Photochemical aerosols in Titan's atmosphere

The optical properties of polymers, produced photolytically from ethylene, which was detected in Titan's atmosphere and from acetylene or hydrogen cyanide which may be present there, were studied experimentally. It is shown that an aerosol consisting of polyethylene provides an excellent fit to the variation of Titan's albedo with wavelength, while polymers of acetylene or hydrogen cyanide do not. This fit seems to remove the requirement of nitrogen-bearing polymers, which was proposed earlier to account for Titan's red coloration. Therefore, Titan's coloration does not necessarily imply the presence of nitrogen in its atmosphere. It is also proposed that above the layer of larger aerosol particles, whose scattering determines the phase function, there are smaller particles of the same material, which act as an absorbing haze to darken and slightly redden the underlying aerosol. This high-altitude haze also causes the observed strong limb-darkening.     

Experimental study of the degradation of polymers: Application to the origin of extended sources in cometary atmospheres

Abstract— This paper presents some preliminary results concerning the degradation of refractory nitrogenated polymers, which could be responsible for the CN extended source in comets. We are studying hexamethylenetetramine (HMT) and HCN polymers. Both compounds have been irradiated or heated to simulate the degradation processes they undergo in the cometary atmosphere. We show that, even if both compounds are quite stable under photolysis, the heating leads to a much more efficient degradation with the formation of HCN, NH₃, and other heavier compounds. Moreover, the thermal degradation of HCN polymers appears to be more efficient than that of HMT. Thus, the HCN polymers seem to be better candidates for the CN extended source. We are now developing a new reactor to quantify the production of gaseous molecules and to detect in situ CN radicals.     

Formaldehyde polymers in comets

The possibility that crystalline formaldehyde polymers are present in cometary dust is discussed. In common with most other parent molecules proposed for comets, (H₂CO) _n is difficult to detect, even if it is present in relatively high concentrations. The optical properties of these polymers in the visual and infrared regions are similar to those of silicate grains, and crystalline formaldehyde polymers provide no emission at 6 cm wavelength. The lifetime of gaseous H₂CO in the solar radiation field is too short, and the expected transitions in the microwave region would be too weak to be detected. However, the available data concerning the physical properties of comets indicate that polymerized formaldehyde cannot be ruled out as a major constituent of cometary material.     

Formaldehyde Polymers in Comets

The possibility that crystalline formaldehyde polymers are present in cometary dust is discussed. In common with most other parent molecules proposed for comets, (H₂CO)_n is difficult to detect, even if it is present in relatively high concentrations. The optical properties of these polymers in the visual and infrared regions are similar to those of silicate grains, and crystalline formaldehyde polymers provide no emission at 6 cm wavelength. The lifetime of gaseous H₂CO in the solar radiation field is too short, and the expected transitions in the microwave region would be too weak to be detected. However, the available data concerning the physical properties of comets indicate that polymerized formaldehyde cannot be ruled out as a major constituent of cometary material. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the origin of HNC in Comet Lee

We have modelled the chemistry occurring in the coma of Comet Lee and have critically evaluated the possible routes leading to HNC. We show that the observed levels of HNC cannot be produced by ion–molecule chemistry, or by reactions of energetic H atoms with HCN. Rather, it appears that HNC is injected into the coma following the photodestruction of an unknown precursor. We discuss the possible nature of the parent of HNC and conclude that photofragmentation of large HCN polymers, such as polyaminocyanomethylene (PACM), is responsible. The degradation of hydrogen cyanide polymers may constitute a common source of HNC in comets, accounting for HNC/HCN ratios in the range measured in Lee and Hyakutake (≈ 0.06–0.12). The high HNC/HCN ratio measured in Hale–Bopp (≈0.2) and its heliocentric variation may, however, require an additional source.     

Production of organic molecules in the outer solar system by proton irradiation: Laboratory simulations

In the past few years considerable attention has been given to the determination of likely compounds that could account for the various colors observed in the outer solar system: and to possible formation mechanisms for these compounds. Many experiments have been done using electrical discharges (Chadha, M. S., et al., 1971, Icarus15, 39) and ultraviolet light (Khare, B. N., and Sagan, C., 1973, Icarus20, 311) on mixtures of CH₄, NH₃, and H₂S, which are most likely the dominant minor constituents of the atmospheres of Jupiter, Saturn, Titan, and possibly the other satellites early in their histories. Colored polymers, usually brownish-red, have been produced in these experiments. With the passage of Pioneer 10 around Jupiter, there is another source of energy worthy of consideration, energetic protons (and electrons). Preliminary experiments to investigate the formation of colored polymers and other interesting molecules by the irradiation of gas mixtures by protons are discussed. Two to four Mev protons were used, with corresponding beam fluxes (as measured at 6R_J from the planet) equivalent to approximately 80 Earth years at Jupiter per hour of exposure. As in the other types of experiments, colored polymers have been produced. An important feature of this work is the presence or absence of absorption at 5 μm in the different materials produced; Titan is quite dark at this wavelength and Io is fairly bright. Such features may provide criteria for accepting or rejecting various materials produced in these experiments as reasonable coloring agents for the outer solar system.     

Titan haze: structure and properties of cyanoacetylene and cyanoacetylene-acetylene photopolymers

The structure and morphological properties of polymers produced photochemically from the UV irradiation of cyanoacetylene and cyanoacetylene mixtures have been examined to evaluate their possible contribution to the haze layers found on Titan. A structural analysis of these polymers may contribute to our understanding of the data returned from the Huygens probe of the Cassini mission that will pass through the atmosphere of Titan in the year 2004. Infrared analysis, elemental analysis, and thermal methods (thermogravimetric analysis, thermolysis, pyrolysis) were used to examine structures of polycyanoacetylenes produced by irradiation of the gas phase HC3N at 185 and 254 nm. The resulting brown to black polymer, which exists as small particles, is believed to be a branched chain of conjugated carbon-carbon double bonds, which, on exposure to heat, cyclizes to form a graphitic structure. Similar methods of analysis were used to show that when HC3N is photolyzed in the presence of Titan's other atmospheric constituents (CH4, C2H6, C2H2, and CO), a copolymer is formed in which the added gases are incorporated as substituents on the polymer chain. Of special significance is the copolymer of HC3N and acetylene (C2H2). Even in experiments where C2H2 was absorbing nearly all of the incident photons, the ratio of C2H2 to HC3N found in the resulting polymer was only 2:1. Scanning electron microscopy was used to visually examine the polymer particles. While pure polyacetylene particles are amorphous spheres roughly 1 micrometer in diameter, polycyanoacetylenes appear to be strands of rough, solid particles slightly smaller in size. The copolymer of HC3N and C2H2 exhibits characteristics of both pure polymers. This is particularly important as pure polyacetylenes do not match the optical constants measured for Titan's atmospheric hazes. The copolymers produced by the incorporation of other minor atmospheric constituents, like HC3N, into the polyacetylenes are expected to have optical constants more comparable to those of the Titan haze.     

Thunderstorms on Jupiter

The presence of a considerable acetylene concentration on Jupiter, despite the fast rate of its photolytic hydrogenation, provides strong evidence for the operation of frequent and powerful thunderstorms in the Jovian atmosphere. Whereas acetylene regeneration can occur only during thunderstorms, the photolytically destroyed ammonia can be regenerated both in thunderstorms and in the low and hot atmospheric levels. A rate of Earthlike lightning strokes 10⁴ times larger than on Earth is inferred from the calculated rate of acetylene's photolytic destruction. The rate of acetylene production by thunder shock waves and the products obtained from its photolytic hydrogenation can account for the large ethane concentration and the absence of ethylene. The yellow-brown acetylene polymer and the ruby-red polymers, obtained from thunder-produced hydrogen cyanide or cyanogen with ammonia, are likely contributor to the Jovian coloration.     

Fast tidal cycling and the origin of life

Replicating prebiotic polymers are thought to predate the emergence of true life-forms. The initial mode of replication, a prerequisite for Darwinian selection, is unknown, but demands an explanation based on local physicochemistry. Dual consideration of the conditions of the early terrestrial surface, with the unusual physicochemical properties of nucleic acids like DNA, could explain the emergence of nucleic acids as key biomolecules. The early impact that produced the Moon, and fast terrestrial rotation, subjected coastal areas 3.9 Ga ago to rapid tidal flooding (dilution) and drying (concentration), with a likely periodicity in the range of 2-6 h, and could have provided a driving force for cyclic replication of early biomolecules. Such a mechanism applies only to molecules capable of association/polymerization at high salt concentration, and of dissociation at low salinity. Nucleic acids meet these criteria. It is suggested that tidal cycling, resembling the polymerase chain reaction (PCR) mechanism, could only replicate and amplify DNA-like polymers. This mechanism suggests constraints on the evolution of extra-terrestrial life.     

Transition from Gaseous Compounds to Aerosols in Titan's Atmosphere

We investigate the chemical transition of simple molecules like C₂H₂ and HCN into aerosol particles in the context of Titan's atmosphere. Experiments that synthesize analogs (tholins) for these aerosols can help illuminate and constrain these polymerization mechanisms. Using information available from these experiments, we suggest chemical pathways that can link simple molecules to macromolecules, which will be the precursors to aerosol particles: polymers of acetylene and cyanoacetylene, polycyclic aromatics, polymers of HCN and other nitriles, and polyynes. Although our goal here is not to build a detailed kinetic model for this transition, we propose parameterizations to estimate the production rates of these macromolecules, their C/N and C/H ratios, and the loss of parent molecules (C₂H₂, HCN, HC₃N and other nitriles, and C₆H₆) from the gas phase to the haze. We use a one-dimensional photochemical model of Titan's atmosphere to estimate the formation rate of precursor macromolecules. We find a production zone slightly lower than 200 km altitude with a total production rate of 4×10⁻¹⁴ g cm⁻² s⁻¹ and a C/N?4. These results are compared with experimental data, and to microphysical model requirements. The Cassini/Huygens mission will bring a detailed picture of the haze distribution and properties, which will be a great challenge for our understanding of these chemical processes.     

Organo-Siliceous Biomolecules and the Infrared Spectrum of the Trapezium Nebula

A close correspondence exists between the infrared properties of a mixed culture of diatoms and the infrared spectrum of dust in the Trapezium nebula. We argue that this correspondence points to a cosmic microbiological system in which organo-siliceous polymers are an abundant constituent. The high content of Si relative to Mg found in the Earth's crust and in Lunar and Martian surface material is readily explained on the basis of accretion of silicon-rich microbiology. This revised version was published online in July 2006 with corrections to the Cover Date.     

Aging of Titan's Aerosols

Titan's haze is composed of aerosols containing long chain polymers of acetylene with some hydrogen cyanide. These polymers have alternating double/single and triple/single bonds, which can open spontaneously or under the action of UV radiation or particle impact. Once opened, they can induce the opening of a double or triple bond in an adjacent chain and link to it. This cross-linking and chain elongation hardens or “ages” the polymer particles, making them less sticky. As observed experimentally and calculated theoretically, newly formed polymer particles grow by collecting other polymer chains and by complete merging into symmetrical spheres. However, when aged, they merely adhere to each other and do not merge. Eventually, when hard enough, they do not even adhere to each other. In this paper we calculate the spontaneous aging process as applied to Titan's atmospheric conditions and find that the surface tension and viscosity of the aerosols below H∼570 km are one order of magnitude harder than when the aerosols formed. Furthermore, UV irradiation and particle impacts reduce both viscosity and surface tension by an additional factor of 10-100. Thus, the aerosol particles expected to be encountered by the descending Huygens probe will, most likely, be quite hard.     

Astrobiological molecularly imprinted polymer sensors

The purpose of the Astrobiological MIP Sensor (AMS) Project is to develop reliable, low-cost, low-mass, low-power consumption detection technologies for in situ analysis of biochemical markers, and other indicators of astrobiological importance. To this end, we are investigating the potential role that molecularly imprinted polymers (MIP) could serve in the recognition of pre-biotic and biotic compounds in planetary, astrobiological and geochemical mission profiles. While MIPs are effective molecular recognition tools, a signal transduction method must be developed so that the recognition of analytes can be realized. In the course of this study, surface plasmon resonance (SPR) will be the detection method of the MIP recognition event. In addition, MIP-coated SPR substrates were subjected to vibration, temperature and radiation testing to demonstrate that they could withstand the rigors of space travel. The methods developed in this study require capture of the biomarkers onto the SPR sensor chip, followed by addition of a MIP. It is the binding of the MIP to the SPR bound analyte that amplifies the SPR signal associated with binding of the low molecular weight analyte. The MIPs, developed in this study are water-soluble processable star polymers while the SPR device used was SensíQ™ by Nomatics. Proof-of-principal experiments were first demonstrated using amino biotin.     

Heliocentric evolution of the degradation of polyoxymethylene: Application to the origin of the formaldehyde (H2CO) extended source in Comet C/1995 O1 (Hale-Bopp)

The H₂CO production rates measured in Comet C/1995 O1 (Hale-Bopp) from radio wavelength observations [Biver, N., and 22 colleagues, 2002a. Earth Moon Planets 90, 5-14] showed a steep increase with decreasing heliocentric distance. We studied the heliocentric evolution of the degradation of polyoxymethylene (formaldehyde polymers: (CH₂O)_n, also called POM) into gaseous H₂CO. POM decomposition can indeed explain the H₂CO density profile measured in situ by Giotto spacecraft in the coma of Comet 1P/Halley, which is not compatible with direct release from the nucleus [Cottin, H., Bénilan, Y., Gazeau, M.-C., Raulin, F., 2004. Icarus 167, 397-416]. We show that the H₂CO production curve measured in Comet C/1995 O1 (Hale-Bopp) can be accurately reproduced by this mechanism with a few percents by mass of solid POM in grains. The steep heliocentric evolution is explained by the thermal degradation of POM at distances less than 3.5 AU. This study demonstrates that refractory organics present in cometary dust can significantly contribute to the composition of the gaseous coma. POM, or POM-like polymers, might be present in cometary grains. Other molecules, like CO and HNC, might also be produced by a similar process.     

Organo-siliceous biomolecules and the infrared spectrum of the Trapezium nebula

A close correspondence exists between the infrared properties of a mixed culture of diatoms and the infrared spectrum of dust in the Trapezium nebula. We argue that this correspondence points to a cosmic microbiological system in which organo-siliceous polymers are an abundant constituent. The high content of Si relative to Mg found in the Earth's crust and in Lunar and Martian surface material is readily explained on the basis of accretion of silicon-rich microbiology.     

Spectrophotometry of Jupiter

Spectrophotometric maps of Jupiter were made between 24 and 27(UT) November 1974 on the McMath Solar Telescope at Kitt Peak National Observatory. We report a comparison between observed scaled reflectivities of the Jovian North Tropical Zone and the North Equatorial Belt at System II longitudes between 195 and 205 degrees. The belt/zone reflectivity ratios between 430 and 740nm are related to the optical transmission curves of the organic and/or sulfur polymers synthesized by Khare and Sagan in a simulated Jovian atmosphere.     

Red clouds in reducing atmospheres

A dark reddish-brown high-molecular-weight polymer is produced by long wavelength ultraviolet irradiation of abundant gases in reducing planetary atmospheres. The polymer i examined by paper chromatography, mass spectrometry, and infrared, visible, and ultraviolet spectroscopy. High carbon-number straight-chain alkanes with NH₂ and, probably, OH and CO groups are identified, along with the previously reported amino acids. There are chemical similarities between this polymer and organic compounds recovered from carbonaceous chondrites and precambrian sediments. The visible and near-ultraviolet transmission spectrum of the polymer shows its absorption optical depth to be redder than λ^?2 and perhaps similar in coloration to the clouds of Jupiter, Saturn, and Titan. The near-ultraviolet absorption coefficient is ～10³ cm^?1, and typical grain sizes ～30 μm. The nitrile content is small, and the polymer should be semitransparent in the 5 μm atmospheric window. Such polymers may be a common constituent of clouds in the outer solar system and on the early Earth.     

CH4/N2/H2-spark hydrophobic tholins: A systematic approach to the characterisation of tholins. Part II

Two different simulation experiments of prebiotic synthesis were carried out in a CH₄/N₂/H₂ atmosphere with spark discharge activation of aqueous aerosols and liquid water. In both cases, a hydrophilic tholin and a hydrophobic tholin were obtained. The methodology developed by our group for the characterisation of hydrophilic tholins [Ruiz-Bermejo, M., Menor-Salván, C., Mateo-Martí, E., Osuna-Esteban, S., Martín-Gago, J.A., Veintemillas-Verdaguer, S., 2008. Icarus 198, 232-241] was used in order to study the hydrophobic tholins. The gas precursors of the tholins from mixtures containing CH₄, with and without H₂, were studied. We propose that the formation of the hydrophobic tholins involves the formation of unsaturated oligomeric hydrocarbon chains from vinyl and acetylene monomers, as well as allene derivatives formed in the gas phase after the incorporation of polar groups into these hydrocarbon chains. Finally, we compare our results concerning hydrophobic tholins with HCN polymers, since it is generally suggested that the polymeric material formed in spark experiments are possible oligomers of HCN, and that Titan’s tholins could be poly-HCN.     

Carbon suboxide and the genetic code

Carbon suboxide (C₃O₂) polymers formed in the primitive atmosphere would have produced an organic compound soup of high concentration on the Earth. Various vestiges of C₃O₂ are found in the present genetic scheme, which might suggest that the living system had formed from the polymer soup.     

Correlation Between the Population of Active Species in Low Pressure Plasmas and Their Reactions with Organic Compounds

The oxidation of four alkanes, two olefins and twelve aromatic compounds in the liquid phase with either high voltage or radio frequency glow discharges has been studied. The results of the liquid-plasma interactions of this work considerably differ from those in homogeneous gas phase. Neither fragmentation products in the traps nor polymers on the reactor walls have been detected. The selectivity of product formation is far superior to plasma oxidations in gas phase, making these techniques attractive for preparative chemistry. Total conversion, i.e., mass transformed against initial mass of substrate, has been studied as a function of the temperature of the liquid and the oxygen flow rate in the reactor. Optical emission spectroscopy has been utilized to detect and monitor the active species in the plasma. At the conditions for maximum oxidation no evidence of O₂(¹_g) has been observed. A correlation between the behavior of the O(³P) population in the discharge and the reactivity of the different compounds, as a function of the oxygen flow rate in the reactor, is presented.