Guy Cernogora

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.

Chemical characterization of Titan's tholins: solubility, morphology and molecular structure revisited

In this work Titans atmospheric chemistry is simulated using a capacitively coupled plasma radio frequency discharge in a N(2)-CH(4) stationnary flux. Samples of Titans tholins are produced in gaseous mixtures containing either 2 or 10% methane before the plasma discharge, covering the methane concentration range measured in Titans atmosphere. We study their solubility and associated morphology, their infrared spectroscopy signature and the mass distribution of the soluble fraction by mass spectrometry. An important result is to highlight that the previous Titans tholin solubility studies are inappropriate to fully characterize such a heterogeneous organic matter and we develop a new protocol to evaluate quantitatively tholins solubility. We find that tholins contain up to 35% in mass of molecules soluble in methanol, attached to a hardly insoluble fraction. Methanol is then chosen as a discriminating solvent to characterize the differences between soluble and insoluble species constituting the bulk tholins. No significant morphological change of shape or surface feature is derived from scanning electron microscopy after the extraction of the soluble fraction. This observation suggests a solid structure despite an important porosity of the grains. Infrared spectroscopy is recorded for both fractions. The IR spectra of the bulk, soluble, and insoluble tholins fractions are found to be very similar and reveal identical chemical signatures of nitrogen bearing functions and aliphatic groups. This result confirms that the chemical information collected when analyzing only the soluble fraction provides a valuable insight representative of the bulk material. The soluble fraction is ionized with an atmospheric pressure photoionization source and analyzed by a hybrid mass spectrometer. The congested mass spectra with one peak at every mass unit between 50 and 800 u confirm that the soluble fraction contains a complex mixture of organic molecules. The broad distribution, however, exhibits a regular pattern of mass clusters. Tandem collision induced dissociation analysis is performed in the negative ion mode to retrieve structural information. It reveals that (i) the molecules are ended by methyl, amine and cyanide groups, (ii) a 27 u neutral moiety (most probably HCN) is often released in the fragmentation of tholin anions, and (iii) an ubiquitous ionic fragment at m/z 66 is found in all tandem spectra. A tentative structure is proposed for this negative ion.

Formation of amino acids and nucleotide bases in a Titan atmosphere simulation experiment.

The discovery of large (>100 u) molecules in Titans upper atmosphere has heightened astrobiological interest in this unique satellite. In particular, complex organic aerosols produced in atmospheres containing C, N, O, and H, like that of Titan, could be a source of prebiotic molecules. In this work, aerosols produced in a Titan atmosphere simulation experiment with enhanced CO (N(2)/CH(4)/CO gas mixtures of 96.2%/2.0%/1.8% and 93.2%/5.0%/1.8%) were found to contain 18 molecules with molecular formulae that correspond to biological amino acids and nucleotide bases. Very high-resolution mass spectrometry of isotopically labeled samples confirmed that C(4)H(5)N(3)O, C(4)H(4)N(2)O(2), C(5)H(6)N(2)O(2), C(5)H(5)N(5), and C(6)H(9)N(3)O(2) are produced by chemistry in the simulation chamber. Gas chromatography-mass spectrometry (GC-MS) analyses of the non-isotopic samples confirmed the presence of cytosine (C(4)H(5)N(3)O), uracil (C(5)H(4)N(2)O(2)), thymine (C(5)H(6)N(2)O(2)), guanine (C(5)H(5)N(5)O), glycine (C(2)H(5)NO(2)), and alanine (C(3)H(7)NO(2)). Adenine (C(5)H(5)N(5)) was detected by GC-MS in isotopically labeled samples. The remaining prebiotic molecules were detected in unlabeled samples only and may have been affected by contamination in the chamber. These results demonstrate that prebiotic molecules can be formed by the high-energy chemistry similar to that which occurs in planetary upper atmospheres and therefore identifies a new source of prebiotic material, potentially increasing the range of planets where life could begin.

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

Abstract SPICAM Light is a small UV-IR instrument selected for Mars Express to recover most of the science that was lost with the demise of Mars 96, where the SPICAM set of sensors was dedicated to the study of the atmosphere of Mars (Spectroscopy for the investigation of the characteristics of the atmosphere of mars). The new configuration of SPICAM Light includes optical sensors and an electronics block. A UV spectrometer (118–320 nm, resolution 0.8 nm) is dedicated to Nadir viewing, limb viewing and vertical profiling by stellar occultation (3.8 kg). It addresses key issues about ozone, its coupling with H2O, aerosols, atmospheric vertical temperature structure and ionospheric studies. An IR spectrometer (1.2– 4.8 μm , resolution 0.4–1 nm) is dedicated to vertical profiling during solar occultation of H2O, CO2, CO, aerosols and exploration of carbon compounds (3.5 kg). A nadir looking sensor for H2O abundances (1.0– 1.7 μm , resolution 0.8 nm) is recently included in the package (0.8 kg). A simple data processing unit (DPU, 0.9 kg) provides the interface of these sensors with the spacecraft. In nadir orientation, SPICAM UV is essentially an ozone detector, measuring the strongest O3 absorption band at 250 nm in the spectrum of the solar light scattered back from the ground. In the stellar occultation mode the UV Sensor will measure the vertical profiles of CO2, temperature, O3, clouds and aerosols. The density/temperature profiles obtained with SPICAM Light will constrain and aid in the development of the meteorological and dynamical atmospheric models, from the surface to 160 km in the atmosphere. This is essential for future missions that will rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow study of the ionosphere through the emissions of CO, CO+, and CO2+, and its direct interaction with the solar wind. Also, 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 SPICAM Light IR sensor is inherited from the IR solar part of the SPICAM solar occultation instrument of Mars 96. Its main scientific objective is the global mapping of the vertical structure of H2O, CO2, CO, HDO, aerosols, atmospheric density, and temperature by the solar occultation. The wide spectral range of the IR spectrometer and its high spectral resolution allow an exploratory investigation addressing fundamental question of the possible presence of carbon compounds in the Martian atmosphere. Because of severe mass constraints this channel is still optional. An additional nadir near IR channel that employs a pioneering technology acousto-optical tuneable filter (AOTF) is dedicated to the measurement of water vapour column abundance in the IR simultaneously with ozone measured in the UV. It will be done at much lower telemetry budget compared to the other instrument of the mission, planetary fourier spectrometer (PFS).

Study of oxygen atom recombination on a Pyrex surface at different wall temperatures by means of time-resolved actinometry in a double pulse discharge technique

The surface recombination probability of oxygen atoms as a function of wall temperature is studied by using a double pulse discharge technique. The main discharge pulse dissociates molecular oxygen and the second pulse, shorter than the main one, excites atoms during the stationary afterglow. The recombination probability is determined from the atomic oxygen density decay during the stationary afterglow of the main pulse (MP). The oxygen atoms are detected by time-resolved optical emission spectroscopy. In order to correlate the oxygen emission lines with the oxygen atom density, argon is used as an actinometer. To scan the whole afterglow of the main discharge pulse, the delay of the probe pulse is uniformly increased in every period following the MP. The evolution of the relative O atom density is deduced from the O emission lines at 777 and 844?nm and from the Ar actinometry line at 750?nm. The wall recombination probability ? on a Pyrex surface ranges from 4.0 ? 10?4 to 1.6 ? 10?2 for wall temperatures from 77 to 460?K.

Atomic oxygen recombination on fused silica: modelling and comparison to low-temperature experiments (300 K)*

This work is devoted to the study of atomic oxygen recombination on a glass surface, mainly in connection with atomic sources development. In this paper we present a non-stationary model for atomic oxygen recombination on a fused silica surface. Kinetics equations for oxygen atoms, taking into account heterogeneous reactions between gaseous atoms and the surface (Eley-Rideal mechanisms), as well as homogeneous processes involving surface migration of adsorbed species (Langmuir-Hinshelwood mechanisms), are solved. Surface reaction coefficients are calculated, and the choice of numerical values for surface parameters is discussed. The solution to the equations is compared to our previous experiments concerning the influence of the surface state on atomic recombination. An estimation is made of surface reaction coefficient values.

Methane decomposition and active nitrogen in a N2-CH4 glow discharge at low pressures

Mass spectrometry and optical emission spectroscopy are used in a N2-xCH4 glow discharge with x = 0.5-2%, at low pressures (1-2 Torr) and small flow rates (6 sccm), in order to determine the CH4 and H2 absolute concentrations and the N2(B 3g) and N2(C 3u) relative concentrations. A kinetic model is developed based on the steady-state solutions to the homogeneous electron Boltzmann equation coupled to a system of rate balance equations for the most populated neutral and ionic species produced, either from active nitrogen and CH4 dissociation or as a result of reactions between radicals from N2 and CH4. It is observed that CH4 is very efficiently decomposed through a sequence of reactions in which at the end HCN and H2 appear as the most abundant products in the discharge. A brown deposition on the tube walls has been detected which is attributed to HCN, in agreement with other investigations of Titans atmosphere, since this species is poorly destroyed in volume. The accordance between theory and experiment is very satisfactory allowing an insight to be obtained into the basic elementary mechanisms in these discharges.

Capacitively coupled plasma used to simulate Titan's atmospheric chemistry

A complex chemistry in Titan’s atmosphere leads to the formation of organic solid aerosols. We use a radio-frequency (RF) capacitively coupled plasma discharge produced in different N2–CH4 mixtures (from 0% to 10% of CH4) to simulate this chemistry. The work presented here was devoted to the study of the plasma discharge. In our experiment, the electron density is measured by the resonant cavity method and is about 10 15 m −3 in pure N2 plasma at 30 W excitation RF power. It decreases by a factor of 2 as soon as CH4 is present in the discharge, even for a proportion as small as 2% of CH4. An optical emission spectroscopy diagnostic is installed on the experiment to study the evolution of the N2 bands and to perform actinometry measurements using Ar lines. This diagnostic allowed us to measure variations in the electron temperature and to show that a decrease in the density of the electrons can be compensated by an increase in their energy. We have also used an experimental setup where the plasma is tuned in a pulsed mode, in order to study the formation of dust particles. We observed variations in the self-bias voltage, the RF injected power and the intensities of the nitrogen bands, which indicated that dust particles were formed. The characteristic dust formation time varied, depending on the experimental conditions, from 4 to 110 s. It was faster for higher pressures and for smaller proportions of CH4 in the gas mixture. (Some figures in this article are in colour only in the electronic version)

Atomic oxygen recombination on fused silica : experimental evidence of the surface state influence

The time post discharge of a low-pressure pulsed dc discharge in pure oxygen is used to investigate the atomic oxygen recombination on fused silica surface. With the intention of studying this recombination for different surface states, we perform before each pulsed experiment a wall treatment by means of dc discharges under different experimental conditions. Then, we monitor the decrease of the atomic oxygen in time post discharge by time resolved VUV resonant absorption spectroscopy. We have shown that it is possible to obtain for a given wall treatment, a pulse after pulse variation of this decrease. We have attributed this variation to a filling of the chemisorption sites. Finally, we have determined the surface reaction probability of atomic oxygen on fused silica surface and we have compared it to published values.

Spectroscopy study and modelling of an afterglow created by a low-pressure pulsed discharge in N2-CH4

Time-resolved emission spectroscopy is used to investigate the relaxation of N2(B 3Πg), N2(C 3Πu) and CN(B 2Σ) states in the time afterglow of a low-pressure N2-CH4 pulsed discharge, with time duration of 1 ms and in the range [CH4]/[N2] = 0-2%. The decays in the relative measured concentrations in the afterglow are interpreted by modelling the relaxation of a set of time-varying kinetic master equations for the various species produced in the discharge, with conditions at the beginning of the afterglow calculated from a time-dependent kinetic model for the pulsed discharge. It is observed that the N2(B 3Πg) state is populated in the afterglow mainly via the reaction N2(A 3Σu+) + N2(X 1Σg+, 5≤v≤14)→N2(B 3Πg) + N2(X 1Σg+, v = 0), since the pulse duration is large enough to populate the N2(X 1Σg+, v) levels at its end and, to a smaller extent, also by pooling of N2(A 3Σu+). The N2(C 3Πu) state is populated by pooling of N2(A 3Σu+) only, whereas the CN(B 2Σ) state is created through reactions involving either N2(A 3Σu+) states or N2(X 1Σg+, v) levels in collisions with CN(X 2Σ+) molecules. The agreement between measured and calculated concentrations of N2(B 3Πg) and N2(C 3Πu) states is very good in pure N2 and it may be considered satisfactory in the case of N2-CH4 mixtures, and for the CN(B 2Σ) state the agreement between theory and experiment is also reasonably good.