Insolation in Titan’s troposphere

Seasonality in Titan’s troposphere is driven by latitudinally varying insolation. We show that the latitudinal distributions of insolation in the troposphere and at the surface, based on Huygens DISR measurements, can be approximated analytically with nonzero extinction optical depths τ, and are not equivalent to that at the top of the atmosphere (τ = 0), as has been assumed previously. This has implications for the temperature distribution and the circulation, which we explore with a simple box model. The surface temperature maximum and the upwelling arm of thermally-direct meridional circulation reach the midlatitudes, not the poles, during summertime.     

Evidence for layered methane clouds in Titan’s troposphere

Layered methane clouds in Titan’s troposphere with an upper methane ice cloud, a lower liquid methane-nitrogen cloud, and a gap in between were suggested from in situ measurements and ground-based observations. Here we report laboratory investigations under conditions that mimic Titan’s troposphere providing a detailed picture of the cloud layers. A solid methane cloud with a nitrogen content of less than 14% and a liquid methane-nitrogen cloud with a nitrogen content of ∼30% form above ∼19 km and below ∼16 km altitude, respectively. Contrary to previous assertions, long-lived supercooled liquid methane-nitrogen droplets can be sustained in the region in between. The results demonstrate that a cloud gap could only form in the presence of high amounts of other traces species (ethane nuclei, tholin particles, etc.).     

Far-infrared opacity sources in Titan’s troposphere reconsidered

We use Cassini far-infrared limb and nadir spectra, together with recent Huygens results, to shed new light on the controversial far-infrared opacity sources in Titan’s troposphere. Although a global cloud of large CH₄ ice particles around an altitude of 30 km, together with an increase in tropospheric haze opacity with respect to the stratosphere, can fit nadir and limb spectra well, this cloud does not seem consistent with shortwave measurements of Titan. Instead, the N₂-CH₄ collision-induced absorption coefficients are probably underestimated by at least 50% for low temperatures.     

On Titan’s Xanadu region

A large, circular marking ∼1800 km across is seen in near-infrared images of Titan. The feature is centered at 10°S, 120°W on Titan, encompasses much of Titan’s western Xanadu region, and has an off-center, quasi-circular, inner margin about 700 km across, with lobate outer margins extending 200-500 km from the inner margin. On the feature’s southern flank is Tui Regio, an area that has very high reflectivity at 5 μm, and is hypothesized to exhibit geologically recent cryovolcanic flows (Barnes, J.W. et al. [2006]. Geophys. Res. Lett. 33), similar to flows seen in Hotei Regio, a cryovolcanic area whose morphology may be controlled by pre-existing, crustal fractures resulting from an ancient impact (Soderblom, L.A. et al. [2009]. Icarus, 204). The spectral reflectivity of the large, circular feature is quite different than that of its surroundings, making it compositionally distinct, and radar measurements of its topography, brightness temperature and volume scattering also suggest that the feature is quite distinct from its surroundings. These and several other lines of evidence, in addition to the feature’s morphology, suggest that it may occupy the site of an ancient impact.     

Titan’s atmosphere from ISO mid-infrared spectroscopy

We have analyzed Titan observations performed by the Infrared Space Observatory (ISO) in the range 7-30 μm. The spectra obtained by three of the instruments on board the mission (the short wavelength spectrometer, the photometer, and the camera) were combined to provide new and more precise thermal and compositional knowledge of Titan’s stratosphere. With the high spectral resolution achieved by the SWS (much higher than that of the Voyager 1 IRIS spectrometer), we were able to detect and separate the contributions of most of the atmospheric gases present on Titan and to determine disk-averaged mole fractions. We have also tested existing vertical distributions for C₂H₂, HCN, C₂H₆, and CO₂ and inferred some information on the abundance of the first species as a function of altitude. From the CH₃D band at 1161 cm⁻¹ and for a CH₄ mole fraction assumed to be 1.9% in Titan’s stratosphere, we have obtained the monodeuterated methane-averaged abundance and retrieved a D/H isotopic ratio of 8.7−1.9+3.2 × 10⁻⁵. We discuss the implications of this value with respect to current evolutionary scenarios for Titan. The ν₅ band of HC₃N at 663 cm⁻¹ was observed for the first time in a disk-averaged spectrum. We have also obtained a first tentative detection of benzene at 674 cm⁻¹, where the fit of the ISO/SWS spectrum at R = 1980 is significantly improved when a constant mean mole fraction of 4 × 10⁻¹⁰ of C₆H₆ is incorporated into the atmospheric model. This corresponds to a column density of ∼ 2 × 10¹⁵ molecules cm⁻² above the 30-mbar level. We have also tested available vertical profiles for HC₃N and C₆H₆ and adjusted them to fit the data. Finally, we have inferred upper limits of a few 10⁻¹⁰ for a number of molecules proposed as likely candidates on Titan (such as allene, acetonitrile, propionitrile, and other more complex gases).     

On the chemical composition of Titan’s dry lakebed evaporites

Titan, the main satellite of Saturn, has an active cycle of methane in its troposphere. Among other evidence for a mechanism of evaporation at work on the ground, dry lakebeds have been discovered. Recent Cassini infrared observations of these empty lakes have revealed a surface composition poor in water ice compared to that of the surrounding terrains—suggesting the existence of organic evaporites deposits. The chemical composition of these possible evaporites is unknown. In this paper, we study evaporite composition using a model that treats both organic solids dissolution and solvent evaporation. Our results suggest the possibility of large abundances of butane and acetylene in the lake evaporites. However, due to uncertainties of the employed theory, these determinations have to be confirmed by laboratory experiments.     

Energy deposition and primary chemical products in Titan’s upper atmosphere

Cassini results indicate that solar photons dominate energy deposition in Titan’s upper atmosphere. These dissociate and ionize nitrogen and methane and drive the subsequent complex organic chemistry. The improved constraints on the atmospheric composition from Cassini measurements demand greater precision in the photochemical modeling. Therefore, in order to quantify the role of solar radiation in the primary chemical production, we have performed detailed calculations for the energy deposition of photons and photoelectrons in the atmosphere of Titan and we validate our results with the Cassini measurements for the electron fluxes and the EUV/FUV emissions. We use high-resolution cross sections for the neutral photodissociation of N₂, which we present here, and show that they provide a different picture of energy deposition compared to results based on low-resolution cross sections. Furthermore, we introduce a simple model for the energy degradation of photoelectrons based on the local deposition approximation and show that our results are in agreement with detailed calculations including transport, in the altitude region below 1200 km, where the effects of transport are negligible. Our calculated, daytime, electron fluxes are in good agreement with the measured fluxes by the Cassini Plasma Spectrometer (CAPS), and the same holds for the measured FUV emissions by the Ultraviolet Imaging Spectrometer (UVIS). Finally, we present the vertical production profiles of radicals and ions originating from the interaction of photons and electrons with the main components of Titan’s atmosphere, along with the column integrated production rates at different solar zenith angles. These can be used as basis for any further photochemical calculations.     

A tropical haze band in Titan’s stratosphere

Inspection of near-infrared images from Cassini’s Imaging Science Subsystem and Visual and Infrared Mapping Spectrometer have revealed a new feature in Titan’s haze structure: a narrow band of increased scattering by haze south of the equator. The band seems to indicate a region of very limited mixing in the lower stratosphere, which causes haze particles to be trapped there. This could explain the sharp separation between the two hemispheres, known as the north-south asymmetry, seen in images. The separation of the two hemispheres can also be seen in the stratosphere above 150 km using infrared spectra measured by Cassini’s Composite Infrared Spectrometer. Titan’s behaviour in the lower tropical stratosphere is remarkably similar to that of the Earth’s tropical stratosphere, which hints at possible common dynamical processes.     

Limits on the size of aerosols from measurements of linear polarization in Titan’s atmosphere

The Descent Imager/Spectral Radiometer (DISR) instrument on the Huygens probe into the atmosphere of Titan yielded information on the size, shape, optical properties, and vertical distribution of haze aerosols in the atmosphere of Titan [Tomasko, M.G., Doose, L., Engel, S., Dafoe, L.E., West, R., Lemmon, M., Karkoschka, E., 2008. Planet. Space Sci. 56, 669-707] from photometric and spectroscopic measurements of sunlight in Titan’s atmosphere. This instrument also made measurements of the degree of linear polarization of sunlight in two spectral bands centered at 491 and 934 nm. Here we present the calibration and reduction of the polarization measurements and compare the polarization observations to models using fractal aggregate particles which have different sizes for the small dimension (monomer size) of which the aggregates are composed. We find that the Titan aerosols produce very large polarizations perpendicular to the scattering plane for scattering near 90° scattering angle. The size of the monomers is tightly constrained by the measurements to a radius of 0.04 ± 0.01 μm at altitudes from 150 km to the surface. The decrease in polarization with decreasing altitude observed in red and blue light is as expected by increasing dilution due to multiple scattering at decreasing altitudes. There is no indication of particles that produce small amounts of linear polarization at low altitudes.     

Atomic and molecular hydrogen budget in Titan’s atmosphere

Using a one-dimensional model, we investigate the hydrogen budget and escape to space in Titan’s atmosphere. Our goal is to study in detail the distributions and fluxes of atomic and molecular hydrogen in the model, while identifying sources of qualitative and quantitative uncertainties. Our study confirms that the escape of atomic and molecular hydrogen to space is limited by the diffusion through the homopause level. The H distribution and flux inside the atmosphere are very sensitive to the eddy diffusion coefficient used above altitude 600 km. We chose a high value of this coefficient 1 × 10⁸ cm² s⁻¹ and a homopause level around altitude 900 km. We find that H flows down significantly from the production region above 500 km to the region [300-500] km, where it recombines into H₂. Production of both H and H₂ also occurs in the stratosphere, mostly from photodissociation of acetylene. The only available observational data to be compared are the escape rate of H deduced from Pioneer 11 and IUE observations of the H torus 1-3 × 10⁹ cm⁻² s⁻¹ and the latest retrieved value of the H₂ mole fraction in the stratosphere: (1.1 ± 0.1) × 10⁻³. Our results for both of these values are at least 50-100% higher, though the uncertainties within the chemical schemes and other aspects of the model are large. The chemical conversion from H to H₂ is essentially done through catalytic cycles using acetylene and diacetylene. We have studied the role of this diacetylene cycle, for which the associated reaction rates are poorly known. We find that it mostly affects C₄ species and benzene in the lower atmosphere, rather than the H profile and the hydrogen budget. We have introduced the heterogenous recombination of hydrogen on the surface of aerosol particles in the stratosphere, and this appears to be a significant process, comparable to the chemical processes. It has a major influence on the H distribution, and consequently on several other species, especially C₃H₄, C₄H₂ and C₆H₆. Therefore, this heterogenous process should be taken into account when trying to understand the stratospheric distribution of these hydrocarbons.     

The structure of Titan’s atmosphere from Cassini radio occultations

We present results from the two radio occultations of the Cassini spacecraft by Titan in 2006, which probed mid-southern latitudes. Three of the ingress and egress soundings occurred within a narrow latitude range, 31-34°S near the surface, and the fourth at 52.8°S. Temperature-altitude profiles for all four occultation soundings are presented, and compared with the results of the Voyager 1 radio occultation (Lindal, G.F., Wood, G.E., Hotz, H.B., Sweetnam, D.N., Eshleman, V.R., Tyler, G.L. [1983]. Icarus 53, 348-363), the HASI instrument on the Huygens descent probe (Fulchignoni, M. et al. [2005]. Nature 438, 785-791), and Cassini CIRS results (Flasar, F.M. et al. [2005]. Science 308, 975-978; Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, C.A. [2008b]. Icarus 194, 263-277). Sources of error in the retrieved temperature-altitude profiles are also discussed, and a major contribution is from spacecraft velocity errors in the reconstructed ephemeris. These can be reduced by using CIRS data at 300 km to make along-track adjustments of the spacecraft timing. The occultation soundings indicate that the temperatures just above the surface at 31-34°S are about 93 K, while that at 53°S is about 1 K colder. At the tropopause, the temperatures at the lower latitudes are all about 70 K, while the 53°S profile is again 1 K colder. The temperature lapse rate in the lowest 2 km for the two ingress (dawn) profiles at 31 and 33°S lie along a dry adiabat except within ∼200 m of the surface, where a small stable inversion occurs. This could be explained by turbulent mixing with low viscosity near the surface. The egress profile near 34°S shows a more complex structure in the lowest 2 km, while the egress profile at 53°S is more stable.     

Probing Titan’s lower atmosphere with acousto-optic tuning

Narrow-band images of Titan were obtained in November 1999 with the NASA/GSFC- built acousto-optic imaging spectrometer (AImS) camera. This instrument utilizes a tunable filter element that was used within the 500- to 1050-nm range, coupled to a CCD camera system. The images were taken with the Mount Wilson 2.54-m (100 in.) Hooker telescope, which is equipped with a natural guide star adaptive optics system. We observed Titan at 830 and 890 nm and at a series of wavelengths across the 940-nm window in Titan’s atmosphere where the methane opacity is relatively low. We determined the absolute reflectivity (I/F) of Titan and fit the values at 940 nm to a Minnaert function at Titan’s equator and at −30° latitude (closer to the subsolar point) and obtained average values for the Minnaert limb-darkening slope, k, of 0.661 ± 0.007 and 0.775 ± 0.018, respectively. Comparison with models suggests that the equatorial value of k is consistent with rain removal of haze in the lower atmosphere. The higher value of k at −30° is consistent with the southern hemisphere being brighter than the equator. However, the fits are not unique. The data and models at 890 are consistent with no limb brightening or darkening at this wavelength either at the equator or at −30°.     

Small-scale composition and haze layering in Titan’s polar vortex

Fine scale layering of haze and composition in Titan’s stratosphere and mesosphere was investigated using visible/UV images from Cassini’s Imaging Science Sub-system (ISS) and IR spectra from Cassini’s Composite Infra-Red Spectrometer (CIRS). Both ISS and CIRS independently show fine layered structures in haze and composition, respectively, in the 150-450 km altitude range with a preferred vertical wavelength of around 50 km. Layers are most pronounced around the north polar winter vortex, although some weaker layers do exist at more southerly latitudes. The amplitude of composition layers in each trace gas profile is proportional to the relative enrichment of that species in the winter polar vortex compared to equatorial latitudes. As enrichment is caused by polar subsidence, this suggests a dynamical origin. We propose that the polar layers are caused by cross-latitude advection across the vortex boundary. This is analogous to processes that lead to ozone laminae formation around Earth’s polar vortices.     

Regional geomorphology and history of Titan’s Xanadu province

Titan’s enigmatic Xanadu province has been seen in some detail with instruments from the Cassini spacecraft. The region contains some of the most rugged, mountainous terrain on Titan, with relief over 2000 m. Xanadu contains evolved and integrated river channels, impact craters, and dry basins filled with smooth, radar-dark material, perhaps sediments from past lake beds. Arcuate and aligned mountain chains give evidence of compressional tectonism, yet the overall elevation of Xanadu is puzzlingly low compared to surrounding sand seas. Lineations associated with mountain fronts and valley floors give evidence of extension that probably contributed to this regional lowering. Several locations on Xanadu’s western and southern margins contain flow-like features that may be cryovolcanic in origin, perhaps ascended from lithospheric faults related to regional downdropping late in its history. Radiometry and scatterometry observations are consistent with a water-ice or water-ammonia-ice composition to its exposed, eroded, fractured bedrock; both microwave and visible to near-infrared (v-nIR) data indicate a thin overcoating of organics, likely derived from the atmosphere. We suggest Xanadu is one of the oldest terrains on Titan and that its origin and evolution have been controlled and shaped by compressional and then extensional tectonism in the icy crust and ongoing erosion by methane rainfall.     

Sounding of Titan’s atmosphere at submillimeter wavelengths from an orbiting spacecraft

An investigation of the capabilities and science goals of a submillimeter-wave heterodyne sounder onboard a Titan orbiter is presented. Based on a model of Titan’s submillimeter spectrum, and including realistic instrumental performances, we show that passive limb observations of Titan’s submillimeter radiation would bring novel and unique information on the dynamical and chemical state of Titan’s atmosphere, particularly in the so far poorly probed 500-900 km region. The 300-360, 540-660 and 1080-1280 GHz spectral ranges appear especially promising, and could be explored with an instrument equipped with a tunable local oscillator system. Vertical temperature profiles can be determined up to ∼1200 km using rotational lines of CH₄, CO, and HCN. Winds can be measured over the 200-1200 km altitude range with an accuracy of 3-5 m/s from Doppler shift measurements of any strong optically thin line. Numerous molecular species are accessible, including H₂O, NH₃, CH₃C₂H, CH₂NH, and several nitriles (HC₃N, HC₅N, CH₃CN, and C₂H₃CN). Many of them are expected to be detectable in a large fraction of the atmosphere and in some cases at all levels, providing an observational link between stratospheric and thermospheric chemistry. Isotopic variants of some of these species can also be measured, providing new measurements of H, C, N, and O isotopic ratios. Mapping of the thermal, wind, and composition fields, best achieved from a polar orbit and with an articulated antenna, would provide a new view of the couplings between chemistry and dynamics over an extended altitude range of Titan’s atmosphere. Additional science goals at Saturn and Enceladus are briefly discussed.     

Transient surface liquid in Titan’s polar regions from Cassini

Cassini RADAR images of Titan’s south polar region acquired during southern summer contain lake features which disappear between observations. These features show a tenfold increases in backscatter cross-section between images acquired one year apart, which is inconsistent with common scattering models without invoking temporal variability. The morphologic boundaries are transient, further supporting changes in lake level. These observations are consistent with the exposure of diffusely scattering lakebeds that were previously hidden by an attenuating liquid medium. We use a two-layer model to explain backscatter variations and estimate a drop in liquid depth of approximately 1-m-per-year. On larger scales, we observe shoreline recession between ISS and RADAR images of Ontario Lacus, the largest lake in Titan’s south polar region. The recession, occurring between June 2005 and July 2009, is inversely proportional to slopes estimated from altimetric profiles and the exponential decay of near-shore backscatter, consistent with a uniform reduction of 4 ± 1.3 m in lake depth.Of the potential explanations for observed surface changes, we favor evaporation and infiltration. The disappearance of dark features and the recession of Ontario’s shoreline represents volatile transport in an active methane-based hydrologic cycle. Observed loss rates are compared and shown to be consistent with available global circulation models. To date, no unambiguous changes in lake level have been observed between repeat images in the north polar region, although further investigation is warranted. These observations constrain volatile flux rates in Titan’s hydrologic system and demonstrate that the surface plays an active role in its evolution. Constraining these seasonal changes represents the first step toward our understanding of longer climate cycles that may determine liquid distribution on Titan over orbital time periods.     

The photochemical products of benzene in Titan’s upper atmosphere

The Cassini spacecraft detected benzene high in Titan’s atmosphere as well as the presence of large mass positive and negative ions. Previous work has suggested that these large mass ions could be composed of fused-ring polycyclic aromatic hydrocarbon compounds. These fused-ring PAHs, such as naphthalene and anthracene, are usually the result of high temperature processes that may not occur in Titan’s thin, cold, upper thermosphere. Here we suggest that a different class of aromatic compounds, polyphenyls, may be a better explanation of the data. Polyphenyls can grow to be large polymeric structures and could condense to form the aerosols seen in Titan’s cloud and hazes. They have similar properties to fused-ring PAHs (for example, electron affinity, ionization potential) and could be the negative ion species seen in the CAPS instrument data from the Cassini spacecraft.     

Determining a tilt in Titan’s north-south albedo asymmetry from Cassini images

Analysis of Titan’s hemispheric brightness asymmetry from mapped Cassini images reveals an axis of symmetry that is tilted with respect to the rotational axis of the solid body. Twenty images taken from 2004 through 2007 show a mean axial offset of 3.8 ± 0.9° relative to the solid body’s pole, directed 79 ± 24° to the west of the sub-solar longitude. These values are consistent with recent measurements of an implied atmospheric spin axis determined from isothermal mapping by [Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, C.A., 2008. Icarus 197, 549-555].     

Organic sedimentary deposits in Titan’s dry lakebeds: Probable evaporite

We report the discovery of organic sedimentary deposits at the bottom of dry lakebeds near Titan’s north pole in observations from the Cassini Visual and Infrared Mapping Spectrometer (VIMS). We show evidence that the deposits are evaporitic, making Titan just the third known planetary body with evaporitic processes after Earth and Mars, and is the first that uses a solvent other than water.