Fractionation of planetary systems is both interesting and puzzling. While the Jovian satellite Io seems to be enriched with sulfur and Europa and Ganymede with ice, Titan - the largest moon of Saturn is enriched with organics, and it is the third body in our solar system known to have significant atmosphere after Earth and Venus. Thus, Titan has at least two confirmed key ingradients for life as we know (energy and organics). Models suggest that Titan's interior is composed of fractionated rocks, minerals, and ice (water), making Titan's interior potentially habitable.

Following our recent communication (Gudipati, M.S. et al. [2013]. Nat. Commun. 4, 1648. http://dx.doi.org/10.1038/ncomms2649) on the discovery of condensed-phase non-volatile polymeric material with similar spectral features as tholins, we present here a comprehensive spectroscopic study of photochemical formation of polymeric material from condensed dicyanoacetylene (C4N2) ice films. C4N2 is chosen as starting material for the laboratory simulations because of the detection of this and similar molecules (nitriles and cyanoacetylenes) in Titan's atmosphere. UV-Vis and infrared spectra obtained during long-wavelength (>300 nm) photon irradiation and subsequent warming of the ice films are used to analyze changes in C4N2 ice, evolution of tholins, and derive photopolymerization mechanisms. Our data analysis revealed that many processes occur during the photolysis of condensed Titan's aerosol analogs, including isomerization and polymerization leading to the formation of long-chain as well as aromatic cyclic polymer molecules. In the light of tremendous new data from the Cassini mission on the seasonal variations in Titan's atmosphere, our laboratory study and its results provide fresh insight into the formation and evolution of aerosols and haze in Titan's atmosphere. (c) 2014 Elsevier Inc. All rights reserved.

Titan, the largest moon of Saturn and similar to Earth in many aspects, has unique orange-yellow colour that comes from its atmospheric haze, whose formation and dynamics are far from well understood. Present models assume that Titan's tholin-like haze formation occurs high in atmosphere through gas-phase chemical reactions initiated by high-energy solar radiation. Here we address an important question: Is the lower atmosphere of Titan photochemically active or inert? We demonstrate that indeed tholin-like haze formation could occur on condensed aerosols throughout the atmospheric column of Titan. Detected in Titan's atmosphere, dicyanoacetylene (C4N2) is used in our laboratory simulations as a model system for other larger unsaturated condensing compounds. We show that C4N2 ices undergo condensed-phase photopolymerization (tholin formation) at wavelengths as long as 355 nm pertinent to solar radiation reaching a large portion of Titan's atmosphere, almost close to the surface.

This is an experimental study of ice grain ejection when trapped gases are released from water ice. When ice is formed by adherence of water molecules at low temperatures, it forms an amorphous structure with many pores, where gas molecules can reside. When further ice layers are formed, the gases are trapped in the ice. Upon its warming-up, the ice structure changes, releasing fractions of the trapped gas. If they do not encounter obstacles, they are released quiescently by dynamic percolation. In a non-dense ice a huge flux of ice grains emanates from the ice, propelled by gas jets and covering its entire surface. When the overlying ice is denser, due to back-migration of water vapor during its sublimation, gas trying to escape from below cannot penetrate the dense ice and breaks it, producing non-circular craters and a chaotic terrain, as observed experimentally and in close encounters with Comets Wild 2, Tempel 1 and Hartley 2. These experimental findings explain several observations of Solar System bodies: ice grain ejection from Comets Temple 1 and Hartley 2. Also explained are the dark jets observed on Triton, where their ejection speed suggests a deep source. On Mars, dark streaks are observed in the southern pole in spring, most likely by plumes carrying dark dust, carried by winds and falling on the surface. As found by us experimentally, only frozen CO2 covered by water ice or mixed with it will work to form jets, whereas pure frozen CO2 will sublimate quiescently.

This investigation extends the work presented by Bell et al. (2010a, 2010b). Using the one-dimensional (1-D) configuration of the Titan Global Ionosphere-Thermosphere Model (T-GITM), we quantify the relative importance of the different dynamical and chemical mechanisms that determine the CH4 escape rates calculated by T-GITM. Moreover, we consider the implications of updated Huygens Gas Chromatograph Mass Spectrometer (GCMS) determinations of both the Ar-40 mixing ratios and N-15/N-14 isotopic ratios in work by Niemann et al. (2010). Combining the GCMS constraints in the lower atmosphere with the Ion Neutral Mass Spectrometer (INMS) measurements in work by Magee et al. (2009), our simulation results suggest that the optimal CH4 homopause altitude is located at 1000 km. Using this homopause altitude, we conclude that topside escape rates of 1.0 x 10(10) CH4 m(-2) s(-1) (referred to the surface) are sufficient to reproduce the INMS methane measurements in work by Magee et al. (2009). These escape rates of methane are consistent with the upper limits to methane escape (1.11 x 10(11) CH4 m(-2) s(-1)) established by both the Cassini Plasma Spectrometer (CAPS) and Magnetosphere Imaging Instrument (MIMI) measurements of Carbon-group ions in the near Titan magnetosphere.

New experimental studies confirmed our previous findings on ice grain ejection during the heating process [1]. Our findings can explain the structure and the mechanism of grain ejection from comets, MBC, the Martian South Polar regions, Enceladus and Triton.

We will discuss photochemical properties of Titan’s organic molecules in the condensed phase as solid aerosols or surface material, from small linear polyyenes (polyacetylenes and polycyanoacetylenes) such as C2H2, C4N2, HC5N, etc. In particular we will focus on photochemistry caused by longer wavelength UV-VIS photons (>250 nm) photons that make it through Titan’s atmosphere to the haze region (~100 km) and on to the surface of Titan.

We will discuss laboratory experiments designed to enhance our understanding the chemical processes on icy solar system bodies, enable interpretation of in-situ and remote-sensing data, and help future missions to icy solar system bodies, such as comets, Europa, Ganymede, Enceladus etc.

We employ a newly developed Navier‐Stokes model, the Titan Global Ionosphere‐Thermosphere Model (T‐GITM) to address the one dimensional (1‐D) coupled composition, dynamics, and energetics of Titan's upper atmosphere. Our main goals are to delineate the details of this new theoretical tool and to present benchmark calibration simulations compared against the Ion‐Neutral Mass Spectrometer (INMS) neutral density measurements. First, we outline the key physical routines contained in T‐GITM and their computational formulation. Then, we compare a series of model simulations against recent 1‐D work by Cui et al. (2008), Strobel (2008, 2009), and Yelle et al. (2008) in order to provide a fiducial for calibrating this new model. In paper 2 and a future paper, we explore the uncertainties in our knowledge of Titan's atmosphere between ∼500 km and 1000 km in order to determine how the present measurements constrain our theoretical understanding of atmospheric structures and processes.

In Bell et al. (2010) (paper 1), we provide a series of benchmark simulations that validate a newly developed Titan Global Ionosphere‐Thermosphere Model (T‐GITM) and calibrate its estimates of topside escape rates with recent work by Cui et al. (2008), Strobel (2009), and Yelle et al. (2008). Presently, large uncertainties exist in our knowledge of the density and thermal structure of Titan's upper atmosphere between the altitudes of 500 km and 1000 km. In this manuscript, we explore a spectrum of possible model configurations of Titan's upper atmosphere that are consistent with observations made by the Cassini Ion‐Neutral Mass Spectrometer (INMS), Composite Infrared Spectrometer, Cassini Plasma Spectrometer, Magnetospheric Imaging Instrument, and by the Huygens Gas Chromatograph Mass Spectrometer and Atmospheric Science Instrument. In particular, we explore the ramifications of multiplying the INMS densities of Magee et al. (2009) by a factor of 3.0, which significantly alters the overall density, thermal, and dynamical structures simulated by T‐GITM between 500 km and 1500 km. Our results indicate that an entire range of topside CH 4 escape fluxes can equivalently reproduce the INMS measurements, ranging from ∼10 8 − 1.86 × 10 13 molecules m −2 s −1 (referred to the surface). The lowest topside methane escape rates are achieved by scaling the INMS densities by a factor of 3.0 and either (1) increasing the methane homopause altitude to ∼1000 km or (2) including a physicochemical loss referred to as aerosol trapping. Additionally, when scaling the INMS densities by a factor of 3.0, we find that only Jeans escape velocities are required to reproduce the H 2 measurements of INMS.

A large fraction of the major unsaturated species (C2H2 C2H4, HCN, and HC3N) with mixing ratios of about 3 × 10−6, 10−7–10−8, 3 × 10−7, and 10−9–10−10 reside in Titan's atmosphere between 150 and 500 km (Vinatier et al., 2009; Coustenis et al., 2007) before they condense near the tropopause. A large flux of medium‐wavelength UV penetrates down to these levels, resulting in the polymerization of these unsaturated compounds and the formation of aerosols. We performed our experiments on aerosol formation at these altitudes where both abundances and solar UV flux are high, bearing in mind that additional photolysis occurs at both higher and lower altitudes. In the gas phase, C2H2 photolysis results in unsaturated C4 species that, on further addition of C2, form the cyclic benzene. These gas‐phase intermediates are consumed when an acetylene‐poor gas mixture is irradiated for a long time, giving rise to larger solid‐state species, mainly by addition of C2, followed by further cyclization. The largest species formed was the condensed 5‐ring benzpyrene, not the 7‐ring coronen molecule, which could have been detected there. Another fraction of the polymers consists of polyvinyl and vinyl acetylene chains, which are cross‐linked due to their labile π electrons and form an insoluble solid matrix. This explains the reduction of the C:H ratio from the condensed aromatics of 1.2 to the measured C:H = 1.013 ± 0.001 of the polymers.

The UV photolysis of gas mixtures containing C2H2, C2H4 and HCN, diluted in N2 and CH4 was studied experimentally. At the onset of irradiation, a pale yellow aerosol appeared, similar to Titan's haze [1]. The chemical composition of the haze particles and of their gas-phase precursors was determined [2]. In the solid-phase, a plethora of aromatics and condensed aromatics (PAHs) were observed such as toluene, phenyl acetylene, naphthalene, biphenyl, phenanthrene and benzopyrene. A considerable fraction of linear polyvinyls was also formed, which was cross- linked to form a matrix whose composition could not be analyzed. In the gas-phase, diacetylene, vinyl acetylene and divinyl were detected as well as benzene, phenyl acetylene and styrene, all of which are precursors of the solid-phase species. The mechanism of formation of the main gas-phase and solid-phase species is addition of acetylene and ring closure. During Titan's lifetime, a layer of aerosols ~40 m thick could have accumulated on the surface, mixed with a similar height of liquid C2H6 and C3H8. References [1] Bar-Nun, A., Kleinfeld, I., Ganor, E. (1988) JGR, 93, 8383-8387. [2] Jacovi, R., Bar-Nun, A., Laufer, D. (2006) DPS meeting #38, #27.08; Bull. Am. Astronom. Soc., 38, 528.

We conducted a set of experiments on changes in the surface and subsurface structure of 200 μm layers of amorphous water ice due to CO2 release from below or liquid C3H8 underneath the ice. These findings can explain the changes observed on the surfaces of Europa and Enceladus, where the surface temperatures are above 80 K. In other experiments, we studied the CO2 release from a few cm thick amorphous ice samples upon warming up. The CO2 ice sublimates, breaking gradually the overlying water ice layer. The gas jets are accompanied by massive ejection of ice grains. The results of our measurements of the density, tensile strengths and thermal inertia are very similar to those obtained by Deep Impact mission on Comet Temple 1 [1, 2]. These findings can explain also the jets of gas and ice grains observed on active comets [3], on Mars' pole and the cryovolcanic activity on Enceladus' south pole [4, 5].

Titan's haze, formed by photolysis of C2H2, C2H4 and HCN, was found experimentally to trap Ar, Kr and Xe with efficiencies of 3.5 x 10(-4), 1.9 x 10(-3) and 6.5 x 10(-2) [noble gas atom]/[carbon atom] in the polymer, respectively. The rate of aerosol formation and settling down of 3 x 10(-13) kg m(-2) s(-1), as inferred from our experiments on CH4 photolysis in the far UV [Podolak, M., Bar-Nun, A., 1979. Icarus 39, 272-276], is sufficient to reduce the mixing ratios of Ar-36 and Ar-40 to their low values of (2.8 +/- 0.3) x 10(-7) and (4.3 +/- 0.1) x 10(-3), respectively, and those of Kr and Xe to below the detection limit of 10(-8). (c) 2008 Elsevier Inc. All rights reserved.