Titan's hydrodynamically escaping atmosphere: Escape rates and the structure of the exobase region

In Strobel [Strobel, D.F., 2008. Icarus, 193, 588-594] a mass loss rate from Titan's upper atmosphere, , was calculated for a single constituent, N₂ atmosphere by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating due to CH₄ absorption. It was estimated, but not proven, that the hydrodynamic mass loss is essentially CH₄ and H₂ escape. Here the individual conservation of momentum equations for the three major components of the upper atmosphere (N₂, CH₄, H₂) are solved in the low Mach number limit and compared with Cassini Ion Neutral Mass Spectrometer (INMS) measurements to demonstrate that light gases (CH₄, H₂) preferentially escape over the heavy gas (N₂). The lightest gas (H₂) escapes with a flux 99% of its limiting flux, whereas CH₄ is restricted to ?75% of its limiting flux because there is insufficient solar power to support escape at the limiting rate. The respective calculated H₂ and CH₄ escape rates are 9.2×¹⁰²⁷ and 1.7×¹⁰²⁷ s−1, for a total of . From the calculated densities, mean free paths of N₂, CH₄, H₂, and macroscopic length scales, an extended region above the classic exobase is inferred where frequent collisions are still occurring and thermal heat conduction can deliver power to lift the escaping gas out of the gravitational potential well. In this region rapid acceleration of CH₄ outflow occurs. With the thermal structure of Titan's thermosphere inferred from INMS data by Müller-Wodarg et al. [Müller-Wodarg, I.C.F., Yelle, R.V., Cui, J., Waite Jr., J.H., 2008. J. Geophys. Res. 113, doi:10.1029/2007JE003033. E10005], in combination with calculated temperature profiles that include sputter induced plasma heating at the exobase, it is concluded that on average that the integrated, globally average, orbit-averaged, plasma heating rate during the Cassini epoch does not exceed ().     

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].     

Titan's corona: The contribution of exothermic chemistry

The contribution of exothermic ion and neutral chemistry to Titan's corona is studied. The production rates for fast neutrals N₂, CH₄, H, H₂, ³CH₂, CH₃, C₂H₄, C₂H₅, C₂H₆, N(⁴S), NH, and HCN are determined using a coupled ion and neutral model of Titan's upper atmosphere. After production, the formation of the suprathermal particles is modeled using a two-stream simulation, as they travel simultaneously through a thermal mixture of N₂, CH₄, and H₂. The resulting suprathermal fluxes, hot density profiles, and energy distributions are compared to the N₂ and CH₄ INMS exospheric data presented in [De La Haye, V., Waite Jr., J.H., Johnson, R.E., Yelle, R.V., Cravens, T.E., Luhmann, J.G., Kasprzak, W.T., Gell, D.A., Magee, B., Leblanc, F., Michael, M., Jurac, S., Robertson, I.P., 2007. J. Geophys. Res., doi:10.1029/2006JA012222, in press], and are found insufficient for producing the suprathermal populations measured. Global losses of nitrogen atoms and carbon atoms in all forms due to exothermic chemistry are estimated to be and .     

N2 escape rates from Pluto's atmosphere

Hydrodynamic escape of N₂ molecules from Pluto's atmosphere is calculated under the assumption of a high density, slow outflow expansion driven by solar EUV heating by N₂ absorption, near-IR and UV heating by CH₄ absorption, and CO cooling by rotational line emission as a function of solar activity. At 30 AU, the N₂ escape rate varies from in the absence of heating, but driven by an upward thermal heat conduction flux from the stratosphere, for lower boundary temperatures varying from 70-100 K. With solar heating varying from solar minimum to solar maximum conditions and a calculated lower boundary temperature, 88.2 K, the N₂ escape rate range is , respectively. LTE rotational line emission by CO reduces the net solar heat input by at most 35% and plays a minor role in lowering the calculated escape rates, but ensures that the lower boundary temperature can be calculated by radiative equilibrium with near-IR CH₄ heating. While an upward thermal conduction heat flux at the lower boundary plays a fundamental role in the absence of heating, with solar heating it is downward at solar minimum, and is, at most, 13% of the integrated net heating rate over the range of solar activity. For the arrival of the New Horizons spacecraft at Pluto in July 2015, predictions are lower boundary temperature, T₀∼81 K, and N₂ escape rate , and peak thermospheric temperature ∼103 K at 1890 km, based on expected solar medium conditions.     

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.     

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).     

Heavy ion escape from Mars, influence from solar wind conditions and crustal magnetic fields

We have used more than 4 years of Mars Express ion data to estimate the escape of heavy ions ( and ) from Mars. To take the limited field of view of the instrument into account, the data has been binned into spatial bins and angular bins to create average distribution functions for different positions in the near Mars space. The net escape flux for the studied low solar activity period, between May 2007 and May 2011, is 2.0 ± 0.2 × 10²⁴ s⁻¹. The escape has been calculated independently for four different quadrants in the Y_MSO − Z_MSO plane, south, dusk, north and dawn. Escape is highest from the northern and dusk quadrants, 0.6 ± 0.1 × 10²⁴ s⁻¹, and smallest from the south and dawn quadrants, 0.4 ± 0.1 × 10²⁴ s⁻¹. The flux ratio of molecular ( and ) to O⁺ ions is 0.9 ± 0.1, averaged over all quadrants. The flux difference between the north and south quadrants is statistically significant, and is presumed to be due to the presence of significant crustal magnetic fields in the southern hemisphere, reducing the outflow. The difference between the dawn and dusk quadrants is likely due to the magnetic tension associated with the nominal Parker angle spiral, which should lead to higher average magnetic tension on the dusk side. The escape increases during periods of high solar wind flux and during times when co-rotating interaction regions (CIR) affect Mars. In the latter case the increase is a factor 2.4-2.9 as compared to average conditions.     

Monte Carlo computations of the escape of atomic nitrogen from Mars

Monte Carlo simulations were carried out to compute the escape flux of atomic nitrogen for the low and high solar activity martian thermospheres. The total escape of atomic nitrogen at low and high solar activities was found to be 3.03×¹⁰⁵ and , respectively. The escape flux of atomic nitrogen at low and high solar activities from photodissociation of N₂ was found to be 2.75×¹⁰⁵ and , respectively. The remainder of the contribution is from dissociative recombination, which is only important at high solar activity were it comprises about 25% of the total escape. The relative contributions to the total N escape flux from thermal motion of the background atmosphere, winds and co-rotation, and photoionization and subsequent solar wind pickup are also considered here. We find that the total predicted escape fluxes are observed to increase by 20 and 25% at low and high solar activities owing to thermal motion of the background atmosphere. At low and high solar activities, we find that the co-rotation and wind velocities combined translate to a maximum transferable energy of ∼0.0103 and 0.0181 eV, respectively, and that the total escape flux contribution from winds and co-rotation is negligible. Photoionization was found to be a minor process only impacting those source atoms produced with energies close to the escape energy, between 1.5 and 2 eV. The contributions to the total escape fluxes at low and high solar activities from photoionization and subsequent solar wind pickup are found to be about 8 and 13%, respectively.     

Parametric analysis of modeled ion escape from Mars

We develop a parametric fit to the results of a detailed magnetohydrodynamic (MHD) study of the response of ion escape rates (O⁺, and ) to strongly varied solar forcing factors, as a way to efficiently extend the MHD results to different conditions. We then use this to develop a second, evolutionary model of solar forced ion escape. We treat the escape fluxes of ion species at Mars as proportional to the product of power laws of four factors - that of the EUV flux R_euv, the solar wind particle density R_ρ, its velocity (squared) R_v², and the interplanetary magnetic field pressure R_B², where forcing factors are expressed in units of the current epoch-averaged values. Our parametric model is: , where ?(i) is the escape flux of ion i. We base our study on the results of just six provided MHD model runs employing large forcing factor variations, and thus construct a successful, first-order parametric model of the MHD program. We perform a five-dimensional least squares fit of this power law model to the MHD results to derive the flux normalizations and the indices of the solar forcing factors. For O⁺, we obtain the values, 1.73 × 10²⁴ s⁻¹, 0.782, 0.251, 0.382, and 0.214, for ?₀, α, β, γ, and δ, respectively. For , the corresponding values are 1.68 × 10²⁴ s⁻¹, −0.393, 0.798, 0.967, and 0.533. For , they are 8.66 × 10²² s⁻¹, −0.427, 1.083, 1.214, and 0.690. The fit reproduces the MHD results to an average error of about 5%, suggesting that the power laws are broadly representative of the MHD model results. Our analysis of the MHD model shows that by itself an increase in R_EUV enhances O⁺ loss, but suppresses the escape of and , whereas increases in solar wind (i.e., in , and R_B², with R_euv constant) favors the escape of heavier ions more than light ions. The ratios of escaping ions detectable at Mars today can be predicted by this parametric fit as a function of the solar forcing factors. We also use the parametric model to compute escape rates over martian history. This second parametric model expresses ion escape functions of one variable (per ion), ?(i) = ?₀(i)(t/t₀)^−ξ(i). The ξ(i) are linear combinations of the epoch-averaged ion escape sensitivities, which are seen to increase with ion mass. We integrate the and oxygen ion escape rates over time, and find that in the last 3.85 Gyr, Mars would have lost about mbars of , and of water (from O⁺ and ) from ion escape.     

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.     

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.     

Optical and thermal infrared observations of six near-Earth asteroids in 2002

We present thermal infrared photometry and spectrophotometry of six Near-Earth Asteroids (NEAs) using the 3.8 m United Kingdom Infrared Telescope (UKIRT) together with quasi-simultaneous optical observations of five NEAs taken at the 1.0 m Jacobus Kapteyn Telescope (JKT). For Asteroid (6455) 1992 HE we derive a rotational period P=2.736±0.002 h, and an absolute visual magnitude H=14.32±0.24. For Asteroid 2002 HK₁₂ we derive . The Standard Thermal Model (STM), the Fast Rotating Model (FRM) and the Near-Earth Asteroid Thermal Model (NEATM) have been fitted to the measured fluxes to derive albedos and effective diameters. The derived geometric albedos and effective diameters are (6455) 1992 HE: pv=0.26±0.08, Deff=3.55±0.5 km; 1999 HF₁: pv=0.18±0.07, ; 2000 ED₁₀₄: pv=0.18±0.05, Deff=1.21±0.2 km; 2002 HK₁₂: , Deff=0.62±0.2 km; 2002 NX₁₈: pv=0.031±0.009, Deff=2.24±0.3 km; 2002 QE₁₅: , Deff=1.94±0.4 km. The limitations of using the NEATM to observe NEAs at high phase angles are discussed.     

Formation of radical species in photolyzed CH4:N2 ices

We report photochemical studies of thin cryogenic ice films composed of N₂, CH₄ and CO in ratios analogous to those on the surfaces of Neptune’s largest satellite, Triton, and on Pluto. Experiments were performed using a hydrogen discharge lamp, which provides an intense source of ultraviolet light to simulate the sunlight-induced photochemistry on these icy bodies. Characterization via infrared spectroscopy showed that C₂H₆ and C₂H₂, and HCO are formed by the dissociation of CH₄ into H, CH₂ and CH₃ and the subsequent reaction of these radicals within the ice. Other radical species, such as C₂, , CN, and CNN, are observed in the visible and ultraviolet regions of the spectrum. These species imply a rich chemistry based on formation of radicals from methane and their subsequent reaction with the N₂ matrix. We discuss the implications of the formation of these radicals for the chemical evolution of Triton and Pluto. Ultimately, this work suggests that , CN, HCO, and CNN may be found in significant quantities on the surfaces of Triton and Pluto and that new observations of these objects in the appropriate wavelength regions are warranted.     

Jupiter's hydrocarbon polar brightening: Discovery of 3-micron line emission from south polar CH4, C2H2, and C2H6

Jupiter exhibits bright H⁺₃ auroral arcs at 3-4 microns that cool the hot (>1000 K) ionosphere above the ∼¹⁰−7 bar level through the infrared bands of this trace constituent. Below the ¹⁰−7 bar level significant cooling proceeds through infrared active bands of CH₄, C₂H₂, and C₂H₆. We report the discovery of 3-micron line emission from these hydrocarbon species in spectra of the jovian south polar region obtained on April 18 and 20, 2006 (UT) with CGS4 on the United Kingdom Infrared Telescope. Estimated cooling rates through these molecules are 7.5×¹⁰−3, 1.4×¹⁰−3, and , respectively, for a total nearly half that of H⁺₃. We derive a temperature of 450 ± 50 K in the ¹⁰−7-¹⁰−5 bar region from the C₂H₂ lines.     

The 1.10- and 1.18-μm nightside windows of Venus observed by SPICAV-IR aboard Venus Express

Observations of the 1.10- and 1.18-μm nightside windows by the SPICAV-IR instrument aboard Venus Express were analyzed to characterize the various sources of gaseous opacity and determine the H₂O mole fraction in the lower atmosphere of Venus. We showed that the line profile model of Afanasenko and Rodin (Afanasenko, T.S., Rodin, A.V. [2007]. Astron. Lett. 33, 203–210) underestimates the CO₂ absorption in the high-wavelength wing of the 1.18-μm window and we derived an empirical lineshape that matches this wing well. An additional continuum opacity is required to reproduce the variation of the 1.10- and 1.18-μm radiances with surface elevation as observed by the VIRTIS-M instrument aboard Venus Express. A constant absorption coefficient of 0.7 ± 0.2 × 10⁻⁹ cm⁻¹ am⁻² best reproduces the observed variation. We compared spectra calculated with different CO₂ and H₂O line lists. We found that the CDSD line list lacks the 5ν₁ + ν₃ series of CO₂ bands, which provide significant opacity in Venus’ deep atmosphere, and we have constructed a composite line list that best reproduces the observations. We also showed for the first time that HDO brings significant absorption at 1140–1190 nm. Using the best representation of the atmospheric opacity we could reach, we retrieved a water vapor mole fraction of ppmv, pertaining to the altitude range 5–25 km. Combined with previous measurements in the 1.74- and 2.3-μm windows, this result provides strong evidence for a uniform H₂O profile below 40 km, in agreement with chemical models.     

Thermal infrared and optical observations of four near-Earth asteroids

We present thermal infrared photometry and spectrophotometry of four near-Earth asteroids (NEAs), namely (433) Eros, (66063) 1998 RO₁, (137032) 1998 UO₁, and (138258) 2000 GD₂, using the United Kingdom Infrared Telescope (UKIRT) in 2002. For two objects, i.e. (433) Eros and (137032) 1998 UO₁, quasi-simultaneous optical observations were also obtained, using the Jacobus Kapteyn Telescope (JKT). For (127032) 1998 UO₁, we obtain a rotation period P=3.0±0.1 h and an absolute visual magnitude HV=16.7±0.4. The Standard Thermal Model (STM), Fast Rotating Model (FRM) and near-Earth asteroid Thermal Model (NEATM) have been fitted to the IR fluxes to determine effective diameters Deff, geometric albedos pv, and beaming parameters η. The derived values are (433) Eros: Deff=23.3±3.5 km (at lightcurve maximum), pv=0.24±0.07, η=0.95±0.19; (66063) 1998 RO₁: , ; (137032) 1998 UO₁: Deff<1.13 km, pv>0.29; (138258) 2000 GD₂: Deff=0.27±0.04 km, , η=0.74±0.15. (66063) 1998 RO₁ is a binary asteroid from lightcurve characteristics [Pravec, P., and 56 colleagues, 2006. Icarus 181, 63-93] and we estimate the effective diameter of the primary (Dp) and secondary (Ds) components: and . The diameter and albedo of (138258) 2000 GD₂ are consistent with the trend of decreasing diameter for S- and Q-type asteroids found by Delbó et al. [Delbó, M., Harris, A.W., Binzel, R.P., Pravec, P., Davies, J.K., 2003. Icarus 166, 116-130]. A possible trend of increasing beaming parameter with diameter for small (less than about 3 km) S- and Q-type asteroids is found.     

Detection of C2HD and the D/H ratio on Titan

We report here the first detection of mono-deuterated acetylene (acetylene-d1, C₂HD) in Titan's atmosphere from the presence of two of its emission bands at 678 and 519 cm⁻¹ as observed in CIRS spectral averages of nadir and limb observations taken between July 2004 and mid-2007. By using new laboratory spectra for this molecule, we were able to derive its abundance at different locations over Titan's disk. We find the C₂HD value () to be roughly constant with latitude from the South to about 45° N and then to increase slightly in the North, as is the case for C₂H₂. Fitting the 678 cm⁻¹ν₅ band simultaneously with the nearby C₂H₂ 729 cm⁻¹ν₅ band, allows us to infer a D/H ratio in acetylene on Titan with an average of the modal values of 2.09±0.45×¹⁰⁻⁴ from the nadir observations, the uncertainties being mainly due to the vertical profile used for the fit of the acetylene band. Although still subject to significant uncertainty, this D/H ratio appears to be significantly larger than the one derived in methane from the CH₃D band (upper limit of 1.5×¹⁰⁻⁴; Bézard, B., Nixon, C.A., Kleiner, I., Jennings, D.E., 2007. Icarus, 191, 397-400; Coustenis, A., Achterberg, R., Conrath, B., Jennings, D., Marten, A., Gautier, D., Bjoraker, G., Nixon, C., Romani, P., Carlson, R., Flasar, M., Samuelson, R.E., Teanby, N., Irwin, P., Bézard, B., Orton, G., Kunde, V., Abbas, M., Courtin, R., Fouchet, Th., Hubert, A., Lellouch, E., Mondellini, J., Taylor, F.W., Vinatier, S., 2007. Icarus 189, 35-62). From the analysis of limb data we infer D/H values of (at 54° S), (at 15° S), (at 54° N) and (at 80° N), which average to a mean value of 1.63±0.27×¹⁰⁻⁴.     

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.     

Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions

The third most abundant species in Titan’s atmosphere is molecular hydrogen with a tropospheric/lower stratospheric mole fraction of 0.001 derived from Voyager and Cassini infrared measurements. The globally averaged thermospheric H₂ mole fraction profile from the Cassini Ion Neutral Mass Spectrometer (INMS) measurements implies a small positive gradient in the H₂ mixing ratio from the tropopause region to the lower thermosphere (∼950-1000 km), which drives a downward H₂ flux into Titan’s surface comparable to the H₂ escape flux out of the atmosphere (∼2 × 10¹⁰ cm⁻² s⁻¹ referenced to the surface) and requires larger photochemical production rates of H₂ than obtained by previous photochemical models. From detailed model calculations based on known photochemistry with eddy, molecular, and thermal diffusion, the tropospheric and thermospheric H₂ mole fractions are incompatible by a factor of ∼2. The measurements imply that the downward H₂ surface flux is in substantial excess of the speculative threshold value for methanogenic life consumption of H₂ (McKay, C.P., Smith, H.D. [2005], Icarus 178, 274-276. doi:10.1016/j.icarus.2005.05.018), but without the extreme reduction in the surface H₂ mixing ratio.