Thermal characterization of Titan's tholins by simultaneous TG–MS, DTA, DSC analysis

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Thermal characterization of Titan's tholins by simultaneous TG-MS, DTA, DSC analysis Delphine Nna-Mvondo, José L. de la Fuente, Marta Ruiz-Bermejo, Bishun Khare, Christopher P. McKay

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S0032-0633(13)00166-9 http://dx.doi.org/10.1016/j.pss.2013.06.025 PSS3562

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Received date: 11 February 2013 Revised date: 21 June 2013 Accepted date: 23 June 2013 Cite this article as: Delphine Nna-Mvondo, José L. de la Fuente, Marta RuizBermejo, Bishun Khare, Christopher P. McKay, Thermal characterization of Titan's tholins by simultaneous TG-MS, DTA, DSC analysis, Planetary and Space Science, http://dx.doi.org/10.1016/j.pss.2013.06.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Thermal characterization of Titan’s tholins by simultaneous TG-MS, DTA, DSC analysis

Delphine Nna-Mvondo a, e, *, José L. de la Fuente b, Marta Ruiz-Bermejo a, Bishun Khare c, d

a

, Christopher P. McKay c

Centro de Astrobiologia (CSIC-INTA), Ctra. de Ajalvir km 4, 28850 Torrejon de Ardoz, Madrid, Spain

b

Instituto Nacional de Técnica Aeroespacial “Esteban Terradas” (INTA), Ctra. de Ajalvir km 4, 28850 Torrejon de Ardoz, Madrid, Spain

c

Space Science Division, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA d

SETI Institute, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA

e

Present address: Laboratoire de Planétologie et Géodynamique, CNRS, UMR6112,

Université de Nantes, 2 rue de la Houssinière, BP92208, 44322 Nantes cedex 3, France.

* Corresponding Author: Laboratoire de Planétologie et Géodynamique, CNRS, UMR6112, Université de Nantes, 2 rue de la Houssinière, BP92208, 44322 Nantes cedex 3, France. Tel.: +33 276645153; Fax: +33 251125268 Delphine E-mail address: [email protected]

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Abstract. Three samples of Titan’s tholins synthesized in laboratory under simulated Titan’s conditions and presenting different degrees of exposure to ambient atmosphere have been used to study in detail their thermal behaviour using thermogravimetry coupled with a mass spectrometer (TG-MS), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The degradation of Titan’s tholins under inert atmosphere follows a three-step consecutive decomposition: a drying stage (>150 ºC) where moisture is desorbed, this stage indicated the high hydrophilicity of the tholins; a second stage, the main pyrolysis stage (150ºC – 575ºC) where endothermic decomposition begins releasing mainly ammonia, HCN, acetonitrile, and methane over a broad temperature range. Few other hydrocarbon fragments such as ethylene and propane are released but no cyclic molecules, aliphatic or aromatic, are observed. The last stage (> 575ºC) is the carbonization of the material leading to a non-crystalline graphitic residue. The thermal degradation under oxygen atmosphere shows the same stages as in argon, with a shift of the thermogravimetric peaks towards lower temperatures indicating a lower thermal stability. The last stage in this case is an oxidative combustion of the char residue. This research concludes that even if Titan tholins, subjected to air contamination for few minutes to several years (varying with the storage conditions) transform to produce different C/N and C/O ratios and thermal stabilities, they undergo the same thermal degradation phases and products. This suggests that the three studied tholins have a similar main chemical structure which does not alter by the air exposure. We discuss on the possible nature of this structure.

Keywords. Tholin, Titan, Thermogravimetry, Mass spectrometry, Chemical structure, Thermal degradation

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1. Introduction

On Titan Saturn’s moon, the complex chemistry occurring in the CH4/N2 atmosphere under UV radiation, high charged particles irradiation from galactic cosmic rays and possibly electric discharges leads to a large variety of organic compounds ranging from simple hydrocarbons and nitriles to high molecular weight molecules forming Titan’s aerosols. Cassini Plasma Spectrometer (CAPS) results support for the possible presence of high molecular weight species in the ionosphere, such as Polycyclic Aromatic Hydrocarbons (PAHs), large complex organics and nitrile compounds from 100 up to 350 Da and very heavy negatively charged ions from 20 to 8000 Da observed at ~1000 km (Waite et al., 2007). In the laboratory, experimental simulations of Titan’s atmospheric conditions are performed to synthesize complex organics, named Titan tholins (Khare et al., 1984b; Sagan and Khare, 1982) which present similar physico-chemical properties as Titan’s aerosols. Particularly, optical properties in the visible range are fairly comparable for laboratory Titan’s tholins and Titan’s aerosols, or even for the component of the reddish surfaces of many icy satellites and small bodies (Khare et al., 2001; McKay et al., 1989; Toon et al., 1992). As a result, Titan tholins are commonly defined as laboratory analogues of Titan’s aerosols. Experimental studies of Titan’s tholins show that they are complex combinations of C–N–H molecules, ranging from low molecular mass molecules up to macromolecules readily soluble in polar solvents and very poorly soluble in nonpolar solvents (Carrasco et al., 2009; Coll et al., 2001; Coll and Raulin, 1998; Kawai et al., 2013; McKay, 1996). Laboratory measurements of molecular weights of Titan’s tholins range from 100 to 800 Da (Imanaka et al., 2004; McDonald et al., 1994; Sarker et al., 2003). Over the past thirty years, considerable efforts have been made to try

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to elucidate the chemical structure and composition of Titan´s tholins. However, so far they are still very poorly chemically identified and further laboratory investigation is clearly needed.

Several experimental works have demonstrated that after conventional flash pyrolysis (Pyr-GC-MS, Pyr-MS) laboratory Titan’s tholins produced a great diversity of organic pyrolysates such as lineal and branched hydrocarbons (saturated and unsaturated), aromatic hydrocarbons (i.e. benzene, PAHs), nitriles and N-heterocycles (i.e. pyrroles, pyridine) (Coll et al., 1998; Coll et al., 1999; Coll et al., 2013; Ehrenfreund et al., 1995; Khare et al., 1984b; McGuigan et al., 2006; Pietrogrande et al., 2001; Szopa et al., 2006). In situ flash pyrolysis analysis of Titan’s atmospheric aerosols has also been performed with the ACP/GC-MS instrument (Aerosol Collector Pyrolyser coupled to a Gas Chromatograph-Mass Spectrometry) onboard the Huygens probe of the CassiniHuygens mission (Israel et al., 2005). Flash pyrolysis is a powerful technique for the study of complex organic molecules on a microscale. However, it is important to note that flash pyrolysis produces intense heating at one selected temperature (normally temperatures between 450-650ºC) over the sample, in a short period of time (high heating rates) in the absence of oxygen. In addition, the chromatographic analysis coupled to the flash pyrolysis (most common used technique) of such multicomponent material as tholins yield complex chromatograms with many peaks, many of them overlapping (doublets or triplets peaks formed by two or three components). This makes very difficult to extract the analytical information and consequently to identify precisely the eluted molecules. Alternative thermal degradation techniques such as thermogravimetry coupled with a mass spectrometer (TG-MS) may be more appropriate than the flash pyrolysis to characterize the chemical structure of tholins. TG-MS relies on slow heating profiles

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from room temperature to about 1000°C. It allows to monitor the release of ion fragments before the polymer completely decomposes. This is important because the organic additives to a polymer-type compound are generally all detected at temperatures below the decomposition temperature of the polymer.

Thermogravimetry (TG) is one of the most commonly used thermal analysis techniques for the characterization of both inorganic and organic materials, including complex prebiotic mixtures such as HCN polymers and hydrophobic tholins (Cataldo et al., 2010; de la Fuente et al., 2011; de la Fuente et al., 2012). This technique provides quantitative results regarding the weight loss of a sample as a function of temperature or time and gives information about the thermal properties. Derivative thermogravimetry (DTG) can be used to investigate the differences between thermograms. However, information about the molecules responsible for the reduction in mass cannot be obtained through this system alone. In order to determinate the compounds evolving from the decomposing materials, some type of evolved gas analysis (EGA) must be used. One of them is the TG coupled with a mass spectrometer (TG-MS). This technique provides several advantages, which include speed and reduced sample handling. This technique gives a time-dependent record of the composition of the gas phase. From TG-MS, curves can be constructed for selected species. All these characteristics make TG-MS very different from that applied using flash pyrolysis and consequently the products and/or reactions that take place in each thermal process may be distinct.

TG-MS technique has not yet been used to chemically characterized Titan's tholins but is increasingly used to characterize polymers. It can provide much more reliable data and complementary information on the chemical structure and thermal

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behaviour than the common flash pyrolisis used until now to characterize Titan’s aerosol analogues. The present work aims to provide new data on the thermal evolution of laboratory Titan’s tholins and the compounds released from the slowly degraded material as a function of temperature and time using TG-MS.

2. Experimental method

2.1. Titan’s tholins synthesis

Three Titan’s tholins were used for this study. The first sample (tholin 1) was produced from December 1993 to March 1994 at Cornell University (Ithaca, New York) subjecting a gas mixture of 10% CH4 in N2 (Linde Division, research grade, 99.99% purity), at continuous flow and ambient temperature (293 K), to a radio frequency (RF) plasma discharge. The gas pressure during the discharge was 0.25 Torr (0.3 mbar) and the plasma was maintained primarily by inductive coupling of RF power at 100 W and 13.56 MHz (Khare et al., 1987). In three months, 21 g of tholin was synthesized and deposited on the wall of the quartz cylindrical reaction chamber. It was collected by scraping from the wall of the reaction chamber and kept in sealed glass vials. In order to compare those aged Titan’s tholins with newly formed samples, two other Titan’s tholins (tholin 2 and tholin 3 samples) were produced during October 2011 at NASA Ames Research Center (Moffett Field, California) from a N2/CH4 gas mixture by cold plasma irradiation with a RF power source (Advanced Energy, RFX 600A, 13.56 MHz frequency) inside a stainless steel simulation chamber, under a continuous gas flow, as described in Imanaka et al. (2004) and Sekine et al. (2008). Such experimental set-up simulates chemical reactions occurring in the upper atmosphere of Titan (> 300 km)

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triggered by the dissociation of methane and nitrogen by the charged particle irradiation from the Saturnian magnetospheric electrons and cosmic rays. The inductive cold plasma in our experiments involves no sparks and is a process very similar to that which occurs along, and at the termini of, charged particle tracks (Thompson et al., 1987). During such cold plasma generation, the electrons are highly superthermal, but the neutral molecules and ions remain near the room temperature. On the contrary, a high voltage, high current arc, spark or a laser-induced breakdown discharges, are hot plasmas, in which the kinetic and excitation temperatures of all species are very high (Roth et al., 1995; Thompson et al., 1991). Consequently, cold plasmas simulate in the laboratory electron and proton irradiation, whereas hot plasmas mimic lightning discharges and meteoritic impact shocks. In this study, a CH4/N2 gas mixture containing 10% methane (Matheson Tri-Gas, Research Purity) was introduced by a mass flow controller at a flow rate of 10–40 sccm (standard cubic centimeter per minute) to maintain the desired pressures. We used 10% CH4 in N2 for the initial gas mixture, which is a larger abundance of CH4 than in Titan’s upper atmosphere (CH4 mole fraction varies from 1.4 × 10-2 in the upper atmosphere to reach 4.9 × 10-2 at the surface, Niemann et al., 2005). In our study, we focused on comparing the thermal behaviour of fresh tholin with aged tholin (synthesized 20 years ago at Cornell University), then having been exposed to ambient air for few minutes up to several years. To be able to compare them, we worked with the same gas composition and energetic source (cold plasma inductively coupled with a RF power) that has been used in previous experimental work of the Cornell University Group (Khare et al., 1987, 1984a, 1984b; Thompson et al., 1991). We note that a recent study of pyrolysis of Titan’s tholins ranging from 2% to 98% CH4 in N2, has resulted in no qualitative change in the volatile pyrolytic products and limited changes in the volatiles relative abundances with

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respect to CH4 concentrations (Coll et al., 2013). As well, Carrasco et al. (2009) have found similar results in the solubility, the infrared spectroscopy signature, the mass distribution of the soluble fraction by mass spectrometry for Titan’s tholins produced in gaseous mixtures containing 2% and 10% methane. Tholins were formed at ambient laboratory temperature (~295 K) at two pressures, 1 Torr (1.3 mbar, tholin 2) and 0.25 Torr (0.3 mbar, tholin 3), corresponding to Titan’s atmospheric pressures at 200 km and 250 km altitude, respectively. The forward and reflected RF powers were monitored with the RF power supply, and the net power was kept at 95 W for all the experimental runs. For each pressure, tholins were deposited onto three Pyrex glass substrates placed on the lower electrode. This particular type of Titan’s aerosol analog synthesized at NASA Ames laboratory is reasonably consistent with the observational constraints of real Titan’s aerosols when comparing their optical properties (Imanaka et al., 2004; Khare et al., 1984a; McKay et al., 1989) and the pyrolysis analysis (Ehrenfreund et al., 1995; Israel et al., 2005; Khare et al., 1984b). After irradiation, tholins particles were collected by scraping from the orange-brownish film deposited on the glass substrates and transferred in chemically inert sealed glass vials to be ready for the analyses described in the next section.

2.2. Analytical techniques

Infrared spectroscopy. The laboratory diffuse reflectance spectra were acquired in the spectral 400 - 4000 cm-1 region using a Fourier Transform Infrared spectrometer (Nicolet, model NEXUS 670) configured with a drift reflectance accessory (Harrick, model Praying Mantis DRP) mounted inside the instrument compartment. A XT-KBr beamsplitter and DTGS/KBr detector was used to measure in the MID-IR region.

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Spectral resolution was 2 cm-1. The amount of sample in the sample holder was consistent with infinite thickness.

Elemental analysis. A few mg of tholin samples were examined for determining their mass fractions of carbon, hydrogen, nitrogen (1 mg tholins, LECO elemental analyzer, model CHNS-932) and of oxygen (2 mg tholins, LECO elemental analyzer, model VTF900). Table 1 presents the results for the elemental analysis for the three Titan’s tholin samples studied. Significant variation in the empirical formula is noted. It is explained as a direct consequence of a change in the tholin elemental composition when it is subjected to air contamination for few minutes up to several years (depending on the storage conditions).

Thermal Analysis. Thermogravimetry (TG), derivative thermogravimetry (DTG) and differential thermal analysis (DTA) measurements were performed with a simultaneous thermal analyzer model SDTQ-600/Thermo Star of TA Instruments. Non-isothermal experiments were carried out under dynamic conditions from room temperature to 1000ºC at a heating rate of 10ºC/min under argon and oxygen atmospheres. The average sample weight was ~ 10 mg, and the argon and oxygen flow rate was 100 ml/min. A coupled TG-MS system using an electron-impact quadrupole mass-selective detector (model Thermostar QMS200 M3) could analyze the main species evolving during the dynamic thermal decomposition of fragmentation processes of all the samples. A differential scanning calorimeter (DSC; Perkin Elmer DSC/ TA7DX, PC series with liquid nitrogen for low temperatures) was also used. The temperature and heat flow were calibrated with common standards, such as indium. Samples (∼ 10 mg) were scanned at 10ºC/min under dry argon (100 ml/min).

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The residue remaining after the thermogravimetric analysis were analyzed by FTIR reflectance spectroscopy.

3. Results and Discussion

The elemental analysis of Titan tholins formed with the tholin-RF system shows the presence of oxygen in the samples due to exposure to the ambient atmosphere during their transfer, handling and even storage. Indeed, after irradiation the three tholins samples were collected while opening the simulation chamber to the ambient atmosphere. Tholin 1 sample, which has the higher content of oxygen (17.55%), was kept in glass vials without further precautions against oxidation for 18 years. Tholin 2 and tholin 3, with lower oxygen content, were kept in sealed glass vials inside a vacuum desiccator filled with silica gel and pumped out with a primary vacuum (10-3 mbar) to preserve the samples from atmospheric moisture.

The infrared reflectance spectra of the three samples (Fig. 1) exhibit the main functional groups observed commonly in Titan’s tholins. The assignments of the absorption bands are presented in Table 2.

3.1. Thermogravimetric and DSC analysis

Representative thermogravimetric curves obtained for the three Titan’s tholin samples under argon atmosphere are shown in Figure 2a. As observed in these curves, the thermal degradation of the three samples shows similar behaviour, however with some differences due to the fact that they were obtained under distinct experimental conditions.

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This figure indicates that the thermal degradation of all samples can be divided into three stages: drying stage (< 150ºC), main pyrolysis stage (150-575ºC) and carbonization (> 575ºC). The first stage, between 25 and 150ºC, involves a mass loss of around 5 wt%, and corresponds to the vaporization of moisture, the desorption of water and the possible emission of volatile organic compounds. This initial step of weight loss at low temperatures indicates the hydrophilicity of these tholins, where in this case the hydrophilic property refers to their tendency to readily absorb air moisture. The degradation starts around 150ºC and distributes over a broad temperature range. This second stage, between 150 and 575ºC, corresponds with a mass loss of about 50 wt% for the sample 1, and ~35 wt% for the samples 2 and 3. This broad stage may therefore be rationalized to be a composite of three thermal events as described below. The final stage occurs between approximately 575 and 1000ºC. This last step is a minor thermal decomposition stage. At 1000ºC, the char residue is another characteristic of these Titan’s tholins, being equal to 27%, 50% and 42% of the initial weights for the samples 1, 2 and 3, respectively. This last result strongly suggests that part of the structure of the three tholins is constituted by a thermally stable component. The IR spectra of the residues of the three tholins after complete thermal degradation were taken in order to have information on the nature of the final degraded components from our Titan tholins (Fig. 3). The residues show optical properties very different from the initial tholins with two main intense absorption bands: one at 1400-1350 cm-1 corresponding to the breathing mode of aromatic rings (disorder of the rings in the material) and another one at 16501640 cm-1 related to C=C stretching mode seen in both aromatic and olefinic organics. Such absorption features can be seen specifically in aromatic structure of non-crystalline graphitic material (Friedel et al., 1971; Friedel and Carlson, 1971). The pyrolytic residues of Titan tholins may therefore have a structure that closely resembles this type of

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amorphous insoluble graphitic carbon and seem to indicate an increase of the unsaturated functional groups in the structure and of the aromaticity compared to the initial nonheated tholins. The formation of this char may be at least partly from a secondary reaction taking place during the thermal analysis in an inert atmosphere. The first derivatives of thermograms were calculated to highlight the inflection points that indicate thermal transitions. These DTG curves are shown in Figure 2b, where a deconvolution of one of these curves, sample 1, into an individual Gaussian peak has been made assuming a linear background over the temperature range of the fitting. The temperatures of the DTG maxima with the corresponding rates of weight loss, dW/dt, for the three samples are collected in Table 3. Figure 2b illustrates more clearly the differences in the thermal decomposition behaviour of the three samples. The first DTG peak appears at ~95ºC, very probably resulting from the desorption of water and/or organic volatile material as was mentioned earlier. At the second stage, three peaks are observed, the first one around 200ºC, the second one appears between 240ºC and 300ºC; and the last one is present at ~405ºC for samples 1 and 2, and is shifted 55ºC higher in temperature for sample 3. A more significant change due to thermal degradation processes takes place from the fourth peak, and continues with a secondary degradation at the last stage, with a peak of maximum loss weight around 670ºC. During the overall thermal decomposition process, the three samples follow globally the same three stages of degradation, however they show some specific significant differences. For tholin 3, synthesized recently at a pressure of 0.3 mbar, its maxima decomposition rate takes place at slightly higher temperatures than the two other samples. Another relevant feature is observed in tholin 2, which has a higher thermal stability at low temperatures (< ca 125ºC) and at high temperatures (> 500ºC), corresponding to the first and last stages, respectively, with the

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final maximum decomposition rate practically negligible. The initial tholin 2 differs from tholin 1 and tholin 3. It shows the lowest oxygen content of the three samples (3.5% only) and its chemical structure appears to be composed of a higher variety of functional groups, with the presence in its infrared spectrum of –CH2- and –CH3- stretching bands. Aliphatic -C≡N band is not seen in the two other tholins. The elemental analysis of tholin 2 clearly indicates that it has absorbed less ambient moisture than tholins 1 and 3 or that it has reacted less with ambient H2O which could explain its higher thermal stability at low temperatures during the first stage of desorption of water. Its higher stability at temperatures above 500ºC indicating that it does not degrade so easily at such elevated temperatures compared to the two other tholins, which could be due to its initial structure having stronger chemical bonds (such as more unsaturated bonds) or that the initial structure is reacting under heat to form stronger bonds leading to a secondary structure more stable. Intramolecular reactions such as the Bergman cyclization or other intramolecular cyclization during heat treatment such as cyclization of nitrile groups could be examples of such thermal reactions, as illustrated in scheme 1 and 2 below. The DSC curve of Titan tholins made under argon atmosphere exhibits different stages in agreement with the previous DTG analysis, with a very broad endotherm between 180 and 970ºC, as shown in Figure 4. Among the three samples, tholin 1 displays regions better defined. The first and well-defined endothermic peak at 95ºC is attributed to the evaporation of absorbed water, and the rest of the observed peaks, at 235, 375, and 725ºC, reflect decomposition. A poorly defined endothermic peak appears at ~490ºC, as observed in tholin 1. The more intense peak is found at higher temperature with an asymmetric form as seen more clearly for tholins 2 and 3, with profiles really similar.

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3.2. Mass spectrometric thermal analysis

MS coupled with TG system has been used to study volatile species of thermal decomposition and fragmentation processes for the three synthesized tholin samples. Dynamic measurements were carried out in an argon atmosphere. Figure 5 presents the ion current for m/z detected in the MS versus temperature. In this inert atmosphere major signals are observed in the range 50-650ºC. A first intense MS peak appears at 95ºC with m/z = 18 which corresponds to H2O+. This data coincide with the loss observed on TG curves and DSC data. The second major signal corresponds to m/z = 17, which is attributed to OH+ or/and ammonia NH3+, with maximum rate at 100ºC (probably for OH+), and a following broad peaks (extended between 150 and 600ºC) centred around 220 and 430ºC. This second broad MS peaks present the same profile than the third major signal for m/z = 16 (NH2+ and/or methane) and do not appear in the profile of m/z = 18. Consequently, we attribute the broad MS peak observed for m/z = 17 between 150ºC and 600ºC to NH3+ while the first peak at 100ºC to OH+. In this inert atmosphere, another major signals are observed at m/z = 28 for the ion C2H4+ (ethylene). This fragment could also be assigned to carbon monoxide CO+, in this case attributed to the air contamination of the tholins. We exclude the contribution of this mass as the presence of air (N2) during the TG-MS analysis since the blank control performed previously to the tholins analysis show a very distinct profile than the signal at m/z = 28 reported here. In addition to the mentioned products, H2O, NH3, C2H4 and CH4, other significant components are released from the three tholins such as: hydrogen cyanide, acetonitrile and isocyanic acid H-N=C=O (or cyanic acid H-O-CN). The HCN evolution with m/z = 26 for CN+ and 27 for HCN+, starts at 130ºC and takes place over the all temperature range under study, i.e. in the overall degradation process suggesting that HCN is most

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probably incorporated into the chemical structure of our Titan tholins. These profiles are very similar to those ion currents corresponding to C+ and CH+ with m/z = 12 and 13, respectively. The release of acetonitrile is followed with the profiles of m/z = 41 and 15 (CH3+), where two peaks around 215 and 450ºC can be observed. The release of gases contributing to m/z = 43 can be associated to isocyanic acid but also can strongly be considered for the hydrocarbon fragments of C3H7+. The release begins at 120ºC, and the ion continues to be released at higher temperatures on a large range, with profiles showing the same tendencies as seen for C+ and CH+. Such behaviour strongly indicates that the C3H7+ component is also part of the chemical structure, like HCN and not having simply condensed or absorbed onto the tholins like H2O which is observed in thermal profiles to be driven off during the other degradation stages. Another relevant signal has an m/z = 44, which can be assigned to CO2+, though propane C3H8+ could also be considered. In the case of tholins 1 and tholins 3, it is very likely that both, CO2+ and C3H8+ are present in the signals. Profiles of both tholins show a significant peak at around 200ºC, probably due to the release of CO2 due to the air exposure and then broader variation beginning around 250ºC - 300ºC, possibly attributed to propane. Tholin 2, showing lower content of oxygen in its initial composition, does not display this intense CO2 peak at 200ºC, and the peak for tholin 3 is much lower than for tholin 1, the sample with the most and longest exposure to air. We disregard the signal at m/z = 44 to be from possible presence of CO2 from ambient air into the TG-MS instrument since the blank control do not reveal any signal for this mass. The mass spectrometric thermal analysis reveals also the non-negligible release of the ion m/z = 29 which can be assigned to the ethyl ion CH3-CH2+. A dominant peak is showing up around 470ºC for tholin 2, in agreement with the results of the infrared analysis indicating the presence of –CH3- and – CH2- moieties in the structure of tholin 2.

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Finally, lower signals of gaseous species with higher molecular mass are observed in this study. They can be mainly due to the tar cracking of the Titan tholins. Those lower signals appear for the m/z = 52 (the formation of –CH=CH–CH=CH–+ and/or NC– CH=CH–+ type structure) and m/z = 54 for butadiene C4H6+, probably with further fragmentations producing signals on the profiles of m/z = 13 (CH+) and 12 (C+). No fragment ions with higher mass than 54 are observed during the thermal degradation process of the three Titan’s tholins. Figure 6 compares the ion current curve for the main thermal decomposition gases for a Titan tholin sample, Tholin 1, with the signals at m/z = 16, 17, 18, 27, 28 and 44. As is shown in this figure, the intensity order of MS signals is m/z = 18 (16 ·10-9) > 17 (4 ·109

) > 28 (1.3 ·10-9) ≈ 16 (1·10-9) ≈ 27 (7.5·10-10) > 44 (2.5·10-10). This indicates that the

predominant thermal decomposable components in the Titan tholins are H2O, ammonia, ethylene, methane, HCN and propane. Some of these results are in agreement with data obtained from the flash pyrolysis of real Titan’s aerosols and laboratory Titan tholins. Indeed, ion fragments at m/z = 17 and m/z = 27 attributed to NH3 and HCN, respectively, have also been observed to be the main pyrolysis products in the in situ analysis of Titan’s atmospheric aerosols carried out by Aerosols Collector and Pyrolyser (ACP) instrument at 600ºC coupled to GC-MS (Israel et al., 2005). These analyses concluded that nitrogen may be incorporated into Titan’s aerosols, NH3 and HCN being fingerprints of the chemical structure itself and not condensed products. HCN has also been extensively reported to be released significantly from laboratory Titan tholin under pyrolysis-GCMS (Coll et al., 1999; Ehrenfreund et al., 1995; Khare et al., 1984b; Pietrogrande et al., 2001; Szopa et al., 2006). In the case of NH3 very few studies have reported it as a Titan tholin pyrolyzate (Ehrenfreund et al., 1995; Pietrogrande et al., 2001) mainly due to the choice of the experimental analytical conditions which were

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unable to measure it. Very recently, Coll and coworkers (2013) have pointed out this issue and by changing the chromatographic parameters to analyse the fragments issued from the pyrolysis of their Titan tholins, they were able to detect it. Methane, ethylene and propane have as well been identified as pyrolysis products of Titan tholins (Coll et al., 1999; McKay et al., 2012; Pietrogrande et al., 2001). Our results strongly agree with ACP analysis that NH3 and HCN are significant part of the chemical structure of Titan tholins. Aliphatic carbons are also present as indicated by the release of methane during the main pyrolysis stage but the thermal degradation of our Titan tholins does not give any signals of aromatic fragment due to the presence of possible simple aromatic hydrocarbons. Our thermal treatment reveals much less products than reported in flash pyrolysis studies of laboratory analogues. The hydrophilic character of Titan tholins evidenced in our work (i.e. their tendency to easily absorb air moisture) is also a feature of other well-known prebiotic polymers, such as HCN polymers (de la Fuente et al., 2011). However, our study resulting in no major chemical changes between the three samples, seems to indicate than the exposure of the Titan tholins to ambient air do not affect the main backbone of their chemical structure, but decreases its thermal stability. Water from moisture seems to behave like a component simply condensed or deposited onto the tholins, maybe interacting with small branched chains, but not reacting actively with their “core” structure to induce its chemical modification. Finally, it should be mentioned that the intensity of m/z signal in this technique is related to the amount of species, but not decisive. It is also determined by different ionized energies of the distinct gas species. Therefore, the quantitative comparison of MS signals of different samples and components is not performed in this paper.

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3.3. Effect of oxygen on the thermal decomposition

The effect of the oxygen on the thermal degradation of the Titan tholins was also analyzed. The tholins thermal oxidative degradation has a different behaviour than in argon degradation. Figures 7a and 7b shows the TG and DTG curves, respectively, for the three samples degraded under an oxygen atmosphere from room temperature to 1000ºC. A detailed analysis of the curves allows us to conclude that, for the three tholin samples, the two first degradation stages, until temperature around 500ºC, are not apparently affected by the type of environment under study. Below this temperature the oxygen notably decreases the stability of the Titan tholins. The first DTG peak, caused by the presence of water is slightly shifted to lower temperatures, and now appears around 80ºC in the thermo-oxidative degradation. At the second stage, where three separated DTG maxima under argon flux can be registered (see figure 2), now under oxygen atmosphere they also appear but less defined and shifted to lower temperatures. This observation suggests that these peaks, which correspond to the main pyrolysis stage, do not arise from the thermally weak structures sensitive to oxidation. On the contrary, above 500ºC, a great difference is observed for the thermal degradation of Titan tholins under oxygen atmosphere compared with that under argon flow. Titan tholins present higher stability in argon than in an oxygen atmosphere at higher temperatures. Besides, the TG analyses of all tholins under oxidative atmosphere provide no residues, the initial tholins is totally degraded at high temperatures. And the change at the third decomposition stage under oxygen flow is very illustrative, with a DTG peak quite narrow, between 500 and 650ºC (versus 575 and 1000ºC in argon). In an inert atmosphere, the DTG maximum is observed around 670ºC, and in this case it appears at lower temperatures, 565ºC. The dW/dT values of this peak significantly

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increase under oxygen atmosphere, as the figure 7b clearly shows. In this range of temperatures decomposition-carbonization reactions normally take place. Thus, the presence of molecular oxygen diminishes the stability of the macrostructure of the Titan tholins, and leads to intense thermo-oxidation processes. This means that the oxygen accelerated the mass loss of these three Titan tholins through oxidation.

3.4 Implications for the understanding of the formation mechanisms of aerosols on Titan and for future in-situ measurement on Titan missions.

During the in-situ exploration of Titan in January 2005, flash pyrolysis at 600ºC has been used in ACP instrument onboard Huygens probe (Israel et al., 2005). Flash pyrolysis, commonly used, has the inconvenient of not allowing to track the products released gradually as a function of the temperature during the heating process, unlike TGMS technique. With TG-MS, because the behaviour of products and fragments can be monitored at each increase of temperature, it provides information on the nature of the pyrolyzate products, if it's a simple condensed product or part of the main structure of the tholin. With common flash pyrolysis, fundamental information on the chemical structure can fail to be noticed. TG-MS yields important and additional structural information on tholins than flash pyrolysis and other techniques such as infrared spectroscopy and nuclear magnetic resonance (NMR) do not provide. DTG curves with its corresponding deconvolution obtained with this technique could be actually good fingerprints of Titan’s tholins. Indeed, previous DTG curves obtained for hydrophobic tholins produced in early Earth’s conditions, i.e under a CH4/N2/H2 atmosphere with the presence of aqueous aerosols and liquid water (de la Fuente et al., 2012), are very different from the curves in our present work. DTG curves for Titan’s tholins are much complex than the ones for

19

early Earth’s tholins. While for Titan’s tholins five DTG peaks are observed here during their three-step degradation process, only two maxima appear during the two-step decomposition of early Earth’s tholins. The thermal degradation TG-MS of early Earth’s tholins also produces signals on the profiles at m/z = 77, 78, 79, 91, 105 indicating the presence of benzene derivatives and that possibly aromatic hydrocarbons form part of their structure. For Titan’s tholins no aromatic fragment appears. Such differences in TGMS results reveal clearly distinct chemical structure for both types of tholins. Thermogravimetric analyzer coupled to a mass spectrometer offer the means of identifying, in real time, small gaseous products like CO2, H2O, CH4 and CO, as well as large organic species, monomers and oligomers but also of giving specific thermograms and fragment signals depending on the nature and structure of the polymer. Moreover, it can be used to monitor continually the weight loss kinetics. We believe it could be a suitable and productive technique for the in-situ analysis of Titan’s aerosols through the atmosphere and onto the surface. Our results on TG-MS detailed study of Titan’s tholins provides significant additional information allowing to constrain the possible formation mechanisms of tholins on Titan: 1. In our thermal analysis, no fragment attributed to benzene or toluene or other monoaromatic hydrocarbons or simplest polycyclic aromatic hydrocarbons (PAHs) such as naphtalene, antracene are observed in the signals at low or high temperature (at high temperature between 600-900ºC it could be the cracking of the PAHs). We do not detect neither the formation of highest PAHs at high temperature > 550 ºC. Consequently, one of the proposed pathway of the formation of tholins via benzene polymerization and PAHs condensation (Wilson and Atreya, 2003) is excluded for our Titan's tholins.

20

2. The same exclusion applies for the formation via the polymerization of acetylene (C2H2) through polyyne formation (Allen et al., 1980; Clarke and Ferris, 1997) because our results do not show a significant presence of hydrocarbons, and C2H2 is not released. Even if the mass 26 is present, its signal has totally the same tendency as the mass 27 and therefore is attributed only to HCN. 3. The polymerization of nitriles could be a very potential chemical pathway as proposed in previous works (Ehrenfreund et al., 1995; Thompson and Sagan, 1989) because of the dominant release of HCN we observe during the overall thermal degradation process. However, the simultaneous significant release of NH3 suggests that the Titan’s tholin material would not be a pure -C≡N polymer but rather similar to a polyaminonitrile structure. The release of simple hydrocarbons such as methane, ethylene and ethyl ion CH3-CH2+ could be due to the presence of aliphatic hydrocarbon chains in the structure but not part of the internal nucleus. They could be present as branched chains of a parent structure. Photochemical model of Wilson and collaborators (2003) proposed that at altitudes around 200 km, there is a significant contribution of nitrile polymerization (but not pure nitrile polymers), being the most dominant mechanism in the upper atmosphere (>1000 km). Our results with laboratory tholins produced by simulation of altitudes around 200 - 250 km appear to agree with this part of the model of Wilson and co-authors (2003). In order to have more indication on the possible formation via polyaminonitrile or other nitrogen-bearing polymers, it would be interesting to perform laboratory studies on the same thermal TG-MS analysis on different polymers such as HCN polymers, polyamines, polyacrylonitrile, polybenzonitrile, alpha-aminonitriles and polyacetonitriles and make an exhaustive comparative study. This is currently one of the objectives in our group.

21

4. Conclusions

This work is the first report on a thermogravimetry-mass spectrometry extensive study of laboratory Titan tholins to explore their thermal stability and to provide new information to infer its possible chemical structure. We used three different tholin samples synthesized under different conditions and revealing very distinct oxygen contents related to their degree of exposition to the ambient atmosphere. From the results we have obtained, the TG curves showed that the mass loss percentages for the decomposition were similar for the three Titan tholins, with only some differences concerning their thermal stability. As well, the chemical products released during the thermal process are the same for the three tholins and their behaviours with increasing temperature do not show significant differences between the samples. Exposure to ambient air and moisture seems not to affect the main structure of the tholin. The oxidative decomposition of our Titan tholins showed the same stages in the same manner in argon atmosphere, with DTG peaks in oxygen shifted towards lower temperatures. These tholins present a high percentage of a thermally stable structure (between 30% and 50% of mass). This structure, possibly of graphitic nature can be originated during the formation of Titan tholins but much possibly forms during their thermal degradation process in inert atmosphere. DTA and DSC measurements revealed the exclusive endothermic nature for all the thermal decomposition processes. From our present results, we strongly point out that Titan’s tholins may be an open-chain molecular structure rather than a ring-shaped structure, which could undergo cycloaromatization after high temperature treatment. The main structure may contain mainly nitrile and amino groups

22

and chains of branched hydrocarbons into this structure. Polymerization of non-pure C≡N structural units such as aminonitriles could give rise to this complex structure. The results shown in this paper represent a non-negligible advancement in the development of a systematic structural characterization of these tholins and in the understanding of their thermal decomposition mechanism and their behaviour with respect to their oxygen contents due to air contamination. Such effective instrumentation and useful methodical characterization could be applied for the next in-situ analysis of Titan’s aerosols.

Acknowledgments

D.N.-M. acknowledges the Spanish Minister of Economy and Competitiveness (MINECO) for financial support under the funded project AYA2009-09288. J.-L.F and M.R.-B. have used the research facilities of Centro de Astrobiología (CAB) and have been supported by Instituto Nacional de Técnica Aeroespacial “Esteban Terradas” (INTA) and the project AYA2009-13920-C02-01 of the Ministerio de Ciencia e Innovación (Spain).

23

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Coll, P., Ramirez, S.I., Navarro-Gonzalez, R., Raulin, F., 2001. Chemical and optical behaviour of tholins, laboratory analogues of Titan aerosols. Advances in Space Research 27, 289-297. Coll, P., Navarro-González, R., Szopa, C., Poch, O., Ramírez, S.I., Coscia, D., Raulin, F., Cabane, M., Buch, A., Israël, G., 2013. Can laboratory tholins mimic the chemistry producing Titan's aerosols? A review in light of ACP experimental results. Planetary and Space Science 77, 91-103. de la Fuente, J.L., Ruiz-Bermejo, M., Menor-Salván, C., Osuna-Esteban, S., 2011. Thermal characterization of HCN polymers by TG–MS, TG, DTA and DSC methods. Polymer Degradation and Stability 96, 943-948. de la Fuente, J.L., Ruiz-Bermejo, M., Menor-Salván, C., Osuna-Esteban, S., 2012. Pyrolysis study of hydrophobic tholins By TG-MS, TG, DTA and DSC methods. Journal of Thermal Analysis and Calorimetry, 1-8. Devasia, R., Nair, C.P.R., Sivadasan, P., Katherine, B.K., Ninan, K.N., 2003. Cyclization reaction in poly(acrylonitrile/itaconic acid) copolymer: An isothermal differential scanning calorimetry kinetic study. Journal of Applied Polymer Science 88, 915920. Ehrenfreund, P., Boon, J.J., Commandeur, J., Sagan, C., Thompson, W.R., Khare, B., 1995. Analytical pyrolysis experiments of Titan aerosol analogues in preparation for the Cassini Huygens mission. Advances in Space Research 15, 335-342. Fitzer, E., Müller, D.J., 1975. The influence of oxygen on the chemical reactions during stabilization of pan as carbon fiber precursor. Carbon 13, 63-69. Friedel, R., Queiser, J., Carlson, G., 1971. Infrared and Raman spectra of intractable carbonaceous substances. Reassignments in coal spectra: American Chemical Society, Division of Fuel Chemistry, Preprints of Papers 15, 123-136.

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Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W., 1984a. Optical constants of organic tholins produced in a simulated Titanian atmosphere from soft X-Ray to microwave frequencies. Icarus 60, 127-137. Khare, B.N., Sagan, C., Thompson, W.R., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W., Shrader, S., Ogino, H., Willingham, T.O., Nagy, B., 1984b. The organic aerosols of Titan. Advances in Space Research 4, 59-68. Khare, B.N., Sagan, C., Thompson, W.R., Arakawa, E.T., Votaw, P., 1987. Solid hydrocarbons aerosols produced in simulated Uranian and Neptunian Stratospheres. Journal of Geophysical Research-Space Physics 92, 15067-15082. McDonald, G.D., Thompson, W.R., Heinrich, M., Khare, B.N., Sagan, C., 1994. Chemical Investigation of Titan and Triton Tholins. Icarus 108, 137-145. McGuigan, M., Waite, J.H., Imanaka, H., Sacks, R.D., 2006. Analysis of Titan tholin pyrolysis products by comprehensive two-dimensional gas chromatography–timeof-flight mass spectrometry. Journal of Chromatography A 1132, 280-288. McKay, C.P., Pollack, J.B., Courtin, R., 1989. The thermal structure of Titan's atmosphere. Icarus 80, 23-53. McKay, C.P., 1996. Elemental composition, solubility, and optical properties of Titan's organic haze. Planetary and Space Science 44, 741-747. McKay, C.P., Khare, B.N., Amin, R., Klasson, M., Kral, T.A., 2012. Possible sources for methane and C2–C5 organics in the plume of Enceladus. Planetary and Space Science 71, 73-79. Niemann, H.B., Atreya, S.K., Bauer, S.J., Carignan, G.R., Demick, J.E., Frost, R.L., Gautier, D., Haberman, J.A., Harpold, D.N., Hunten, D.M., Israel, G., Lunine, J.I., Kasprzak, W.T., Owen, T.C., Paulkovich, M., Raulin, F., Raaen, E., Way,

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S.H., 2005. The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe. Nature 438, 779-784. Pietrogrande, M.C., Coll, P., Sternberg, R., Szopa, C., Navarro-Gonzalez, R., VidalMadjar, C., Dondi, F., 2001. Analysis of complex mixtures recovered from space missions: Statistical approach to the study of Titan atmosphere analogues (tholins). Journal of Chromatography A 939, 69-77. Roth, J.R., Tsai, P.P., Liu, C., Laroussi, M., Spence, P.D., 1995. One atmosphere, uniform glow discharge plasma. U.S. Patent 5,414,324, issued May 9, 1995. Sagan, C., Khare, B.N., 1982. The organic clouds of Titan. Origins of life 12, 280. Sarker, N., Somogyi, A., Lunine, J.I., Smith, M.A., 2003. Titan aerosol analogues: Analysis of the nonvolatile tholins. Astrobiology 3, 719-726. Sekine, Y., Imanaka, H., Matsui, T., Khare, B.N., Bakes, E.L.O., McKay, C.P., Sugita, S., 2008. The role of organic haze in Titan's atmospheric chemistry I. Laboratory investigation on heterogeneous reaction of atomic hydrogen with Titan tholin. Icarus 194, 186-200. Szopa, C., Cernogora, G., Boufendi, L., Correia, J.J., Coll, P., 2006. PAMPRE: A dusty plasma experiment for Titan's tholins production and study. Planetary and Space Science 54, 394-404. Thompson, W.R., Henry, T., Khare, B., Flynn, L., Schwartz, J., Sagan, C., 1987. Light hydrocarbons from plasma discharge in H2‐He‐CH4: First results and Uranian auroral chemistry. Journal of Geophysical Research: Space Physics 92, 1508315092. Thompson, W.R., Sagan, C., 1989. Atmospheric formation of organic heteropolymers from N2+CH4: Structural suggestions for amino acid and oligomer precursors. Origins of life and evolution of the biosphere 19, 503-504.

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Thompson, W.R., Todd, J.H., Schwartz, J.M., Khare, B., Sagan, C., 1991. Plasma discharge in N2 + CH4 at low pressures: Experimental results and applications to Titan. Icarus 90, 57-73. Toon, O.B., McKay, C.P., Griffith, C.A., Turco, R.P., 1992. A physical model of Titan's aerosols. Icarus 95, 24-53. Waite, J.H., Young, D.T., Cravens, T.E., Coates, A.J., Crary, F.J., Magee, B., Westlake, J., 2007. The Process of Tholin Formation in Titan's Upper Atmosphere. Science 316, 870-875. Wilson, E., Atreya, S., 2003. Chemical sources of haze formation in Titan's atmosphere. Planetary and Space Science 51, 1017-1033. Xue, T.J., McKinney, M.A., Wilkie, C.A., 1997. The thermal degradation of polyacrylonitrile. Polymer Degradation and Stability 58, 193-202.

29

TABLES Table 1. Laboratory Titan tholins analyzed in this work. Experimental conditions and elemental analyses are reported. p, initial pressure; T, temperature; t, irradiation time. Distinct differences in the empirical formula among the three tholins are noted. Titan tholin

Initial gas mixture

samples

P

T

[mbar]

[K]

t

C

[days, hours] [%]

N

H

O

C/N

[%]

[%]

[%]

C/H

C/O

Empirical

Ref.

Formula

Tholin 1

10%CH4, 90% N2

0.3

293

90 d

38.99

38.48

4.98

17.55

1.18

0.65

2.96

C6H9N5O2

Khare et al. (1987)

Tholin 2

10%CH4, 90% N2

1.3

295

3 d, 18 h

61.22

28.47

6.74

3.53

2.50

0.75

23.12

C23H30N9O

This study

Tholin 3

10%CH4, 90% N2

0.3

295

3 d, 18 h

45.60

36.95

5.15

8.68

1.44

0.74

7.00

C14H19N10O This study

Table 2. Asssigment of absorption features in the infrared reflectance spectra of Titan tholin samples. The main vibrational modes (in bold) are present in all the three Titan tholins. The other vibrational modes (in regular font) are observed only in tholin 2. Frequency (ν) cm1

Wavelength (λ) µm

3500-3100

2.86 - 3.22

–NH– stretching –NH2– asymmetric stretching –NH2– symmetric stretching Absorbed H2O

2962

3.38

–CH3– asymmetric stretching

2935

3.41

–CH2– asymmetric stretching

2877

3.47

–CH3–symmetric stretching

2243

4.46

–C≡N stretching (aliphatic nitriles)

2184, 2180,2176

4.59, 4.58, 4.60

–C≡N

Vibrational bands

stretching

(conjugated

nitriles) 1700-1650

5.88 - 6.06

C=N stretching or NH2 bending

30

Table 3. Characteristic temperatures for the thermal decomposition of synthesized Titan tholins in argon atmosphere, DTG maxima with the corresponding rates of weight loss, dW/dT. Tholin

Tmax1

dW1/dT

Tmax2

dW2/dT

Tmax3

dW3/dT

Tmax4

dW4/dT

Tmax5

dW5/dT

samples

(ºC)

(wt%/ºC)

(ºC)

(wt%/ºC)

(ºC)

(wt%/ºC)

(ºC)

(wt%/ºC)

(ºC)

(wt%/ºC)

1

101

-0.054

201

-0.169

241

-0.168

401

-0.121

647

-0.070

2

93.7

-0.029

184

-0.040

280

-0.077

408

-0.143

674

-0.029

3

93.1

-0.057

220

-0.069

302

-0.087

457

-0.095

693

-0.067

31

FIGURES CAPTIONS

Figure 1. Infrared reflectance spectra of Titan tholin samples used for this study. For each spectrum the baseline has been corrected to 100% in order to compare the absorption of the three samples. All the main vibrational modes are present in all the three Titan Tholins. The sample tholin 2 shows three additional vibrational modes listed in Table 2.

Figure 2. (a) TG and (b) DTG curves for Titan tholin samples. Heating rate was 10 ºC/min under an argon atmosphere.

Figure 3. IR reflectance spectra of the residues after heating at 1000º C of Titan tholin samples used for this study. For each spectrum the baseline has been corrected to 100% in order to compare the absorption of the three samples.

Figure 4. DSC curves for Titan tholin samples. Heating rate was 10 ºC/min under argon atmosphere. Figure 5. Ion intensity curves for Titan tholin samples, heating at 10 ºC/min in argon atmosphere. Figure 6. (a) DTG curve and (b) MS signal comparison of m/z = 17, 18, 27, 42 and 44 for Titan tholin sample 1, heating at 10 ºC/min in argon atmosphere. Figure 7. (a) TG and (b) DTG curves for Titan tholin samples under an oxygen atmosphere. Heating rate was 10 ºC/min.

32

SCHEMES CAPTIONS

Scheme 1. Example of thermal cyclization of nitrile groups: thermal degradation of polyacrylonitrile (PAN) (Devasia et al., 2003; Fitzer and Müller, 1975; Grassie and McGuchan, 1970; Xue et al., 1997).

Scheme 2. Bergman cycloaromatization reaction during heating treatment (Bergman, 1973; John and Tour, 1994).

33

FIGURES Figure 1.

34

Figure 2.

35

Figure 3.

36

Figure 4.

37

Figure 5.

38

Figure 6.

39

Figure 7.

40

Schemes 1 and 2.

41

Highlights • • • • •

We report on the first thermogravimetry‐mass spectrometry study of Titan tholins.  Exposure to ambient air appears not to alter the tholin basic chemical structure.  No aromatic fragment is observed during the thermal degradation of Titan tholins.  Our results point toward a Titan tholin structure similar to a polyaminonitrile.  Derivative thermogravimetry curves could be good fingerprints of Titan aerosols. 

42

FIGURES CAPTIONS

Figure 1. Infrared reflectance spectra of Titan tholin samples used for this study. For each spectrum the baseline has been corrected to 100% in order to compare the absorption of the three samples. All the main vibrational modes are present in all the three Titan Tholins. The sample tholin 2 shows three additional vibrational modes listed in Table 2.

Figure 2. (a) TG and (b) DTG curves for Titan tholin samples. Heating rate was 10 ºC/min under an argon atmosphere.

Figure 3. IR reflectance spectra of the residues after heating at 1000º C of Titan tholin samples used for this study. For each spectrum the baseline has been corrected to 100% in order to compare the absorption of the three samples.

Figure 4. DSC curves for Titan tholin samples. Heating rate was 10 ºC/min under argon atmosphere.

Figure 5. Ion intensity curves for Titan tholin samples, heating at 10 ºC/min in argon atmosphere.

Figure 6. (a) DTG curve and (b) MS signal comparison of m/z = 17, 18, 27, 42 and 44 for Titan tholin sample 1, heating at 10 ºC/min in argon atmosphere.

Figure 7. (a) TG and (b) DTG curves for Titan tholin samples under an oxygen atmosphere. Heating rate was 10 ºC/min

43

SCHEMES CAPTIONS

Scheme 1. Example of thermal cyclization of nitrile groups: thermal degradation of polyacrylonitrile (PAN) (Devasia et al., 2003; Fitzer and Müller, 1975; Grassie and McGuchan, 1970; Xue et al., 1997).

Scheme 2. Bergman cycloaromatization reaction during heating treatment (Bergman, 1973; John and Tour, 1994).

44

Table 1. Laboratory Titan tholins analyzed in this work. Experimental conditions and elemental analyses are reported. p, initial pressure; T, temperature; t, irradiation time. Distinct differences in the empirical formula among the three tholins are noted. p

T

gas mixture

[mbar]

[K]

Tholin 1

10%CH4, 90% N2

0.3

Tholin 2

10%CH4, 90% N2

Tholin 3

10%CH4, 90% N2

Titan tholin samples

Initial

t

C

N

H

O

C/N

C/H

C/O

[days, hours] [%]

[%]

[%]

[%]

293

90 d

38.99

38.48

4.98

17.55

1.18

0.65

2.96

C6H9N5O2

Khare et al. (1987)

1.3

295

3 d, 18 h

61.22

28.47

6.74

3.53

2.50

0.75

23.12

C23H30N9O

This study

0.3

295

3 d, 18 h

45.60

36.95

5.15

8.68

1.44

0.74

7.00

C14H19N10O This study

Empirical

Ref.

Formula

45

Table 2. Assigment of absorption features in the infrared reflectance spectra of Titan tholin samples. The main vibrational modes (in bold) are present in all the three Titan Tholins. The other vibrational modes (in regular font) are observed only in tholin 2. Frequency (ν) cm-1

Wavelength (λ) µm

3500-3100

2.86 - 3.22

–NH– stretching –NH2– asymmetric stretching –NH2– symmetric stretching Absorbed H2O

2962

3.38

–CH3– asymmetric stretching

2935

3.41

–CH2– asymmetric stretching

2877

3.47

–CH3–symmetric stretching

2243

4.46

–C≡N stretching (aliphatic nitriles)

2184, 2180,2176

4.59, 4.58, 4.60

–C≡N stretching (conjugated nitriles)

1700-1650

5.88 - 6.06

C=N stretching or NH2 bending

Vibrational bands

46

Table 3. Characteristic temperatures for the thermal decomposition of synthesized Titan tholins in argon atmosphere, DTG maxima with the corresponding rates of weight loss, dW/dT. Tholin

Tmax1

dW1/dT

Tmax2

dW2/dT

Tmax3

dW3/dT

Tmax4

dW4/dT

Tmax5

dW5/dT

sample

(ºC)

(wt%/ºC)

(ºC)

(wt%/ºC)

(ºC)

(wt%/ºC)

(ºC)

(wt%/ºC)

(ºC)

(wt%/ºC)

1

101

-0.054

201

-0.169

241

-0.168

401

-0.121

647

-0.070

2

93.7

-0.029

184

-0.040

280

-0.077

408

-0.143

674

-0.029

3

93.1

-0.057

220

-0.069

302

-0.087

457

-0.095

693

-0.067

47