The atmosphere of the early Earth and the origin of the organic matter

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The early Earth atmosphere

All experimental and modeling studies on the reactivity of the early atmosphere, focused on the formation of organic compounds, are pointed out the importance of the atmospheric composition on the efficient of this reactivity. Constraining the composition of the atmosphere of the early Earth during the Hadean and the Archean is an important subject of investigation in planetology. If different points are still subject to debate, a scenario has been established for this formation process and constrains the Earth atmospheric composition at this period.

Early atmosphere formation and evolution

Two origins are considered for the formation of early planetary atmospheres: primary and secondary. Primary atmospheres have a Sun-like composition and are mainly made of H2 and He. This is typically the case of the atmospheres of giant planets in the Solar System. Secondary atmospheres are coming from degassing processes of volatiles from planets interiors (Zahnle et al., 2010). A schematic representation of the early stages of the Earth formation is given in Figure 8 originating from Gargaud et al., 2012a.
After the end of the Earth’s accretion, the atmosphere was probably originating from the solar nebula and mainly composed of H2 and He. This primary atmosphere has been lost in space though different mechanisms as hydrodynamic escape and impact erosion during the Late Veneer. The latter event has contributed to enrich the Earth in volatiles (Albarede, 2009; Lammer et al., 2008) and contributed to the C-H-O-N-S Earth’s content (Marty, 2012). This primary atmosphere has been replaced by a secondary atmosphere from degassing processes. The composition of the gases emitted is controlled by the oxidation state of the magma (Hirschmann, 2012). Today, the Earth’s mantle is oxidized and its oxidation state has not changed since 3.9 Ga (Delano, 2001) and probably 4.4 Ga (Trail, 2011). The resulting atmosphere had a similar composition to actual composition of volcanic emissions, which are manly made of CO2 and H2O and in minor proportion N2 and SO2. Because of the important surface temperature at this time of the Earth history, volatiles emitted and notably H2O and CO2 were in gaseous phase in the atmosphere. This involve that the atmosphere contained at least the equivalent of the ocean water content corresponding to a partial pressure of H2O of 300 bars and contained the CO2 now trapped as carbonates and corresponding to a minimum partial pressure of 40 bars (Gargaud et al., 2012a). After ~100 Ma, the surface temperature decreased sufficiently to allow the water condensation and the formation of oceans. The measurement of the isotopic composition of Jack Hills zircons dated back to 4.4 Ga has highlighted the presence of a hydrosphere during their formation involving that water was condensed and the oceans formed 4.4 Ga ago. After the condensation of oceans, CO2 remained the major constituent with at least 40 bars providing an important greenhouse effect resulting in a surface temperature higher than 200 °C. The formation of oceans allowed starting the CO2 sequestration process. CO2 dissolved in water could react and precipitate as calcium carbonate CaCO3. Once subduction started on Earth, carbonated could be recycled into the mantle providing a long-term sequestration of CO2 and permitting a decrease of the greenhouse effect and so of the surface temperature (Nakamura and Kato, 2004).
These different steps of the early formation of the Earth’s atmosphere and its evolution have led to the atmospheric composition existing at the end of the Hadean and during the Archean until the GOE.

Constraints on the atmospheric composition of the early Earth

It is now widely accepted that the mechanism of the formation of the Earth atmosphere has led to a primitive atmosphere mainly made of N2, CO2 and H2O. However, the exact proportions of these species and the proportion of trace gases in the atmosphere as well as 21 their evolution with time are not clearly determined. This is particularly true for the Hadean, for which no rock has been preserved (Bowring and Williams, 1999). Nevertheless, different constraints are given from geological indices for the Archean (details are given for each species below) and different scenarios are proposed in climate studies. Moreover, these constraints must be replaced in the context of the environment of the Earth 4.5 Ga ago, notably in the context of a young Sun, which have a different spectrum than today as developed in the newt section.

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).

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Elemental analysis

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.

Table of contents :

Chapter I The atmosphere of the early Earth and the origin of the organic matter
I.1 Context
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
I.4 Conclusion
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.1 Introduction
III.2 Experimental methods and protocols
III.3 Results
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 Discussions
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
III.5 Conclusion
Chapter IV CH4 influence on the early Earth atmospheric chemistry
IV.1 Introduction
IV.2 Experimental methods and protocols
IV.3 Results
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
IV.5 Conclusion
Chapter V Effect of CO on the Titan’s atmospheric reactivity
V.1 Introduction
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 Results
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?
V.5 Conclusion
Chapter VI Chemical evolution of Titan’s tholins
VI.1 Introduction
VI.2 Simulation of aging processes in the thermosphere
VI.2.1 Experimental methods and protocols
VI.2.2 Results
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 Results
VI.3.2.a C2H2 reactivity
VI.3.2.b CH3CN reactivity
VI.3.3 Importance for the atmosphere of Titan
VI.4 Conclusion
General conclusion
Table of figures and tables


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