Simulated Titan Research Articles

On the Formation and Accumulation of Solid Carbon Particles in High-Enthalpy Flows Mimicking Re-Entry in the Titan Atmosphere

The problem relating to the formation of solid particles enabled by hypersonic re-entry in methane-containing atmospheres (such as that of Titan) has been tackled in the framework of a combined experimental–numerical approach implemented via a three-level analysis hierarchy. First experimental tests have been conducted using a wind tunnel driven by an industrial arc-heated facility operating with nitrogen as working gas (the SPES, i.e., the Small Planetary Entry Simulator). The formation of solid phases as a result of the complex chemical reactions established in such conditions has been detected and quantitatively measured with high accuracy. In a second stage of the study, insights into the related formation process have been obtained by using multispecies models relying on the NASA CEA code and the Direct Simulation Monte Carlo (DSMC) method. Through this approach the range of flow enthalpies in which carbonaceous deposits can be formed has been identified, obtaining good agreement with the experimental findings. Finally, the deposited substance has been analyzed by means of a set of complementary diagnostic techniques, i.e., SEM, spectroscopy (Raman, FTIR, UV–visible absorption and fluorescence), GC–MS and TGA. It has been found that carbon produced by the interaction of the simulated Titan atmosphere with a solid probe at very high temperatures can be separated into two chemically different fractions, which also include “tholins”.

UV–Vis Light-induced Aging of Titan’s Haze and Ice

Abstract The study of the photochemical aging of aerosols is an important tool for understanding Titan’s stratosphere/troposphere composition and evolution, particularly the haze. Laboratory simulations of the photoreactivity of the haze aerosol analogs provide insight into the photochemical evolution of Titan’s atmosphere at and below the haze layers. Here we use experimental simulations to investigate the evolution of the laboratory analogs of these organic aerosols under ultraviolet (UV)–visible (Vis) photons, which make it through the haze layers during their sedimentation process. We present experimental results for the aging of Titan’s aerosol analogs obtained from two dominant nitrogen-containing organics, HC3N and HCN, under simulated Titan atmospheric conditions (photons and temperature). We report that volatile nitriles condensed on haze particles could be incorporated through photochemistry and provide one such sink mechanism for nitrile compounds. We provide laboratory evidence that the organic aerosols could photochemically evolve during their sedimentation through Titan’s atmosphere.

A qualitative study of the retention and release of volatile gases in JSC-1A lunar soil simulant at room temperature under ultrahigh vacuum (UHV) conditions

We conducted a qualitative study to simulate the flux of volatile gases expected to occur at the lunar surface due to cometary impact or lunar outgassing events. A small sample cell containing 8.8g of JSC-1A lunar soil simulant in a vacuum system with a base pressure of 1.5×10−8Torr was exposed to various gases using dynamic pressure dosing at room temperature to observe any retention of those gases as a function of the exposure times, temperatures and pressures used. Gases included pure argon, a five-component gas mixture (H2, He, Ne, N2, Ar), a simulated Mars atmospheric mixture (CO2, N2, Ar, CO, O2), and a simulated Titan mixture (N2, CH4). Results at exposure pressures of approximately 1.5×10−8Torr above background showed no observable retention of rare gases, slight retention of molecular gases, but surface retention of the triatomic gas CO2 occurred at room temperature with a time to reach equilibrium of greater than 10min, which was an unanticipated result. Despite several bakeouts and months under ultrahigh vacuum (UHV) conditions, trace levels of atmospheric gases continued to evolve from the simulant. Mechanical and optical probing of the simulant surface increased this latent gas evolution, particularly for CO2 and CO, with some evidence also for the release of CH4. We assert our results are, by analogy, applicable to protocols and instrumentation needed for conducting analytical chemistry aboard future landed lunar missions.

A comprehensive NMR structural study of Titan aerosol analogs: Implications for Titan’s atmospheric chemistry

Titan has a thick atmosphere composed primarily of nitrogen and methane. Complex organic chemistry induced by solar ultraviolet radiation and energetic particles, takes place in Titan’s upper atmosphere, producing an optically thick reddish brown carbon based haze encircling this moon. The chemistry in Titan’s atmosphere and its resulting chemical structures are still not fully understood in spite of a great many efforts being made. In our previous work, we have investigated the structure of the 13C and 15N labeled, simulated Titan haze aerosols (tholin) by NMR and identified several dominant small molecules in the tholin. Here we report our expanded structural investigation of the bulk of the tholin by more comprehensive NMR study. The NMR results show that the tholin materials are dominated by heavily nitrogenated compounds, in which the macromolecular structures are highly branched polymeric or oligomeric compounds terminated in methyl, amine, and nitrile groups. The structural characteristic suggest that the tholin materials are formed via different copolymerization or incorporation mechanisms of small precursors, such as HCN, CH2NH, NH3 and C2H2. This study helps to understand the formation process of nitrogenated organic aerosols in Titan’s atmosphere and their prebiotic implications.

Surface-active substances in a laboratory simulated Titan׳s organic haze: Prebiotic microstructures

Titan, the largest satellite of Saturn, is a key planetary body for astrobiological studies due to its active organic chemistry, hydrocarbon lakes and possible subsurface water-ammonia liquids. We have investigated the physicochemical properties of organic compounds synthesized in a simulated Titan atmosphere. A laboratory analog of Titan׳s aerosols, called tholin, was produced by irradiation of a nitrogen/methane gas mixture. The primary aim was to determine whether tholin represent possible sources of surface-active substances that could have been involved in the formation of prebiotic structures. A tholin sample was extracted with chloroform-methanol and the chloroform soluble material was separated by two-dimensional thin layer chromatography. Fluorescence excited by UV light was used to identify the major components on the plates. After being scraped from the TLC plate, the components were eluted as specific fractions and investigated by surface chemical methods, FTIR, scanning electron microscopy and cyclic voltammetry. Fractions 1 and 2 were strongly fluorescent and surface active, producing films at air-water interfaces. When exposed to aqueous phases, components in fraction 1 form spherical microstructures resembling prebionts. The prebionts are precursor structures that might have evolved into the first living cells.

Dissolution of benzene, naphthalene, and biphenyl in a simulated Titan lake

We constructed a laboratory apparatus capable of measuring the saturation equilibrium concentration (csat) and dissolution rate constants (keff) of organic solutes in ethane at 94K. We determined a csat of 18.5±1.9mg L−1, 0.159±0.003mg L−1, and 0.039±0.006mg L−1 for benzene, naphthalene, and biphenyl, respectively. The derived csat and keff can be used to predict the dissolution behavior of the materials in ethane under Titan conditions. The aromatic materials dissolved relatively quickly in liquid ethane at 94K, reaching saturation in less than 2h. The dissolution characteristics of benzene in ethane at 94K are compared to those of terrestrial karst-forming materials in water at 298K, and are used to constrain Titan surface processes. We discuss the implications of our measurements on the formation of karst on Titan, the concentration of organics in Titan’s lakes, and the formation of evaporite deposits during lake evaporation.

Amino Acid Precursors from a Simulated Lower Atmosphere of Titan: Experiments of Cosmic Ray Energy Source with 13C- and 18O-Stable Isotope Probing Mass Spectrometry

The organic haze of aerosols that shrouds the Saturnian moon Titan has previously been studied by both observations and laboratory simulation experiments. Here we report the abiotic formation of amino acid precursors in complex organic molecules during experimental simulation of the environment near Titan's surface with proton irradiation. Pyrolysis of the organic molecules formed in the simulated Titan atmosphere by proton irradiation at 600°C yielded compounds that contained HCN and NH₃ (m/z = 27 and 17). These experimental results are consistent with the molecular information obtained by pyrolysis gas chromatography/mass spectrometry (pyrolysis GC/MS) of samples collected by the Huygens probe to Titan. Scanning electron microscopy (SEM) and three-dimensional atomic force microscopy (AFM) images of the irradiation products reveal nanometer-scale filaments and globules in complex amorphous structures (approximately 1000 Da). Isotope probing experiments by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) show that oxygen atoms were incorporated into the racemic amino acids by hydrolysis of ¹⁸O-labeled water. We suggest that the amino acid precursors possibly formed after water hydrolysis, as suggested in a previous observational study (C. A. Griffith, T. Owen, T. R. Geballe, J. Rayner, and P. Rannou, Science, 2003, 300, 628). We propose that cosmic rays are a significant and effective energy source for producing complex organics and amino acid precursors in Titan's atmospheric haze.

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

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 behavior 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°C) where moisture is desorbed, this stage indicated the high hydrophilicity of the tholins; a second stage, the main pyrolysis stage (150–575°C) 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 (>575°C) 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 toward 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 studied three tholins have a similar main chemical structure which does not alter by the air exposure. We discuss on the possible nature of this structure.

Experimental simulations of CH4 evaporation on Titan

We present the first experimental results on the evaporation of liquid CH4 under simulated Titan surface conditions similar to those observed at the Huygens landing site. An average evaporation rate of (3.1 ± 0.6) × 10−4 kg s−1 m−2 at 94 K and 1.5 bar was measured. While our results are generally higher than previous models based on energy balance, they show an excellent match with a theoretical mass transfer approach. Indeed, we find that evaporation in the Titan environmental chamber is predominantly diffusion driven and affected by the buoyancy of lighter CH4 in the heavier N2 atmosphere. After correcting for the difference in gravity of Earth and Titan, the resulting evaporation rate is (1.6 ± 0.3) × 10−4 kg s−1m−2 (or 1.13 ± 0.3 mm hr−1). Using our experimental evaporation rates, we determine that the low‐latitude storm recently observed by Cassini ISS would have resulted in a maximum evaporated mass of (5.4 ± 1.2) × 1010 kg of CH4 equivalent to a 2.4 ± 0.5 m thick layer over 80 days. Based on our results, a sufficient amount of CH4 can accumulate in the otherwise arid equatorial regions to produce transient ponds and liquid flows.

NMR identification of hexamethylenetetramine and its precursor in Titan tholins: Implications for Titan prebiotic chemistry

The reddish brown organic haze surrounding Titan has been investigated using methods including remote observation, direct exploration (the Cassini mission) and laboratory simulations, but its formation mechanism and the contributing chemical structures and prebiotic potential are still not well understood. We report here the structural investigation of the 13C and 15N labeled, simulated Titan haze aerosol (tholin) using solution-state NMR. These spectra demonstrate a material composed of a mixture of moderate polymer and small molecules. Hexamethylenetetramine (HMT) is identified as the major small molecule component in the Titan tholin and its precursor (1,3,5-hexahydrotriazine) is also detected. We discuss the formation mechanism of HMT and its implications for Titan and early Earth prebiotic chemistry.

Structural Investigation of Titan Tholins by Solution-State 1H, 13C, and 15N NMR: One-Dimensional and Decoupling Experiments

Titan, the largest moon of Saturn, is enveloped in a reddish brown organic haze. Titan haze is presumed to be formed from methane and nitrogen (CH(4) and N(2)) in Titan's upper atmosphere through energetic photochemistry and particle bombardment. Though Titan haze has been directly investigated using methods including the Cassini mission, its formation mechanism and the contributing chemical structures and prebiotic potential are still not well developed. We report here the structural investigation of the (13)C and (15)N labeled, simulated Titan haze aerosol (tholin) by solution-state NMR. The one-dimensional (1)H, (13)C, and (15)N NMR spectra and decoupling experiments indicate that the tholin sample contains amine, nitrile, imine, and N-heteroaromatic compounds of tremendous import in understanding complex organic chemistry in anaerobic, extraterrestrial environments.

Chemical composition of simulated Titan's midatmospheric aerosols

A large fraction of the major unsaturated species (C2H2 C2H4, HCN, and HC3N) with mixing ratios of about 3 × 10−6, 10−7–10−8, 3 × 10−7, and 10−9–10−10 reside in Titan's atmosphere between 150 and 500 km (Vinatier et al., 2009; Coustenis et al., 2007) before they condense near the tropopause. A large flux of medium‐wavelength UV penetrates down to these levels, resulting in the polymerization of these unsaturated compounds and the formation of aerosols. We performed our experiments on aerosol formation at these altitudes where both abundances and solar UV flux are high, bearing in mind that additional photolysis occurs at both higher and lower altitudes. In the gas phase, C2H2 photolysis results in unsaturated C4 species that, on further addition of C2, form the cyclic benzene. These gas‐phase intermediates are consumed when an acetylene‐poor gas mixture is irradiated for a long time, giving rise to larger solid‐state species, mainly by addition of C2, followed by further cyclization. The largest species formed was the condensed 5‐ring benzpyrene, not the 7‐ring coronen molecule, which could have been detected there. Another fraction of the polymers consists of polyvinyl and vinyl acetylene chains, which are cross‐linked due to their labile π electrons and form an insoluble solid matrix. This explains the reduction of the C:H ratio from the condensed aromatics of 1.2 to the measured C:H = 1.013 ± 0.001 of the polymers.

Nonequilibrium Radiation Intensity Measurements inSimulated Titan Atmospheres

This paper details the experimental work conducted at the University of Queensland to measure the nonequilibrium radiation intensity behind a shock in simulated Titan atmospheres, as would be seen during planetary entry. Radiation during Titan entry is more important at lower speeds (about 5-6 km/s) than other planetary entries due to the formation of cyanogen in above equilibrium concentrations in the shock layer, which is a highly radiative species. The experiments were focused on measuring the nonequilibrium radiation emitted from cyanogen between the wavelength range of 310-450 nm. This paper includes experimental results for radiation and spectra found in the postshock region of the flow. Experiments have been conducted at various ambient pressures, shock speeds, and chemical compositions. This leads to a comprehensive benchmark data set for Titan entry, which will be useful for validation of theoretical models. Spectra were recorded at various axial locations behind the shock, enabling the construction of radiation profiles for Titan entry. Furthermore, wavelength profiles can also be constructed to identify various radiating species, in this case, predominately cyanogen violet. Furthermore, this paper includes comparisons with experiments performed at NASA Ames Research Center on their electric arc-driven shock tube in Titan compositions. Excellent quantitative agreement has been obtained between the two facilities.

Optical properties of Titan and early Earth haze laboratory analogs in the mid-visible

Scattering and absorption of sunlight by aerosols are integral to understanding the radiative balance of any planetary atmosphere covered in a haze, such as Titan and possibly the early Earth. One key optical parameter of an aerosol is its refractive index. We have simulated both Titan and early Earth organic haze aerosols in the laboratory and measured the real and imaginary portion of their refractive index at λ = 532 nm using cavity ringdown aerosol extinction spectroscopy. This novel technique allows analysis on freely-floating particles minutes after formation. For our Titan analog particles, we find a real refractive index of n = 1.35 ± 0.01 and an imaginary refractive index k = 0.023 ± 0.007, and for the early Earth analog particles we find n = 1.81 ± 0.02 and k = 0.055 ± 0.020. The Titan analog refractive index has a smaller real and similar imaginary refractive index compared to most previous laboratory measurements of Titan analog films, including values from Khare et al. (Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W. [1984]. Icarus 60, 127–137). These newly measured Titan analog values have implications for spacecraft retrievals of aerosol properties on Titan. The early Earth analog has a significantly higher real and imaginary refractive index than Titan analogs reported in the literature. These differences suggest that, for a given amount of aerosol, the early Earth analog would act as a stronger anti-greenhouse agent than the Titan analog.

Laboratory experiments as support to the built up of Titan's theoretical models and interpretation of Cassini-Huygens data

AbstractTo interpret the concentrations of the products measured in Titan's atmosphere and to better understand the associated chemistry, many theoretical models have been developed so far. Unfortunately, large discrepancies are still found between theoretical and observational data. A critical examination of the chemical scheme included in these models points out some problems regarding the reliability of the description of critical reaction pathways as well as the accuracy of kinetic parameters. Laboratory experiments can be used to reduce these two sources of uncertainty. It can be:i) experimental simulations: in our laboratory (LISA), representative Titan's simulation experiments are planned to be carried out in a reactor where the initial gas mixture will be exposed, for the first time, to both electrons and photons. Thus, the chemistry between N atoms and CH3, CH2, CH fragments, issued from electron dissociation of N2 and photo-dissociation of CH4 respectively, will be initiated. Thank to a time resolved technique, we will be able to analyse “in situ”, qualitatively and quantitatively, the stable species as well as the short life intermediates. Then, the implied chemistry will be determined precisely, and consequently, its description will be refined in theoretical models. The current status of this program will be given.ii) specific experiments: they are devoted, for example, to determine kinetic rate constants and low temperature VUV spectra that will be used to feed models and to interpret observational data. Such experiments performed in LISA and in Rennes' laboratory concern polyynes and cyanopolyynes as these compounds could link the gaseous and the solid phase in planetary atmosphere. Results concerning C4H+ hydrocarbons kinetic rate constants and VUV cross section of HC3N and HC5N will be detailed.

陽子線照射により生成したタイタン「ソーリン」のキャラクタリゼーション

陽子線照射により生成したタイタン「ソーリン」のキャラクタリゼーション

Energy dissipation of possible Titan lightning strokes

The search for lightning on Saturn's satellite Titan is one scientific target of the Cassini/Huygens mission. Although Voyager 1 did not detect any radio emissions caused by Titan lightning during its flyby in November 1980, one cannot generally rule out their existence, because of low flash rates or ionospheric radiation blockade. Recently, Tokano et al. (Planet. Space Sci. 49 (2001a) 539) have developed a thundercloud model in Titan's troposphere favoring the existence of Titan lightning due to negatively charged clouds causing temporary electric fields sufficient to initiate cloud-to-ground lightning strokes. In the present investigation we estimate the amount of energy dissipation of such lightning strokes by electrostatic energy considerations similarly to those by Cooray (J. Geophys. Res. 102(D17) (1997) 21,401). The analysis is based on the cloud charge distribution given by Tokano et al. (2001a), which has a monopole structure or a dipole structure depending on the electrification mechanism. It consists of horizontally homogenous charge layers, whose charge densities depend on the altitude above ground. As results we get the typical charge lowered in a possible Titan lightning stroke and the amount of energy dissipation. For a simulated Titan monopolar cloud charged by electron attachment we found that cloud-to-ground strokes lower about 30C of charge and dissipate energies about 1010J. For the modelled bipolar clouds charged by collisional charging these values are a few C of lowered charge and about 108–109J of dissipated energy, which are quite similar to typical Earth values. These energies are substantially higher than the energies suggested by Desch and Kaiser (Nature 343 (1990) 442), who concluded from the Voyager data that discharges might be frequent but weak (<106J). We shortly discuss the detection capability of the Cassini/RPWS (Radio and Plasma Wave Science experiment) for possible Titan lightning strokes taking into account the wave attenuation in the frequency range up to 16MHz during the propagation through Titan's ionosphere as calculated by Schwingenschuh et al. (Adv. Space Res. 28(10) (2001) 1505).

Titan aerosol analogues: analysis of the nonvolatile tholins.

Laboratory experiments that produced tholins in a simulated Titan atmosphere were conducted. We report the first systematic analyses of these compounds using Fourier-transform ion cyclotron resonance mass spectrometry. The findings suggest surprising simplicity and nonrandomness in the mass distribution and regularity in species clusters. The degree of unsaturation generally increased with increasing molecular weight in a predictable fashion, and nitrogen is proposed as the dominant carrier of unsaturation. In detected compounds with a general formula of C(x)H(y)N(z), the carbon to nitrogen ratio (x/z) varied only slightly within a narrow limit, and decreased with increasing molecular weights. These compounds are of potential prebiotic interest since they sediment to the surface of Titan, and would dissolve readily in transient aqueous pools that might be generated from time to time by impacts and volcanic

Corona chemistry in Titan.

The atmosphere of Titan is constantly bombarded by galactic cosmic rays and Saturnian magnetospheric electrons causing the formation of free electrons and primary ions, which are then stabilized by ion cluster formation and charging of aerosols. These charged particles accumulate in drops in cloud regions of the troposphere. Their abundance can substantially increase by friction, fragmentation or collisions during convective activity. Charge separation occurs with help of convection and gravitational settling leading to development of electric fields within the cloud and between the cloud and the ground. Neutralization of these charged particles leads to corona discharges which are characterized by low current densities. We have therefore, experimentally studied the corona discharge of a simulated Titan's atmosphere (10% methane and 2% argon in nitrogen) at 500 Torr and 298 K by GC-FTIR-MS techniques. The main products have been identified as hydrocarbons (ethane, ethyne, ethene, propane, propene+propyne, cyclopropane, butane, 2-methylpropane, 2-methylpropene, n-butane, 2-butene, 2,2-dimethylpropane, 2-methylbutane, 2-methylbutene, n-pentane, 2,2-dimethylbutane, 2-methylpentane, 3-methylpentane, n-hexane, 2,2-dimethylhexane, 2,2-dimethylpentane, 2,2,3-trimethylbutane, 2,3-dimethylpentane and n-heptane), nitriles (hydrogen cyanide, cyanogen, ethanenitrile, propanenitrile, 2-methylpropanenitrile and butanenitrile) and a highly branched hydrocarbon deposit. We present the trends of hydrocarbons and nitriles formation as a function of discharge time in an ample interval and have derived their initial yields of formation. The results clearly demonstrate that a complex organic chemistry can be initiated by corona processes in the lower atmosphere. Although photochemistry and charged particle chemistry occurring in the stratosphere can account for many of the observed hydrocarbon species in Titan, the predicted abundance of ethene is too low by a factor of 10 to 40. While some ethene will be produced by charged-particle chemistry, the production of ethene by corona processes and its subsequent diffusion into the stratosphere appears to be an adequate source. Because little UV penetrates to the lower atmosphere to destroy the molecules formed there, the corona-produced species may be long-lived and contribute significantly to the composition of the lower atmosphere and surface.

Titan’s Atmosphere and Surface from Infrared Spectroscopy and Imagery

AbstractWe have performed observations of Titan in the near-infrared since 1991, using both spectroscopy and imagery. A higher geometric albedo is observed near the Greatest Eastern Elongation than near the Greatest Western Elongation or Conjunctions. The surface is within reach for telescopes at 1.075, 1.28, 1.6 and 2.0 micron, so we simulated Titan’s geometric albedo in order to derive the surface albedo at all longitudes. Titan’s leading hemisphere appears brighter than the trailing one by 20-25% at 1.08; 1.28 and 1.6 μm, and by 35% at 2.0 μm, with spectra compatible with H2O everywhere. We have then simulated the presence of a mountain on Titan covering 50% of the leading hemisphere disk, varying its height and introducing a higher surface albedo when required. We find that a relief can not reproduce the whole geometric albedo differences observed between the two hemispheres. Some other additional component bright at 1.0 and 2.0 μm must then be invoked. Candidates are tholins or CH4, which can exist on the top of a mountain if the temperature is lower than 90 K.