A methodology to construct a reduced chemical scheme for 2D-3D photochemical models: Application to Saturn

We present a methodology to build a reduced chemical scheme adapted to the study of hydrocarbons in the atmospheres of giant planets and Titan. As an example, we have built a reduced chemical scheme, containing only 25 compounds and 46 reactions (including photolysis), which is well adapted to compute the abundance of the main hydrocarbons observed so far in the atmosphere of Saturn (CH₃, CH₄, C₂H₂, C₂H₄, C₂H₆, CH₃C₂H, C₃H₈ and C₄H₂). This scheme gives similar results, within the error bars of the model, as a 1D photochemical model using an initial chemical scheme containing 90 compounds and more than 600 reactions. As a consequence, such a methodology can be used to build a reduced scheme well adapted to future 2D (or 3D) photochemical models and GCMs.     

Identification of a new band system of isotopic CO2 near 3.3 μm: Implications for remote sensing of biomarker gases on Mars

We present the discovery of a new vibrational band system of isotopic CO₂ (carbon dioxide) near 3.3 μm, with multiple strong P, Q and R lines in the prime spectral region used to search for Mars CH₄ (methane). The band system was discovered on Mars using high-resolution spectrometers (λ/δλ>40,000, CSHELL and NIRSPEC) at telescopes (NASA-IRTF and Keck-2) atop Mauna Kea, HI. The observed line intensities and frequencies agree very well with values predicted by a vibrational band model that we developed using known parameters for the molecular levels involved. Using this model, we synthesized spectra for different observing conditions (from Space and ground-based telescopes) and for different spectral resolving powers (5000 to 40,000). Although the total atmospheric burden on Mars is more than 150 times smaller than on Earth, the greater mixing ratio of CO₂ ensures that its column abundance on Mars is almost 20 times greater than on Earth. Thus, weak telluric CO₂ band systems appear much stronger on Mars. Many molecules of possible biological and geothermal interest have strong signatures at these wavelengths, in particular hydrocarbons owing to their strong ro-vibrational CH stretching modes. For example, the new isotopic CO₂ band-system encompasses lines of CH₄, C₂H₆ (ethane), CH₃OH (methanol) and H₂O (water). Implications for previous and future searches of biomarker gases are presented.     

Temporally varying ethylene emission on Jupiter

Ethylene (C₂H₄) emission has been measured in the poles and equator of Jupiter. The 949 cm⁻¹ spectra were recorded with a high resolution spectrometer at the McMath-Pierce telescope at Kitt Peak in October-November 1998 and at the Infrared Telescope Facility at Mauna Kea in June 2000. C₂H₄ is an important product of methane chemistry in the outer planets. Knowledge of its abundance can help discriminate among the various proposed sets of CH₄ photolysis branching ratios at Ly-α, and determine the relative importance of the reaction pathways that produce C₂H₂ and C₂H₆. In the equatorial region the C₂H₄ emission is weak, and we were only able to detect it at high air-mass, near the limb. We derive a peak equatorial molar abundance of C₂H₄ of 4.5×¹⁰⁻⁷-1.7×¹⁰⁻⁶ near 2.2×¹⁰⁻³ mbar, with a total column of 5.7×¹⁰¹⁴-2.2×¹⁰¹⁵ molecules cm⁻² above 10 mbar depending upon choice of thermal profile. We observed enhanced C₂H₄ emission from the poles in the regions where auroras are seen in X-ray, UV, and near infrared images. In 2000 we measured a short-term change in the distribution of polar C₂H₄ emission; the emission in the north IR auroral “hot spot” decreased by a factor of three over a two-day interval. This transient behavior and the sensitivity of C₂H₄ emission to temperature changes near its contribution peak at 5-10 microbar suggests that the polar enhancement is primarily a thermal effect coupled with vertical transport. Comparing our observations from Kitt Peak and Mauna Kea shows that the C₂H₄ emission of the northern non-“hot spot” auroral regions did not change over the three-year period while that in the southern polar regions decreased.     

High-resolution 3-μm spectra of Jupiter: Latitudinal spectral variations influenced by molecules, clouds, and haze

We present latitudinally-resolved high-resolution (R = 37,000) pole-to-pole spectra of Jupiter in various narrow longitudinal ranges, in spectral intervals covering roughly half of the spectral range 2.86-3.53 μm. We have analyzed the data with the aid of synthetic spectra generated from a model jovian atmosphere that included lines of CH₄, CH₃D, NH₃, C₂H₂, C₂H₆, PH₃, and HCN, as well as clouds and haze. Numerous spectral features of many of these molecular species are present and are individually identified for the first time, as are many lines of and a few unidentified spectral features. In both polar regions the 2.86-3.10-μm continuum is more than 10 times weaker than in spectra at lower latitudes, implying that in this wavelength range the single-scattering albedos of polar haze particles are very low. In contrast, the 3.24-3.53 μm the weak polar and equatorial continua are of comparable intensity. We derive vertical distributions of NH₃, C₂H₂ and C₂H₆, and find that the mixing ratios of NH₃ and C₂H₆ show little variation between equatorial and polar regions. However, the mixing ratios of C₂H₂ in the northern and southern polar regions are ∼6 and ∼3 times, respectively, less than those in the equatorial regions. The derived mixing ratio curves of C₂H₂ and C₂H₆ extend up to the 10⁻⁶ bar level, a significantly higher altitude than most previous results in the literature. Further ground-based observations covering other longitudes are needed to test if these mixing ratios are representative values for the equatorial and polar regions.     

Infrared polar brightening on Jupiter: III. Spectrometry from the Voyager 1 IRIS experiment

Spectra from the Voyager 1 IRIS experiment confirm the existence of enhanced infrared emission near Jupiter's north magnetic pole in March 1979. The spectral characteristics of the enhanced emission are consistent with a Planck source function. A temperature-pressure profile is derived for the region near the north magnetic pole, from which quantitative abundance estimates of minor species are made. Some species previously detected on Jupiter, including CH₃D, C₂H₂ and C₂H₆, have been observed again near the pole. Newly discovered species, not previously observed on Jupiter, include C₂H₄, C₃H₄, and C₆H₆. All of these species except CH₃D appear to have enhanced abundances at the north polar region with respect to midlatitudes. Upper limits are determined for C₄H₂ and C₃H₈. The quantitative results are compared with model calculations based on ultraviolet results from the IUE satellite. The plausibility of the C₆H₆ identification in discussed in terms of the literature on C₂H₂ polymerization. The relation of C₆H₆ to cuprene is also discussed.     

Analysis of Titan's neutral upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements

In this paper we present an in-depth study of the distributions of various neutral species in Titan's upper atmosphere, between 950 and 1500 km for abundant species (N₂, CH₄, H₂) and between 950 and 1200 km for other minor species. Our analysis is based on a large sample of Cassini/INMS (Ion Neutral Mass Spectrometer) measurements in the CSN (Closed Source Neutral) mode, obtained during 15 close flybys of Titan. To untangle the overlapping cracking patterns, we adopt Singular Value Decomposition (SVD) to determine simultaneously the densities of different species. Except for N₂, CH₄, H₂ and ⁴⁰Ar (as well as their isotopes), all species present density enhancements measured during the outbound legs. This can be interpreted as a result of wall effects, which could be either adsorption/desorption of these molecules or heterogeneous surface chemistry of the associated radicals on the chamber walls. In this paper, we provide both direct inbound measurements assuming ram pressure enhancement only and abundances corrected for wall adsorption/desorption based on a simple model to reproduce the observed time behavior. Among all minor species of photochemical interest, we have firm detections of C₂H₂, C₂H₄, C₂H₆, CH₃C₂H, C₄H₂, C₆H₆, CH₃CN, HC₃N, C₂N₂ and NH₃ in Titan's upper atmosphere. Upper limits are given for other minor species.The globally averaged distributions of N₂, CH₄ and H₂ are each modeled with the diffusion approximation. The N₂ profile suggests an average thermospheric temperature of 151 K. The CH₄ and H₂ profiles constrain their fluxes to be and , referred to Titan's surface. Both fluxes are significantly higher than the Jeans escape values. The INMS data also suggest horizontal/diurnal variations of temperature and neutral gas distribution in Titan's thermosphere. The equatorial region, the ramside, as well as the nightside hemisphere of Titan appear to be warmer and present some evidence for the depletion of light species such as CH₄. Meridional variations of some heavy species are also observed, with a trend of depletion toward the north pole. Though some of the above variations might be interpreted by either the solar-driven models or auroral-driven models, a physical scenario that reconciles all the observed horizontal/diurnal variations in a consistent way is still missing. With a careful evaluation of the effect of restricted sampling, some of the features shown in the INMS data are more likely to be observational biases.     

Spectroscopy of Comet Hale-Bopp in the infrared

High resolution infrared spectra of Comet C/1995 O1 (Hale-Bopp) were obtained during 2-5 March 1997 UT from the NASA Infrared Telescope Facility on Mauna Kea, Hawaii, when the comet was at r≈1.0 AU from the Sun pre-perihelion. Emission lines of CH₄, C₂H₆, HCN, C₂H₂, CH₃OH, H₂O, CO, and OH were detected. The rotational temperature of CH₄ in the inner coma was T_rot=110±20 K. Spatial profiles of CH₄, C₂H₆, and H₂O were consistent with release solely from the nucleus. The centroid of the CO emission was offset from that of the dust continuum and H₂O. Spatial profiles of the CO lines were much broader than those of the other molecules and asymmetric. We estimate the CO production rate using a simplified outflow model: constant, symmetric outflow from the peak position. A model of the excitation of CO that includes optical depth effects using an escape probability method is presented. Optical depth effects are not sufficient to explain the broad spatial extent. Using a parent+extended-source model, the broad extent of the CO lines can be explained by CO being produced mostly (∼90% on 5 March) from an extended source in the coma. The CO rotational temperature was near 100 K. Abundances relative to H₂O (in percent) were 1.1±0.3 (CH₄), 0.39±0.10 (C₂H₆), 0.18±0.04 (HCN), 0.17±0.04 (C₂H₂), 1.7±0.5 (CH₃OH), and 37-41 (CO, parent+extended source). These are roughly comparable to those obtained for other long-period comets also observed in the infrared, though CO appears to vary.     

Methane in Oort cloud comets

We detected CH₄ in eight Oort cloud comets using high-dispersion (λ/Δλ∼2×10⁴) infrared spectra acquired with CSHELL at NASA's IRTF and NIRSPEC at the W.M. Keck Observatory. The observed comets were C/1995 O1 (Hale-Bopp), C/1996 B2 (Hyakutake), C/1999 H1 (Lee), C/1999 T1 (McNaught-Hartley), C/1999 S4 (LINEAR), C/2000 WM₁ (LINEAR), C/2001 A2 (LINEAR), and 153/P Ikeya-Zhang (C/2002 C1). We detected the R0 and R1 lines of the ν₃ vibrational band of CH₄ near 3.3 μm in each comet, with the exception of McNaught-Hartley where only the R0 line was measured. In order to obtain production rates, a fluorescence model has been developed for this band of CH₄. We report g-factors for the R0 and R1 transitions at several rotational temperatures typically found in comet comae and relevant to our observations. Using g-factors appropriate to T_rot as determined from HCN, CO and/or H₂O and C₂H₆, CH₄ production rates and mixing ratios are presented. Abundances of CH₄/H₂O are compared among our existing sample of comets, in the context of establishing their place of origin. In addition, CH₄ is compared to native CO, another hypervolatile species, and no correlation is found among the comets observed.     

On the Dynamics of the Jovian Ionosphere and Thermosphere: II. The Measurement of H3 Vibrational Temperature, Column Density, and Total Emission

We present the first reported measurements of the intensity of a “hotband” transition for the H₃⁺ molecular ion in the northern auroral/polar region of Jupiter. This transition is identified as the R(3, 4⁺) line of the (2v₂(l=0)→v₂) hotband, with a wavelength of 3.94895 μm. This is the first time such a transition has been measured outside the laboratory, and the wavelength as measured on Jupiter is within the experimental accuracy of the lab measurement. This detection makes it possible to investigate H₃⁺ transitions that simultaneously originate from different vibrational levels. We use the intensity ratio between this line and the Q(1, 0⁻) fundamental transition to derive effective vibrational temperatures, column densities, and total emission parameters as a function of position across the auroral/polar region. Effective temperatures range from ∼900 to ∼1250 K; an increase in average temperature during our observing run of ∼100 K is noted. The derived temperatures are toward the high end or in excess of the auroral temperature range that has been reported in the literature to date. The relationship among emission intensity, temperature, and density is shown to be complex. This may reflect the nonthermalization of the vibrational levels at the gas densities prevailing in the jovian thermosphere. An alternative analysis allowing for this effect is presented. But this approach requires thermospheric temperatures to be ∼1500 K at the level that the majority of H₃⁺ is being produced, higher than has previously been proposed.     

Total internal partition sums to support planetary remote sensing

Total internal partition sums are determined from 65 to 3010 K for ¹³C¹⁸O₂, ¹³C¹⁸O¹⁷O, ¹²CH₃D, ¹³CH₃D, H¹²C¹²CD, ¹³C¹²CH₆, ¹²CH₃⁷⁹Br, ¹²CH₃⁸¹Br, ¹²CF₄, H¹²C¹²C¹²C¹²CH, H¹²C¹²C¹²C¹⁴N, H¹²C¹²C¹³C¹⁴N, H¹²C¹³C¹²C¹⁴N, H¹³C¹²C¹²C¹⁴N, H¹²C¹²C¹²C¹⁵N, D¹²C¹²C¹²C¹⁴N, ¹⁴N¹²C¹²C¹⁴N, ¹⁵N¹²C¹²C¹⁵N, ¹²C³²S, ¹²C³³S, ¹²C³⁴S, ¹³C³²S, H₂, HD, ³²S¹⁶O, ³²S¹⁸O, ³⁴S¹⁶O, ¹²C₃H₄, ¹²CH₃, ¹²C³²S₂, ³²S¹²C³⁴S, ¹³C³²S₂, and ³²S¹²C³³S. These calculations complete the partition sum data needed for additional isotopologues in HITRAN2008 and also extend the partition sums to molecules of astrophysical interest. These data, at 25 K steps, are incorporated into a FORTRAN code (TIPS_2011.for) that can be used to rapidly generate the data at any temperature in the range 70-3000 K.     

The molecular composition of Comet C/2007 W1 (Boattini): Evidence of a peculiar outgassing and a rich chemistry

We measured the chemical composition of Comet C/2007 W1 (Boattini) using the long-slit echelle grating spectrograph at Keck-2 (NIRSPEC) on 2008 July 9 and 10. We sampled 11 volatile species (H₂O, OH^∗, C₂H₆, CH₃OH, H₂CO, CH₄, HCN, C₂H₂, NH₃, NH₂, and CO), and retrieved three important cosmogonic indicators: the ortho-para ratios of H₂O and CH₄, and an upper-limit for the D/H ratio in water. The abundance ratios of almost all trace volatiles (relative to water) are among the highest ever observed in a comet. The comet also revealed a complex outgassing pattern, with some volatiles (the polar species H₂O and CH₃OH) presenting very asymmetric spatial profiles (extended in the anti-sunward hemisphere), while others (e.g., C₂H₆ and HCN) showed particularly symmetric profiles. We present emission profiles measured along the Sun-comet line for all observed volatiles, and discuss different production scenarios needed to explain them. We interpret the emission profiles in terms of release from two distinct moieties of ice, the first being clumps of mixed ice and dust released from the nucleus into the sunward hemisphere. The second moiety considered is very small grains of nearly pure polar ice (water and methanol, without dark material or apolar volatiles). Such grains would sublimate only very slowly, and could be swept into the anti-sunward hemisphere by radiation pressure and solar-actuated non-gravitational jet forces, thus providing an extended source in the anti-sunward hemisphere.     

Near-infrared spectrophotometry of Titan

Detailed analysis of the R(5) manifold of Titan's 3ν₃ CH₄ band confirms that the column abundance of Titan's spectroscopically visible atmosphere is greater than 1.6 kmamagats. This agrees with the value estimated from the strength of Titan's 3ν₃ CH₄Q branch and is at least 25 times the value for the column abundance of Mars' atmosphere. Moreover, the enhanced strength of the weaker CH₄ lines in Titan's spectrum relative to Saturn's spectrum suggests that CH₄ constitutes a significant fraction of this bulk.Recently discovered strong, unidentified absorptions in Titan's spectrum at 1.05–1.06 μm have been compared with laboratory spectra of a number of gases including CH₄, C₂H₄, C₂H₆, and C₃H₈ with negative results. These comparisons, however, have not excluded the possibility that these features arise from a very large quantity of CH₄ or from an isotope of CH₄. The fundamental transition of the responsible molecule may affect the interpretation of Titan's 8–14 μm spectrum since its wavelength may lie in this window. Comparison with Uranus' spectrum suggests that the visible abundance of this molecule in Titan's atmosphere may be much greater than in Uranus' relatively clear, deep atmosphere.Spectra of features at λ8150.7 and λ8272.7 attributed possibly to H₂ have been obtained at high resolution also during the apparitions of 1971, 1972, and 1973. These are presented for comparison with the results of the 1970 apparition. The existence of the λ8150.7 feature is established definitively but further observations are needed to establish whether the λ8272.7 feature exists beyond doubt.     

The atmospheric composition and structure of Jupiter and Saturn from ISO observations: a preliminary review

Infrared spectra of Jupiter and Saturn have been recorded with the two spectrometers of the Infrared Space Observatory (ISO) in 1995–1998, in the 2.3–180 μm range. Both the grating modes (R=150–2000) and the Fabry-Pérot modes (R=8000–30,000) of the two instruments were used. The main results of these observations are (1) the detection of water vapour in the deep troposphere of Saturn; (2) the detection of new hydrocarbons (CH₃C₂H, C₄H₂, C₆H₆, CH₃) in Saturn’s stratosphere; (3) the detection of water vapour and carbon dioxide in the stratospheres of Jupiter and Saturn; (4) a new determination of the D/H ratio from the detection of HD rotational lines. The origin of the external oxygen source on Jupiter and Saturn (also found in the other giant planets and Titan in comparable amounts) may be either interplanetary (micrometeoritic flux) or local (rings and/or satellites). The D/H determination in Jupiter, comparable to Saturn’s result, is in agreement with the recent measurement by the Galileo probe (Mahaffy, P.R., Donahue, T.M., Atreya, S.K., Owen, T.C., Niemann, H.B., 1998. Galileo probe measurements of D/H and ³He/⁴He in Jupiters atmosphere. Space Science Rev. 84 251–263); the D/H values on Uranus and Neptune are significantly higher, as expected from current models of planetary formation.     

Latitudinal variation of Saturn photochemistry deduced from spatially-resolved ultraviolet spectra

We obtained spatially-resolved ultraviolet spectra of Saturn in 1994 with the Faint Object Spectrometer and Goddard High Resolution Spectrograph of the Hubble Space Telescope. We observed four areas on the planet at 15° N, 33° S, 41° S, and 52° S, with a field-of-view of less than 2 × 2 arcsec², compared to the 16-arcsec planet diameter. The wavelength range, 1550-2300 Å, encompasses absorption from major hydrocarbons (C₂H₆, C₂H₄, C₂H₂, CH₃C₂H, C₄H₂) and water. We find global hydrocarbon abundances and a C₂H₂ vertical distribution compatible with infrared observations, in contrast with previous analyses of ultraviolet spectra. The stratospheric haze opacity decreases from polar region to the equator. Saturn mid-latitudes are photochemically distinct from the rest of the planet. At 33° S, the spectrum requires either (1) a distinctly different C₂H₂ vertical distribution or (2) a locally enhanced water abundance. At 41° S, the hydrocarbon abundance exhibits a local minimum, within a global trend of increasing abundance from equator to pole. This global trend may result from an increased abundance of short-lived hydrocarbons such as C₄H₂. Photochemical models predict a depletion of hydrocarbon molecules in the presence of stratospheric water [Moses et al., 2000. Icarus 143, 166-202]. These results are consistent with a localized influx of water, in the form of high charge to mass ratio particles, flowing into Saturn's atmosphere at latitudes magnetically linked to the rings.     

Interpretation of the carbon abundance in Saturn measured by Cassini

Spectral observations of Saturn from the far infrared spectrometer aboard the Cassini spacecraft [Flasar, F.M., et al., 2005. Temperatures, winds, and composition in the Saturnian system. Science 307, 1247-1251] have revealed that the C/H ratio in the planet is in fact about twice higher than previously derived from ground based observations and in agreement with the C/H value derived from Voyager IRIS by Courtin et al. [1984. The composition of Saturn's atmosphere at northern temperate latitudes from Voyager IRIS spectra - NH₃, PH₃, C₂H₂, C₂H₆, CH₃D, CH₄, and the Saturnian D/H isotopic ratio. Astrophys. J. 287, 899-916]. The implications of this measurement are reanalyzed in the present report on the basis that volatiles observed in cometary atmospheres, namely CO₂, CH₄, NH₃ and H₂S may have been trapped as solids in the feeding zone of the planet. CH₄ and H₂S may have been in the form of clathrate hydrates while CO₂ presumably condensed in the cooling solar nebula. Carbon may also have been incorporated in organics. Conditions of temperature and pressure ease the hydratation of NH₃. Such icy grains were included in planetesimals which subsequently collapsed into the hydrogen envelope of the planet, then resulting in C, N and S enrichments with respect to the solar abundance. Our calculations are consistent, within error bars, with observed elemental abundances on Saturn provided that the carbon trapped in planetesimals was mainly in the form of CH₄ clathrate and CO₂ ice (and maybe as organics) while nitrogen was in the form of NH₃ hydrate. Our approach has implications on the possible pattern of noble gases in Saturn, since we predict that contrary to what is observed in Jupiter, Ar and Kr should be in solar abundance while Xe might be strongly oversolar. The only way to verify this scenario is to send a probe making in situ mass spectrometer measurements. Our scenario also predicts that the ¹⁴N/¹⁵N ratio should be somewhat smaller in Saturn than measured in Jupiter by Galileo.     

Remote sensing D/H ratios in methane ice: Temperature-dependent absorption coefficients of CH3D in methane ice and in nitrogen ice

The existence of strong absorption bands of singly deuterated methane (CH₃D) at wavelengths where normal methane (CH₄) absorbs comparatively weakly could enable remote measurement of D/H ratios in methane ice on outer Solar System bodies. We performed laboratory transmission spectroscopy experiments, recording spectra at wavelengths from 1 to 6 μm to study CH₃D bands at 2.47, 2.87, and 4.56 μm, wavelengths where ordinary methane absorption is weak. We report temperature-dependent absorption coefficients of these bands when the CH₃D is diluted in CH₄ ice and also when it is dissolved in N₂ ice, and describe how these absorption coefficients can be combined with data from the literature to simulate arbitrary D/H ratio absorption coefficients for CH₄ ice and for CH₄ in N₂ ice. We anticipate these results motivating new telescopic observations to measure D/H ratios in CH₄ ice on Triton, Pluto, Eris, and Makemake.     

Enceladus' plume: Compositional evidence for a hot interior

The combined observations of Saturn's moon Enceladus by the Cassini CAPS, INMS and UVIS instruments detected water vapor geysers in which were present molecular nitrogen (N₂), carbon dioxide (CO₂), methane (CH₄), propane (C₃H₈), acetylene (C₂H₂), and several other species, together with all of the decomposition products of water. We propose that the presence of N₂ in the plume indicates thermal decomposition of ammonia, and hence high temperatures in the interior of the moon (e.g., 500 to 800 K). Such an environment also appears to be suitable for the production of methane (CH₄) from carbon monoxide (CO), or carbon dioxide (CO₂). The presence of C₂H₂ and C₃H₈ strongly suggest that catalytic reactions took place within a very hot environment. The internal environment of Enceladus is inferred to be or have been favorable for aqueous, catalytic chemistry. This permits the synthesis of many complex organic compounds that could be detected in future Cassini observations.     

The C/C isotopic ratio in Titan hydrocarbons from Cassini/CIRS infrared spectra

We have analyzed infrared spectra of Titan recorded by the Cassini Composite Infrared Spectrometer (CIRS) to measure the isotopic ratio ¹²C/¹³C in each of three chemical species in Titan's stratosphere: CH₄, C₂H₂ and C₂H₆. This is the first measurement of ¹²C/¹³C in any C₂ molecule on Titan, and the first measurement of ¹²CH₄/¹³CH₄ (non-deuterated) on Titan by remote sensing. Our spectra cover five widely-spaced latitudes, 65° S to 71° N and we have searched for both latitude variability of ¹²C/¹³C within a given species, and also for differences between the ¹²C/¹³C in the three gases. For CH₄ alone, we find (1-σ), essentially in agreement with the ¹²CH₄/¹³CH₄ measured by the Huygens Gas Chromatograph/Mass Spectrometer instrument (GCMS) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784]: 82.3±1.0, and also with measured values in H¹³CN and ¹³CH₃D by CIRS at lower precision [Bézard, B., Nixon, C., Kleiner, I., Jennings, D., 2007. Icarus 191, 397-400; Vinatier, S., Bézard, B., Nixon, C., 2007. Icarus 191, 712-721]. For the C₂ species, we find in C₂H₂ and 89.8±7.3 in C₂H₆, a possible trend of increasingly value with molecular mass, although these values are both compatible with the Huygens GCMS value to within error bars. There are no convincing trends in latitude. Combining all fifteen measurements, we obtain a value of , also compatible with GCMS. Therefore, the evidence is mounting that ¹²C/¹³C is some 8% lower on Titan than on the Earth (88.9, inorganic standard), and lower than typical for the outer planets (88±7 [Sada, P.V., McCabe, G.H., Bjoraker, G.L., Jennings, D.E., Reuter, D.C., 1996. Astrophys. J. 472, 903-907]). There is no current model for this enrichment, and we discuss several mechanisms that may be at work.     

The spectra of Uranus and Neptune at 8–14 and 17–23 μm

An array spectrometer was used on the nights of 1985 May 30–June 1 to observe the disks of Uranus and Neptune in the spectral regions 7–14 and 17–23 μm with effective resolution elements ranging from 0.23 to 0.87 μm. In the long-wavelength region, the spectra are relatively smooth with the broad S(1) H₂ collision-induced rotation line showing strong emission for Neptune. In the short-wavelength spectrum of Uranus, an emission feature attributable to C₂H₂ with a maximum stratospheric mixing ratio of 9 × 10⁻⁹ is apparent. An upper limit of 2 × 10⁻⁸ is placed on the maximum stratospheric mixing ratio of C₂H₆. The spectrum of Uranus is otherwise smooth and quantitatively consistent with the opacity provided by H₂ collision-induced absorption and spectrally continuous stratospheric emission, as would be produced by aerosols. Upper limits to detecting the planet near 8 μm indicate a CH₄ stratospheric mixing ratio of 1 × 10⁻⁵ or less, below a value consistent with saturation equilibrium at the temperature minimum. In the short-wavelength spectrum of Neptune, strong emission features of CH₄ and C₂H₆ are evident and are consistent with local saturation equilibrium with maximum stratospheric mixing ratios of 0.02 and 6 × 10⁻⁶, respectively. Emission at 8–10 μm is most consistent with a [CH₃D]/[CH₄] volume abundance ratio of 5 × 10⁻⁵. The spectrum of Neptune near 13.5 μm is consistent with emission by stratospheric C₂H₂ in local saturation equilibrium and a maximum mixing ratio of 9 × 10⁻⁷. Radiance detected near 10.5 μm could be attributed to stratospheric C₂H₄ emission for a maximum mixing ratio of approximately 3 × 10⁻⁹. Quantitative results are considered preliminary, as some absolute radiance differences are noted with respect to earlier observations with discrete filters.     

Meridional variations of C2H2 and C2H6 in Jupiter's atmosphere from Cassini CIRS infrared spectra

Hydrocarbons such as acetylene (C₂H₂) and ethane (C₂H₆) are important tracers in Jupiter's atmosphere, constraining our models of the chemical and dynamical processes. However, our knowledge of the vertical and meridional variations of their abundances has remained sparse. During the flyby of the Cassini spacecraft in December 2000, the Composite Infrared Spectrometer (CIRS) instrument was used to map the spatial variation of emissions from 10 to 1400 cm⁻¹ (1000-7 μm). In this paper we analyze a zonally averaged set of CIRS spectra taken at the highest (0.48 cm⁻¹) resolution, firstly to infer atmospheric temperatures in the stratosphere at 0.5-20 mbar via the ν₄ band of CH₄, and in the troposphere at 150-400 mbar, via the H₂ absorption at 600-800 cm⁻¹. Stratospheric temperatures at 5 mbar are generally warmer in the north than the south by 7-8 K, while tropospheric temperatures show no such asymmetry. Both latitudinal temperature profiles however do show a pattern of maxima and minima which are largely anti-correlated between the two levels. We then use the derived temperature profiles to infer the vertical abundances of C₂H₂ and C₂H₆ by modeling tropospheric absorption (∼200 mbar) and stratospheric emission (∼5 mbar) in the C₂H₂ν₅ and C₂H₆ν₉ bands, and also emission of the acetylene (ν₄+ν₅)−ν₄ hotband (∼0.1 mbar). Acetylene shows a distinct north-south asymmetry in the stratosphere, with 5 mbar abundances greatest close to 20° N and decreasing from there towards both poles by a factor of ∼4. At 200 mbar in contrast, acetylene is nearly flat at a level of ∼3×¹⁰⁻⁹. Additionally, the abundance gradient of C₂H₂ between 10 and 0.1 mbar is derived, based on interpolated temperatures at 0.1 mbar, and is found to be positive and uniform with latitude to within errors. Ethane at both 5 and 200 mbar shows increasing VMR towards polar regions of ∼1.75 towards 70° N and ∼2.0 towards 70° S. An explanation for the meridional trends is proposed in terms of a combination of photochemistry and dynamics. Poleward, the decreasing UV flux is predicted to decrease the abundances of C₂H₂ and C₂H₆ by factors of 2.7 and 3.5, respectively, at latitude 70°. However, the lifetime of C₂H₆ in the stratosphere (3×¹⁰¹⁰ s at 5 mbar) is much longer than the dynamical timescale for meridional mixing inferred from Comet SL-9 debris (5-50×¹⁰⁸ s), and therefore the rising abundance towards high latitudes likely indicates that meridional mixing dominates over photochemical effects. For C₂H₂, the opposite occurs, with the relatively short photochemical lifetime (3×¹⁰⁷ s), compared to meridional mixing times, ensuring that the expected photochemical trends are visible.