Context: simulate ionospheric chemistry with a plasma
In the solar system, chemistry occurring in planetary atmosphere is principally driven by dissociation and ionization by solar photons of atmospheric constituents. In particular, the activation of molecular nitrogen, which is the main constituent of the early Earth and Titan’s atmospheres, need VUV photons with wavelengths below 100 nm (Peng et al., 2013). Such a process, which could occur in the ionosphere of the early Earth, is difficult to reproduce in the laboratory. Indeed, VUV radiations below 100 nm are difficult to produce and need specific facility as synchrotron. Deuterium lamp used as source of photons in experimental simulations presents emissions bands in the VUV with maxima at 121.6 nm (Lyman-α) and 160 nm (Trainer et al., 2006). Typical windows used in photochemical experiments are made of LiF or MgF2 which absorb radiation below ~110 nm.
In this thesis, I used an experimental device, named PAMPRE, which is described in detail below, to reproduce complex organic chemistry initiated by VUV solar photons in the upper atmospheres of the early Earth and Titan. This experiment uses an electrical discharge as energy source. The modeling of the used plasma has been done in pure N2 (Alves et al., 2012). The Electrons Energy Distribution Function (EEDF) presents a maximum at 2 eV and a relatively populated tail for electron energy above 4 eV and up to 14 eV. However, to facilitate comparison between the EEDF and the young Sun spectrum, the energy of the electrons has been converted in the corresponding wavelength. Figure 11 presents a comparison between the calculated young Sun spectrum (panel a)) and the converted EEDF in the 89 nm (14 eV) to 250 nm (5 eV) (panel b)).
In situ mass spectrometry
The first diagnosis available on the PAMPRE reactor to study the gaseous phase composition is mass spectrometry. In situ monitoring of the gaseous phase were achieved with a Pfeiffer QME 200 quadrupole mass spectrometer (QMS). The mass spectrometer covers a mass range between 1 and 100 u with a resolution of 100 at m/z 100. Gases are transferred in the spectrometer through a capillary tube (0.8 mm internal diameter) which is long enough to keep the pressure below 10-5 mbar in the spectrometer when the pressure inside the reactor is 0.9 mbar. Only neutral stable species can be detected with this technique. In the spectrometer, neutral species are ionized by electron impact at 70 eV. In this thesis two acquisition methods have been used with the QMS. First one named ‘scan analog’ provides a mass spectrum for a selected mass range, i.e. the intensity of the peak in function of the m/z ratio of this peak. This method has been used for the detection and the identification of species in the reactive gaseous mixture. This diagnosis is useful because of the sensitivity of the mass spectrometer as well as important dynamic, which allow detecting an important number of species formed in the reactor.
A blank spectrum of the isolated mass spectrometer is presented in Figure 14. The signature of residual air is visible on this figure. The residual air signature corresponds to the vacuum limit of the MS pumping, at 3.10-8 mbar. Nitrogen has contribution at m/z 14 (N+), 15 (15N+), 28 (N2+), 29 (15N14N+), 30 (15N2). Oxygen has contribution at m/z 16 (O+) and 32 (O2+). Carbon dioxide has contribution at 12 (C+), 16 (O+), 28 (CO+) and 44 (CO2+). Finally, water has contribution at m/z 1 (H+), 16 (O+), 17 (OH+), 18 (H2O+), 19 (HDO+), 20 (D2O+).
In situ infrared absorption spectroscopy
In complement to the in situ mass spectrometry, I used the in situ infrared spectroscopy. The sensitivity of the IR spectrometer is lower than the sensitivity of the QMS because of the low path length of the IR beam in the reactor and only the major products are detected. However, these products can be detected without ambiguity because they have specific absorption bands in the infrared. Since the detected species has absorption bands without overlapping other species, it is possible to estimate their concentration using the Beer-Lambert law where the absorbance 𝐴(𝜆) at a given wavelength is defined by: 𝐴(𝜆)=𝜀(𝜆)×𝑙×[𝐶] (1).
Where 𝜀(𝜆) is the absorption cross section of the molecule at a given wavelength, l is the path length of the beam through the gas cell and [𝐶] is the concentration of absorbing molecules in the reactor. [𝐶]=𝐴(𝜆)𝜀(𝜆)×𝑙 (2).
However, contrary to mass spectrometry, the long acquisition time (~10 min depending the acquisition parameters) of one spectrum with the IR spectrometer does not allow to realize kinetic monitoring of the gaseous species in the reactor. I used a Thermo Scientific Nicolet 6700 Fourier Transform Infrared Spectrometer (FTIR). A schema of the FTIR setup on the PAMPRE reactor is presented in Figure 15.
Ex situ Gas Chromatography coupled to Mass Spectrometry (GC-MS)
In complement of mass spectrometry and infrared spectroscopy, a third diagnosis used to analyze the gaseous phase is the ex situ Gas Chromatography coupled to Mass Spectrometry (GC-MS). GC-MS is an analytical technique that allows separating and identifying chemical species. Firstly, the species are separated depending of their elution time on the chromatographic column. The column outlet is connected to a mass spectrometer, which allows the identification of each separated species. As for IR spectroscopy, GC-MS permits a better identification than mass spectrometry and with a better sensitivity than IR spectroscopy. However, the configuration used for GC-MS analysis in our experiments does not permit to realize an absolute quantification of the gas trapped.
An external or an internal cryogenic trapping is used to accumulate volatile products in order to analyze the gaseous phase composition by GC-MS. The two cases are described in the corresponding chapters.
Gases trapped are directly injected through a six port gas valve. The GC-MS used is a Thermo Scientific trace GC-ultra with a Thermo Scientific ITQ 900 mass spectrometer. The MS is composed of an ion trap using a 70 eV ionization system. For the gas separation, the column is a MXT-QPLOT (Restek, 30 m long, 0.25 mm internal diameter and 10 μm stationary phase thickness). The column temperature is set with an isothermal initial plate at 30 °C during 5 min, then the temperature is increased with a gradient of 5 °C/min up to 190 °C and kept at this final temperature for 5 min. Helium is used as the carrier gas (>99.9995 purity) at a constant 1mL/min flow rate. A blank is done before each sample analysis.
Self-bias voltage of the polarized electrode: in situ detection of tholins formation
This diagnosis is used to detect appearance of dust in the plasma discharge. A self-bias voltage (Vdc) appears at the driven electrode of a RF CCP discharge if both electrodes differ in size when a blocking capacitor is inserted between the RF generator and the electrode (Bogaerts et al., 2002). In our discharge configuration, the polarized electrode is disk-shaped, when the grounded electrode one is the cylindrical confining grid. As a matchbox containing a tunable capacitor is placed between the RF generator and the electrodes, and the surfaces or the electrodes are different, a Vdc exists in the used discharge configuration. The self-bias voltage Vdc is negative in our case. The variation is related to changes in the electrons density and temperature. It has already been shown that the appearance of dust in the discharge induces Vdc perturbations (Praburam and Goree, 1996; Samsonov and Goree, 1999). These perturbations are explained by the fact that, during their growth, the solid particles attach some electrons present in the plasma. It results in a decrease of the electrons density and a sharp variation of Vdc (Alcouffe et al., 2010; Cavarroc et al., 2006; Mikikian and Boufendi, 2004). Vdc is measured using a 4-channels numerical oscilloscope (Tektronix TDS2004B).
An elemental composition analysis of the tholins samples produced in PAMPRE is achieved to determine the C, N, H and O molar percentages of these tholins. This technique gives quantitative information about the constitution of tholins in these four elements. The analysis is achieved using a Thermo Scientific Flash 2000 Series CHNS/Oxygen Automatic Elemental Analyzer. In our organic materials, carbon, nitrogen and hydrogen were measured by flash combustion under a helium flow. The sample (1-2 mg) is weighted in Tin capsules (a capsule of silver for O analysis) and placed in the Thermo Scientific MAS200R auto sampler at a preset time, and then dropped into an oxidation or reduction reactor kept at a temperature of 900-1000 °C. The gases released, i.e. CO2, H2O and N2, are separated on a chromatographic column and detected by a highly sensitive thermal conductivity detector (TCD). Oxygen determination is achieved through the same protocol but with a specific oxygen pyrolysis reactor. CO is then measured using a TCD. Elemental analysis for C, N, H and O are realized on a same PAMPRE sample divided in two subsamples: one for C, N and H analysis and one for O analysis.
Infrared analyses of tholins by ATR
An infrared analyses of solid samples produced with PAMPRE is achieved by Attenuated Total reflectance (ATR). This technique is used to determine the absorbance spectrum of solid material and to identify its main absorption bands. However, this technique does not give a quantitative analysis because of the dependence of the spectrum to the amount of solid deposited on the crystal surface and to the pressure applied on the solid.
A sample is disposed on the surface of a prism with a high reflection index (ATR crystal). The infrared beam entering in the prism is totally reflected inside it and an evanescent wave can escape the crystal though the upper face and passed then through the sample. The wave is then attenuated in the absorbance area of the sample depending on the chemical functions present in the sample. The resulting signal is measured by a Deuterium TriGlycine Sulfate (DTGS) detector in the 1200-4500 cm-1 range. Each spectrum is a co-addition of 500 spectra recorded with a resolution of 4 cm-1.
The Acquabella experiment
The Acquabella experimental setup is used in this thesis to study the chemical evolution of tholins produced in the upper atmosphere of Titan during their sedimentation through this atmosphere. Particularly, this setup is used to simulate processes, which could occur in the lower atmosphere where the conditions of pressure and temperature could allow a condensation of species on the surface of aerosols. The aerosols coated with condensed species can then evolve when exposed to the solar photons flux.
Cryogenic trapping of the gaseous products for an in situ analysis
In order to concentrate the gas species product in PAMPRE during the experiment, I used an internal cryogenic trapping following a method described in section III.2. After 4 hours of trapping at 173 K, the plasma is switched off; the reactor is pumped and then isolated. The plasma box is warmed up to room temperature. The pressure in the reactor is measured for different temperature during this warm up. The evolution of the pressure in the reactor is presented in Figure 22. During the increase of the plasma box temperature, there is an increase of the pressure in the reactor. The pressure increases slowly in a first time to reach 0.1 mbar at 230 K and then the pressure increases more importantly to reach finally 2 mbar at 294 K after 12 hours of warming and, which corresponds to the room temperature.
The case of ammonia: NH3 and hydrogen cyanide: HCN
Ammonia is of great interest for prebiotic chemistry. The signature of ammonia in mass spectrometry is principally at m/z = 17 with the ion NH3+. This is one of the principal peaks of the mass spectrum at room temperature in Figure 24. However, water has also a contribution at this mass due to OH+ fragment. Here, I used the infrared spectroscopy to remove this ambiguity.
Figure 25 presents an infrared spectrum recorded at 300 K. This is a spectra center on the 700-1200 cm-1 range where species with a strong absorption is visible. This species is identified as NH3 according to the NIST database and the Hitran database. The presence of a doublet at 930 cm-1 and 967 cm-1 is characteristic of NH3 and allows identification unambiguously. This is possible to quantify the concentration of ammonia in the reactor using equation (2).
Reactive species consumption in the plasma
The consumption of CO2 and CH4 is monitored using in situ mass spectrometry. The carbon dioxide consumption is measured using a time-tracking of CO2+ at m/z 44. In order to prevent any overlapping of CH4 ion signal with O+ signal at m/z 16; the methane consumption is measured using a time tracking of CH3+ (m/z 15) after a correction of the contribution of 15N+ based on laboratory calibration.
Figure 29 presents the evolution of the CO2 mixing ratio when the plasma is switched on for three different mixing ratios: 1 %, 5 % and 10 %. The initial concentration of CO2 is known and it is possible to calibrate the m/z 44 signal intensity in order to obtain the evolution of the in situ CO2 concentration. The concentration obtained has been multiply for 1 % and 5 % of CO2 by a factor 10 and a factor 2 respectively. The CO2 mixing ratio decreases when the plasma is switched on and reaches a steady-state after a few minutes of transient regime.
Table of contents :
Chapter I The atmosphere of the early Earth and the origin of the organic matter
I.2 The origin of the organic matter on the early Earth
I.2.1 The exogenous sources
I.2.2 The endogenous sources
I.2.2.a The hydrothermal vents
I.2.2.b The atmosphere
I.3 The early Earth atmosphere
I.3.1 Early atmosphere formation and evolution
I.3.2 Constraints on the atmospheric composition of the early Earth
I.3.2.a The context of the young Sun
I.3.2.b Nitrogen N2
I.3.2.c Water H2O
I.3.2.d Carbon dioxide CO2
I.3.2.e Other gaseous compounds
Chapter II Materials and Methods
II.1 The PAMPRE experimental setup
II.1.1 Context: simulate ionospheric chemistry with a plasma
II.1.2 The experimental device
II.1.3 Gas phase analysis
II.1.3.a In situ mass spectrometry
II.1.3.b In situ infrared absorption spectroscopy
II.1.3.c Ex situ Gas Chromatography coupled to Mass Spectrometry (GC-MS)
II.1.4 Solid phase analysis
II.1.4.a Self-bias voltage of the polarized electrode: in situ detection of tholins formation
II.1.4.b Elemental analysis
II.1.4.c Infrared analyses of tholins by ATR
II.2 The Acquabella experiment
II.2.1 The experimental device
II.2.2 Infrared absorption spectroscopy
II.2.3 UV-visible absorption spectroscopy
Chapter III CO2: An efficient source of carbon for an atmospheric organic growth
III.2 Experimental methods and protocols
III.3.1 Analysis of the gaseous phase composition
III.3.1.a Reactive species consumption in the plasma: the first step for the product formation
III.3.1.b Global in situ analysis of the gaseous phase composition by mass spectrometry
III.3.1.c Cryogenic trapping of the gaseous products for an in situ analysis
III.3.1.d Cryogenic trapping of the gaseous products for an ex situ analysis by GC-MS .
III.3.2 Analysis of the solid phase produced in the reactive medium
III.3.3 Effect of the CO2 initial amount
III.4.1 Impact of high altitudes water vapor formation on the early Earth
III.4.1.a Water content in the early Earth water atmosphere
III.4.1.b Formation of high altitudes clouds
III.4.1.c Effect of the water atmospheric profile
III.4.2 Solid organic aerosols formations
Chapter IV CH4 influence on the early Earth atmospheric chemistry
IV.2 Experimental methods and protocols
IV.3.1 Analysis of the gaseous phase composition
IV.3.1.a Reactive species consumption in the plasma
IV.3.1.b In situ analysis of the gaseous phase composition by mass spectrometry
IV.3.1.c Cryogenic trapping of the gaseous products for an in-situ analysis
IV.3.1.d Influence of the CO2 initial amount on the gaseous phase
IV.3.2 Analysis of the solid phase produced in the reactive medium
IV.4 Importance of methane for the formation of organic compounds in the atmosphere of the early Earth
Chapter V Effect of CO on the Titan’s atmospheric reactivity
V.2 Experimental method and protocols
V.2.1 In situ mass spectrometry
V.2.2 Ex situ GC-MS analysis: cold trapping principle
V.3.1 Effect of CO on the kinetics
V.3.2 Effects of CO on the steady-state
V.3.3 Oxygen incorporation
V.3.3.a Elemental analysis of tholins
V.3.3.b Analysis of the gaseous phase
V.4 Titan’s atmospheric reactivity in presence of CO: a natural example of the early Earth atmospheric reactivity?
Chapter VI Chemical evolution of Titan’s tholins
VI.2 Simulation of aging processes in the thermosphere
VI.2.1 Experimental methods and protocols
VI.2.2.a General observations of the infrared signature evolution of Titan’s aerosols analogues
VI.2.2.b Wavelength dependence of the irradiation effect on the tholins
VI.2.2.c Expected effect on aerosols in Titan’s atmosphere
VI.3 Simulation of tropospheric/stratospheric aging processes
VI.3.1 Experimental methods and protocols
VI.3.2.a C2H2 reactivity
VI.3.2.b CH3CN reactivity
VI.3.3 Importance for the atmosphere of Titan