The influence of methane, acetylene and carbon dioxide on the crystallization of supercooled ethane droplets in Titan's clouds

Spectroscopic studies of the homogeneous and heterogeneous crystallization of supercooled ethane aerosol droplets performed under conditions representative of those in Titan's lower atmosphere are presented. Pure ethane aerosols and both internally- and externally-mixed ensembles of ethane/acetylene and ethane/carbon dioxide aerosols were generated in a bath gas cooling cell and their freezing dynamics monitored using infrared absorption spectroscopy. A detailed overview of the spectroscopic signatures of pure ethane and mixed ethane/acetylene and ethane/carbon dioxide aerosols and their phase-dependence is provided. The ice-nucleating efficiencies of acetylene and carbon dioxide aerosols were compared, as were the efficiencies of freezing via an immersion or contact freezing mechanism. The spectral data provided will be of significant use for remote sensing applications, while the nucleation studies have important consequences for models of Titan's ethane clouds.     

Microphysical modeling of ethane ice clouds in titan’s atmosphere

A time-dependent microphysical model is used to study the evolution of ethane ice clouds in Titan’s atmosphere. The model simulates nucleation, condensational growth, evaporation, coagulation, and transport of particles. For a critical saturation of 1.15 (a lower limit, determined by laboratory experiments), we find that ethane clouds can be sustained between altitudes of 8 and 50 km. Growth due to coalescence is inefficient, limiting the peak in the size distribution (by number) to 10 μm. These clouds vary with a period of about 20 days. This periodicity disappears for higher critical saturation values where clouds remain subvisible. Rainout of ethane due to methane cloud formation raises the altitude of the ethane cloud bottom to near the tropopause and may eliminate ethane clouds entirely if methane cloud formation occurs up to 30 km. However, clouds formed above the troposphere from other gases in Titan’s atmosphere could be sustained even with rainout up to 30 km. Although the optical depth of ethane clouds above 20 km is typically low, short-lived clouds with optical depths of order 0.1-1 can be created sporadically by dynamically driven atmospheric cooling. Ethane cloud particles larger than 25 μm can fall to the surface before total evaporation. However, ethane clouds remain only a small sink for tholin particles. At the peak of their cycle, the optical depth of ethane clouds could be comparable to that of tholin in the near-infrared, resulting in a 5% increase in Titan’s albedo for wavelengths between 1 and 2 μm. A number of factors limit our ablility to predict the ethane cloud properties. These factors include the mixing time in the troposphere, the critical saturation ratio for ethane ice, the existence of a surface reservoir of ethane, the magnitude and timing of dynamically driven temperature perturbations, and the abundance and life cycle of methane clouds.     

Ethane aerosol phase evolution in Titan's atmosphere

Strong experimental evidence is presented that the northern polar cloud observed in Titan's atmosphere by the Cassini orbiter (VIMS) was indeed composed of ethane aerosol as proposed by Griffith et al. [2006. Science 313, 1620-1622]. We report on the condensation and phase behavior of ethane aerosol under atmospheric conditions of Titan (145 hPa, 40 km altitude, 70-90 K, 10-30 ppm ethane in nitrogen). The results were obtained in an in-situ collisional cooling experiment combined with Fourier-transform infrared (FTIR) spectroscopy. Apart from the liquid phase, three crystalline phases (solid I, solid II, metastable) and the transitions into each other have been observed in the ethane aerosol. The phases were found to have a significant effect on the particles' IR spectra, their growth dynamics and the final size of the aerosols which varies between 0.5 and 4 μm (compared to 1-3 μm observed on Titan). This has strong implications on the ethane vapor pressure, precipitation and optical aerosol detection.     

A search for ethane on Pluto and Triton

We present here a search for solid ethane, C₂H₆, on the surfaces of Pluto and Triton, based on near-infrared spectral observations in the H and K bands (1.4-2.45 μm) using the Very Large Telescope (VLT) and the United Kingdom Infrared Telescope (UKIRT). We model each surface using a radiative transfer model based on Hapke theory (Hapke, B. [1993]. Theory of Reflectance and Emittance Spectroscopy. Cambridge University Press, Cambridge, UK) with three basic models: without ethane, with pure ethane, and with ethane diluted in nitrogen. On Pluto we detect weak features near 2.27, 2.405, 2.457, and 2.461 μm that match the strongest features of pure ethane. An additional feature seen at 2.317 μm is shifted to longer wavelengths than ethane by at least 0.002 μm. The strength of the features seen in the models suggests that pure ethane is limited to no more than a few percent of the surface of Pluto. On Triton, features in the H band could potentially be explained by ethane diluted in N₂, however, the lack of corresponding features in the K band makes this unlikely (also noted by Quirico et al. (Quirico, E., Doute, S., Schmitt, B., de Bergh, C., Cruikshank, D.P., Owen, T.C., Geballe, T.R., Roush, T.L. [1999]. Icarus 139, 159-178)). While Cruikshank et al. (Cruikshank, D.P., Mason, R.E., Dalle Ore, C.M., Bernstein, M.P., Quirico, E., Mastrapa, R.M., Emery, J.P., Owen, T.C. [2006]. Bull. Am. Astron. Soc. 38, 518) find that the 2.406-μm feature on Triton could not be completely due to ¹³CO, our models show that it could not be accounted for entirely by ethane either. The multiple origin of this feature complicates constraints on the contribution of ethane for both bodies.     

Methane, ethane, and mixed clouds in Titan's atmosphere: Properties derived from microphysical modeling

Theoretical arguments point to and recent observations confirm the existence of clouds in Titan's atmosphere, yet we possess very little data on their particle size, composition and formation mechanism. A time-dependent microphysical model is used to study the evolution of ice clouds in Titan's atmosphere. The model simulates nucleation, condensational growth, evaporation, coagulation, and transport of particles in a column of atmosphere. A variety of cloud compositions are studied, including pure ethane clouds, pure methane clouds, and mixed methane-ethane clouds (all with tholin cores). The abundance of methane cloud particles may be limited by the number of ethane coated tholin nuclei rather than the number of tholins with hydrocarbon coatings. However, even the condensation of methane onto these relatively sparse ethane/tholin cloud particles is sufficient to keep the methane close to saturation. Typical methane supersaturations are of order 0.06 on the average. For simulations which take into account recent lab measurements indicating it is relatively easy for methane to nucleate onto tholin particles without an ethane-layer present, the three types of clouds (methane, ethane, and mixed) exist simultaneously. Pure methane clouds are the most abundant cloud type and serve to lower the supersaturation to about 0.04. Cloud production does not require a continuous surface source of methane. However, clouds produced by mean motions are not the visible methane clouds seen in recent Cassini and ground-based observations. Ethane clouds in the troposphere almost instantaneously nucleate methane to form mixed clouds. However, a thin ethane ‘haze’ remains just above the tropopause for some scenarios and the mixed clouds at the tropopause remain ?50% ethane by mass. Also, evaporation of methane from the mixed cloud particles near the surface leaves a thicker layer of ethane cloud particles at ∼10 km. Nevertheless, the precipitation rate of methane to Titan's surface is between 0.001 and 0.5 cm/terrestrial-year, depending on various initial conditions such as critical saturation, size and abundance of cloud condensation nuclei, surface sources and eddy diffusion.     

Ethane ices in the outer Solar System: Spectroscopy and chemistry

We report recent experiments on ethane ices made at temperatures applicable to the outer Solar System. New near- and mid-infrared data for crystalline and amorphous ethane, including new spectra for a seldom-studied solid phase that exists at 35-55 K, are presented along with radiation-chemical experiments showing the formation of more-complex hydrocarbons.     

Laboratory studies of methane and ethane adsorption and nucleation onto organic particles: Application to Titan's clouds

Titan, Saturn's largest moon, has a thick nitrogen/methane atmosphere. The temperature and pressure conditions in Titan's atmosphere are such that the methane vapor should condense near the tropopause to form clouds. Several ground-based measurements have observed sparse cloud-like features in Titan's atmosphere, while the Cassini mission to Saturn has provided large scale images of the clouds. However, Titan's cloud formation conditions remain poorly constrained. Heterogeneous nucleation (from the vapor phase onto a solid or liquid aerosol surface) greatly enhances cloud formation relative to homogeneous nucleation. In order to elucidate the cloud formation mechanism near the tropopause, we have performed laboratory measurements of the adsorption of methane and ethane onto solid organic particles (tholins) representative of Titan's photochemical haze. We find that monolayers of methane adsorb onto tholin particles at saturation ratios less than unity. We also find that solid methane nucleates onto the adsorbed methane at a saturation ratio of S=1.07±0.008. This implies that Titan's methane clouds should form easily. This is consistent with recent measurements of the column of methane ruling out excessive methane supersaturation. In addition, we find ethane adsorbs onto tholin particles in a metastable phase prior to nucleation. However, ethane nucleation onto the adsorbed ethane occurs at a relatively high saturation ratio of S=1.36±0.08. These findings are consistent with the recent report of polar ethane clouds in Titan's lower stratosphere.     

The surface energy balance at the Huygens landing site and the moist surface conditions on Titan

The Huygens Probe provided a wealth of data concerning the atmosphere of Titan. It also provided tantalizing evidence of a small amount of surface liquid. We have developed a detailed surface energy balance for the Probe landing site. We find that the daily averaged non-radiative fluxes at the surface are 0.7 W m^?2, much larger than the global average value predicted by McKay et al. (1991) of 0.037 W m^?2. Considering the moist surface, the methane and ethane detected by the Probe from the surface is consistent with a ternary liquid of ethane, methane, and nitrogen present on the surface with mole fractions of methane, ethane, and nitrogen of 0.44, 0.34, and 0.22, respectively, and a total mass load of ～0.05 kg m^?2. If this liquid is included in the surface energy balance, only a small fraction of the non-radiative energy is due to latent heat release (～10^?3 W m^?2). If the amount of atmospheric ethane is less than 0.6×10^?5, the surface liquid is most likely evaporating over timescales of 5 Titan days, and the moist surface is probably a remnant of a recent precipitation event. If the surface liquid mass loading is increased to 0.5 kg m^?2, then the liquid lifetime increases to ～56 Titan days. Our modeling results indicate a dew cycle is unlikely, given that even when the diurnal variation of liquid is in equilibrium, the diurnal mass variation is only 3% of the total liquid. If we assume a high atmospheric mixing ratio of ethane (>0.6×10^?5), the precipitation of liquid is large (38 cm/Titan year for an ethane mixing ratio of 2×10^?5). Such a flux is many orders of magnitude in excess of the photochemical production rate of ethane.     

Impact of the seasonal variations of composition on the temperature field of Titan’s stratosphere

We investigate the role of seasonal variations of Titan’s stratospheric composition on the temperature. We use a general circulation model coupled with idealized chemical tracers that reproduce variations of ethane (C₂H₆), acetylene (C₂H₂), and hydrogen cyanide (HCN). Enhancement of the mole fractions of these compounds, at high latitudes in the winter hemisphere relative to their equatorial values, induces a relative decrease in temperature above approximately 0.2 mbar, with a peak amplitude around −20 K, and a relative increase in temperature below, around 1 mbar, with a peak amplitude around +7 K. These thermal effects are mainly due to the variations of the cooling to space induced by the varying distributions. The ethane, acetylene, and hydrogen cyanide variations affect the cooling rates in a similar way, with the dominant effect being due to ethane, though its latitudinal variations are small.     

High spectral resolution infrared studies of Titan: Winds, temperature, and composition

The Cassini Huygens mission provides a unique opportunity to combine ground-based and spacecraft investigations to increase our understanding of chemical and dynamical processes in Titan’s atmosphere. Spectroscopic measurements from both vantage points enable retrieving global wind structure, temperature structure, and atmospheric composition. An updated analysis of Titan data obtained with the NASA Goddard Space Flight Center’s Infrared Heterodyne Spectrometer (IRHS) and Heterodyne Instrument for Planetary Wind and Composition (HIPWAC) prior to and during the Cassini Huygens mission is compared to retrievals from measurements with the Cassini Composite Infrared Spectrometer (CIRS). IRHS/HIPWAC results include the first direct stratospheric wind measurements on Titan, constraints on stratospheric temperature, and the study of atmospheric molecular composition. These results are compared to CIRS retrievals of wind and temperature profile from thermal mapping data and ethane abundance at 10-15° South latitude, near the equatorial region. IRHS/HIPWAC wind results are combined with other direct techniques, stellar occultation measurements, and CIRS results to explore seasonal variability over nearly one Titan year and to provide an empirical altitude profile of stratospheric winds, varying from ∼50 to 210 m/s prograde. The advantage of fully resolved line spectra in species abundance measurements is illustrated by comparing the possible effect on retrieved ethane abundance by blended spectral features of other molecular constituents, e.g., acetylene (C₂H₂), ethylene (C₂H₄), allene (C₃H₄), and propane (C₃H₈), which overlap the ν₉ band of ethane, and are not resolved at lower spectral resolution. IR heterodyne spectral resolution can discriminate weak spectral features that overlap the ν₉ band of ethane, enabling ethane lines alone to be used to retrieve abundance. Titan’s stratospheric mean ethane mole fraction (8.6±3 ppmv) retrieved from IRHS/HIPWAC emission line profiles (resolving power λ?Δλ∼10⁶) is compared to past values obtained from lower resolution spectra and from CIRS measurements (resolving power λ?Δλ∼2×10³) and more compatible recent analysis. Results illustrate how high spectral resolution ground-based studies complement the spectral and spatial coverage and resolution of moderate spectral resolution space-borne spectrometers.     

Transport of nonmethane hydrocarbons to Jupiter's troposphere by descent of smog particles

Ethane and other C₂ and C₃ nonmethane hydrocarbons were observed by the Galileo Probe Mass Spectrometer to have a deep minimum in mixing ratio near and below the tropopause, rising to much larger values at and below the 15 bar level. This minimum requires that these hydrocarbons condense on smog particles at low temperatures. The large amounts at deeper levels were produced in the pumping system of the instrument and any contribution from the ambient atmosphere cannot be resolved, although evaporation from the smog particles is proposed and incorporated in a model. In the stratosphere some of the ethane is released by photon-stimulated desorption. The other light nonmethane hydrocarbons are not modeled, but can be expected to show the same behavior as ethane.     

Photolysis of methane and the ionosphere of Uranus

Photochemical calculations for Uranus predict an extensive region of condensation of acetylene, ethane and methane in the vicinity of the temperature inversion layer. This could explain why ethane was not detected on Uranus, unlike Neptune which has a much warmer inversion layer. Subsequent snow-out of the condensibles is expected to result in reduced visibility in the troposphere. Ionospheric calculations for the equatorial region to be probed by Voyager, indicate peak electron concentrations on the order of 5×10³ cm^?3, if dynamical effects are important. Upper limit to the electron peak is 3×10⁴ cm^?3. Exospheric temperatures as high as 200–250K are conceivable.     

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

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

Sub-brown dwarfs as seats of life based on non-polar solvents: Thermodynamic restrictions

Sub-brown dwarfs (SBD) might originate either around a star or in solitary fashion. These bodies can retain atmospheres composed of molecular gases, which, upon cooling, have basal pressures of tens of bars or more. Pressure-induced opacity of these gases prevents such a body from eliminating its internal radioactive heat and its surface temperature can exceed the melting point of the life-supporting solvent for an extended period of time. Earth life uses water as a solvent but synthesis of observational data makes it possible to conceive chemical reactions that might support life involving non-carbon compounds, occurring in solvents other than water. In this paper a non-polar solvent is considered: ethane. Thermodynamic requirements to be fulfilled by a hypothetic gas constituent of a life-supporting SBD atmosphere are studied. Three gases are analyzed: nitrogen, carbon dioxide and methane. For thermodynamic reasons carbon dioxide is excluded from the list of candidate gases. We show that bodies with ethane oceans are possible in interstellar space. This may happen on SBD of (significantly) smaller or larger mass than the Earth. Generally, in case of SBD smaller in size than the Earth, the atmosphere exhibits a convective layer near the surface and a radiative layer at higher altitudes while the atmosphere of SBDs larger in size than Earth does not exhibit a convective layer. The prescribed thermodynamic state of ethane on the surface has some influence on the features of the atmosphere. The atmospheric mass of a life-hosting SBD of Earth size is two or three orders of magnitude larger than the mass of Earth atmosphere.     

Comment on “Transport of nonmethane hydrocarbons to Jupiter's troposphere by descent of smog particles” by Donald M. Hunten [Icarus 194 (2008) 616-622]

The downward transport of nonmethane hydrocarbons, condensed onto solid photochemically produced particles termed “smust,” may indeed be an important process in the methane-rich atmospheres of Jupiter and Titan. However, evidence supporting this mechanism on Jupiter [Hunten, D.M., 2008. Icarus 194, 616-622] is considerably weakened by three new considerations: the ethane mixing ratio does not increase with depth or show a 1-bar minimum, atmospheric characteristics measured by the probe throughout its descent are representative of much higher altitudes in the “normal” jovian atmosphere, and transport models must consider the lower boundary condition imposed by deep thermochemical destruction of nonmethane hydrocarbons. Additionally, ethane was not the most abundant nonmethane hydrocarbon detected by the Galileo Probe Mass Spectrometer (GPMS) near 11 bar, reinforcing previously published findings that some (if not all) of the nonmethane hydrocarbons detected by the GPMS were of instrumental rather than atmospheric origin.     

Detection of CO and Ethane in Comet 21P/Giacobini-Zinner: Evidence for Variable Chemistry in the Outer Solar Nebula

Ethane and carbon monoxide were detected in a short-period comet of probable Kuiper Belt origin. Ethane is substantially less abundant compared with Hyakutake and Hale-Bopp, two comets from the giant-planet region of the solar nebula, suggesting a heliocentric gradient in ethane in precometary ices. It is argued that processing by X-rays from the young Sun may be responsible.     

Meridional variations in stratospheric acetylene and ethane in the southern hemisphere of the saturnian atmosphere as determined from Cassini/CIRS measurements

These are the first results from nadir studies of meridional variations in the abundance of stratospheric acetylene and ethane from Cassini/CIRS data in the southern hemisphere of Saturn. High resolution, 0.5 cm⁻¹, CIRS data was used from three data sets taken in June-November 2004 and binned into 2° wide latitudinal strips to increase the signal-to-noise ratio. Tropospheric and stratospheric temperatures were initially retrieved to determine the temperature profile for each latitude bin. The stratospheric temperature at 2 mbar increased by 14 K from 9° to 68° S, including a steep 4 K rise between 60° and 68° S. The tropospheric temperatures showed significantly more meridional variation than the stratospheric ones, the locations of which are strongly correlated to that of the zonal jets. Stratospheric acetylene abundance decreases steadily from 30 to 68° S, by a factor of 1.8 at 2.0 mbar. Between 18° and 30° S the acetylene abundance increases at 2.0 mbar. Global values for acetylene have been calculated as (1.9±0.19)×¹⁰⁻⁷ at 2.0 mbar, (2.6±0.27)×¹⁰⁻⁷ at 1.6 mbar and (3.1±0.32)×¹⁰⁻⁷ at 1.4 mbar. Global values for ethane are also determined and found to be (1.6±0.25)×¹⁰⁻⁵ at 0.5 mbar and (1.4±0.19)×¹⁰⁻⁵ at 1.0 mbar. Ethane abundance in the stratosphere increases towards the south pole by a factor of 2.5 at 2.0 mbar. The increase in stratospheric ethane is especially pronounced polewards of 60° S at 2.0 mbar. The increase of stratospheric ethane towards the south pole supports the presence of a meridional wind system in the stratosphere of Saturn.     

Modification of Jupiter’s stratosphere three weeks after the 2009 impact

Infrared spectroscopy sensitive to thermal emission from Jupiter’s stratosphere reveals effects persisting 23 days after the impact of a body in late July 2009. Measurements obtained on 2009 August 11 UT at the impact latitude of 56°S (planetocentric), using the Goddard Heterodyne Instrument for Planetary Wind and Composition mounted on the NASA Infrared Telescope Facility, reveal increased ethane abundance and the effects of aerosol opacity. An interval of reduced thermal continuum emission at 11.744 μm is measured ∼60-80° towards planetary east of the impact site, estimated to be at 305° longitude (System III). Retrieved stratospheric ethane mole fraction in the near vicinity of the impact site is enhanced by up to ∼60% relative to quiescent regions at this latitude. Thermal continuum emission at the impact site, and somewhat west of it, is significantly enhanced in the same spectra that retrieve enhanced ethane mole fraction. Assuming that the enhanced continuum brightness near the impact site results from thermalized aerosol debris blocking contribution from the continuum formed in the upper troposphere and indicating the local temperature, then continuum emission by a haze layer can be approximated by an opaque surface inserted at the 45-60 mbar pressure level in the stratosphere in an unperturbed thermal profile, setting an upper limit on the pressure and therefore a lower limit on the altitude of the top of the impact debris at this time. The reduced continuum brightness east of the impact site can be modeled by an opaque surface near the cold tropopause, which is consistent with a lower altitude of ejecta/impactor-formed opacity. The physical extent of the observed region of reduced continuum implies a minimum average velocity of 21 m/s transporting material prograde (planetary east) from the impact.     

Influence of peroxyacetyl nitrate (PAN) on odd nitrogen in the troposphere and lower stratosphere

Nonmethane hydrocarbon breakdown in the atmosphere produces aldehydes of which a fraction are transferred into peroxyacetyl nitrates (PAN) in the presence of NO and NO₂. Since ethane is destroyed photochemically primarily above 1 km, PAN can be introduced into the upper troposphere and lower stratosphere without the need to be transported from the boundary layer where most hydrocarbons are destroyed and where PAN may be lost due to thermal decomposition and heterogeneous loss. Mixing ratios of ethane in the lower troposphere increase by a factor of 4–8 from equatorial to northern mid-latitudes. This difference is directly translatable into a PAN latitude gradient. At mid-latitudes the concentration of PAN below 20 km is 0.1 ppb comparable to and in some instances larger than predicted HO₂NO₂ mixing ratios. Like HO₂NO₂ and HNO₃, PAN serves as a reservoir for odd nitrogen.