Radiation chemistry in the Jovian stratosphere: laboratory simulations

Low-pressure continuous-flow laboratory simulations of plasma induced chemistry in H2/He/CH4/NH3 atmospheres show radiation yields of hydrocarbons and nitrogen-containing organic compounds that increase with decreasing pressure in the range 2-200 mbar. Major products of these experiments that have been observed in the Jovian atmosphere are acetylene (C2H2), ethylene (C2H4), ethane (C2H6), hydrogen cyanide (HCN), propane (C3H8), and propyne (C3H4). Major products that have not yet been observed on Jupiter include acetonitrile (CH3CN), methylamine (CH3NH2), propene (C3H6), butane (C4H10), and butene (C4H8). Various other saturated and unsaturated hydrocarbons, as well as other amines and nitriles, are present in these experiments as minor products. We place upper limits of 10(6)-10(9) molecules cm-2 sec-1 on production rates of the major species from auroral chemistry in the Jovian stratosphere, and calculate stratospheric mole fraction contributions. This work shows that auroral processes may account for 10-100% of the total abundances of most observed organic species in the polar regions. Our experiments are consistent with models of Jovian polar stratospheric aerosol haze formation from polymerization of acetylene by secondary ultraviolet processing.     

Observations of CH4, C2H6, and C2H2 in the stratosphere of Jupiter

We have performed high-resolution spectral observations at mid-infrared wavelengths of CH4 (8.14 micrometers), C2H6 (12.16 micrometers), and C2H2 (13.45 micrometers) on Jupiter. These emission features probe the stratosphere of the planet and provide information on the carbon-based photochemical processes taking place in that region of the atmosphere. The observations were performed using our cryogenic echelle spectrometer CELESTE, in conjunction with the McMath-Pierce 1.5-m solar telescope between November 1994 and February 1995. We used the methane observations to derive the temperature profile of the jovian atmosphere in the 1-10 mbar region of the stratosphere. This profile was then used in conjunction with height-dependent mixing ratios of each hydrocarbon to determine global abundances for ethane and acetylene. The resulting mixing ratios are 3.9(+1.9)(-1.3) x 10(-6) for C2H6 (5 mbar pressure level), and 2.3 +/- 0.5 x 10(-8) for C2H2 (8 mbar pressure level), where the quoted uncertainties are derived from model variations in the temperature profile which match the methane observation uncertainties.     

Polycyclic aromatic hydrocarbons in the atmospheres of Titan and Jupiter

Polycyclic aromatic hydrocarbons (PAHs) are important components of the interstellar medium and carbonaceous chondrites, but have never been identified in the reducing atmospheres of the outer solar system. Incompletely characterized complex organic solids (tholins) produced by irradiating simulated Titan atmospheres reproduce well the observed UV/visible/IR optical constants of the Titan stratospheric haze. Titan tholin and a tholin generated in a crude simulation of the atmosphere of Jupiter are examined by two-step laser desorption/multiphoton ionization mass spectrometry. A range of two- to four-ring PAHs, some with one to four alkylation sites are identified, with net abundance approximately 10(-4) g g-1 (grams per gram) of tholins produced. Synchronous fluorescence techniques confirm this detection. Titan tholins have proportionately more one- and two-ring PAHs than do Jupiter tholins, which in turn have more four-ring and larger PAHs. The four-ringed PAH chrysene, prominent in some discussions of interstellar grains, is found in Jupiter tholins. Solid state 13C NMR spectroscopy suggests approximately equal to 25% of the total C in both tholins is tied up in aromatic and/or aliphatic alkenes. IR spectra indicate an upper limit in both tholins of approximately equal to 6% by mass in benzenes, heterocyclics, and PAHs with more than four rings. Condensed PAHs may contribute at most approximately 10% to the observed detached limb haze layers on Titan. As with interstellar PAHs, the synthesis route of planetary PAHs is likely to be via acetylene addition reactions.     

Galileo Photopolarimetry of Jupiter at 678.5 nm

Brightness and linear polarization measurements at 678.5 nm for four south-north strips of Jupiter are studied. These measurements were obtained in 1997 by the Galileo photopolarimeter/radiometer. The observed brightness exhibits latitudinal variations consistent with the belt/zone structure of Jupiter. The observed degree of linear polarization is small at low latitudes and increases steeply toward higher latitudes. No clear correlations were observed between the degree of linear polarization and the brightness. The observed direction of polarization changes from approximately parallel to the local scattering plane at low latitudes to perpendicular at higher latitudes. For our studies, we used atmospheric models that include a haze layer above a cloud layer. Parameterized scattering matrices were employed for the haze and cloud particles. On a pixel-wise basis, the haze optical thickness and the single-scattering albedo of the cloud particles were derived from the observed brightness and degree of linear polarization; results were accepted only if they were compatible with the observed direction of polarization. Using atmospheric parameter values obtained from Pioneer 10 and 11 photopolarimetry for the South Tropical Zone and the north component of the South Equatorial Belt, this analysis yielded acceptable results for very few pixels, particularly at small phase angles. However, for almost all pixels, acceptable results were found when the parameterized scattering matrix of the cloud particles was adjusted to produce more negative polarization for single scattering of unpolarized light, especially at large scattering angles, similar to some laboratory measurements of ammonia ice crystals. Using this adjusted model, it was found that the derived latitudinal variation of the single-scattering albedo of the cloud particles is consistent with the belt/zone structure, and that the haze optical thickness steeply increases toward higher latitudes.     

The abundance of acetylene in the atmosphere of Jupiter

The Q and R branches of the C₂H₂ ν₅ fundamental, observed in emission in an aircraft spectrum of Jupiter near 750 cm^?1, have been analyzed with the help of an improved line listing for this band. The line parameters have been certified in the laboratory with the same interferometer used in the Jovian observations. The maximum mixing ratio of C₂H₂ is found to be between 5 × 10^?8 and 6 × 10^?9, depending on the form of its vertical distribution and the temperature structure assumed for the lower stratosphere. Most consistent with observations of both Q and R branches are: (1) distributions of C₂H₂ with a constant mixing ratio in the stratosphere and a cutoff at a total pressure of 100 mbar or less, and (2) the assumption of a temperature at 10^?2 bar which is near 155°K.     

Search for HD in Jupiter and Saturn

Upper limits are placed on the D/H ratio in the observed portions of the atmospheres of Jupiter and Saturn from observations at high S/N over the region of the 5-0 R(1) line of HD. The upper limits of 4 × 10^?5 and 6 × 10^?5 D/H on Jupiter and Saturn, respectively, are not inconsistent with present models for abundance ratios in the primitive solar nebula or with other estimates of this quantity from observations.     

Photodissociation of cyanoacetylene: application to the atmospheric chemistry of Titan.

The quantum yield and reaction threshold for the photochemical dissociation of cyanoacetylene into a hydrogen atom and the cyanoethynyl radical have been determined. The quantum yield at 185 nm is approximately 0.09. The threshold is approximately 240 nm. Combination of this data with literature values shows that production of excited-state cyanoacetylene is the major primary process resulting from irradiation between 185 and 254 nm. Also determined are the relative rate constants for the abstraction of a hydrogen atom from hydrogen, methane, and ethane by the cyanoethynyl radical (k(H2):k(CH4):k(C2H6) = 1:9.3:63). Implications of these results for the proposal that hydrogen abstraction plays an important role in the conversion of methane to ethane and in the protection of unsaturated compounds from photoconsumption in the atmosphere of Titan are discussed.     

Dependence of the effective rate constants for the hydrogenation of CO on the temperature and composition of the surface

Experiments on the hydrogenation of CO on pure CO and CO-H₂O mixed ice have been performed at temperatures between 8 and 20 K. We obtained temperature and compositional dependence of the effective reaction rate constants. Results indicate that hydrogenation proceeds efficiently on pure solid CO and CO-H₂O mixed ice at temperatures below 10 and 20 K, respectively. Rate constants for pure CO decreased significantly at 12 K compared to those obtained with CO-H₂O mixed ice. Hydrogenation of CO at temperatures greater than 12 K were catalyzed by the H₂O adjacent to the CO. The importance of the experimental results for some relevant astrophysical environments has also been outlined.     

A transmission electron microscope study of exsolution and coarsening in iron‐bearing clinopyroxene from synthetic analogues of chondrules

Abstract— The microstructure of Fe‐rich clinopyroxene from synthetic analogues of chondrules was studied by transmission electron microscopy. The samples were cooled at various rates from 1455 °C to the quench temperature of 1000 °C. Slow cooling at rates below approximately 50–60 °C/h leads to the development of coherent pigeonite/augite exsolution lamallae on (001). A final wavelength of 19.6 ± 1.1 nm was obtained at a cooling rate of 10 °C/h, and 17.4 ± 2.4 ran at a cooling rate of 50 °C/h. Faster cooling at rates between approximately 50 and 450 °C/h yields only modulated structures with a wavelength on the order of 17–19 nm for the (001) orientation. Coherent exsolution lamellae on (001) in clinopyroxene occur in chondrules of H, L, LL, and CV chondrites, indicating that slow cooling of chondrules at subsolidus temperatures is a widespread phenomenon. The variation of the lamellar wavelength observed in natural chondrules corresponds to a variation of the subsolidus cooling rates between ～0.1 and 50 °C/h. The low cooling rates at subsolidus temperatures deduced from the microstructure of Fe‐rich clinopyroxene point to nonlinear cooling, with cooling rates decreasing with decreasing temperature.     

Five-micrometer measurements of Uranus and Neptune

We have obtained 5-μm brightness temperatures and brightness temperature upper limits for Uranus and Neptune which are substantially lower than those of Jupiter and Saturn and which correspond to a geometric albedo of approximately 0.01, in agreement with results reported by F. C. Gillet and G. H. Rieke (1977, Astrophys. J.218, L141–L144). Phospine and CH₃D, which are observed at 5 μm on Jupiter and Saturn, are discussed as possible sources of opacity at 5 μm in the atmospheres of Uranus and Neptune.     

A three-dimensional model of moist convection for the giant planets II: Saturn's water and ammonia moist convective storms

Moist convective storms constitute a key aspect in the global energy budget of the atmospheres of the giant planets. Among them, Saturn is known to develop the largest scale convective storms in the Solar System, the Great White Spots (GWS) which occur rarely and have been detected once every 30 years approximately. On the average, Saturn seems to show much less convective storms than Jupiter with smaller size and reduced frequency and intensity. Here we present detailed simulations of the onset and development of storms at the Equator and mid-latitudes of Saturn. These are the regions where most of the recent convective activity of the planet has been observed. We use a 3D anelastic model with parameterized microphysics (Hueso and Sánchez-Lavega, 2001, Icarus 151, 257) studying the onset and evolution of water and ammonia moist convective storms up to sizes of a few hundred km. Water storms, while more difficult to initiate than in Jupiter, can be very energetic, arriving to the 150 mbar level and developing vertical velocities on the order of 150 m s⁻¹. Ammonia storms develop easier but with a much smaller intensity unless very large abundances of ammonia (10 times solar) are present in Saturn's atmosphere. The Coriolis forces play a major role in the morphology and properties of water based storms.     

Dynamo region and the equatorial electrojet in the Jovian atmosphere

The existence of the dynamo region is identified in the atmosphere of Jupiter. It is found that the dynamo region extends from an altitude of 130 km (0.153 mbar) to 330 km (0.027 μbar) reckoned from zero altitude corresponding to 43.8 mbar pressure level. Physical features of the equatorial electrojet in the ionosphere of Jupiter are modelled in detail. The Jovian equatorial electrojet has a maximum eastward current density of about 1.5 Akm^?2 at an altitude of 270km (0.33 μbar) with a latitudinal half width of about ±11°. The thickness of the equatorial half width is 100 km in altitude range. The type I instability in the electrojet can exist only if the electron streaming velocity exceeds the value of about 250 m s^?1.     

On the C/H and D/H ratios in the atmospheres of Jupiter and Saturn

Using a method defined in a previous paper [M. Combes and T. Encrenaz, Icarus39 1–27 (1979)], we reestimated the C/H ratio in the atmospheres of Jupiter and Saturn by the measurements of the weak visible CH₄ bands, the CH₄ 3ν₃ band, and the (3-0) and (4-0) quadrupole bands of H₂. In the case of Jupiter we conclude that the C/H ratio is enriched by a factor ranging from 1.7 to 3.6 relative to the solar value. In the case of Saturn, our derived C/H value ranges from 1.2 to 3.2 times the solar value. The Jovian D/H ratio derived from this study is 1.2 × 10^?5 < D/H < 3.1 × 10^?5. The value derived for the D/H ratio on Saturn is not precise enough to be conclusive.     

The 12C/13C ratio in jupiter from the Voyager infrared investigation

The ¹²C/¹³C ratio in Jupiter has been derived from the analysis of the ν₄ band of CH₄ in the spectra recorded by the Voyager 1 IRIS experiment. It is found to be 160_?55⁺⁴⁰, i.e., 1.8_?0.6^+0.4 times the terrestrial value. Instrumental noise as well as systematic sources of error were taken into account for the estimate of the uncertainty. No plausible theory predicts such a difference between the values of the ¹²C/¹³C ratio in the inner solar system and in Jupiter. However, values of this ratio in the solar neighborhood 4.5 by ago inferred—through the use of models of chemical evolution of the Galaxy —from recent interstellar medium measurements are compatible with the present determination in Jupiter. The Jovian value, rather than the terrestrial one, could then be representative of the ratio in the primitive solar nebula.     

Hydrocarbon photochemistry in the upper atmosphere of Jupiter

The hydrocarbon photochemistry in the upper atmosphere of Jupiter is investigated using a one-dimensional, photochemical-diffusive, and diurnally averaged model. The important chemical cycles and pathways among the major species are outlined and a standard model for the North Equatorial Belt region is examined in detail. It is found that several traditionally dominant chemical pathways among the C and C2 species are replaced in importance by cycles involving C-C4 species. The pressure and altitude profiles of mixing ratios for several observable hydrocarbon species are compared with available ultraviolet- and infrared-derived abundances. The results of sensitivity studies on the standard model with respect to variations in eddy diffusion profile, solar flux, atomic hydrogen influx, latitude, temperature, and important chemical reaction rates are presented. Measured and calculated airglow emissions of He at 584 angstroms and H at 1216 angstroms are also used to provide some constraints on the range of model parameters. The relevance of the model results to the upcoming Galileo mission is briefly discussed. The model is subject to considerable improvement; there is a great need for laboratory measurements of basic reaction rates and photodissociation quantum yields, even for such simple species as methylacetylene and allene. Until such laboratory measurements exist there will be considerable uncertainty in the understanding of the C3 and higher hydrocarbons in the atmospheres of the jovian planets.     

Titan's stratospheric temperature asymmetry: a radiative origin?

During the 1981 Voyager encounter, Titan's stratosphere exhibited a large thermal asymmetry, with high northern latitudes being colder than comparable southern latitudes. Given the short radiative time constant, this asymmetry would not be expected at the season of the Voyager observations (spring equinox), if the infrared and solar opacity sources were distributed symmetrically. We have investigated the radiative budget of Titan's stratosphere, using two selections of Voyager IRIS spectra recorded at symmetric northern and southern latitudes. In the region 0.1-1 mbar, temperatures are 7 K colder at 50 degrees N than at 53 degrees S and the difference reaches approximately 13 K at 5 mbar. On the other hand, the northern region is strongly enriched in nitriles and hydrocarbons, and the haze optical depth derived from the continuum emission between 8 and 15 micrometers is twice as large as in the south. Cooling rate profiles have been computed at the two locations, using the gas and haze abundances derived from the IRIS measurements. We find that, despite lower temperatures, the cooling rate profiles in the pressure range 0.15-5 mbar are 20 to 40% larger in the north than in the south, because of the enhanced concentrations of infrared radiators. Because the northern hemisphere appears darker than the southern one in the Voyager images, enhanced solar heating is also expected to take place at 50 degrees N. Solar heating rate profiles have been calculated, with two different assumptions on the origin of the hemispheric asymmetry. In the most likely case where it results from a variation in the absorbance of the haze material, the heating rates are found to be 12-15% larger at the northern location than at the southern one, a smaller increase than that in the cooling rates. If the lower albedo in the north results from an increase in the particle number density, a 55 to 75% difference is found for the pressure range 0.15-5 mbar, thus larger than that calculated for the cooling rates. Considering the uncertainties in the haze model, dynamical heat transport may significantly contribute to the meridional temperature gradients observed in the stratosphere. On the other hand, the latitudinal variation in gas and haze composition may be sufficient to explain the entire temperature asymmetry observed, without invoking a lag in the thermal response of the atmosphere due to dynamical inertia.     

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.     

Optical constants of Titan tholins at mid-infrared wavelengths (2.5–25 μm) and the possible chemical nature of Titan’s haze particles

Complex organic materials may exist as haze layers in the atmosphere of Titan and as dark coloring agents on icy satellite surfaces. Laboratory measurements of optical constants of plausible complex organic materials are necessary for quantitative evaluation from remote sensing observations, and to document the existence of complex organic materials in the extraterrestrial environments. The recent Cassini VIMS and CIRS observations provide new constraints on Titan’s haze properties in the mid-infrared wavelength region. Here, we present the optical constants (2.5–25 μm) of Titan tholins generated with cold plasma irradiation of a N₂/CH₄ (90/10) gas mixture at pressures of 0.26 mbar, 1.6 mbar, and 23 mbar. Our new optical constants of three types of Titan tholins suggest that no single Titan tholin in this study fulfills all the observational constraints of the Titan haze material. The discrepancy remains a challenge for future modeling and laboratory efforts that aim toward a better understanding of Titan’s haze material.     

High-resolution spectroscopy of Saturn at 3 microns: CH4, CH3D, C2H2, C2H6, PH3, clouds, and haze

We report observation and analysis of a high-resolution 2.87-3.54 μm spectrum of the southern temperate region of Saturn obtained with NIRSPEC at Keck II. The spectrum reveals absorption and emission lines of five molecular species as well as spectral features of haze particles. The ν₂+ν₃ band of CH₃D is detected in absorption between 2.87 and 2.92 μm; and we derived from it a mixing ratio approximately consistent with the Infrared Space Observatory result. The ν₃ band of C₂H₂ also is detected in absorption between 2.95 and 3.05 μm; analysis indicates a sudden drop in the C₂H₂ mixing ratio at 15 mbar (130 km above the 1 bar level), probably due to condensation in the low stratosphere. The presence of the ν₃+ν₉+ν₁₁ band of C₂H₆ near 3.07 μm, first reported by Bjoraker et al. [Bjoraker, G.L., Larson, H.P., Fink, U., 1981. Astrophys. J. 248, 856-862], is confirmed, and a C₂H₆ condensation altitude of 10 mbar (140 km) in the low stratosphere is determined. We assign weak emission lines within the 3.3 μm band of CH₄ to the ν₇ band of C₂H₆, and derive a mixing ratio of 9±4×¹⁰⁻⁶ for this species. Most of the C₂H₆ 3.3 μm line emission arises in the altitude range 460-620 km (at ∼μbar pressure levels), much higher than the 160-370 km range where the 12 μm thermal molecular line emission of this species arises. At 2.87-2.90 μm the major absorber is tropospheric PH₃. The cloud level determined here and at 3.22-3.54 is 390-460 mbar (∼30 km), somewhat higher than found by Kim and Geballe [Kim, S.J., Geballe, T.R., 2005. Icarus 179, 449-458] from analysis of a low resolution spectrum. A broad absorption feature at 2.96 μm, which might be due to NH₃ ice particles in saturnian clouds, is also present. The effect of a haze layer at about 125 km (∼12 mbar level) on the 3.20-3.54 μm spectrum, which was not apparent in the low resolution spectrum, is clearly evident in the high resolution data, and the spectral properties of the haze particles suggest that they are composed of hydrocarbons.     

Saturn's latitudinal C2H2 and C2H6 abundance profiles from Cassini/CIRS and ground-based observations

Hydrocarbons in the upper atmosphere of Saturn are known, from Voyager, ground-based, and early Cassini results, to vary in emission intensity with latitude. Of particular interest is the marked increase in hydrocarbon line intensity near the south pole during southern summer, as the increased line intensity cannot be simply explained by the increased temperatures observed in that region since the variations between C₂H₂ and C₂H₆ emission in the south pole region are different. In order to measure the latitudinal variations of hydrocarbons in Saturn's southern hemisphere we have used 3 cm⁻¹ resolution Cassini CIRS data from 2006 and combined this with measurements from the ground in October 2006 at NASA's IRTF using Celeste, an infrared high-resolution cryogenic grating spectrometer. These two data sets have been used to infer the molecular abundances of C₂H₂ and C₂H₆ across the southern hemisphere in the 1-10 mbar altitude region. We find that the latitudinal acetylene profile follows the yearly average mean daily insolation except at the southern pole where it peaks in abundance. Near the equator (5° S) the C₂H₂ abundance at the 1.2 mbar level is (1.6±0.19)×¹⁰−7 and it decreases by a factor of 2.7 from the equator toward the pole. However, at the pole (∼87° S) the C₂H₂ abundance jumps to (1.8±0.3)×¹⁰−7, approximately the equatorial value. The C₂H₆ abundance near the equator at the 2 mbar level is (0.7±0.1)×¹⁰−5 and stays approximately constant until mid-latitudes where it increases gradually toward the pole, attaining a value of (1.4±0.4)×¹⁰−5 there. The increase in ethane toward the pole with the corresponding decrease in acetylene is consistent with southern hemisphere meridional winds [Greathouse, T.K., Lacy, J.H., Bézard, B., Moses, J.I., Griffith, C.A., Richter, M.J., 2005. Icarus 177, 18-31]. The localized increase in acetylene at the pole provides evidence that there is dynamical transport of hydrocarbons from the equator to the southern pole.