Acetylene Abundance Research Articles

Hydrocarbon chemistry in the inner regions of planet-forming disks

Context. The analysis of the mid-infrared spectra helps understanding the composition of the gas in the inner, dense and warm terrestrial planet forming region of disks around young stars. ALMA has detected hydrocarbons in the outer regions of the planet forming disk and Spitzer detected C2H2 in the inner regions. JWST-MIRI provides high spectral resolution observations of C2H2 and a suite of more complex hydrocarbons are now reported. Interpreting the fluxes observed in the spectra is challenging and radiation thermo-chemical codes are needed to properly take into account the disk structure, radiative transfer, chemistry and thermal balance. Various disk physical parameters like the gas-to-dust ratio, dust evolution including radial drift, dust growth and settling can affect the fluxes observed in the mid-IR. Still, thermo-chemical disk models were not always successful in matching all observed molecular emission bands simultaneously. Aims. The goal of this project is two-fold. Firstly, we analyse the warm carbon chemistry in the inner regions of the disk, namely within 10 au, to find pathways forming C2H2 potentially missing from the existing chemical networks. Secondly, we analyse the effect of the new chemistry on the line fluxes of acetylene. Methods. We used the radiative thermo-chemical disk code called PRODIMO to expand the hydrocarbon chemistry that occurs in a typical standard T Tauri disks. We used the UMIST and the KIDA rate databases for collecting reactions for the species. We included a number of three-body and thermal decomposition reactions from the STAND2020 network. We also included isotopomers for the species that were present in the databases. The chemistry was then analysed in the regions that produce observable features in the mid-infrared spectra. We studied the effect of expanding the hydrocarbon chemistry on the mid-infrared spectra. Results. Acetylene is formed via two pathways in the surface layers of disks: neutral-neutral and ion-neutral. They proceed via the hydrogenation of C or C+, respectively. Thus, the abundances of C, C+, H and H2 affect the formation of C2H2. Therefore, also the formation of H2 indirectly affects the abundance of acetylene. Chemisorbed H is more efficient in forming H2 compared to physisorbed H at warm temperatures and hence increases the abundance of C2H2. Conclusions. We provide a new extended warm chemical network that considers up to eight carbon atom long species, while also taking into account different isotopomers and can form the building blocks of PAHs: C6H6. For a standard T Tauri disk with a canonical value of gas-to-dust mass, the line fluxes increase only by a factor of less than 2. JWST is now detecting hydrocarbons such as methane, acetylene, and C4H2 in disks with a high C/O ratio. Hence, this new extended warm hydrocarbon network will aid in interpreting the observed mid-infrared fluxes.

Reaction kinetic modelling of tar cracking in a non-thermal plasma reactor: Model evaluation and reaction mechanism investigation

Gasification is a process that converts waste into syngas, offering various applications such as fuel, energy, hydrogen production, or further chemicals synthesis. However, tar formation hinders its application due to fouling, corrosion, and catalyst deactivation. Non-thermal plasma (NTP) has emerged as a promising method for tar reforming and cracking, demonstrated through surrogate compounds at the laboratory scale. The scope of this study is to investigate the reaction mechanisms in NTP tar cracking using a zero-dimensional (0D) plasma reactor model with Arrhenius kinetics, aiming to elucidate complex pathways, validate against experimental data, and contribute to the understanding of the reactions. The modelling work utilises a global model to represent NTP. This work collects various reaction kinetics and validates predictions against experimental data. The approach begins with heptane modelling, which is later expanded into (1) toluene in hydrogen gas and (2) a tar mixture (phenol, toluene, and naphthalene) in an oxygen-steam mixture. The model predicts external experimental results with good accuracy, especially at temperatures ranging from 600 to 800 K. The results reveal that radical collisions have a more significant impact on the process than electron impact reactions. Toluene and naphthalene ring-opening reactions occurred when a reactive functional group attacks aromatic double bonds. Finally, the model estimates tar cracking in syngas environment. The low oxygen environment results in secondary tar formation and an abundance of acetylene at the output. Lastly, the assumptions adopted in this study are verified by investigating the impact of plasma and electron temperatures, along with a comparative review of current methodologies in NTP chemistry modelling.

On the formation and spectral signatures of magnesacyclopropene (c-MgC[formula omitted]H[formula omitted

Magnesium’s divalent nature and likely importance as a building block for planet formation lends it credence to being a reasonable replacement for oxygen in organometallic species found in the interstellar medium (ISM). With the natural abundance of acetylene and magnesium and the presence of propylene oxide in the ISM, a natural next-step for astrochemical exploration of such organometallic molecules would be magnesacyclopropene (c-MgC2H2). The present work utilizes a combination of highly accurate quantum chemical quartic force field approaches to provide fundamental anharmonic frequencies to assist in the observation of the 11A1 and 13B2 electronic states of c-MgC2H2 and potential energy surface (PES) scans of Mg dissociation to assist in possible formation pathways of this molecule. The 13B2ω7 mode is less than 0.5 cm−1 above previous theory implying that the current work is accurate and reliable. The 13B2 electronic state is lower in energy than the 11A1 by approximately 10.3 kcal/mol, while the 11A1 electronic state is more rovibrationally observable with three fundamental vibrational frequencies having intensities of greater than 100 km/mol and a dipole moment of 5.47 D. The PES scan of the Mg dissociation shows a flatter well for the 13B2, rather than the 11A1, electronic state offering overlap between the vibronic levels. While radiative association is likely required for formation of c-MgC2H2, the triplet surface is rather flat reducing the energy gaps between vibrational levels. Additionally, the singlet surface in many ways mirrors the triplet providing for both likely and low-energy transitions between the states.

Jupiter in the Ultraviolet: Acetylene and Ethane Abundances in the Stratosphere of Jupiter from Cassini Observations between 0.15 and 0.19 μm

Abstract At wavelengths between 0.15 and 0.19 μm, the far-ultraviolet spectrum of Jupiter is dominated by the scattered solar spectrum, attenuated by molecular absorptions primarily by acetylene and ethane, and to a lesser extent ammonia and phosphine. We describe the development of our radiative transfer code that enables the retrieval of abundances of these molecular species from ultraviolet reflectance spectra. As a proof-of-concept we present an analysis of Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of the disk of Jupiter during the 2000/2001 flyby. The ultraviolet-retrieved acetylene abundances in the upper stratosphere are lower than those predicted by models based solely on infrared thermal emission from the mid-stratosphere observed by the Composite Infrared Spectrometer (CIRS), requiring an adjustment to the vertical profiles above 1 mbar. We produce a vertical acetylene abundance profile that is compatible with both CIRS and UVIS, with reduced abundances at pressures <1 mbar: the 0.1 mbar abundances are 1.21 ± 0.07 ppm for acetylene and 20.8 ± 5.1 ppm for ethane. Finally, we perform a sensitivity study for the JUICE ultraviolet spectrograph, which has extended wavelength coverage out to 0.21 μm, enabling the retrieval of ammonia and phosphine abundances, in addition to acetylene and ethane.

Uranus in Northern Midspring: Persistent Atmospheric Temperatures and Circulations Inferred from Thermal Imaging

Abstract We present results from mid-infrared imaging of Uranus at wavelengths of 13.0 and 18.7 μm, sensing emission from the stratosphere and upper troposphere, acquired using the VISIR instrument at the Very Large Telescope, 2018 September 4–October 20. Using a combination of inverse and forward modeling, we analyze these northern midspring (L s ∼ 46°) images and compare them to archival data to assess seasonal changes since the 1986 southern solstice and subsequent equinox. We find the data are consistent with little change (<0.3 K) in the upper tropospheric temperature structure, extending the previous conclusions of Orton et al. well past equinox, with only a subtle increase in temperature at the emerging north pole. Additionally, spatial-temporal variations in 13 μm stratospheric emission are investigated for the first time, revealing meridional variation and a hemispheric asymmetry not predicted by models. Finally, we investigate the nature of the stratospheric emission and demonstrate that the observed distribution appears related and potentially coupled to the underlying tropospheric emission six scale heights below. The observations are consistent with either midlatitude heating or an enhanced abundance of acetylene. Considering potential mechanisms and additional observations, we favor a model of acetylene enrichment at midlatitudes resulting from an extension of the upper tropospheric circulation, which appears capable of transporting methane from the troposphere, through the cold trap, and into the stratosphere for subsequent photolysis to acetylene.

Particle formation in premixed ethylene-benzene flames: An experimental and modeling study

In this work soot formation was studied in laminar premixed flames of binary ethylene-benzene mixtures varying throughout the composition range from pure ethylene to pure benzene keeping constant the equivalence ratio (φ = 2) and obtaining a very similar maximum temperature (Tmax around 1750 K). In such way, it was possible to study for the first time the effect of binary aliphatic-aromatic fuel mixtures composition on the sooting behavior in comparable combustion conditions. In-situ optical techniques (laser induced incandescence and fluorescence) and ex-situ particle size distribution (PSD) measured downstream of the flame front, as well as modeling by means of a multi-sectional method, were applied. PSD profiles showed that particles with sizes less than 10 nm decrease as benzene percentage in the feed mixture increases, disappearing for benzene percentages above 30%. Conversely, large aggregates grow towards sizes larger than 100 nm when benzene concentration is increased. A non-linear effect of the benzene content in the binary fuel mixture on soot particle concentration was observed by laser induced incandescence, and confirmed by the multi-sectional model. In particular soot formation was found to increase more than linear up to 50% then leveled off to reincrease linearly from 80% to 100%. On the contrary, particles smaller than 10 nm at the end of the flame rapidly decreased for benzene percentages larger than 30%. From reaction rate analysis, the formation of gas-phase polycyclic aromatic hydrocarbons (PAH) and high-molecular mass aromatics precursors was found to be significantly large already for fuel mixtures featured by low benzene amounts (from 10 up to 40–50%). The enhanced aromatic precursor formation, combined with the abundance of acetylene mainly coming from the dehydrogenation of ethylene as predominant component of the binary fuel mixture, appeared to be responsible for the non-linear effect of ethylene-benzene composition on particle formation, particularly significant up to 40–50% of benzene.This finding has a considerable importance as regards the exploitation of highly-aromatic fuels as well as to foresee the soot emission for effect of the aromatic presence in natural and synthetic fuels used in practical combustion systems.

VULCAN: An Open-source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres

Abstract We present an open-source and validated chemical kinetics code for studying hot exoplanetary atmospheres, which we name VULCAN. It is constructed for gaseous chemistry from 500 to 2500 K, using a reduced C–H–O chemical network with about 300 reactions. It uses eddy diffusion to mimic atmospheric dynamics and excludes photochemistry. We have provided a full description of the rate coefficients and thermodynamic data used. We validate VULCAN by reproducing chemical equilibrium and by comparing its output versus the disequilibrium-chemistry calculations of Moses et al. and Rimmer & Helling. It reproduces the models of HD 189733b and HD 209458b by Moses et al., which employ a network with nearly 1600 reactions. We also use VULCAN to examine the theoretical trends produced when the temperature–pressure profile and carbon-to-oxygen ratio are varied. Assisted by a sensitivity test designed to identify the key reactions responsible for producing a specific molecule, we revisit the quenching approximation and find that it is accurate for methane but breaks down for acetylene, because the disequilibrium abundance of acetylene is not directly determined by transport-induced quenching, but is rather indirectly controlled by the disequilibrium abundance of methane. Therefore we suggest that the quenching approximation should be used with caution and must always be checked against a chemical kinetics calculation. A one-dimensional model atmosphere with 100 layers, computed using VULCAN, typically takes several minutes to complete. VULCAN is part of the Exoclimes Simulation Platform (ESP; exoclime.net) and publicly available at https://github.com/exoclime/VULCAN.

The excitation of circumstellar C2in carbon-rich post-AGB object IRAS 22272+5435

Context. Observations of the circumstellar envelopes of post-asymptotic giant branch (post-AGB) stars provide information about their mass-loss history during the AGB phase and about the chemistry inside envelopes after that phase. Aims. We analyze visual observations of C2 molecule in absorption and millimeter and sub-millimeter observations of CO molecule in emission with the aim of determining the physical and chemical conditions in the circumstellar shell of the carbon-rich post-AGB star IRAS 22272+5435. Methods. We determined the column densities and excitation structure of C2 from equivalent width analysis of the molecular absorptions. The thermal and density structure of post-AGB shell (AGB remnant) is constrained by multilevel radiative transfer modeling of CO emission lines. The chemical structure of the envelope was computed and then used in the multilevel radiative transfer in C2 for comparison with observed column densities. Results. We estimate the column density of C2 to be 3.2 × 1015 cm−2. From the chemical model we estimate peak abundance of C2 as 6.8 × 10−6 relative to nucleon density. The absorption of molecular lines originate in the ring between 5 and 10 × 1016 cm. The excitation temperature of the lowest levels of 58 K is consistent with the gas kinetic temperature derived from the CO modeling. The initial abundance of the parent molecule C2H2 inferred from the analysis is found to be 1.27 × 10−5 relative to nucleon density. Conclusions. C2 molecule is a promising tool for probing the temperature structure of the envelopes of post-AGB objects and indirectly for determining the chemical abundance of acetylene.

Meridional variations in stratospheric acetylene and ethane in the southern hemisphere of the saturnian atmosphere as determined from Cassini/CIRS measurements

These are the first results from nadir studies of meridional variations in the abundance of stratospheric acetylene and ethane from Cassini/CIRS data in the southern hemisphere of Saturn. High resolution, 0.5 cm −1, CIRS data was used from three data sets taken in June–November 2004 and binned into 2° wide latitudinal strips to increase the signal-to-noise ratio. Tropospheric and stratospheric temperatures were initially retrieved to determine the temperature profile for each latitude bin. The stratospheric temperature at 2 mbar increased by 14 K from 9° to 68° S, including a steep 4 K rise between 60° and 68° S. The tropospheric temperatures showed significantly more meridional variation than the stratospheric ones, the locations of which are strongly correlated to that of the zonal jets. Stratospheric acetylene abundance decreases steadily from 30 to 68° S, by a factor of 1.8 at 2.0 mbar. Between 18° and 30° S the acetylene abundance increases at 2.0 mbar. Global values for acetylene have been calculated as ( 1.9 ± 0.19 ) × 10 −7 at 2.0 mbar, ( 2.6 ± 0.27 ) × 10 −7 at 1.6 mbar and ( 3.1 ± 0.32 ) × 10 −7 at 1.4 mbar. Global values for ethane are also determined and found to be ( 1.6 ± 0.25 ) × 10 −5 at 0.5 mbar and ( 1.4 ± 0.19 ) × 10 −5 at 1.0 mbar. Ethane abundance in the stratosphere increases towards the south pole by a factor of 2.5 at 2.0 mbar. The increase in stratospheric ethane is especially pronounced polewards of 60° S at 2.0 mbar. The increase of stratospheric ethane towards the south pole supports the presence of a meridional wind system in the stratosphere of Saturn.

OH and halogen atom influence on the variability of non-methane hydrocarbons in the Antarctic Boundary Layer

Measurements of C2‐C8 non-methane hydrocarbons (NMHCs) have been made in situ at Halley Base, Antarctica (75 ◦ 35 � S, 26 ◦ 19 � W) from February 2004 to February 2005 as part of the Chemistry of the Antarctic Boundary Layer and the Interface with Snow (CHABLIS) experiment. The data show long- and short-term variabilities in NMHCs controlled by the seasonal and geographic dependence of emissions and variation in atmospheric removal rates and pathways. Ethane, propane, iso-butane, n-butane and acetylene abundances followed a general OH-dependent sinusoidal seasonal cycle. The yearly averages were 186, 31, 3.2, 4.9 and 19 pptV, respectively, lower than those which were reported in some previous studies. Superimposed on a seasonal cycle was shorter-term variability that could be attributed to both synoptic airmass variability and localized loss processes due to other radical species. Hydrocarbon variability during periods of hour-to-day-long surface O3 depletion in late winter/early spring indicated active halogen atom chemistry estimated to be in the range 1.7 × 10 3 ‐3.4 × 10 4 atom cm −3 for Cl and 4.8 × 10 6 ‐9.6 × 10 7 atom cm −3 for Br. Longer-term negative deviations from sinusoidal behaviour in the late August were indicative of NMHC reaction with a persistent [Cl] of 2.3 × 10 3 atom cm −3 . Maximum ethene and propene of 157 and 179 pptV, respectively, were observed in the late February/early March, consistent with increased oceanic biogenic emissions; however, their presence was significant year-round (June‐August concentrations of 17.1 ± 18.3 and 7.9 ± 20.0 pptV, respectively).

OH and halogen atom influence on the variability of non-methane hydrocarbons in the Antarctic Boundary Layer

Measurements of C2–C8 non-methane hydrocarbons (NMHCs) have been made in situ at Halley Base, Antarctica (75◦35°S, 26◦19°W) from February 2004 to February 2005 as part of the Chemistry of the Antarctic Boundary Layer and the Interface with Snow (CHABLIS) experiment. The data show long- and short-term variabilities in NMHCs controlled by the seasonal and geographic dependence of emissions and variation in atmospheric removal rates and pathways. Ethane, propane, iso-butane, n-butane and acetylene abundances followed a general OH-dependent sinusoidal seasonal cycle. The yearly averages were 186, 31, 3.2, 4.9 and 19 pptV, respectively, lower than those which were reported in some previous studies. Superimposed on a seasonal cycle was shorter-term variability that could be attributed to both synoptic airmass variability and localized loss processes due to other radical species. Hydrocarbon variability during periods of hour-to-day-long surface O3 depletion in late winter/early spring indicated active halogen atom chemistry estimated to be in the range 1.7 × 103–3.4 × 104 atom cm−3 for Cl and 4.8 × 106–9.6 × 107 atom cm−3 for Br. Longer-term negative deviations from sinusoidal behaviour in the late August were indicative of NMHC reaction with a persistent [Cl] of 2.3×103 atom cm−3.Maximum ethene and propene of 157 and 179 pptV, respectively, were observed in the late February/early March, consistent with increased oceanic biogenic emissions; however, their presence was significant year-round (June–August concentrations of 17.1 ± 18.3 and 7.9 ± 20.0 pptV, respectively).

Meridional variations of temperature, C 2H 2 and C 2H 6 abundances in Saturn's stratosphere at southern summer solstice

Measurements of the vertical and latitudinal variations of temperature and C 2H 2 and C 2H 6 abundances in the stratosphere of Saturn can be used as stringent constraints on seasonal climate models, photochemical models, and dynamics. The summertime photochemical loss timescale for C 2H 6 in Saturn's middle and lower stratosphere ( ∼ 40 – 10 , 000 years, depending on altitude and latitude) is much greater than the atmospheric transport timescale; ethane observations may therefore be used to trace stratospheric dynamics. The shorter chemical lifetime for C 2H 2 ( ∼ 1 – 7 years depending on altitude and latitude) makes the acetylene abundance less sensitive to transport effects and more sensitive to insolation and seasonal effects. To obtain information on the temperature and hydrocarbon abundance distributions in Saturn's stratosphere, high-resolution spectral observations were obtained on September 13–14, 2002 UT at NASA's IRTF using the mid-infrared TEXES grating spectrograph. At the time of the observations, Saturn was at a L S ≈ 270 ° , corresponding to Saturn's southern summer solstice. The observed spectra exhibit a strong increase in the strength of methane emission at 1230 cm −1 with increasing southern latitude. Line-by-line radiative transfer calculations indicate that a temperature increase in the stratosphere of ≈ 10 K from the equator to the south pole between 10 and 0.01 mbar is implied. Similar observations of acetylene and ethane were also recorded. We find the 1.16 mbar mixing ratio of C 2H 2 at − 1 ° and − 83 ° planetocentric latitude to be 9.2 −3.8 +6.4 × 10 −7 and 2.5 −1.0 +1.8 × 10 −7 , respectively. The C 2H 2 mixing ratio at 0.12 mbar is found to be 1.0 −0.3 +0.5 × 10 −5 at − 1 ° planetocentric latitude and 2.6 −0.9 +1.3 × 10 −6 at − 83 ° planetocentric latitude. The 2.3 mbar mixing ratio of C 2H 6 inferred from the data is 7.5 −1.7 +2.3 × 10 −6 and 1.0 −0.2 +0.3 × 10 −5 at − 1 ° and − 83 ° planetocentric latitude, respectively. Further observations, creating a time baseline, will be required to completely resolve the question of how much the latitudinal variations of C 2H 2 and C 2H 6 are affected by seasonal forcing and/or stratospheric circulation.

HST observation of the atmospheric composition of Jupiter’s equatorial region: evidence for tropospheric C 2H 2☆

This paper presents the first detailed analysis of acetylene absorption features observed longward of 190.0 nm in a jovian spectrum by the Faint Object Spectrograph on board the Hubble Space Telescope. The presence of two features located near 207.0 nm can only be explained by a substantial abundance of acetylene in the upper troposphere. Using a Rayleigh–Raman radiative transfer model, it was determined that the acetylene vertical profile is characterized by a decrease in the mole fraction with increasing pressure in the upper stratosphere, a minimum around 14 to 29 mbar, followed by an increase to about 1.5 × 10 −7 in the upper troposphere. Longward of 220 nm, the relatively high contrast of Raman features to the continuum precludes the existence of an optically significant amount of clouds from 150 to 500 mbar unless they are highly reflective. Instead, the reflectivity at these long wavelengths is determined by stratospheric, not tropospheric, scatterers and absorbers. Analysis of the data also suggests that ammonia is extremely undersaturated at pressures below 700 mbar. However, no firm conclusions can be reached because of the uncertainties surrounding its cross section longward of 217.0 nm, which are due to vibrationally excited states.

Parameters of warm molecular clouds from methyl acetylene observations

The results of a survey of 63 Galactic star-forming regions in the 6K–5K and 5K–4K methyl acetylene lines at 102.5 and 85.5 GHz are presented. Fourty-three sources were detected at 102.5 GHz, and twenty-five at 85.5 GHz. Emission was detected toward molecular clouds with kinetic temperatures of 20–60 K (so-called “warm clouds”). The CH3CCH abundances in these clouds are about several ×10−9. Five sources (NGC 2264, G30.8-0.1, G34.26+0.15, DR 21(OH), S140) were mapped using the maximum-entropy method. The sizes of the mapped clouds fall in the range 0.1–1.7 pc, and the clouds have virial masses of 90–6200 M⊙ and densities between 6×104 and 6×105 cm−3. The CH3CCH sources coincide spatially with the CO and CS sources. Chemical-evolution simulations show that the typical methyl acetylene abundances in the observed clouds correspond to ages of ≈6×104 years.

An upper limit to the acetylene abundance toward BN in the Orion molecular cloud.

We report a search for the acetylene (C2H2) nu 3 infrared vibration-rotation absorption near 3 micrometers toward the Becklin-Neugebauer source in the Orion molecular cloud. The relative abundance of C2H2/CO in the quiescent gas (9 km s-1 component) is less than 3 x 10(-3) (5 sigma).

Formation of polycyclic aromatic hydrocarbons in circumstellar envelopes

Production of polycyclic aromatic hydrocarbons in carbon-rich circumstellar envelopes was investigated using a kinetic approach. A detailed chemical reaction mechanism of gas-phase PAH formation and growth, containing approximately 100 reactions of 40 species, was numerically solved under the physical conditions expected in cool stellar winds. The chemistry is based on studies of soot production in hydrocarbon pyrolysis and combustion. Several first-ring and second-ring cyclization processes were considered. A linear lumping algorithm was used to describe PAH growth beyond the second aromatic ring. PAH production using this mechanism was examined with respect to a grid of idealized constant velocity stellar winds as well as several published astrophysical models. The basic result is that the onset of PAH production in the interstellar envelopes is predicted to occur within the temperature interval of 1100 to 900 K. The absolute amounts of the PAHs formed, however, are very sensitive to a number of parameters, both chemical and astrophysical, whose values are not accurately known. Astrophysically meaningful quantities of PAHs require particularly dense and slow stellar winds and high initial acetylene abundance. It is suggested that most of the PAHs may be produced in a relatively small fraction of carbon-rich red giants.

Infrared polar brightenings on Jupiter: V. A thermal equilibrium model for the north polar hot spot

Infrared hydrocarbon emissions from Jupiter's north polar region, recorded using the Voyager IRIS instrument, determine spatial and other properties of the north polar hot spot. Emission at 7.8 μm by stratospheric methane reveals that the peak is asymmetric with respect to system III longitude. A thermal equilibrium model exploits this asymmetry to derive an estimate for the zonal wind velocity in the stratosphere. The same model predicts accurately the observed asymmetry in acetylene emission at 13.6 μm, but requires that the acetylene abundance be enhanced in the hot spot. Ethane, in contrast, appears to be depleted. Energetic charged particles are the most probable cause of these effects; their energies determine the altitude of the hot spot in the stratosphere, estimated here to be between the 1-mbar and 1-μbar pressure levels.

Observations of Neptune and Uranus below 2000 Å with the IUE

New observations of Neptune and improved data on Uranus below 2000 Å, obtained with the International Ultraviolet Explorer satellite, are presented. For Uranus, there is excellent agreement between the derived abundance of acetylene (C 2H 2) from the ultraviolet and from independent infrared data. A mixing ratio of 1 × 10 −8 is found. Comparison with Voyager ultraviolet data indicates that the mixing ratio of C 2H 2 increases with increasing altitude in the range 1–15 mbar. For Neptune, the derived C 2H 2 abundances from IUE and from infrared spectra are different by orders of magnitude. Since the abundance derived from the infrared is incompatible with the new ultraviolet data, it appears that some aspects of the infrared modeling require modification.

The abundances of ethane and acetylene in the atmospheres of Jupiter and Saturn

Infrared spectra near 780 cm −1 of Jupiter and Saturn have been obtained to determine the stratospheric abundances of ethane (C 2H 6) and acetylene (C 2H 2). Atmospheric models using Voyager thermal profiles and density profiles with constant mixing ratios result in the mixing ratios, X(C 2H 2) = 1.0(±0.3) × 10 −7 and X(C 2H 6) = 5.5(±1.5) × 10 −6 for Jupiter. The results for Saturn are X(C 2 H 2) = 3.0(±1.0) × 10 −7 and X(C 2H 6) = 7.0(±1.5) × 10 −6. The ratio of ethane to acetylene, n[C 2H 6]/ n[C 2H 2], is found to be insensitive to model atmosphere assumptions. The ratio is 55 ± 31 for Jupiter and 23 ± 12 for Saturn from models with uniform mixing ratios. Atmospheric models with densitu profiles adapted from the theoretical photochemical models also result in a higher ratio of ethane to acetylene (by a factor of 2 at the 1-mbar level) on Jupiter. The lower abundance of acetylene on Jupiter suggests that the rate of vertical transport in the stratosphere may be more rapid on Saturn than on Jupiter.

The photochemistry of hydrocarbons in Titan's atmosphere

Results of the far uv photolysis of methane diluted 1000 times with argon are presented for temperatures of 300 and 200°K. It was found that ethylene and acetylene reached a temperature-dependent steady state, while ethane and propane continued to rise linearly with time. The experimental ethylene column abundance in a photochemical steady state was extrapolated to the temperatures expected in the methane photolysis layer on Titan. This, together with Gillett's [(1975) Astrophys. J. 201] observations, make it possible to set a lower limit to the altitude of the top of Titan's aerosol layer, below which the ethylene is protected from photolysis. We find this to be 4.8 or 7.2 scale heights above the surface, for mean temperatures in the photolysis layer of 130 and 160°K, respectively. For 130°K, the ethylene abundance above the inversion base agrees well with Gillett's observations, and leads to an aerosol cloud top coincident with the inversion base. In order to transport ethane to the cold (78°K) “surface” with a flux sufficiently large so that the rate of condensation equals the rate of production, an eddy diffusion coefficient at the surface of at least 10 2 cm 2 sec −1 is required. The experimental results lead to an acetylene abundance in a photochemical steady state whicch is ∼2 orders of magnitude smaller than the one suggested by Strobel [(1974) Icarus 21]. Since propane is one of the major products, the possibility of observing it at 13.4 μm is suggested.