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Catherine de Bergh,
Régis Courtin, Bruno Bézard,
Athéna Coustenis,
Emmanuel Lellouch,
Mathieu Hirtzig,
Pascal Rannou,
Pierre Drossart,
Alain Campargue,
Samir Kassi,
Le Wang,
Vincent Boudon,
Andrei Nikitin,
Vladimir Tyuterev
[show abstract]
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ABSTRACT: In this paper we apply a recently released set of methane line
parameters (Wang et al., 2011) to the modeling of Titan spectra in the
1.58 μm window at both low and high spectral resolution. We first
compare the methane absorption based on this new set of methane data to
that calculated from the methane absorption coefficients derived in situ
from DISR/Huygens (Tomasko et al., 2008a; Karkoschka and Tomasko, 2010)
and from the band models of Irwin et al. (2006) and Karkoschka and
Tomasko (2010). The Irwin et al. (2006) band model clearly
underestimates the absorption in the window at
temperature-pressure conditions representative of Titan's
troposphere, while the Karkoschka and Tomasko (2010) band model gives an
acceptable agreement in the whole window, overestimating the absorption
by about 15% in the range 6300-6500 cm-1. We also
find that the transmittance of Titan's atmosphere is in excellent
agreement with that calculated from the Tomasko et al. (2008a)
coefficients after reducing them by about 7%. Synthetic spectra computed
with spectral resolutions of 1.2 cm-1 (R˜5400)
and 0.35 cm-1 (R˜18000) are then compared with
two high-resolution Earth-based measurements of Titan's albedo obtained
in 1982 and 1993 (with KPNO/FTS and IRTF/CSHELL). The new set of methane
line parameters leads to an excellent match of all the CH3D
and CH4 absorption features in these spectra, and permits us
to derive a ratio of
CH3D/CH4=(4.5±1.0)×10-4
- hence a D/H ratio in methane for Titan of
(1.13±0.25)×10-4 - and a CO mole
fraction of 40±10 ppm (from the KPNO/FTS dataset) and 51±7
ppm (from the IRTF/CSHELL dataset). We also infer constraints on the
far-wing lineshape of methane lines of the 2ν3 band. We
finally present two other examples of models of Titan's spectrum using
the new line parameters, one potentially useful for future
higher-resolution (R=40,000) observations, another one applicable to the
ongoing low-resolution (R˜100) observations by Cassini VIMS. We
show that the aerosol model of Tomasko et al. (2008b) produces too much
intensity at low phase angle compared to a VIMS spectrum recorded near
the Huygens site and we propose a slightly revised model that reproduces
this observation.
Planetary and Space Science 01/2012; 61:85-98. · 2.22 Impact Factor
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Conor A Nixon,
Richard K Achterberg,
Nicholas A Teanby,
Patrick G J Irwin,
Jean-Marie Flaud,
Isabelle Kleiner,
Alix Dehayem-Kamadjeu,
Linda R Brown,
Robert L Sams, Bruno Bézard,
Athena Coustenis,
Todd M Ansty,
Andrei Mamoutkine,
Sandrine Vinatier,
Gordon L Bjoraker,
Donald E Jennings,
Paul N Romani,
F Michael Flasar
[show abstract]
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ABSTRACT: In this paper we describe the first quantitative search for several molecules in Titan's stratosphere in Cassini CIRS infrared spectra. These are: ammonia (NH3), methanol (CH3OH), formaldehyde (H2CO), and acetonitrile (CH3CN), all of which are predicted by photochemical models but only the last of which has been observed, and not in the infrared. We find non-detections in all cases, but derive upper limits on the abundances from low-noise observations at 25 degrees S and 75 degrees N. Comparing these constraints to model predictions, we conclude that CIRS is highly unlikely to see NH3 or CH3OH emissions. However, CH3CN and H2CO are closer to CIRS detectability, and we suggest ways in which the sensitivity threshold may be lowered towards this goal.
Faraday Discussions 01/2010; 147:65-81; discussion 83-102. · 5.00 Impact Factor
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Bruno Bézard
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ABSTRACT: Our present knowledge of the composition and chemistry of Titan's stratosphere is reviewed. Thermal measurements by the Cassini spacecraft show that the mixing ratios of all photochemical species, except ethylene, increase with altitude at equatorial and southern latitudes, reflecting transport from a high-altitude source to a condensation sink in the lower stratosphere. Most compounds are enriched at latitudes northward of 45 degrees N, a consequence of subsidence in the winter polar vortex. This enrichment is much stronger for nitriles and complex hydrocarbons than for ethane and acetylene. Titan's chemistry originates from breakdown of methane due to photodissociation in the upper atmosphere and catalytical reactions in the stratosphere, and from destruction of nitrogen both by UV photons and electrons. Photochemistry also produces haze particles made of complex refractory material, albeit at a lower rate than ethane, the most abundant gas product. Haze characteristics (vertical distribution, physical and spectral properties) inferred by several instruments aboard Cassini/Huygens are discussed here.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 12/2008; 367(1889):683-95. · 2.77 Impact Factor
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[show abstract]
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ABSTRACT: The giant planets, Venus, Titan, and, to a lesser extent, Mars, have thick planetary atmospheres that can be probed very efficiently
by high-resolution infrared spectroscopy. They are strong infrared emitters, both through thermal emission and reflection
of solar radiation, and many of the molecules and radicals detected in their atmospheres have intense rovibrational transitions
in this spectral range. We will review some of the most recent ground-based infrared observations of these atmospheres made
at high spectral resolution and what we have learnt from them. We will then show that, with the high spatial and spectral
resolutions of the CRIRES and VISIR instruments at the VLT, it will be possible, in particular, to search for still undetected
species, study the atmospheric circulation at altitude levels otherwise unaccessible, study auroral phenomena in giant planets’
atmospheres, measure precisely some isotopic ratios, investigate the photochemistry and formation of hazes in Titan and the
giant planets, map and monitor the chemistry in the deep atmosphere of Venus, and measure the effects of cometary impacts
in the giant planets.
01/2006: pages 513-527;
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[show abstract]
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ABSTRACT: Infrared spectroscopic observations of planets and Saturn's satellite Titan with the Infrared Space Observatory led to many
significant discoveries that improved our understanding on the formation, physics and chemistry of these objects. The prime
results achieved by ISO are: (1) a new and consistent determination of the D/H ratios on the giant planets and Titan; (2)
the first precise measurement of the 15N/14N ratio in Jupiter, a valuable indicator of the protosolar nitrogen isotopic ratio; (3) the first detection of an external
oxygen flux for all giant planets and Titan; (4) the first detection of some stratospheric hydrocarbons (CH3, C2H4, CH3C2H, C4H2, C6H6); (5) the first detection of tropospheric water in Saturn; (6) the tentative detection of carbonate minerals on Mars; (7)
the first thermal lightcurve of Pluto.
Space Science Reviews 01/2005; 119(1):123-139. · 3.61 Impact Factor
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[show abstract]
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ABSTRACT: We present the results of the Infrared Space Observatory Short Wavelength Spectrometer (ISO-SWS) observations of Jupiter related to ammonia. We focus on two spectral regions; the first one (the 10-μm region), ranging from 9.5 to 11.5 μm, probes atmospheric levels between 1 and 0.2 bar, while the second one (the 5-μm window), ranging from 4.8 to 5.5 μm, sounds the atmosphere between 8 and 2 bar. The two spectral windows cannot be fitted with the same ammonia vertical distribution. From the 10-μm region we infer an ammonia distribution of about half the saturation profile above the 1-bar level, where the N/H ratio is roughly solar. A totally different picture is derived from the 5-μm window, where we determine an upper limit of 3.7×10−5 at 1 bar and find an increasing NH3 abundance at least down to 4 bar. This profile is similar to that measured by the Galileo probe. The discrepancy between the two spectral regions most likely arises from the spatial heterogeneity of Jupiter, the 5-μm window sounding dry areas unveiled by a locally thin cloud cover (the 5-μm hot spots), and the 10-μm region probing the mean jovian atmosphere above 1 bar. The 15NH3 mixing ratio is measured around 400 mbar from ν2 band absorptions in the 10-μm region. We find the atmosphere of Jupiter highly depleted in 15N at this pressure level [(15N/14N)[formula]=1.9+0.9−1.0)×10−3, while (15N/14N)⊕=3.68×10−3]. It is not clear whether this depletion reveals the global jovian 15N/14N ratio. Instead an isotopic fractionation process, taking place during the ammonia cloud condensation, is indicated as a possible mechanism. A fractionation coefficient α higher than 1.08 would explain the observed isotopic ratio, but the lack of laboratory data does not allow us to decide unambiguously on the origin of the observed low 15N/14N ratio.
Icarus. 11/1999;
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ABSTRACT: We present a radiative equilibrium model for extrasolar giant planets applied to 51 Peg b. The atmospheric model extends from 10−5 to ∼10 bar and is limited at the bottom by an optically thick cloud of silicate (Mg2SiO4 or MgSiO3) or iron (Fe) particles. Rayleigh scattering at short wavelengths and absorption by the H2–H2 and H2–He continuum and molecular bands of H2O, CO, and CH4 are included. Atmospheric heating and cooling result, respectively, from absorption of stellar flux and from infrared thermal emission. The solution temperature profiles do not show any temperature inversion, in contrast with the giant planets of the solar system. The lapse rate is subadiabatic at all levels above the cloudtop, justifying the use of radiative equilibrium. We find that, under thermochemical equilibrium, CO dominates over CH4 at all levels. The effective temperature is in the range 1150–1270 K and the Bond albedo in the range 0.15–0.42, depending on the location and reflectivity of the lower cloud deck. Mg2SiO4 or Fe clouds are weakly reflective in contrast to a MgSiO3 cloud. The thermal emission spectrum prevails over the stellar reflected component below 13,000–15,000 cm−1 (λ>0.7 μm); it shows various windows, between the H2O and CO bands, with the brightest centered at 2550 cm−1 (3.9 μm). The most prominent CO and CH4 bands occur around 2100 (4.7 μm) and 3030 cm−1 (3.3 μm. Assuming a jovian abundance for PH3, the bands around 2300 (4.3 μm) and 2450 cm−1 (4.1 μm) are clearly visible in absorption at a resolving power of ∼100. We investigated the detectability of the minor species CO and CH4 at a resolution of 4000–6000, adequate to separate the planet's absorption features from their stellar and telluric counterparts. Considering photon noise from the star as the only noise source, we find that S/N ratios of about 8 are reached on the molecular features in the 4.7- and 3.3-μm regions after ∼10 h of integration on an 8-m telescope. On the other hand, the planet-to-star contrast is as low as (2–4)×10−4.
Earth Moon and Planets 12/1997; · 0.67 Impact Factor
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Proceedings of the International Astronomical Union 202:277.
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Sandrine Vinatier, Bruno Bézard,
Remco de Kok,
Carrie M. Anderson,
Robert E. Samuelson,
Conor A. Nixon,
Andrei Mamoutkine,
Ronald C. Carlson,
Donald E. Jennings,
Ever A. Guandique,
Gordon L. Bjoraker,
F. Michael Flasar,
Virgil G. Kunde
[show abstract]
[hide abstract]
ABSTRACT: We have analyzed the continuum emission of limb spectra acquired by the Cassini/CIRS infrared spectrometer in order to derive information on haze extinction in the 3–0.02 mbar range (∼150–350 km). We focused on the 600–1420 cm−1 spectral range and studied nine different limb observations acquired during the Cassini nominal mission at 55°S, 20°S, 5°N, 30°N, 40°N, 45°N, 55°N, 70°N and 80°N. By means of an inversion algorithm solving the radiative transfer equation, we derived the vertical profiles of haze extinction coefficients from 17 spectral ranges of 20-cm−1 wide at each of the nine latitudes. At a given latitude, all extinction vertical profiles retrieved from various spectral intervals between 600 and 1120 cm−1 display similar vertical slopes implying similar spectral characteristics of the material at all altitudes. We calculated a mean vertical extinction profile for each latitude and derived the ratio of the haze scale height (Hhaze) to the pressure scale height (Hgas) as a function of altitude. We inferred Hhaze/Hgas values varying from 0.8 to 2.4. The aerosol scale height varies with altitude and also with latitude. Overall, the haze extinction does not show strong latitudinal variations but, at 1 mbar, an increase by a factor of 1.5 is observed at the north pole compared to high southern latitudes. The vertical optical depths at 0.5 and 1.7 mbar increase from 55°S to 5°N, remain constant between 5°N and 30°N and display little variation at higher latitudes, except the presence of a slight local maximum at 45°N. The spectral dependence of the haze vertical optical depth is uniform with latitude and displays three main spectral features centered at 630 cm−1, 745 cm−1 and 1390 cm−1, the latter showing a wide tail extending down to ∼1000 cm−1. From 600 to 750 cm−1, the optical depth increases by a factor of 3 in contrast with the absorbance of laboratory tholins, which is generally constant. We derived the mass mixing ratio profiles of haze at the nine latitudes. Below the 0.4-mbar level all mass mixing ratio profiles increase with height. Above this pressure level, the profiles at 40°N, 45°N, 55°N, at the edge of the polar vortex, display a decrease-with-height whereas the other profiles increase. The global increase with height of the haze mass mixing ratio suggest a source at high altitudes and a sink at low altitudes. An enrichment of haze is observed at 0.1 mbar around the equator, which could be due to a more efficient photochemistry because of the strongest insolation there or an accumulation of haze due to a balance between sedimentation and upward vertical drag.
Icarus.
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Catherine De Bergh,
Régis Courtin, Bruno Bézard,
Athena Coustenis,
Emmanuel Lellouch,
Mathieu Hirtzig,
Pierre Drossart,
Alain Campargue,
Samir Kassi,
Le Wang,
Vincent Boudon,
Andrei Nikitin,
Vl-G. Tyuterev
International Workshop: Spectroscopy of Methane and Planetary Applications.
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Athena Coustenis,
Richard K. Achterberg,
Barney J. Conrath,
Donald E. Jennings,
André Marten,
Daniel Gautier,
Conor A. Nixon,
F. Michael Flasar,
Nick A. Teanby, Bruno Bézard, [......],
Fred W. Taylor,
Patrick G.J. Irwin,
Thierry Fouchet,
Augustin Hubert,
Glenn S. Orton,
Virgil G. Kunde,
Sandrine Vinatier,
Jacqueline Mondellini,
Mian M. Abbas,
Regis Courtin
[show abstract]
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ABSTRACT: We have analyzed data recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft during the Titan flybys T0–T10 (July 2004–January 2006). The spectra characterize various regions on Titan from 70° S to 70° N with a variety of emission angles. We study the molecular signatures observed in the mid-infrared CIRS detector arrays (FP3 and FP4, covering roughly the 600–1500 cm−1 spectral range with apodized resolutions of 2.54 or 0.53 cm−1). The composite spectrum shows several molecular signatures: hydrocarbons, nitriles and CO2. A firm detection of benzene (C6H6) is provided by CIRS at levels of about 3.5×10−9 around 70° N. We have used temperature profiles retrieved from the inversion of the emission observed in the methane ν4 band at 1304 cm−1 and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. No longitudinal variations were found for these gases. Little or no change is observed generally in their abundances from the south to the equator. On the other hand, meridional variations retrieved for these trace constituents from the equator to the North ranged from almost zero (no or very little meridional variations) for C2H2, C2H6, C3H8, C2H4 and CO2 to a significant enhancement at high northern (early winter) latitudes for HCN, HC3N, C4H2, C3H4 and C6H6. For the more important increases in the northern latitudes, the transition occurs roughly between 30 and 50 degrees north latitude, depending on the molecule. Note however that the very high-northern latitude results from tours TB–T10 bear large uncertainties due to few available data and problems with latitude smearing effects. The observed variations are consistent with some, but not all, of the predictions from dynamical-photochemical models. Constraints are set on the vertical distribution of C2H2, found to be compatible with 2-D equatorial predictions by global circulation models. The D/H ratio in the methane on Titan has been determined from the CH3D band at 1156 cm−1 and found to be . Implications of this deuterium enrichment, with respect to the protosolar abundance on the origin of Titan, are discussed. We compare our results with values retrieved by Voyager IRIS observations taken in 1980, as well as with more recent (1997) disk-averaged Infrared Space Observatory (ISO) results and with the latest Cassini–Huygens inferences from other instruments in an attempt to better comprehend the physical phenomena on Titan.
Icarus.
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ABSTRACT: Thirteen lines of the CO band near 4.7 μm have been observed on a jovian hot spot at a resolution of 0.045 cm−1. The measured line profiles indicate that the CO mole fraction is 1.0±0.2 ppb around the 6-bar level and is larger in the upper troposphere and/or stratosphere. An external source of CO providing an abundance of 4+3−2×1016 molecules cm−2 is implied by the observations in addition to the amount deposited at high altitude by the Shoemaker–Levy 9 collision. From a simple diffusion model, we estimate that the CO production rate is (1.5–10)×106 molecules cm−2 s−1 assuming an eddy diffusion coefficient around the tropopause between 300 and 1500 cm2 s−1. Precipitation of oxygen atoms from the jovian magnetosphere or photochemistry of water vapor from meteoroidal material can only provide a negligible contribution to this amount. A significant fraction of the CO in Jupiter's upper atmosphere may be formed by shock chemistry due to the infall of kilometer- to subkilometer-size Jupiter family comets. Using the impact rate from Levison et al. (2000, Icarus143, 415–420) rescaled by Bottke et al. (2002, Icarus156, 399–433), this source can provide the observed stratospheric CO only if the eddy diffusion coefficient around the tropopause is 100–300 cm2 s−1. Higher values, ∼700 cm2 s−1, would require an impact rate larger by a factor of 5–10, which cannot be excluded considering uncertainties in the distribution of Jupiter family comets. Such a large rate is indeed consistent with the observed cratering record of the Galilean satellites (Zahnle et al. 1998, Icarus136, 202–222). On the other hand, the ∼1 ppb concentration in the lower troposphere requires an internal source. Revisiting the disequilibrium chemistry of CO in Jupiter, we conclude that rapid vertical mixing can provide the required amount of CO at ∼6 bar for a global oxygen abundance of 0.2–9 times the solar value considering the uncertainties in the convective mixing rate and in the chemical constants.
Icarus.
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Sandrine Vinatier, Bruno Bézard,
Thierry Fouchet,
Nick A. Teanby,
Remco de Kok,
Patrick G.J. Irwin,
Barney J. Conrath,
Conor A. Nixon,
Paul N. Romani,
F. Michael Flasar,
Athena Coustenis
[show abstract]
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ABSTRACT: Limb spectra recorded by the Composite InfraRed Spectrometer (CIRS) on Cassini provide information on abundance vertical profiles of C2H2, C2H4, C2H6, CH3C2H, C3H8, C4H2, C6H6 and HCN, along with the temperature profiles in Titan's atmosphere. We analyzed two sets of spectra, one at 15° S (Tb flyby) and the other one at 80° N (T3 flyby). The spectral range 600–1400 cm−1, recorded at a resolution of 0.5 cm−1, was used to determine molecular abundances and temperatures in the stratosphere in the altitude range 100–460 km for Tb and 170–495 km for T3. Both temperature profiles show a well defined stratopause, at around 310 km (0.07 mbar) and 183 K at 13° S, and 380 km (0.01 mbar) with 207 K at 80° N. Near the north pole, stratospheric temperatures are colder and mesospheric temperatures are warmer than near the equator. C2H2, C2H6, C3H8 and HCN display vertical mixing ratio profiles that increase with height at 15° S and 80° N, consistent with their formation in the upper atmosphere, diffusion downwards and condensation in the lower stratosphere, as expected from photochemical models. The CH3C2H and C4H2 mixing ratios also increase with height at 15° S. But near the north pole, their profiles present an unexpected minimum around 300 km, observed for the first time thanks to the high vertical resolution of the CIRS limb data. C2H4 is the only molecule having a vertical abundance profile that decreases with height at 15° S. At 80° N, it also displays a minimum of its mixing ratio around the 0.1-mbar level. For C6H6, an upper limit of 1.1 ppb (in the 0.3–10 mbar range) is derived at 15° S, whereas a constant mixing ratio profile of is inferred near the north pole. At 15° S, the vertical profile of HCN exhibits a steeper gradient than other molecules, which suggests that a sink for this molecule exists in the stratosphere, possibly due to haze formation. All molecules display a more or less pronounced enrichment towards the north pole, probably due, in part, to subsidence of air at the north (winter) pole that brings air enriched in photochemical compounds from the upper atmosphere to lower levels.
Icarus.
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Sandrine Vinatier, Bruno Bézard,
Conor A. Nixon,
Andrei Mamoutkine,
Ronald C. Carlson,
Donald E. Jennings,
Ever A. Guandique,
Nick A. Teanby,
Gordon L. Bjoraker,
F. Michael Flasar,
Virgil G. Kunde
[show abstract]
[hide abstract]
ABSTRACT: Observations of the Composite InfraRed Spectrometer (CIRS) during the entire nominal Cassini mission (2004–2008) provide us with an accurate global view of composition and temperature in the middle atmosphere of Titan (between 100 and 500 km). We investigated limb spectra acquired at resolution at nine different latitudes between 56°S and 80°N, with a better sampling in the northern hemisphere where molecular abundances and temperature present strong latitudinal variations. From this limb data acquired between February 2005 and May 2008, we retrieved the vertical mixing ratio profiles of C2H2, C2H4, C2H6, C3H8, CH3C2H, C4H2, C6H6, HCN, HC3N and CO2. We present here for the first time, the latitudinal variations of the C2H6, C3H8, CO2, C2H4 and C6H6 vertical mixing ratios profiles. Some molecules, such as C2H6 or C3H8 present little variations above their condensation level. The other molecules (except CO2) show a significant enhancement of their mixing ratios poleward of 50°N. C2H4 is the only molecule whose mixing ratio decreases with height at latitudes below 46°N. Regions depleted in C2H2, HCN and C4H2 are observed around 400 km (0.01 mbar) and 55°N. We also inferred a region enriched in CO2 located between 30 and 40°N in the 2–0.7 mbar pressure range. At 80°N, almost all molecules studied here present a local minimum of their mixing ratio profiles near 300 km (∼0.07 mbar), which is in contradiction with Global Circulation Models that predict constant-with-height vertical profiles due to subsidence at the north pole.
Icarus.
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ABSTRACT: Limb and nadir spectra acquired by Cassini/CIRS (Composite InfraRed Spectrometer) are analyzed in order to derive, for the first time, the meridional variations of diacetylene (C4H2) and methylacetylene (CH3C2H) mixing ratios in Saturn’s stratosphere, from 5 hPa up to 0.05 hPa and 80°S to 45°N. We find that the C4H2 and CH3C2H meridional distributions mimic that of acetylene (C2H2), exhibiting small-scale variations that are not present in photochemical model predictions. The most striking feature of the meridional distribution of both molecules is an asymmetry between mid-southern and mid-northern latitudes. The mid-southern latitudes are found depleted in hydrocarbons relative to their northern counterparts. In contrast, photochemical models predict similar abundances at north and south mid-latitudes. We favor a dynamical explanation for this asymmetry, with upwelling in the south and downwelling in the north, the latter coinciding with the region undergoing ring shadowing. The depletion in hydrocarbons at mid-southern latitudes could also result from chemical reactions with oxygen-bearing molecules.Poleward of 60°S, at 0.1 and 0.05 hPa, we find that the CH3C2H and C4H2 abundances increase dramatically. This behavior is in sharp contradiction with photochemical model predictions, which exhibit a strong decrease towards the south pole. Several processes could explain our observations, such as subsidence, a large vertical eddy diffusion coefficient at high altitudes, auroral chemistry that enhances CH3C2H and C4H2 production, or shielding from photolysis by aerosols or molecules produced from auroral chemistry. However, problems remain with all these hypotheses, including the lack of similar behavior at lower altitudes.Our derived mean mixing ratios at 0.5 hPa of (2.4 ± 0.3) × 10−10 for C4H2 and of (1.1 ± 0.3) × 10−9 for CH3C2H are compatible with the analysis of global-average ISO observations performed by Moses et al. (Moses, J.I., Bézard, B., Lellouch, E., Gladstone, G.R., Feuchtgruber, H., Allen, M. [2000a]. Icarus 143, 244–298). Finally, we provide values for the ratios [CH3C2H]/[C2H2] and [C4H2]/[C2H2] that can constrain the coupled chemistry of these hydrocarbons.
Icarus.
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ABSTRACT: We report the detection of benzene (c-C6H6) in the upper atmospheres of Jupiter at midlatitudes and Saturn (disk average). Observations with the Short-Wavelength Spectrometer of the Infrared Space Observatory reveal emission from the Q-branch of benzene around 14.84 μm on these two planets but not on Uranus and Neptune. Column densities have been derived through radiative transfer calculations assuming various mixing ratio profiles. The inferred abundances are 9+4.5−7.5×1014 molecules cm−2 above the 50-mbar level on Jupiter and 4.7+2.1−1.1×1013 molecules cm−2 above the 10-mbar level on Saturn. Upper limits have been derived for Uranus and Neptune. Results are compared with existing chemical models.
Icarus.
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ABSTRACT: We report the detection of H13CN and HC15N in mid-infrared spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard Cassini, along with the determination of the 12C/13C and 14N/15N isotopic ratios. We analyzed two sets of limb spectra recorded near 13–15° S (Tb flyby) and 83° N (T4 flyby) at 0.5 cm−1 resolution. The spectral range 1210–1310 cm−1 was used to retrieve the temperature profile in the range 145–490 km at 13° S and 165–300 km at 83° N. These two temperature profiles were then incorporated in the atmospheric model to retrieve the abundance profile of H12C14N, H13CN and HC15N from their bands at 713, 706 and 711 cm−1, respectively. The HCN abundance profile was retrieved in the range 90–460 km at 15° S and 165–305 km at 83° N. There is no evidence for vertical variations of the isotopic ratios. Constraining the isotopic abundance profiles to be proportional to the HCN one, we find at 15° S, and at 83° N, two values that are statistically consistent. A combination of these results yields a 12C/13C value equal to 75±12. This global result, as well as the 15° S one, envelop the value in Titan's methane (82.3±1) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779–784] measured at 10° S and is slightly lower than the terrestrial inorganic standard value (89). The 14N/15N isotopic ratio is found equal to at 15° S and at 83° N. Combining the two values yields 14N/15N = 56 ± 8, which corresponds to an enrichment in 15N of about 4.9 compared with the terrestrial ratio. These results agree with the values obtained from previous ground-based millimeter observations [Hidayat, T., Marten, A., Bézard, B., Gautier, D., Owen, T., Matthews, H.E., Paubert, G., 1997. Icarus 126, 170–182; Marten, A., Hidayat, T., Biraud, Y., Moreno, R., 2002. Icarus 158, 532–544]. The 15N/14N ratio found in HCN is ∼3 times higher than in N2 [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779–784], which implies a large fractionation process in the HCN photochemistry.
Icarus.
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ABSTRACT: We report the detection of 13CH3D in Titan's stratosphere from Cassini/CIRS infrared spectra near 8.7 μm. Fitting simultaneously the ν6 bands of both 13CH3D and 12CH3D and the ν4 band of CH4, we derive a D/H ratio equal to and a 12C/13C ratio in deuterated methane of , consistent with that measured in normal methane.
Icarus.
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ABSTRACT: We have analyzed nine Voyager 1 infrared spectral averages covering Titan's disk from 53°S to 70°N. By use of radiative transfer modeling we have determined the thermal profiles and mean molecular abundances in the stratosphere of the species with signatures in the region 200-1500 cm-1. Temperature latitudinal variations were found in accordance with Flasar and Conrath (1990, Icarus 85, 346-354). A maximal temperature decrease of 17 K at the 0.4-mbar level (225 km of altitude) is observed between 5°S (the warmest region) and 70°N, whereas the temperature drops only by ∼3 K from 5° to 53°S. Mean molecular fractions, associated with atmospheric levels between 4 and 9 mbar, were derived from the best fit of the infrared data. The CO2 abundance remains constant from pole to pole within error bars. HCN shows a steady increase from south to north (total enhancement of > 30). For all the other molecules, variations in composition exist mainly between the equator and the north polar region. Ethane, acetylene, and propane show a moderate enrichment by about a factor of two. C4H2, C2H4, C3H4 show significantly higher mole fractions at latitudes > 50°N (by factors of ∼7-15). C2N2 and HC3N, undetected southward of 50°N, show at least an order of magnitude enhancement near the north pole. The stratospheric haze opacity at wavenumbers larger than 600 cm-1 was found to show a north-to-south enhancement of ∼2.5 ± 0.3. Coldest temperatures, found at high northern latitudes, are associated with enhanced gas concentration and haze opacity, and this may be caused by more efficient radiative cooling (Bézard, B., A. Coustenis, and C. P. McKay 1995, Icarus 113,267-276). The observed latitudinal variations in hydrocarbons and nitriles may be related to seasonal and spatial variations of the solar flux (Yung, Y. L. 1987, Icarus 72, 468-472). The present results set constraints for the future development of 2-D seasonal photochemical models.
Icarus.
