F. Raulin

The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe.

Saturns largest moon, Titan, remains an enigma, explored only by remote sensing from Earth, and by the Voyager and Cassini spacecraft. The most puzzling aspects include the origin of the molecular nitrogen and methane in its atmosphere, and the mechanism(s) by which methane is maintained in the face of rapid destruction by photolysis. The Huygens probe, launched from the Cassini spacecraft, has made the first direct observations of the satellites surface and lower atmosphere. Here we report direct atmospheric measurements from the Gas Chromatograph Mass Spectrometer (GCMS), including altitude profiles of the constituents, isotopic ratios and trace species (including organic compounds). The primary constituents were confirmed to be nitrogen and methane. Noble gases other than argon were not detected. The argon includes primordial 36Ar, and the radiogenic isotope 40Ar, providing an important constraint on the outgassing history of Titan. Trace organic species, including cyanogen and ethane, were found in surface measurements.

Experimental laboratory simulation of Titan's atmosphere: aerosols and gas phase

The discovery that Titan, the largest satellite of Saturn, has an atmosphere and that methane is a significant constituent of it, was the starting point for a systematic study of Titan’s atmospheric organic chemistry. Since then, the results from numerous ground-based observations and two flybys of Titan, by Voyager I and II, have led to experimental laboratory simulation studies and photochemical and physical modeling. All these works have provided a more detailed picture of Titan. We report here a continuation of such a study performing an experimental laboratory simulation of Titan’s atmospheric chemistry, and considering the two physical phases involved: gases and aerosols. Concerning the gaseous phase, we report the first detection of C4N2 and we propose possible atmospheric abundances for 70 organic compounds on Titan’s upper atmosphere. Concerning the solid phase, we have characterized aerosol analogues synthesized in conditions close to those of Titan’s environment, using elemental analysis, pyrolysis, solubility studies and infrared spectroscopy. # 1999 Elsevier Science Ltd. All rights reserved.

Titan's atmosphere and hypothesized ocean: A reanalysis of the Voyager 1 radio-occultation and IRIS 7.7-μm data

Voyager1 radio-occultation refractivity profiles of Titan in the 0–200 km altitude range are reanalyzed in order to derive limiting profiles for the equatorial thermal structure, taking into account uncertainties on the mean molecular weight m. The major atmospheric constituents are assumed to be N2 and CH4 (already detected) and Ar which is a plausible additional constituent when m is greater than 28. The stratospheric abundance of CH4 is assumed to be limited by saturation at the tropopause. Spectra calculated from the infrared thermal profiles in the CH4 ν4 band at 7.7 μm are compared with the Voyager IRIS observations. This allows us to constrain both the abundances of major atmospheric components and the stratospheric temperatures. The methane mole fraction is between 0.5 and 3.4% in the stratosphere but may be as high as 21% at the surface, whereas the atmospheric Ar/N2 ratio can be anything between 0 and 0.27. The temperature is between 92.5 and 101°K at the ground level, and between 70.5 and 74.5°K at the tropopause. The maximum uncertainty (±4°K) occurs between 150 and 200 km altitude. A “nominal” temperature and composition profile of Titans atmosphere between the ground and 200 km is given with tabulated values. The implications of composition and surface temperature uncertainties on the abundances of the expected major oceanic constituents are discussed, as well as the consequences for the solubility of noncodenssable atmospheric constituents (H2 and CO) and organic compounds raining out from the atmosphere.

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.

Cometary organic chemistry: a review from observations, numerical and experimental simulations

Abstract This paper is a review dealing with the organic chemistry of comets. It describes how the chemical composition of comets can provide information about the chemistry of the interstellar medium, and the formation of the solar system. We discuss to what extent they could have brought to Earth the ingredients essential to the emergence of life: water and prebiotic compounds. We review all molecules which have been detected or tentatively detected in comets by remote sensing or in-situ observations, inputs of theoretical models, and all other organic species expected to be present from the results of experimental simulations. This compilation yields a list of more than a hundred molecules which can be used as a reference for the preparation of experiments developed for the Rosetta and Deep Space 4 cometary missions. We point out that further experiments are necessary to investigate the connections between the solid and gaseous phases of comets, especially studying the photodegradation of high molecular weight compounds which could be present in the nuclei.

The limitations on organic detection in mars-like soils by thermal volatilization-gas chromatography-MS and their implications for the viking results

The failure of Viking Lander thermal volatilization (TV) (without or with thermal degradation)–gas chromatography (GC)–MS experiments to detect organics suggests chemical rather than biological interpretations for the reactivity of the martian soil. Here, we report that TV–GC–MS may be blind to low levels of organics on Mars. A comparison between TV–GC–MS and total organics has been conducted for a variety of Mars analog soils. In the Antarctic Dry Valleys and the Atacama and Libyan Deserts we find 10–90 μg of refractory or graphitic carbon per gram of soil, which would have been undetectable by the Viking TV–GC–MS. In iron-containing soils (jarosites from Rio Tinto and Panoche Valley) and the Mars simulant (palogonite), oxidation of the organic material to carbon dioxide (CO2) by iron oxides and/or their salts drastically attenuates the detection of organics. The release of 50–700 ppm of CO2 by TV–GC–MS in the Viking analysis may indicate that an oxidation of organic material took place. Therefore, the martian surface could have several orders of magnitude more organics than the stated Viking detection limit. Because of the simplicity of sample handling, TV–GC–MS is still considered the standard method for organic detection on future Mars missions. We suggest that the design of future organic instruments for Mars should include other methods to be able to detect extinct and/or extant life.

An intense stratospheric jet on Jupiter.

The Earths equatorial stratosphere shows oscillations in which the east–west winds reverse direction and the temperatures change cyclically with a period of about two years. This phenomenon, called the quasi-biennial oscillation, also affects the dynamics of the mid- and high-latitude stratosphere and weather in the lower atmosphere. Ground-based observations have suggested that similar temperature oscillations (with a 4–5-yr cycle) occur on Jupiter, but these data suffer from poor vertical resolution and Jupiters stratospheric wind velocities have not yet been determined. Here we report maps of temperatures and winds with high spatial resolution, obtained from spacecraft measurements of infrared spectra of Jupiters stratosphere. We find an intense, high-altitude equatorial jet with a speed of ∼140 m s-1, whose spatial structure resembles that of a quasi-quadrennial oscillation. Wave activity in the stratosphere also appears analogous to that occurring on Earth. A strong interaction between Jupiter and its plasma environment produces hot spots in its upper atmosphere and stratosphere near its poles, and the temperature maps define the penetration of the hot spots into the stratosphere.

Organic chemistry in the oceans of Titan

Abstract On Titan, most of the organics present in the atmosphere must condense in the lower stratosphere and be solid near the surface, except methane, ethane, propane, propene and 1-butene which must be liquid and could form oceans containing large fractions of dissolved N2. The solid organics, depending on their density relatively to this liquid, can accumulate at the surface or at the bottom of the oceans. In addition, depending on their solubility in this liquid and on their atmospheric flux down to the surface, they can dissolve partly or totally in the oceans. From this stage, Chemical Evolution on Titan must have followed a way very different from the terrestrial one, involving physical chemical processes in a cryogenic apolar solvent mainly composed of CH4-C2H6-N2, in place of organic chemistry in water. Systematic study of the volumic mass and solubility of organics in such a cryogenic mixture of various compositions, at 94 K, is presented, using thermodynamic modelling. The results suggest that the oceans of Titan could be free of any “icebergs” of organic compounds. These oceans could be very rich in dissolved organics, with relatively high concentrations, in the range 1–10 −6 M. In addition, the concentration of several of the organic solutes should be constant, buffered by a bottom layer of the corresponding compound in the solid phase.

Titan's hypothesized ocean properties: the influence of surface temperature and atmospheric composition uncertainties

Abstract Surface temperature and atmospheric composition inferred from a recent reanalysis of Voyager 1 measurements (E. Lellouch et al. 1989, Icarus 79, 328–349) are used to constrain several properties of Titans ocean. Thermodynamic equilibrium is assumed between atmosphere and ocean. The ocean is considered as a nonideal solution. Depending on atmospheric parameters, the main ocean features can vary drastically. Assuming a 92.5°K surface temperature, a 1.5-atm surface pressure, and an argon free atmosphere, a 1.55% methane mole fraction in the low troposphere implies an ocean composed of more than 90% ethane (and heavier alkanes), about 7.3% methane, and 1.8% nitrogen. At 101°K for the same surface pressure with an atmosphere containing 17% argon and 21.1% methane, the ocean is composed of 5% ethane and higher alkanes, 83.4% methane, 6% nitrogen, and 5.6% argon. Other components are quantitatively negligible. The solubilities and thicknesses of precipitate for several minor oceanic components are derived and presented (Table II). Thicknesses of solid deposits formed at the bottom of the ocean are not very dependent on surface temperature and atmospheric composition. The ocean can be an important CO reservoir, able to hold from 0.25 time (ethane rich ocean at 92.5°K) up to 11 times (methane rich ocean at 101°K) the atmospheric CO inventory.

Organic chemistry in Titan's atmosphere: New data from laboratory simulations at low temperature

Many experiments have already been carried out to simulate organic chemistry on Titan, the largest satellite of Saturn. They can provide fruitful information on the nature of minor organic constituents likely to be present in Titans atmosphere, both in gas and aerosol phases. Indeed, all the organic compounds but one already detected in Titans atmosphere have been identified in simulation experiments. The exception, C4N2, as well as other compounds expected in Titan from theoretical modeling, such as other N-organics, and polyynes, first of all C6H2, have never been detected in experimental simulation thus far. All these compounds are thermally unstable, and the temperature conditions used during the simulation experiments were not appropriate. We have recently started a new program of simulation experiments with temperature conditions close to that of Titans environment. It also uses dedicated analytical techniques and procedures compatible with the analysis of organics only stable at low temperatures, as well solid products of low stability in the presence of O2 and H2O. Spark discharge of N2-CH4 gas mixtures was carried out at low temperature in the range 100-150 K. Products were analysed by FTIR, GC and GC-MS techniques. GC-peaks were identified by their mass spectrum, and, in most cases, by comparison of the retention time and mass spectrum with standard ones. We report here the first detection in Titan simulation experiments of C6H2 and HC5N. Their abundance is a few percent relative to C4H2 and HC3N, respectively. Preliminary data on the solid products indicate an elemental composition corresponding to (H11C11N)n. These results open new prospects in the modeling of Titans haze making.