Thomas W. Scattergood

Production of organic compounds in plasmas: A comparison among electric sparks, laser-induced plasmas, and UV light

The chemistry in planetary atmospheres that is induced by processes associated with high-temperature plasmas is of broad interest because such processes may explain many of the chemical species observed. There are at least two important phenomena that are known to generate plasmas (and shocks) in planetary atmospheres: lightning and meteor impacts. For both phenomena, rapid heating of atmospheric gases leads to formation of a high-temperature plasma which emits radiation and produces shock waves that propagate through the surrounding atmosphere. These processes initiate chemical reactions that can transform simple gases into more complex compounds. In order to study the production of organic compounds in plasmas (shocks), various mixtures of N2, CH4, and H2, modeling the atmosphere of Titan, were exposed to discrete sparks, laser-induced plasmas (LIP), an ultraviolet radiation. The yields of HCN and several simple hydrocarbons were measured by gas chromatography and compared to those calculated from a simple quenched thermodynamic equilibrium model. The agreement between experiment and theory was fair for HCN and C2H2. However, the agreement for C2H6 and the other hydrocarbons was poor, indicating that a more comprehensive theory is needed. Our experiments suggest that photolysis by ultraviolet light from the plasma is an important process in the synthesis. This was confirmed by the photolysis of gas samples exposed to the light but not to the shock waves emitted by the sparks. Hence, the results of these experiments demonstrate that the thermodynamic equilibrium theory does not adequately model lightning and meteor impacts and that photolysis must be included. Finally, the similarity in yields between the spark and the LIP experiments suggest that LIP provide valid and clean simulations of lightning and meteor impacts and that photolysis must be included. Finally, the similarity in yields between the spark and the LIP experiments suggests that LIP provide valid and clean simulations of lightning in planetary atmospheres.

Titan's aerosols. I - Laboratory investigations of shapes, size distributions, and aggregation of particles produced by UV photolysis of model Titan atmospheres

Abstract Titans aerosols are believed to have significant effects on the physical and radiative properties of its atmosphere. To investigate the physical properties of model Titan aerosols, experiments in which acetylene, ethylene, and hydrogen cyanide were photolyzed separately and as a mixture by ultraviolet light have been performed. In general, the individual particles formed were spherical, apparently amorphous, and quite sticky. When 1 Torr of C2H2 (in 55 Torr N2) was photolyzed, the average diameter of the individual particles was about 0.6 μm and most ( ≈ 4 5 ) of the particles were found in nonspherical near-linear aggregates. The mean diameter of the particles decreased to 0.4 μm for 0.1 Torr C2H2 and increased to 0.8 μm for 10 Torr C2H2. Aerosols formed from photolysis of C2H4 were physically similar to those formed from C2H2. Photolysis of HCN rapidly produced particles that apparently did not grow to sizes (>0.09 μm) large enough to be collected and imaged. The formation of particles from acetylene was observed within minutes in our experiments, but was slowed by about a factor of 4 when ethylene and hydrogen cyanide were added.

On the sources of ultraviolet absorption in spectra of Titan and the outer planets

Abstract In response to the observations of the ultravioler deficiencies shown by all of the outer planets and Titan, models have been proposed to explain the low albedos by absorption by particles in the upper atmospheres of these objects. These particles are generally believed to be photochemically formed from gases in the upper atmospheres, primarily methane and hydrogen. Such processes may also be operative on Titan. The results of some laboratory experiments of the proton irradiation of mixtures of gases including CH 4 H 2 , NH 3 , etc., have shown that liquid and solid materials are produced that are strong ultraviolet absorbers. However, the material produced from the CH 4 + H 2 mixture was colorless, indicating that species containing elements other than carbon and hydrogen are necessary for the production of color. Two such elements are nitrogen (as NH 3 or N 2 ) and sulfur (as H 2 S) and colored materials have been produced from such mixtures. None of these materials has spectral properties identical to those shown by the planets. Therefore it is necessary that mixtures (and/or cloud layers) of the photochemical materials be present.

Lightning production of hydrocarbons and HCN on Titan - Laboratory measurements

Many hydrocarbon species have been detected in the atmosphere of Titan. It is possible that lightning activity is occurring in the troposphere and that it contributes to the hydrocarbon inventory. Measurements of the chemical yields of hydrogen cyanide, acetylene, ethylene, ethane, and propane from simulated lightning discharges are reported. A comparison of the experimental results with those based on thermodynamic equilibrium assumptions shows significant disagreement and implies that theories based solely on thermodynamic equilibrium are inadequate. Although photochemistry and charged particle chemistry occurring in the stratosphere can account for many of the observed hydrocarbon species, the predicted abundance of ethylene is too low by a factor of 10 to 40. While some ethylene will be produced by charged-particle chemistry, the production of ethylene by lightning and its subsequent diffusion into the stratosphere appears to be an adequate source.

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

Abstract 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, Icarus 15, 39) and ultraviolet light (Khare, B. N., and Sagan, C., 1973, Icarus 20, 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 6RJ 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.

The role of cometary particle coalescence in chemical evolution

Important prebiotic organic compounds might have been transported to Earth in dust or produced in vapor clouds resulting from atmospheric explosions or impacts of comets. These compounds coalesced in the upper atmosphere with particles ejected from craters formed by impacts of large objects. Coalescence during exposure to UV radiation concentrated organic monomers and enhanced formation of oligomers. Continuing coalescence added material to the growing particles and shielded prebiotic compounds from prolonged UV radiation. These particles settled into the lower atmosphere where they were scavenged by rain. Aqueous chemistry and evaporation of raindrops containing nomomers in high temperature regions near the Earths surface also promoted continued formation of oligomers. Finally, these oligomers were deposited in the oceans where continued prebiotic evolution led to the most primitive cell. Results of our studies suggest that prebiotic chemical evolution may be an inevitable consequence of impacting comets during the late accretion of planets anywhere in the universe if oceans remained on those planetary surfaces.

The physical nature of Titan's aerosols: laboratory simulations.

The atmosphere of Titan is known to contain aerosols, as evidenced by the Voyager observations of at least three haze layers. Such aerosols can have significant effects on the reflection spectrum of Titan and on the chemistry and thermal structure of its atmosphere. To investigate some of these effects, laboratory simulations of the chemistry of Titans atmosphere have been done. The results of these studies show that photolysis of acetylene, ethylene, and hydrogen cyanide, known constituents of Titans atmosphere, yields sub-micron sized spheres, with mean diameters ranging from 0.4 to 0.8 microns, depending on the pressures of the reactant gases. Most of the spheres are contained in near-linear aggregates. The formation of the aggregates is consistent with models of Titans reflection spectrum and polarization, which are best fit with non-spherical particles. At room temperature, the particles are very sticky, but their properties at low temperatures on Titan are presently not known.

Laboratory experiments in the study of the chemistry of the outer planets

The investigation of chemical evolution of bodies in our solar system has, in the past, included observations, theoretical modeling, and laboratory simulations. Of these programs, the last one has been the most criticized due to the inherent difficulties in accurately recreating alien environments in the laboratory. Processes such as wall reactions and changes in chemistry due to difficulties in achieving realistic conditions of temperature, pressure, composition, and energy flux may yield results which are not truly representative of the systems being modeled. However, many laboratory studies have been done which have yielded data useful in planetary science. Gross simulations of atmospheric chemistry have placed constraints on the nature of complex molecules expected in planetary atmospheres. More precise studies of specific chemical processes have provided information about the sources and properties of product gases and aerosols. Determinations of basic properties such as spectral features and reaction rate constants yield data useful in the interpretation of observations and in computational modeling. Alone, and in conjunction with modeling, laboratory experiments will continue to be used to further our understanding of the outer solar system, and some experiments that need to be done are listed.

On the abundance of NO2 in the Martian atmosphere

Abstract Spectrophotometric scans of Mars and the Moon in the region 4000–5000 A were obtained and ratioed. No evidence of any absorption greater than 3% is visible in the Martian spectrum. Using our own laboratory spectra of NO 2 as well as the published work of Hall and Blacet (1952) we confirm Marshalls (1964) upper limit of 8 μm atmospheres (0.0008 cm amagat) for the abundance of NO 2 in the atmosphere of Mars.

Exobiology research on Space Station Freedom

The Gas-Grain Simulation Facility (GGSF) is a multidisciplinary experiment laboratory being developed by NASA at Ames Research Center for delivery to Space Station Freedom in 1998. This facility will employ the low-gravity environment of the Space Station to enable aerosol experiments of much longer duration than is possible in any ground-based laboratory. Studies of fractal aggregates that are impossible to sustain on Earth will also be enabled. Three research areas within exobiology that will benefit from the GGSF are described here. An analysis of the needs of this research and of other suggested experiments has produced a list of science requirements which the facility design must accommodate. A GGSF design concept developed in the first stage of flight hardware development to meet these requirements is also described.