A complex prebioticlike chemistry

In the atmosphere of Titan, CH4 chemistry is coupled with N2 chemistry resulting in the formation of many organics - hydrocarbons and N-containing organic compounds - in the gas and particulate phases. These compounds are hydrocarbons, nitriles, and complex refractory organics. Several photochemical models describing the chemical and physical pathways involved in the chemical evolution of the atmosphere of Titan and the resulting vertical concentration profiles of the different molecules involved have been published during the last 20 years. For a review, see the most recent publications and the included references (Lebonnois et al., 2001; Wilson and Atreya, 2004; Hebrard et al., 2005).

The whole chemistry starts with the dissociation of N2 and CH4 through electron and photon impacts. The primary processes allow the formation of C2H2 and HCN in the high atmosphere. These molecules play a key role in the general chemical scheme: once they are formed, they diffuse down to the lower levels where they allow the formation of higher hydrocarbons and nitriles. Additional CH4 dissociation probably also occurs in the low stratosphere through photocatalytic processes involving C2H2 and polyynes.

Another approach to the study of organic chemistry on Titan that is complementary to photochemical modelling is to develop simulation experiments in the laboratory. These experiments seem to mimick the real processes well, since experiments, carried out in particular at the Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), produced all the gas phase organic species already detected in Titan's atmosphere, within the right orders of magnitude of relative concentration for most of them. Such observations demonstrate the validity of these experimental simulations. The experiments also produce many other organics which can be assumed also to be present in Titan's atmosphere. Thus, simulation experiments appear to be a very useful guide for further searches (by both remote sensing and in situ observations). The gas phase as well as the aerosol phases are affected by such an extrapolation.

In the gas phase, more than 150 different organic molecules have been detected in the simulation experiments (Coll et al., 1998, 1999a). These global simulations of Titan's atmospheric chemistry use an open reactor and a low pressure N2-CH4 gas mixture. The energy source is a cold plasma discharge producing mid-energy electrons (around 1-10 eV). The gas phase endproducts (molecules) are analysed by IRFTS (infrared Fourier transform spectroscopy) and GC-MS techniques; the transient species (radicals and ions) are determined by on-line ultraviolet-visible spectroscopy. The evolution of the system is also theoretically described using coupled physical and chemical (ions and neutrals) models. The identified organic products are mainly hydrocarbons and nitriles. The absence at a detectable level of molecules carrying amino groups, like amines, with the exception of ammonia, must be highlighted. These experiments have allowed the detection of all gaseous organic species observed on Titan, including C4N2 (Coll et al., 1999b). Among the other organics formed in these experiments and not yet detected in Titan's atmosphere, one should note the presence of polyynes (C4H2, C6H2, C8H2) and probably cyanopolyyne HC4CN. These compounds are also included in photochemical models of Titan's atmosphere, where they could play a key role in the chemical schemes allowing the transition from gas phase products to aerosols. Also of astrobiological interest is the formation of organic compounds with asymmetric carbon such as

Experiments on N2-CH4 mixtures including CO at the 100 ppm level (Bernard et al., 2002; Coll et al., 2003) show the incorporation of oxygen atoms in the produced organics, with an increasing diversity of the products (more than 200 were identified). The main oxygen-containing organic compound is neither formaldehyde nor methanol, as expected from theoretical models (both thermodynamic and kinetic), but oxirane (also called ethylene oxide), (CH2)2O. Oxirane thus appears to be a good candidate to search for in Titan's atmosphere. These studies also show the formation of ammonia at noticeable concentrations, opening new avenues in the chemical schemes of Titan's atmosphere.

Simulation experiments also produce solid organics, as mentioned above, usually called tholins (Sagan and Khare, 1979). These 'Titan tholins' are supposed to be laboratory analogues of Titan's aerosols, those tiny solid particles which are present in Titan's atmosphere and mask the surface of the satellite in the visible. They have been extensively studied since the first work by Sagan and Khare more than 20 years ago (Khare et al., 1984; 1986 and included references). These laboratory analogues show very different properties depending on the experimental conditions (Cruikshank et al., 2005). For instance, the average C/N ratio of the product varies from less than 1 to more than 11, in the published reports. More recently, dedicated experimental protocols allowing a simulation closer to the real conditions have been developed at LISA using low pressure and low temperature (Coll et al., 1998, 1999a) and recovering the laboratory tholins without oxygen contamination (from the air of the laboratory) in a glove box purged with pure N2. Representative laboratory analogues of Titan's aerosols have thus been obtained and their complex refractive indices have been determined (Ramirez et al., 2002), with - for the first time - error bars. These data can be seen as a new point of reference for modellers who compute the properties of Titan's aerosols. Systematic studies have been carried out on the influence of the pressure of the starting gas mixture on the elemental composition of the tholins. They show that two different chemical-physical regimes are involved in the processes, depending on the pressure, with a transition pressure around 1 mbar (Bernard et al., 2002; Imanaka et al., 2004).

The molecular composition of the Titan tholins is still poorly known. Several possibilities have been considered such as HCN polymers or oligomers, HCN-C2H2 cooligomers, HC3N polymers, HC3N-HCN cooligomers (Tran et al., 2003 and included references). However, it is well established that they are made of macromolecules of largely irregular structure. Gel filtration chromatography of the water soluble fraction of Titan tholins shows an average molecular mass of about 500-1000 Da (McDonald etal., 1994). Information on the chemical groups included in their structure has been obtained from their infrared and ultraviolet spectra and from analysis by pyrolysis-GC-MS techniques (Ehrenfreund et al., 1995; Coll et al., 1998; Imanaka et al., 2004; and included references). The data show the presence of aliphatic and benzenic hydrocarbon groups, of CN, NH2, and C = NH groups. Direct analysis by chemical derivatization techniques before and after hydrolysis allowed the identification of amino acids or their precursors (Khare et al., 1986). Their optical properties have been determined (Khare et al., 1984; McKay, 1996; Ramirez et al., 2002; Tran et al., 2003; Imanaka et al., 2004), because of their importance for retrieving observational data related to Titan. Finally, it is obviously of astrobiological interest to mention that Stoker et al. (1990) demonstrated the nutritious properties of Titan tholins for microorganisms.

Nevertheless, there is still a need for improved experimental laboratory simulations to better mimic the chemical evolution of the atmosphere, including the dissociation of dinitrogen by electron impact with energies close to the case of Titan's atmosphere, and the dissociation of methane through photolysis processes. Such an experiment is currently under development at LISA, with the SETUP (Simulation Exprimentale et Théorique Utile à la Planétologie) programme which, in a dedicated low-temperature flow reactor, couples N2 dissociation by electron and CH4 photodissociation by two-photon (248 nm) laser irradiation, and theoretical studies, in order to improve the chemical schemes. The preliminary results demonstrate the dissociation of methane through the two-photon process (Romanzin et al., 2005).

Several organic compounds have already been detected in Titan's stratosphere (Table 14.3). The list includes hydrocarbons (with both saturated and unsaturated chains) and nitrogen-containing organic compounds, exclusively nitriles, as expected from laboratory simulation experiments. Most of these detections were performed by Voyager observations, with the exception of the C2 hydrocarbons which were observed in spectra in the 1970s (see Strobel (1974)), acetonitrile, which was detected by ground observation in the millimetre wavelength, and water and benzene, which were tentatively detected by ISO. Since the arrival of Cassini in the Saturn system, the presence of water and benzene has been unambiguously confirmed by CIRS. In addition, the direct analysis of the ionosphere by INMS during the low-altitude Cassini fly-bys of Titan shows the presence of many organic species at detectable levels (Figure 14.6), in spite of the very high altitude (1100-1300 km) (Waite et al., 2007).

Surprisingly, GC-MS on board Huygens has not detected a large variety of volatile organic compounds in the low atmosphere. The mass spectra collected during the descent show that the medium and low stratosphere and the troposphere are poor in volatile organic species, with the exception of methane. Condensation of these species on the aerosol particles is a probable explanation for these atmospheric characteristics (Niemann et al., 2005). These particles, for which no direct data on the chemical composition were available before, have been analysed by the aerosol collector and pyrolyser (ACP) instrument. ACP was designed to collect the aerosols during the descent of the Huygens probe on a filter in two different regions of the atmosphere. Then the filter was heated in a closed oven at different temperatures and the produced gases were analysed by the GC-MS instrument. The results show that the aerosol particles are made of refractory organics: non-volatile compounds which are only vapourized after degradation at high temperature. The ACP data also show that these organics release HCN and NH3 during pyrolysis (Israel et al., 2005). This strongly supports the tholin hypothesis: from these new and first in situ measurement data it seems very likely that the aerosol particles are

Table 14.3. Main composition of Titan's stratosphere, trace components already detected and comparison with the products of laboratory simulation experiments (maj. = major product; ++: abundance smaller by one order of magnitude; +: abundance smaller by two orders of magnitude.

Compounds

Stratosphere mixing ratio E = equator; N = north pole

Production in simulation experiments

Main constituents

Nitrogen N2

0.98

Methane CH4

0.02

Hydrogen H2

0.0006-0.00014

Hydrocarbons

Ethane C2H6

1.3 x 10-5

E

maj.

Acetylene C2H2

2.2 x 10-6

E

maj.

Propane C3H8

7.0 x 10-7

E

++

Ethylene C2H4

9.0 x 10-8

E

++

Propyne C3H4

1.7 x 10-8

N

+

Diacetylene C4H2

2.2 x 10-8

N

+

Benzene C6H6

few 10-9

+

N-organics

Hydrogen cyanide HCN

6.0 x 10-7

N

maj.

Cyanoacetylene HC3N

7.0 x 10-8

N

++

Cyanogen C2N2

4.5 x 10-9

N

+

Acetonitrile CH3CN

few 10-9

++

Dicyanoacetylene C4N2

Solid phase

N

+

O-compounds

Carbon monoxide CO

2.0 x 10-5

Carbon dioxide CO2

1.4 x 10-8

E

Water H2O

few 10-9

made of a refractory organic nucleus, covered with condensed volatile compounds (Figure 14.7). The nature of the pyrolysates provides information on the molecular structure of the refractory complex organics: it indicates the potential presence of nitrile groups (-CN), amino groups (-NH2, -NH- and -N<) and/or imino groups (-C = N-).

Furthermore, comparison of the data obtained for the first (mainly stratospheric particles) and second (midtroposphere) samplings indicate that the aerosol composition is homogeneous (Israel et al., 2005). This also fits with some of the data obtained by the descent imager and radial spectrometer, DISR, relative to the

Fig. 14.6. Mass spectrum of Titan's ionosphere at an altitude of about 1200 km. The spectrum shows the signature of organic compounds including up to seven carbon atoms. (Image courtesy of NASA/JPL/University of Michigan.)
Fig. 14.7. Model of the chemical composition of Titan's aerosol from the Huygens-ACP data.

aerosol particle which indicates a fairly constant size distribution of the particles with altitude (with a mean dimension of the order of 1 ^m).

These particles sediment down to the surface where they likely form a deposit of complex refractory organic and frozen volatile compounds. DISR collected the infrared reflectance spectra of the surface with the help of a lamp, illuminating the

Fig. 14.8. The surface of Titan as seen by the Huygens DISR camera. Image courtesy of ESA/NASA/JPL/University of Arizona.

surface before the Huygens probe touched down. These data show the presence of water ice, but no clear evidence - so far - of tholins. The presence of water ice is also suggested by the data of the SSP instrument (Zarnecki et al., 2005). Its accelerometer measurements can be interpreted as due to the presence of small water ice pebbles on the surface where Huygens has landed, in agreement with the DISR surface pictures (Figure 14.8). On the other hand, GC-MS was able to analyse the atmosphere near the surface for more than one hour after the touch down. The corresponding mass spectra show the clear signature of many organics, including cyanogen, C3 and C4 hydrocarbons, and benzene, indicating that the surface is much richer in volatile organics than the low stratosphere and the troposphere (Niemann et al., 2005). These observations are in agreement with the hypothesis that in the low atmosphere of Titan, most of the organic compounds are in the condensed phase.

Thus, altogether, these new data show the diversity of the locations at which organic chemistry is taking place on Titan. Surprisingly the high atmosphere looks very active, with neutral and ion organic processes; the high stratosphere, where many organic compounds have been detected also shows an active organic chemistry in the gas phase. In the lower atmosphere this chemistry seems mainly concentrated in the condensed phase. Titan's surface is probably covered with frozen volatile organics together with refractory, tholin-like organic materials.

Irradiating effects of cosmic rays reaching Titan's surface may induce additional organic syntheses, particularly if these materials are partly dissolved in some small liquid bodies made of low molecular weight hydrocarbons (mainly methane and ethane). This could indeed allow the additional formation of reactive compounds such as azides as well as the polymerization of HCN (Raulin etal., 1995). Moreover, the interface between the liquid phase and the solid deposits at the surface may include sites of catalytic activity favourable to these additional chemical reactions.

In spite of the surface temperatures, even the presence of liquid water is not excluded. Cometary impacts on Titan may melt surface water ice, offering possible episodes as long as about 1000 years of liquid water (Artemieva and Lunine, 2003). This provides conditions for short terrestrial-like prebiotic syntheses at relatively low temperatures. Low temperatures reduce the rate constants of prebiotic chemical reactions, but may increase the concentration of reacting organics by the eutectic effect which increases the rate of the reaction. In addition, the possible presence of a water-ammonia ocean in the depths of Titan, as expected from models of its internal structure (Tobie et al., 2006, and included references), may also provide an efficient way to convert simple organics into complex molecules, and to reprocess chondritic organic matter into prebiotic compounds. These processes may have occurred very efficiently at the beginning of Titan's history (with even the possibility of the water-ammonia ocean exposed to the surface) allowing a CHNO prebiotic chemistry that resulted in the evolution of compounds of terrestrial biological interest.

Even if these liquid water scenarios are false, the possibility of a pseudo biochemistry evolving in the absence of a noticeable number of O atoms cannot be ruled out, with an N-chemistry, based on 'ammono analogues' replacing the O-chemistry (Raulin and Owen, 2002). Such alternatives of terrestrial biochemistry where, in particular, the water solvent could be replaced by ammonia or other N-compounds have been re-examined by Benner (2002) and by Schulze-Makuch and Irwin (2004).

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