myr−1·cm−2 and could imply fluvial concentration to explain the nitrogen (1.4 ± 0.7 g N·Myr−1·cm−2) detected as nitrite (NO2 −) and nitrate (NO3 −) by Curiosity at Yellowknife Bay. One possible explanation is that the nitrogen detected in the lacustrine sediments at Gale was deposited entirely on the crater's surface and was subsequently dissolved and transported by superficial and ground waters to the lake during favorable wet climatic conditions. The nitrogen content sharply decreases in younger sediments of the Murray formation suggesting a decline of H2 in the atmosphere and the rise of oxidizing conditions causing a shortage in the supply to putative microbial life., We acknowledge the NASA Mars Science Laboratory Program, Centre National d'Études Spatiales, the Universidad Nacional Autónoma de México (PAPIIT IN109416, IN111619, and PAPIME PE103216), and the Consejo Nacional de Ciencia y Tecnología de México (CONACyT 220626) for their support.

Pyrolysis of soil or rock samples is the preferred preparation technique used on Mars to search for organic molecules up today. During pyrolysis, oxichlorines present in the soil of Mars release oxidant species that alter the organic molecules potentially contained in the samples collected by the space probes. This process can explain the difficulty experienced by in situ exploration probes to detect organic materials in Mars soil samples until recently. Within a few months, the Curiosity rover should reach and analyze for the first time soils rich in sulfates which could induce a different behavior of the organics during the pyrolysis compared with the types of soils analyzed up today. For this reason, we systematically studied the pyrolysis of organic molecules trapped in magnesium sulfate, in the presence or absence of calcium perchlorate. Our results show that organics trapped in magnesium sulfate can undergo some oxidation and sulfuration during the pyrolysis. But these sulfates are also shown to protect organics trapped inside the crystal lattice and/or present in fluid inclusions from the oxidation induced by the decomposition of calcium perchlorate, and probably other oxychlorine phases currently detected on Mars. Trapped organics may also be protected from degradation processes induced by other minerals present in the sample, at least until these organics are released from the pyrolyzed sulfate mineral (~700 °C in our experiment). Hence, we suggest magnesium sulfate as one of the minerals to target in priority for the search of organic molecules by the Curiosity and ExoMars 2018 rovers.

In preparation for the Russian Luna Resurs mission we combined our compact time of flight mass spectrometer (TOF MS) with a chemical pre separation of the species by gas chromatography (GC). Coupled measurements with both instruments were successfully performed with the prototype of the mass spectrometer and a flight like gas chromatograph. The system was tested with two test gas mixtures a mixture of hydrocarbons and a mixture of noble gases. Due to its capability to record mass spectra over the full mass range at once with high sensitivity and a dynamic range of up to 10(6) within 1 s the TOF MS system is a valuable extension of the GC analytical system. Based on the measurements with calibration gases performed with the combined GC MS prototype and under assumption of mean characteristics for the Moon's regolith the detection limit for volatile species in a soil sample is estimated to 2.10( 10) by mass for hydrocarbons and 2.10( 9) by mass for noble gases. (C) 2015 Elsevier Ltd. All rights reserved.

[1] A single scoop of the Rocknest aeolian deposit was sieved (

Spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAV) is a suite of three spectrometers in the UV and IR range with a total mass of 13.9 kg flying on the Venus Express (VEX) orbiter, dedicated to the study of the atmosphere of Venus from ground level to the outermost hydrogen corona at more than 40,000 km. It is derived from the SPICAM instrument already flying on board Mars Express (MEX) with great success, with the addition of a new IR high-resolution spectrometer, solar occultation IR (SOIR), working in the solar occultation mode. The instrument consists of three spectrometers and a simple data processing unit providing the interface of these channels with the spacecraft. A UV spectrometer (118–320 nm, resolution 1.5 nm) is identical to the MEX version. It is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation. In nadir orientation, SPICAV UV will analyse the albedo spectrum (solar light scattered back from the clouds) to retrieve SO2, and the distribution of the UV-blue absorber (of still unknown origin) on the dayside with implications for cloud structure and atmospheric dynamics. On the nightside, γ and δ bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode the UV sensor will measure the vertical profiles of CO2, temperature, SO2, SO, clouds and aerosols. The density/temperature profiles obtained with SPICAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (not, vert, similar60 km) to 160 km in the atmosphere. This is essential for future missions that would rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO+, and CO2+, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere. The SPICAV VIS-IR sensor (0.7–1.7 μm, resolution 0.5–1.2 nm) employs a pioneering technology: an acousto-optical tunable filter (AOTF). On the nightside, it will study the thermal emission peeping through the clouds, complementing the observations of both VIRTIS and Planetary Fourier Spectrometer (PFS) on VEX. In solar occultation mode this channel will study the vertical structure of H2O, CO2, and aerosols. The SOIR spectrometer is a new solar occultation IR spectrometer in the range λ=2.2–4.3 μm, with a spectral resolution λ/Δλ>15,000, the highest on board VEX. This new concept includes a combination of an echelle grating and an AOTF crystal to sort out one order at a time. The main objective is to measure HDO and H2O in solar occultation, in order to characterize the escape of D atoms from the upper atmosphere and give more insight about the evolution of water on Venus. It will also study isotopes of CO2 and minor species, and provides a sensitive search for new species in the upper atmosphere of Venus. It will attempt to measure also the nightside emission, which would allow a sensitive measurement of HDO in the lower atmosphere, to be compared to the ratio in the upper atmosphere, and possibly discover new minor atmospheric constituents.

Comets harbor the most pristine material in our solar system in the form of ice, dust, silicates, and refractory organic material with some interstellar heritage. The evolved gas analyzer Cometary Sampling and Composition (COSAC) experiment aboard Rosetta's Philae lander was designed for in situ analysis of organic molecules on comet 67P/Churyumov-Gerasimenko. Twenty-five minutes after Philae's initial comet touchdown, the COSAC mass spectrometer took a spectrum in sniffing mode, which displayed a suite of 16 organic compounds, including many nitrogen-bearing species but no sulfur-bearing species, and four compounds—methyl isocyanate, acetone, propionaldehyde, and acetamide—that had not previously been reported in comets.

The Sample Analysis at Mars (SAM) instrument on board the Mars Science Laboratory Curiosity rover is designed to conduct inorganic and organic chemical analyses of the atmosphere and the surface regolith and rocks to help evaluate the past and present habitability potential of Mars at Gale Crater. Central to this task is the development of an inventory of any organic molecules present to elucidate processes associated with their origin, diagenesis, concentration, and long-term preservation. This will guide the future search for biosignatures. Here we report the definitive identification of chlorobenzene (150–300 parts per billion by weight (ppbw)) and C2 to C4 dichloroalkanes (up to 70 ppbw) with the SAM gas chromatograph mass spectrometer (GCMS) and detection of chlorobenzene in the direct evolved gas analysis (EGA) mode, in multiple portions of the fines from the Cumberland drill hole in the Sheepbed mudstone at Yellowknife Bay. When combined with GCMS and EGA data from multiple scooped and drilled samples, blank runs, and supporting laboratory analog studies, the elevated levels of chlorobenzene and the dichloroalkanes cannot be solely explained by instrument background sources known to be present in SAM. We conclude that these chlorinated hydrocarbons are the reaction products of Martian chlorine and organic carbon derived from Martian sources (e.g., igneous, hydrothermal, atmospheric, or biological) or exogenous sources such as meteorites, comets, or interplanetary dust particles. Key Points First in situ evidence of nonterrestrial organics in Martian surface sediments Chlorinated hydrocarbons identified in the Sheepbed mudstone by SAM Organics preserved in sample exposed to ionizing radiation and oxidative condition

SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) Light, a light-weight (4.7 kg) UV–IR instrument to be flown on Mars Express orbiter, is dedicated to the study of the atmosphere and ionosphere of Mars. A UV spectrometer (118–320 nm, resolution 0.8 nm) is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation (3.8 kg). It addresses key issues about ozone, its coupling with H2O, aerosols, atmospheric vertical temperature structure and ionospheric studies. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO+, and CO 2 + , and its direct interaction with the solar wind. An IR spectrometer (1.0–1.7 μm, resolution 0.5–1.2 nm) is dedicated primarily to nadir measurements of H2O abundances simultaneously with ozone measured in the UV, and to vertical profiling during solar occultation of H2O, CO2, and aerosols. The SPICAM Light near-IR sensor employs a pioneering technology acousto-optical tunable filter (AOTF), leading to a compact and light design. Overall, SPICAM Light is an ideal candidate for future orbiter studies of Mars, after Mars Express, in order to study the interannual variability of martian atmospheric processes. The potential contribution to a Mars International Reference Atmosphere is clear.

Many organic molecules are present in interstellar clouds and might have been carried to the early Earth by comets and meteorites during the heavy bombardment phase in the first few hundred million years of the solar system. It has been suggested that extraterrestrial organic material may represent a part of the organic material available for the origin of life. Until samples, brought by future space missions, are available on Earth, in situ measurements are the only way to get unaltered samples for analysis. The analytical technique has to be robust, sensitive and non-specific due to the large scope of target molecules. Currently the only flight qualified technique of analysis of organic molecules in space is gas chromatography (GC). The main objective of this work is to present the capabilities of a GC based subsystem, developed in our laboratory and including sample treatment and capillary column analysis, for the in situ analysis of volatile and refractory organics in atmospheric (Titan) and/or surface or sub-surface samples (Mars, comets).

International audience; Mars Science Laboratory is a large instrumented vehicle (rover) built by NASA that landed on Mars in August 2012 (launched November 2011) and will operate for more than 1 Martian year (i.e., 2 Earth years). This rover, also called Curiosity, characterizes and interprets the geology of the landing region at all scales, assesses the biological potential of encountered targets, investigates planetary processes of relevance to past habitability, and characterizes radiation arriving at Mars’ surface. Aboard MSL, an analytical laboratory is especially devoted to atmospheric analysis and search for organics in the ground.

As part of the development of the European Space Agency Rosetta space mission to investigate a cometary nucleus, the selection of columns dedicated to the gas chromatographic subsystem of the Cometary Sampling and Composition (COSAC) experiment was achieved. Once the space probe launched, these columns will be exposed to the harsh environmental constraints of space missions: vibrations, radiation (by photons or energetic particles), space vacuum, and large temperature range. In order to test the resistance of the flight columns and their stationary phases, the columns were exposed to these rough conditions reproduced in the laboratory. The comparison of the analytical performances of the columns, evaluated prior and after the environmental tests, demonstrated that all the columns withstand space constraints, and that their analytical properties were preserved. Therefore, all the selected capillary columns, even having porous layer or chiral stationary phases, were qualified for space exploration.

International audience; In order to obtain mineralogical information, the Mars Sample Return mission, in 2003 and 2005, will include sub-surface sampling. One expects that the effects of UV radiations and oxidizing agents are attenuated at depths corresponding to the bottom of the drilled cavity. In this case, such sampling will also allow to determine if life is or has been present at the surface of Mars. We propose to perform a preliminary in situ analysis of Martian samples: SAM ( Sample Analysis on Mars) on MSR 2005 lander. Such an analysis, performed on some parts of dedicated samples, will have the advantage of producing a ground-truth for the Earth-based analysis. Indeed, MSR 2003/2005 samples will be delivered to Earth laboratories in 2008, or 2009, depending on the duration of the quarantine, and it is not stated that some of their properties will remain unaltered during these three to 6 years. Moreover, there is a possibility that Martian sub-surface drilling delivers redundant samples: SAM could help to select, amongst the samples, which ones deserve to be sent to Earth, and, should the occasion arise, what are their unexpected peculiarities. To analyze inorganics and organics sampled at various depths, we propose to use the pyrolysis/chemical derivatization/gas chromatography/mass spectrometry technique. This experiment may be performed using techniques already developed in the frame of other planetary (ACP/HUYGENS) or cometary (COSAC/ROSETTA) missions, which can easily be adapted to the proposed mission objective, according to the three following topics: search for organics, search for inorganics and isotope characterization.

Titan's 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. Titan's 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.

The aim of this work is to deduce physical characteristics of the aerosols that compose the thin detached haze layer of Titan, for which the composition and origin are currently unknown. We have used four images at three different phase angles and three different wavelengths where the detached haze clearly appears. The low optical path in the layer permits assumptions that greatly simplify the treatment and allow us to consider the scattered intensity as the product of the scattering cross section, the phase function at the considered phase angle, and the number of aerosols. Physical arguments are given to support fractal aerosols in the detached haze, and we investigate a wide range of monomer radii, monomer numbers, and imaginary refractive indexes. This study yields information about the spectral variation of the refractive index for the detached haze tholins that are clearly different from those for the main haze. On the other hand, no firm values are given for the monomer radius and number, but these two parameters are strongly linked through a simple relation.

Clouds of dust particles and aggregates in the solar system have been known, at least for a decade, to exhibit similar linear polarization phase curves, with a faint negative minimum at a small phase angle, and a wide maximum near 90°. More recently, data inversion techniques and observational campaigns have allowed us to obtain accurate polarization phase curves for interplanetary and cometary dust. The differences pointed out between the characteristics of these curves, together with their wavelength dependence, indicate some differences in the properties of the dust particles. Computational or laboratory tools, which unambiguously link some of the observed trends to these properties, are of major interest to interpret the observational data in terms of origin and evolution of the dust particles.

Existing models of scattering by fractal aggregates of spheres are generally accurate but somewhat complicated and time consuming. Therefore, they are not easily usable for atmospheric studies that need intensive computations. We propose here a simple and fast model, based on Mie scattering and a set of empirical rules, that determines the scattering and absorption cross sections as well as the intensity phase function of isotropic fractal aggregates of identical spheres. This model is 10–50 times faster than the mean field model it is based on, and is easily derivable from a regular Mie code. The parameters of the present model are optimized for values of the real refractive index between 1.2 and 2.2, for imaginary refractive index less than 1 and for the product of real refractive index and spherical size parameter less than 10. This range was chosen as representative of typical particles in planetary atmospheres, for example, Titans aerosols.

Dust particles in the solar system (e.g. atmospheric hazes, cometary or interplanetary dust, regolith) are likely to be irregular aggregates whose light scattering properties (phase functions of polarization) are drastically different from those of Mie spheres. However, the observation of the light they scatter may provide informations on their physical properties. If the mechanisms which lead to aggregation are invariant with time, the aggregates are likely to be fractal particles made up from individual monomers. Computations, developped in relation with the CODAG experiment, are performed using a Discrete Dipole Approximation, and each monomer is described by one or more dipoles. When the particles are formed from a few monomers made up of numerous dipoles, the polarimetric response of the aggregate is similar to the one of the constituent monomer. When the particles are formed from many monomers made up of individual dipoles, the phase curves are similar to those observed in the solar system. Our calculations suggest that dust particles have a fractal dimension of the order of 2 (Ballistic Cluster-Cluster Aggregation), and that the values of the real and imaginary part of the complex refractive index of the constituent material are high. Those results are in agreement with laboratory measurements on samples representative of astronomical organics and minerals.

Light scattering measurements are needed to provide links between the currently available observations of scattering by dust in the solar system, and the still unknown properties of the clouds of dust particles and aggregates. The fragility, the fluffiness, and the size of the aggregates make it almost impossible to obtain realistic measurements throught computational models or classical laboratory measurements. In order to measure the scattering by dust particles and their aggregates, we have conceived the Light Scattering (LS) experiment for the COsmic Dust AGgregation (CODAG) module. This small and compact instrument has been developed to operate under microgravity conditions, jointly with the CODAG Sounding Rocket experiment (Blum et al. , 1998), during ESA rocket flights. The purpose of the experiment is to determine accurately the intensity and the polarization phase functions of the particles that are dispersed in a low pressure chamber, and to document the temporal evolution of their scattering properties, while an aggregation process representative of the solar system formation is starting and spreading out. The instrumental principle relies on simultaneous measurements, performed on a large number of phase angles, from back to forward scattering, of the two polarized components of the scattered intensity.

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.

Dust agglomerates/particles in the solar system (e.g. regoliths, cometary comae, interplanetary dust) are irregular aggregates whose scattering properties (phase functions of the intensity and polarization of scattered light) are drastically different from those of Mie spheres. However, the observation of the light they scatter may provide information on their physical properties. If the mechanism which makes up the aggregates is invariant with time, they are likely to be fractal particles, i.e. clusters formed of individual monomers. Computations have been made using a Discrete Dipole Approximation with hundreds of monomers; comparison with the observations suggests that regoliths or dust clouds have a fractal dimension of the order of 2, and consist of fluffy particles formed by Ballistic Cluster-Cluster aggregation. To built the reference database required for deriving the physical properties of these fluffy dust aggregates, and to avoid any modification of the particles properties by levitation techniques, it has been suggested to measure the scattering properties of numerous samples under microgravity conditions. Results obtained during parabolic flights show some discrepancy at large phase angles between the results obtained under Earth's gravity and microgravity conditions, the latter being more reminiscent of solar system observations.

The collisional mechanisms of Titan's aerosols may lead to a fractal structure in which the aerosols are built by the aggregation of spherical submicrometer particles (monomers). In this initial study of the problem, the optical behavior of these aggregates is modeled assuming that each monomer radiates a dipole field in response to the incident radiation including the radiated fields of all the other elements in the aggregate. This dipole approximation, valid if the monomer radius is smaller than the wavelength, is used to calculate the scattering and extinction efficiencies of such aerosol particles, which are assumed to be composed of tholins. By applying the two-stream approximation for radiative transfer to the vertical distribution of aerosols obtained by microphysical modeling, we compute the geometric albedo of Titan. Computed values and observational values of the albedo are compared for wavelengths from 0.22 to 1.0 μm, and the effects of parameters, such as the fractal dimension of aerosols, their formation altitude or mass production rate, and, in addition, the methane abundance, are investigated. The hypothesized fractal structure of particles can explain both the visible and the UV albedos. In previous models these measurements could only be matched simultaneously under the assumption of a bimodal population. For a fractal dimension Df ≈ 2 in the settling region, corresponding to a growth governed by cluster-cluster aggregation, the computed albedo in the near-UV range matches the observations. A good fit between measurement and calculated albedo is obtained, for a formation altitude Z0 = 535 km, over the whole wavelength range by adjusting the absorption coefficient of the particles within a factor of two from that of tholins and a production rate between 0.2 and 1.5 times 3.5 × 10-13 kg m-2 sec-1. Lower formation altitudes, like our preferred case, Z0 = 385 km, cannot be investigated in the UV range due to limitations of the dipolar approximation, but we expect this case to give the same behavior.

In physics, fractal objects are basically finite. This means that their geometrical features must be corrected by natural cut-offs. In the important example of aggregates of small units, the scaling behaviours break down both for small length-scales (reflecting the typical size of the monomers) and for large length-scales (due to the finite extent of the aggregate). These cut-off functions are either ignored in the theoretical studies, or they are modelled by a simple exponential function. In this paper we show that this simple form is not the generic case and that some physical properties depend quantitatively on the precise form of these cut-offs. Explicit analytical and numerical models, mainly connected to the cluster-cluster aggregation model, are studied from this perspective. All of them exhibit roughly the same form of cut-off function. We discuss the sensitivity to these functions of some optical properties of importance in light scattering experiments.

The evolution of Titan's aerosols is studied from their production altitude down to the ground using a stochastic approach. A background aerosol distribution is assumed, obtained from previous Eulerian modelling, and the evolution of a “tagged” particle, released near the formation altitude, is followed by simulating in a random way its growth through coagulation with particles of the background distribution. The two distinct growth stages, proposed by Cabane et al. (1992) to explain the formation of monomers and subsequent aggregates, are confirmed. The first stage may be divided into two parts. Firstly, within ≈1 terrestrial day, particles grow mainly through collisions with larger particles. They reach their size of monomer (rm ≈0.09 µm) through typically one to five such collisions. Secondly, within a few terrestrial days to ≈1 terrestrial month, particles evolve mainly by collisions with continuously created small particles and acquire their compact spherical structure. In the second stage, whose duration is ≈30 terrestrial years, or one Titan's seasonal cycle, particles grow by cluster-cluster aggregation during their fall through the atmosphere and reach, at low stratospheric levels, a typical radius of 0.4–0.5 µm.

Recent developments in our understanding of the morphology (shape and size) of haze aerosols in Titan's 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 Titan's haze, the formation altitude of aerosols is found to lie in the range from 350 to 400 km.

Microphysical modeling of Titan's aerosols has been developed in order to estimate the distribution and chemical composition of the particles in the low atmosphere. It includes condensation, diffusion, coagulation and sedimentation processes, and it uses up-to-date data relating to the vertical thermal and chemical atmospheric structure. The main results indicates that, down to a few km above the surface, the aerosol clouds would be constituted of particles of mean radius increasing with decreasing altitude, with a solid core of several 10 microns, mainly composed of nitriles, and covered by a thick layer of C1–C2 hydrocarbons. These results have important implications on future in situ aerosol analysis experiments, like Cassini's ACP experiment.

International audience; In this paper, we present the results obtained by the general circulation model developed at the Laboratoire de Météorologie 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 ($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.

ACP’s main objective is the chemical analysis of the aerosols in Titan’s atmosphere. For this purpose, it will sample the aerosols during descent and prepare the collected matter (by evaporation, pyrolysis and gas products transfer) for analysis by the Huygens Gas Chromatograph Mass Spectrometer (GCMS). A sampling system is required for sampling the aerosols in the 135–32 km and 22–17 km altitude regions of Titan’s atmosphere. A pump unit is used to force the gas flow through a filter. In its sampling position, the filter front face extends a few mm beyond the inlet tube. The oven is a pyrolysis furnace where a heating element can heat the filter and hence the sampled aerosols to 250 °C or 600 °C. The oven contains the filter, which has a thimble-like shape (height 28 mm). For transferring effluent gas and pyrolysis products to GCMS, the carrier gas is a labeled nitrogen 15N2, to avoid unwanted secondary reactions with Titan’s atmospheric nitrogen.

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.

Biemann 1 calls into question our preliminary interpretations of our experimental results2. A comparison of laboratory and flight measurements should settle the uncertainties he raises. In addition to evaluating instrumental characteristics such as the 'piston effect', a technique we used for injecting oven-gas content for gas chromatography–mass spectrometry (GC–MS), such studies should enable a wide range of possible compositions for Titan's aerosols to be investigated. Our aerosol collection and pyrolysis (ACP) measurements in Titan's atmosphere can then be revisited.

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).

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.

International audience; In 1976, the Viking landers carried out the most comprehensive search for organics and microbial life in the martian regolith. Their results indicate that Mars' surface is lifeless and, surprisingly, depleted in organics at part-per-billion levels. Several biology experiments on the Viking landers gave controversial results that have since been explained by the presence of oxidizing agents on the surface of Mars. These oxidants may degrade abiotic or biological organics, resulting in their nondetection in the regolith. As several exploration missions currently focus on the detection of organics on Mars (or will do so in the near future), knowledge of the oxidative state of the surface is fundamental. It will allow for determination of the capability of organics to survive on a geological timescale, the most favorable places to seek them, and the best methods to process the samples collected at the surface. With this aim, we review the main oxidants assumed to be present on Mars, their possible formation pathways, and those laboratory studies in which their reactivity with organics under Mars-like conditions has been evaluated. Among the oxidants assumed to be present on Mars, only four have been detected so far: perchlorate ions (ClO4-) in salts, hydrogen peroxide (H2O2) in the atmosphere, and clays and metal oxides composing surface minerals. Clays have been suggested as catalysts for the oxidation of organics but are treated as oxidants in the following to keep the structure of this article straightforward. This work provides an insight into the oxidizing potential of the surface of Mars and an estimate of the stability of organic matter in an oxidizing environment.