An investigation of Pluto’s troposphere using stellar occultation light curves and an atmospheric radiative-conductive-convective model

We use a radiative-conductive-convective model to assess the height of Pluto’s troposphere, as well as surface pressure and surface radius, from stellar occultation data from the years 1988, 2002, and 2006. The height of the troposphere, if it exists, is less than 1 km for all years analyzed. Pluto has at most a planetary boundary layer and not a troposphere. As in previous analyses of Pluto occultation light curves, we find that the surface pressure is increasing with time, assuming that latitude and longitude variations in Pluto’s atmosphere are negligible. The surface pressure is found to be slightly higher ( μbar in 1988,  μbar in 2002, and 18.5 ± 4.7 μbar in 2006) than in our previous analyses with the troposphere excluded. The surface radius is determined to be . Comparison of the minimum reduced chi-squared values between the best-fit radiative-conductive-convective (i.e., troposphere-included) model and best-fit radiative-conductive (i.e., troposphere-excluded) shows that the troposphere-included model is only a slightly better fit to the data for all 3 years. Uncertainties in the small-scale physical processes of Pluto’s lower atmosphere and consequently the functional form of the model troposphere lend more confidence to the troposphere-excluded results.     

On the Vertical Thermal Structure of Pluto's Atmosphere

A radiative–conductive model for the vertical thermal structure of Pluto's atmosphere is developed with a non-LTE treatment of solar heating in the CH₄3.3 μm and 2.3 μm bands, non-LTE radiative exchange and cooling in the CH₄7.6 μm band, and LTE cooling by CO rotational line emission. The model includes the effects of opacity and vibrational energy transfer in the CH₄molecule. Partial thermalization of absorbed solar radiation in the CH₄3.3 and 2.3 μm bands by rapid vibrational energy transfer from the stretch modes to the bending modes generates high altitude heating at sub-microbar pressures. Heating in the 2.3 μm bands exceeds heating in 3.3 μm bands by approximately a factor of 6 and occurs predominantly at microbar pressures to generate steep temperature gradients ∼10–20 K km⁻¹forp> 2 μbar when the surface or tropopause pressure is ∼3 μbar and the CH₄mixing ratio is a constant 3%. This calculated structure may account for the “knee” in the stellar occultation lightcurve. The vertical temperature structure in the first 100 km above the surface is similar for atmospheres with Ar, CO, and N₂individually as the major constituent. If a steep temperature gradient ∼20 K km⁻¹is required near the surface or above the tropopause, then the preferred major constituent is Ar with 3% CH₄mixing ratio to attain a calculated ratio ofT/(= 3.5 K amu⁻¹) in agreement with inferred values from stellar occultation data. However, pure Ar and N₂ices at the same temperature yield an Ar vapor pressure of only ∼0.04 times the N₂vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with N₂as the dominant constituent. One possibility is a 3 μbar N₂atmosphere with 0.3% CH₄that has 106 K isothermal region (T/= 3.8 K amu⁻¹) and ∼8 K km⁻¹surface/tropopause temperature gradient. Another possibility would be a higher surface pressure ∼10 μbar with a scattering haze forp> 2 μbar. Our model with appropriate adjustments in the CH₄density profile to Triton's inferred profile yields a temperature profile consistent with the UVS solar occultation data (Krasnopolsky, V. A., B. R. Sandel, and F. Herbert 1992.J. Geophys. Res.98, 3065–3078.) and ground-based stellar occultation data (Elliot, J. L., E. W. Dunham, and C. B. Olkin 1993.Bull. Am. Astron. Soc.25, 1106.).     

Constraints on bulk composition,seasonal variation,and global dynamics of Pluto's atmosphere

Cosmic abundance, vapor pressure, and molecular weight considerations restrict the likely gas candidates for Pluto's atmosphere to Ne, N₂, CO, O₂, and Ar, in addition to the already detected CH₄. The vapor pressures and cosmic abundances of these gases indicate that all except Ne should be saturated in Pluto's atmosphere. The vapor pressure of Ne is so high that the existence of solid or liquid Ne on Pluto's surface is very unlikely; cosmic abundance arguments imply that Ne cannot attain saturation in Pluto's atmosphere. At both perihelion, N₂ should dominate the saturated gases. CO₂ should have the next highest mixing ratio, followed by O₂ and Ar. CH₄ should have the smallest mixing ratio. Because vapor pressures of these gases vary with temperature at diverse rates, the bulk and constituent mixing ratios of Pluto's atmosphere should vary with season. Between perihelion and aphelion, the column abundance of CH₄ may change by a factor of 260 while that of N₂ changes by only a factor of 52. The potential seasonal variation of Pluto's atmosphere was investigated by considering the behavior of these gases when individually mixed with CH₄. The effects of diurnal and latitudinal variation of insolation and eclipses on the atmosphere also were investigated. Seasonal effects are shown to dominate. It was shown that the atmospheric bulk may not be a minimum near aphelion but rather at intermediate distances from the Sun during summer/winter inadequate ice deposits may allow the atmosphere to collapse by freezing out over winter latitudes. If the atmosphere does not collapse, its weight is sufficient to keep it distributed uniformly around Pluto's surface. In this case, the atmosphere tends to regulate the surface temperature to a seasonally dependent value which is uniform over the globe.Finally, the likely global circulation regimes for each model atmosphere as a function of temperature were investigated and it was concluded that if CH₄, O₂, or CO dominates the atmosphere, Pluto will exhibit cyclic variations between an axially symmetric circulation system at perihelion and a baroclinic wave regime at aphelion. However, if N₂ dominates, as is likely, the wave regime should hold continuously. If the atmosphere collapses to a thin halo during summer/winter seasons, only a weak, symmetric circulation should occur.     

Titania's radius and an upper limit on its atmosphere from the September 8, 2001 stellar occultation

On September 8, 2001 around 2 h UT, the largest uranian moon, Titania, occulted Hipparcos star 106829 (alias SAO 164538, a V=7.2, K0 III star). This was the first-ever observed occultation by this satellite, a rare event as Titania subtends only 0.11 arcsec on the sky. The star's unusual brightness allowed many observers, both amateurs or professionals, to monitor this unique event, providing fifty-seven occultations chords over three continents, all reported here. Selecting the best 27 occultation chords, and assuming a circular limb, we derive Titania's radius: (1-σ error bar). This implies a density of using the value derived by Taylor [Taylor, D.B., 1998. Astron. Astrophys. 330, 362-374]. We do not detect any significant difference between equatorial and polar radii, in the limit , in agreement with Voyager limb image retrieval during the 1986 flyby. Titania's offset with respect to the DE405 + URA027 (based on GUST86 theory) ephemeris is derived: ΔαTcos(δT)=−108±13 mas and ΔδT=−62±7 mas (ICRF J2000.0 system). Most of this offset is attributable to a Uranus' barycentric offset with respect to DE405, that we estimate to be: and ΔδU=−85±25 mas at the moment of occultation. This offset is confirmed by another Titania stellar occultation observed on August 1st, 2003, which provides an offset of ΔαTcos(δT)=−127±20 mas and ΔδT=−97±13 mas for the satellite. The combined ingress and egress data do not show any significant hint for atmospheric refraction, allowing us to set surface pressure limits at the level of 10-20 nbar. More specifically, we find an upper limit of 13 nbar (1-σ level) at 70 K and 17 nbar at 80 K, for a putative isothermal CO₂ atmosphere. We also provide an upper limit of 8 nbar for a possible CH₄ atmosphere, and 22 nbar for pure N₂, again at the 1-σ level. We finally constrain the stellar size using the time-resolved star disappearance and reappearance at ingress and egress. We find an angular diameter of 0.54±0.03 mas (corresponding to projected at Titania). With a distance of 170±25 parsecs, this corresponds to a radius of 9.8±0.2 solar radii for HIP 106829, typical of a K0 III giant.     

Titan solar occultation observed by Cassini/VIMS: Gas absorption and constraints on aerosol composition

A solar occultation by Titan's atmosphere has been observed through the solar port of the Cassini/VIMS instrument on January 15th, 2006. Transmission spectra acquired during solar egress probe the atmosphere in the altitude range 70 to 900 km at the latitude of 71° S. Several molecular absorption bands of CH₄ and CO are visible in these data. A line-by-line radiative transfer calculation in spherical geometry is used to model three methane bands (1.7, 2.3, 3.3 μm) and the CO 4.7 μm band. Above 200 km, the methane 2.3 μm band is well fit with constant mixing ratio between 1.4 and 1.7%, in agreement with in situ and other Cassini measurements. Under 200 km, there are discrepancies between models and observations that are yet fully understood. Under 480 km, the 3.3 μm CH₄ band is mixed with a large and deep additional absorption. It corresponds to the C-H stretching mode of aliphatic hydrocarbon chains attached to large organic molecules. The CO 4.7 μm band is observed in the lower stratosphere (altitudes below 150 km) and is well fit with a model with constant mixing ratio of 33±10 ppm. The continuum level of the observed transmission spectra provides new constraints on the aerosol content of the atmosphere. A model using fractal aggregates and optical properties of tholins produced by Khare et al. [Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W., 1984. Icarus 60, 127-137] is developed. Fractal aggregates with more than 1000 spheres of radius 0.05 μm are needed to fit the data. Clear differences in the chemical composition are revealed between tholins and actual haze particles. Extinction and density profiles are also retrieved using an inversion of the continuum values. An exponential increase of the haze number density is observed under 420 km with a typical scale height of 60 km.     

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.     

The first detection of propane on Saturn

We report the first detection of propane, C₃H₈, in Saturn's stratosphere. Observations taken on September 8, 2002 UT at NASA's IRTF using TEXES, show multiple emission lines due to the 748 cm⁻¹ν₂₁ band of C₃H₈. Using a line-by-line radiative transfer code, we are able to fit the data by scaling the propane vertical mixing ratio profile from the photochemical model of Moses et al. [2000. Icarus 143, 244-298]. Multiplicative factors of 0.7 and 0.65 are required to fit the −20° and −80° planetocentric latitude spectra. The resultant profiles are characterized by a 5 mbar mixing ratio of 2.7±0.8×¹⁰⁻⁸ at −20° and at −80° latitude. These results suggest that the time scale for meridional circulation lies between the net photochemical lifetimes of C₂H₂ and C₃H₈, ≈30-600 years.     

Saturn A ring surface mass densities from spiral density wave dispersion behavior

We have undertaken an analysis of the Voyager photopolarimeter (PPS) stellar occultation data of Saturn's A ring. The Voyager PPS observed the bright star δ Scorpii as it was occulted by Saturn's main rings during the spacecraft flyby of the Saturn system in 1981. The occultation measurement produced a ring profile with radial resolution of approximately 100 m, and radial structure is evident in the profile down to the resolution limit. We have applied an autoregressive technique to the data for estimating the power spectrum as a function of radius, which has allowed us to identify 40 spiral density waves in Saturn's A ring, associated with the strongest torques due to forcing from the moons. The majority of the detected waves are observed to disperse linearly in regions beginning a few kilometers from the resonance location. We have used the dispersion behavior for those waves to calculate local surface mass densities in the vicinity of each wave. We find that the inner three-quarters of the A ring (up to the beginning of the Encke gap) has an average surface mass density of , while the outer region has an average surface mass density of . The two regions have different mean surface mass densities with a significance of approximately 0.999993, as estimated with a T-statistic, which corresponds to about 4.5σ. While the mean optical depth of the A ring increases slightly with increasing distance from Saturn, we find that it is not significantly correlated with the surface mass density; the two quantities having a linear Pearson's correlation coefficient of r_corr≈−0.03. The variation of mass density, independent of PPS optical depth, is consistent with previous conjectures that the particle size distribution and composition are not constant across the entire A ring, particularly in the very outer portion. We estimate the mass of Saturn's A ring from our surface mass density estimates as 4.9×10²¹ gm, or 8.61×10⁻⁹ of the mass of Saturn, roughly equivalent to the mass of a 110-km diameter icy satellite. This mass is almost 25% smaller than estimates from previous studies, but is well within the expected errors of the derived mass densities. We also identified three previously unstudied features which exhibit linear dispersion. The strongest of these features is tentatively identified as the Janus 13:11 density wave. The other two features do not fall near any known satellite resonances and may represent density waves created by previously undetected satellites.     

The methane abundance and structure of Uranus' cloud bands inferred from spatially resolved 2006 Keck grism spectra

Grism spectra of Uranus obtained at the Keck Observatory in 2006, using the NIRC2 instrument and adaptive optics, provide new constraints on the vertical structure of Uranus' cloud bands and on the volume mixing ratio of methane. The best model fits to H-band spectra (1.49-1.635 μm) are found for a methane volume mixing ratio of 1.0 ± 0.25% for latitudes near 43° S and 1-1.6% for latitudes of 12° S and 33° N. Analysis of the J-band spectra are confused by discrepancies between short-wave and long-wave sides of the 1.28 μm window region. The short-wave side of the window (1.23-1.30 μm) is best fit with 1.6% CH₄, but if the fitted spectral range is extended to include the long-wave side of the window (1.2-1.34 μm), the best fit CH₄ mixing ratio is 4% or more, although many small scale spectral features are poorly fit over this range even at high methane mixing ratios, suggesting that models of methane opacity may be inconsistent in this spectral region. Most of the latitudinal variability of the H-band spectra can be fit with clouds near 2-3 and 6-8 bar, with cloud reflectivity of the deeper layer increasing from ∼2% at 33° N to 3-4% in the southern hemisphere. This layer is most likely made of H₂S particles and appears weakly reflective because it is optically thin and possibly also contaminated by absorbing materials. The reflectivity of the 2-3-bar cloud increases from 0.5% at 33° N to ∼1% at the bright band centered near 43° S, where the upper cloud is a little higher (pressure is 10% lower) and ∼25% more reflective than at nearby latitudes. The bright band is also associated with lowering of the deep cloud pressure, by ∼1.4 bar. The bright band parameters are roughly consistent with those obtained from 1975 disk-averaged spectra, obtained when the southern hemisphere was more exposed to the Sun. The lack of significant cloud particle contributions near 1.2 bar, where occultation results suggested a methane cloud, is confirmed by both spectra and HST imaging observations.     

Morphology and variability of the Titan ringlet and Huygens ringlet edges

We present a forward modeling approach for determining, in part, the ring particle spatial distribution in the vicinity of sharp ring or ringlet edges. Synthetic edge occultation profiles are computed based on a two-parameter particle spatial distribution model. One parameter, h, characterizes the vertical extent of the ring and the other, δ, characterizes the radial scale over which the ring optical depth transitions from the background ring value to zero. We compare our synthetic occultation profiles to high resolution stellar occultation light curves observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) for occultations by the Titan ringlet and Huygens ringlet edges.More than 100 stellar occultations of the Huygens ringlet and Titan ringlet edges were studied, comprising 343 independent occultation cuts of the edges of these two ringlets. In 237 of these profiles the measured light-curve was fit well with our two-parameter edge model. Of the remaining edge occultations, 69 contained structure that could only be fit with extremely large values of the ring-plane vertical thickness (h > 1 km) or by adopting a different model for the radial profile of the ring optical depth. An additional 37 could not be fit by our two-parameter model.Certain occultations at low ring-plane incidence angles as well as occultations nearly tangent to the ring edge allow the direct measurement of the radial scale over which the particle packing varies at the edge of the ringlet. In 24 occultations with these particular viewing geometries, we find a wide variation in the radial scale of the edge. We are able to constrain the vertical extent of the rings at the edge to less than ∼300 m in the 70% of the occultations with appropriate viewing geometry, however tighter constraints could not be placed on h due to the weaker sensitivity of the occultation profile to vertical thickness compared to its sensitivity to δ.Many occultations of a single edge could not be fit to a single value of δ, indicating large temporal or azimuthal variability, although the azimuthal variation in δ with respect to the longitudes of various moons in the system did not show any discernible pattern.     

The atmosphere of Io from Pioneer 10 radio occultation measurements

The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S-band radio occultation measurements of its atmosphere. The dayside entry measurements revealed an ionosphere having a peak density of about 6 × 10⁴ elcm^?3 at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na⁺ and 495 K if N₂⁺ is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9 × 10³ elcm^?3 at an altitude of 50 km. The topside plasma temperature is 160 K for N₂^? and 131 K for Na⁺. If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10¹¹?10¹² cm³, corresponding to a surface pressure of less than 10^?8 to 10^?9 bars. Two measurements of its radius were also obtained yielding a value of 1830 km for the entry and 192 km for the exit. The discrepancy between these values may indicate an ephemeris uncertainty of about 45 km. The two measurements yield an average radius of 1875 km, which is not in agreement with the results of the Beta Scorpii stellar occultation.     

Buoyancy waves in Pluto’s high atmosphere: Implications for stellar occultations

We apply scintillation theory to stellar signal fluctuations in the high-resolution, high signal/noise, dual-wavelength data from the MMT observation of the 2007 March 18 occultation of P445.3 by Pluto. A well-defined high wavenumber cutoff in the fluctuations is consistent with viscous-thermal dissipation of buoyancy waves (internal gravity waves) in Pluto’s high atmosphere, and provides strong evidence that the underlying density fluctuations are governed by the gravity-wave dispersion relation.     

Observations of C2H6 and C2H2 in the stratosphere of Saturn

We have performed high-resolution spectral observations at mid-infrared wavelengths of C₂H₆ (12.16 μm), and C₂H₂ (13.45 μm) on Saturn. These emission features probe the stratosphere of the planet and provide information on the hydrocarbon 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 in November and December 1994. We used Voyager IRIS CH₄ observations (7.67 μm) to derive a temperature profile on the saturnian atmosphere for the region of the stratosphere. This profile was then used in conjunction with height-dependent volume mixing ratios of each hydrocarbon to determine global abundances for ethane and acetylene. Our ground-based measurements indicate abundances of for C₂H₆ (1.0 mbar pressure level), and for C₂H₂ (1.6 mbar pressure level). We also derived new mixing ratios from the Voyager mid-latitude IRIS observations; 8.6±0.9×¹⁰⁻⁶ for C₂H₆ (0.1-3.0 mbar pressure level), and 1.6±0.2×¹⁰⁻⁷ for C₂H₂ (2.0 mbar pressure level).     

Titan's hydrodynamically escaping atmosphere

The upper atmosphere of Titan is currently losing mass at a rate , by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating by CH₄ absorption. The hydrodynamic mass loss is essentially CH₄ and H₂ escape. Their combined escape rates are restricted by power limitations from attaining their limiting rates (and limiting fluxes). Hence they must exhibit gravitational diffusive separation in the upper atmosphere with increasing mixing ratios to eventually become major constituents in the exosphere. A theoretical model with solar EUV heating by N₂ absorption balanced by HCN rotational line cooling in the upper thermosphere yields densities and temperatures consistent with the Huygens Atmospheric Science Investigation (HASI) data [Fulchignoni, M., and 42 colleagues, 2005. Nature 438, 785-791], with a peak temperature of ∼185-190 K between 3500-3550 km. This model implies hydrodynamic escape rates of and , or some other combination with a higher H₂ escape flux, much closer to its limiting value, at the expense of a slightly lower CH₄ escape rate. Nonthermal escape processes are not required to account for the loss rates of CH₄ and H₂, inferred by the Cassini Ion Neutral Mass Spectrometer (INMS) measurements [Yelle, R.V., Borggren, N., de la Haye, V., Kasprzak, W.T., Niemann, H.B., Müller-Wodarg, I., Waite Jr., J.H., 2006. Icarus 182, 567-576].     

Studies of Planetary Atmospheres by Stellar Occultations: Application to Mars and Venus

We consider the application of the stellar occultation method to the studies of planetary atmospheres and its history and briefly describe the instruments designed for such measurements (SPICAM/Mars-96, GOMOS/ENVISAT). In comparison with solar occultations, this method allows the profiles to be measured almost at any time of the day and at any location of the planet, irrespective of the orbit of the spacecraft from which observations are carried out. Based on the measuring characteristics of the SPICAM-Light UV spectrometer for the spectral range 118–320 nm with a resolution of 0.9 nm (for the ESA Mars Express Mission; launched in June 2003), we simulate the capabilities of the method to study the Martian atmosphere. In stellar occultation measurements, the stellar spectrum changes because of the absorption by CO₂ and O₃, other gases, and aerosols. The profiles of the CO₂ and O₃ density (and, hence, the temperature) and the aerosol content can be restored by solving the inverse problem. Observations of bright stars (no fewer than 30) three to five times in a turn allow us to measure the atmospheric density at altitudes 10–150 km with an accuracy of about 2% and the temperature at altitudes 20–130 km with an accuracy of 3 K. Ozone is measured with an accuracy of several percent at altitudes 25–40 km or lower, depending on the conditions. Optically thin clouds and hazes, particularly on the nightside where no measurements are possible in reflected light, can be studied. The SPICAV experiment, which is similar to SPICAM-Light, is part of the Venus Express (to be launched in 2005) scientific payload. On Venus, stellar occultations can be used to measure the atmospheric temperature and density above clouds at altitudes up to 130–150 km and to study the SO₂ profile. The results of our simulations can be easily extended to instruments with different measuring characteristics.     

The vertical structure of Titan's upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements

Data acquired by the Ion Neutral Mass Spectrometer (INMS) on the Cassini spacecraft during its close encounter with Titan on 26 October 2004 reveal the structure of its upper atmosphere. Altitude profiles of N₂, CH₄, and H₂, inferred from INMS measurements, determine the temperature, vertical mixing rate, and escape flux from the upper atmosphere. The mean atmospheric temperature in the region sampled by the INMS is 149±3 K, where the variance is a consequence of local time variations in temperature. The CH₄ mole fraction at 1174 km is 2.71±0.1%. The effects of diffusive separation are clearly seen in the data that we interpret as an eddy diffusion coefficient of , that, along with the measured CH₄ mole fraction, implies a mole fraction in the stratosphere of 2.2±0.2%. The H₂ distribution is affected primarily by upward flow and atmospheric escape. The H₂ mole fraction at 1200 km is 4±1×¹⁰⁻³ and analysis of the altitude profile indicates an upward flux of , referred to the surface. If horizontal variations in temperature and H₂ density are small, this upward flux also represents the escape flux from the atmosphere. The CH₄ density exhibits significant horizontal variations that are likely an indication of dynamical processes in the upper atmosphere.     

Latitudinal transport by barotropic waves in Titan's stratosphere.: II. Results from a coupled dynamics-microphysics-photochemistry GCM

We present a 2D general circulation model of Titan's atmosphere, coupling axisymmetric dynamics with haze microphysics, a simplified photochemistry and eddy mixing. We develop a parameterization of latitudinal eddy mixing by barotropic waves based on a shallow-water, longitude-latitude model. The parameterization acts locally and in real time both on passive tracers and momentum. The mixing coefficient varies exponentially with a measure of the barotropic instability of the mean zonal flow. The coupled GCM approximately reproduces the Voyager temperature measurements and the latitudinal contrasts in the distributions of HCN and C₂H₂, as well as the main features of the zonal wind retrieved from the 1989 stellar occultation. Wind velocities are consistent with the observed reversal time of the North-South albedo asymmetry of 5 terrestrial years. Model results support the hypothesis of a non-uniform distribution of infrared opacity as the cause of the Voyager temperature asymmetry. Transport by the mean meridional circulation, combined with polar vortex isolation may be at the origin of the latitudinal contrasts of trace species, with eddy mixing remaining restricted to low latitudes most of the Titan year. We interpret the contrasts as a signature of non-axisymmetric motions.     

The 15 August 1980 occultation by the Uranian system: Structure of the rings and temperature of the upper atmosphere

A stellar occultation by Uranus and its rings was observed on August 15, 1980, from the European Southern Observatory (Chile), at the 3.6-m telescope equipped with an infrared (2.2 μm) photometer. The recording presents the best signal-to-noise ratio obtained since the discovery of the Uranian rings in March 1977. The nine rings were observed, and the profiles of rings α, β, and ? were resolved, the ring α exhibiting a double structure. Strong diffraction peaks are visible in the γ ring profile suggesting an opaque ring with very sharp edges. A broad and faint structure extends outward from the η ring, on a radial scale of about 55 km. Apart from the ring occultations, unexplained sharp and deep events were recorded, and no interpretation is possible until future observations are made. Furthermore, the stellar light curve during the immersion of the star behind the planet provides (via an inversion computation) the temperature profile of the upper atmosphere of Uranus. The temperature is close to 145 ± 10°K at the 3 × 10^?2-mbar pressure level and is nearly constant (155 ± 15°K) in the pressure interval from 10^?2 to 10^?3 mbar. The thermal inversion is as strong as the inversion on Neptune but is located at higher altitudes. This high stratospheric temperature is consistent with the upper limit of the brightness temperature at 8 μm only if CH₄ follows its saturation law.     

Detection of CH3D on Titan

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.     

NN detectability in Pluto’s atmosphere

Based on the vapor pressure behavior of Pluto’s surface ices, Pluto’s atmosphere is expected to be predominantly composed of N₂ gas. Measurement of the N₂ isotopologue ¹⁵N/¹⁴N ratio within Pluto’s atmosphere would provide important clues to the evolution of Pluto’s atmosphere from the time of formation to its present state. The most straightforward way of determining the N₂ isotopologue ¹⁵N/¹⁴N ratio in Pluto’s atmosphere is via spectroscopic observation of the ¹⁴N¹⁵N gas species. Recent calculations of the 80–100 nm absorption behavior of the ¹⁴N₂ and ¹⁴N¹⁵N isotopologues by Heays et al. (Heays, A.N. et al. [2011]. J. Chem. Phys. 135, 244301), Lewis et al. (Lewis, B.R., Heays, A.N., Gibson, S.T., Lefebvre-Brion, H., Lefebvre, R. [2008]. J. Chem. Phys. 129, 164306); Lewis et al. (Lewis, B.R., Gibson, S.T., Zhang, W., Lefebvre-Brion, H., Robbe, J.-M. [2005]. J. Chem. Phys. 122, 144302), and Haverd et al. (Haverd, V.E., Lewis, B.R., Gibson, S.T., Stark, G. [2005]. J. Chem. Phys. 123, 214304) show that the peak magnitudes of the ¹⁴N₂ and ¹⁴N¹⁵N absorption bandhead cross-sections are similar, but the locations of the bandhead peaks are offset in wavelength by ∼0.05–0.1 nm. These offsets make the segregation of the ¹⁴N₂ and ¹⁴N¹⁵N absorption signatures possible. We use the most recent N₂ isotopologue absorption cross-section calculations and the atmospheric density profiles resulting from photochemical models developed by Krasnopolsky and Cruickshank (Krasnopolsky, V.A., Cruickshank, D.P. [1999]. J. Geophys. Res. 104, 21979–21996) to predict the level of solar light that will be transmitted through Pluto’s atmosphere as a function of altitude during a Pluto solar occultation. We characterize the detectability of the isotopic absorption signature per altitude assuming ¹⁴N¹⁵N concentrations ranging from 0.1% to 2% of the ¹⁴N₂ density and instrumental spectral resolutions ranging from 0.01 to 0.3 nm. Our simulations indicate that optical depth of unity is attained in the key ¹⁴N¹⁵N absorption bands located between 85 and 90 nm at altitudes ∼1100–1600 km above Pluto’s surface. Additionally, an ¹⁴N¹⁵N isotope absorption depth ∼4–15% is predicted for observations obtained at these altitudes at a spectral resolution of ∼0.2–0.3 nm, if the N₂ isotopologue ¹⁵N/¹⁴N percent ratio is comparable to the 0.37–0.6% ratio observed at Earth, Titan and Mars. If we presume that the predicted absorption depth must be at least 25% greater than the expected observational uncertainty, then it follows that a statistically significant detection of these signatures and constraint of the N₂ isotopologue ¹⁴N/¹⁵N ratio within Pluto’s atmosphere will be possible if the attainable observational signal-to noise (S/N) ratio is ?9. The New Horizons (NH) Mission will be able to obtain high S/N, 0.27–0.35 nm full-width half-max 80–100 nm spectral observations of Pluto using the Alice spectrograph. Based on the NH/Alice specifications we have simulated 0.3 nm spectral resolution solar occultation spectra for the 1100–1600 km altitude range, assuming 30 s integration times. These simulations indicate that NH/Alice will obtain spectral observations within this altitude range with a S/N ratio ∼25–50, and should be able to reliably detect the ¹⁴N¹⁵N gas absorption signature between 85 and 90 nm if the ¹⁴N¹⁵N concentration is ∼0.3% or greater. This, additionally, implies that the non-detection of the ¹⁴N¹⁵N species in the 1100–1600 km range by NH/Alice may be used to reliably establish an upper limit to the N₂ isotopologue ¹⁵N/¹⁴N ratio within Pluto’s atmosphere. Similar results may be derived from 0.2 to 0.3 nm spectral resolution observations of any other N₂-rich Solar System or exoplanet atmosphere, provided the observations are attained with similar S/N levels.