Titan's Lower Atmosphere Research Articles

Prediction and Rationalization of Abundant C-N-H Molecules in Different Environments.

The extreme chemical diversity of CmNnHk molecules is at the same time very important (central in organic chemistry) and difficult to rationalize in the sense that some molecules are abundant and easy to synthesize, while others are rare and difficult to make. Using the recently developed criteria of molecular "magicity", combined with evolutionary structure prediction and quantum-chemical calculations, we study these molecules in a wide range of compositions (0 ≤ m ≤ 13, 0 ≤ n ≤ 4, and 0 ≤ k ≤ 14). "Magic" molecules are defined as those that are lower in energy than any isochemical mixture of molecules with the nearest compositions. The predicted "magic" molecules are in good agreement with compounds found in versatile environments (interstellar and circumstellar media, Titan's lower atmosphere, and crude oil fractions) and in experimental chemistry. This work shows the predictive power of our approach, capable of predicting and explaining stable molecules in complex systems.

The Propagation of Gravity Waves in Titan's Stratosphere

AbstractGravity waves (GWs) are ubiquitous and important dynamical processes in planetary atmospheres. But their properties and impact on the lower atmosphere remain unclear for most of planets due to the lack of data. The recent in situ observation from Huygens reveals GW activity in Titan's lower stratosphere. This paper investigates the upward propagation of GWs from Titan's lower atmosphere and their thermal effects using a full‐wave model. We reproduce the observed temperature perturbations with a superposition of three GW solutions with λx = 50 km and λz = 5.3, 8.9, and 28 km, with the longer wavelength one overlooked in previous studies. The simulation suggests that the propagation of GWs in Titan's lower atmosphere is almost nondissipative and thus has no wave‐induced thermal effect on the stratosphere, which is very different from the planetary thermosphere. The temperature minimum at the tropopause leads to a rapid local GW growth, which may be the primary reason for the significant GW signals above 60 km. We also find that the zonal wind may filter out the majority of GWs except for those traveling perpendicular to the wind direction, which can propagate upward to above 100 km, as observed by the Huygens probe.

The low electrical conductivity of Titan's lower atmosphere

Titan's near-surface atmosphere, with a relatively high density and with weak ion production by cosmic rays or other sources, likely has a lower electrical conductivity than the Earth's, possibly <1 fS/m. Direct measurements by the Relaxation Probe on Huygens constrain it to be <0.5 pS/m, and extrapolation from higher altitudes suggests orders of magnitude lower than this. We investigate why several published models indicate much higher conductivity: it appears that aerosol removal of electrons may have been underestimated, and photoelectron emission from aerosols may have been overestimated. We propose a simple analytic description of the negative conductivity profile of the form Log10(σ) = A + B*z*exp(−[z/C]^D) with A = -15, B = 0.15, C = 100 km, D = 1.5 where z is the altitude (0–150 km) and σ is the negative ion/electron conductivity in S/m.Some local enhancement of conductivity at the surface may occur due to radiocarbon in recent surface deposits, and a significant perturbation may result from the radioisotope power source on the Dragonfly rotorcraft lander. We consider, and cannot dismiss completely, the hypothesis that the high conductivity of surface material measured by the Huygens probe, and its sudden reduction 11 min after landing, may have been associated with a wind change altering the abundance of nearby ions produced by radiation from the probe's Radioisotope Heater Units.

Remarks about the data processing of the Relaxation Probe on the Huygens experiment

On January 14, 2005, the Relaxation Probe (RP) on the Huygens experiment measured the electrical conductivity in the lower atmosphere of Titan, the largest Saturnian satellite, from 140 km to 40 km of altitude. The Huygens experiment provided a unique opportunity to compare results from the Mutual Impedance Probe (MIP) with the Relaxation Probe (RP) sensor. A geometrical effect related to the size of the electrode and the Debye length of charged species was previously included in the data processing of the signals of the RP. Now, better theoretical estimations of the resistances, enable us to present critical remarks for the data processing of the RP and the different causes which may contaminate the data: a) the model of the RP, b) the fluid structure interaction, c) the stability of the reference potential of the gondola, d) the floating potential of the parachute or a balloon. It is hoped that our remarks will help future planetary or terrestrial missions.

Propane clusters in Titan’s lower atmosphere: insights from a combined theory/laboratory study

ABSTRACT In spite of the considerable advances made by Cassini–Huygens in our understanding of Titan, many questions endure. In particular, the detailed processes that lead to the formation of haze aerosols in Titan’s atmosphere, found in high concentrations at low altitudes, are not well identified. Hydrocarbons, which are abundant constituents of Titan’s cold atmosphere originating from photochemical processes, may simultaneously condense on the surface of existing aerosols, nucleate and grow to generate new aerosol seeds. The relative importance of the various processes depends on several factors, including the saturation ratio. The dynamics of hydrocarbon condensation and nucleation in Titan’s atmosphere remains poorly known. Aiming to progress on these issues, we investigate here the kinetics of propane dimer formation at low temperature through state-of-the-art laboratory experiments combined with theoretical calculations. Our results provide an estimate of the rate coefficients, which are then employed to evaluate the abundance of propane dimers in the lower atmosphere of Titan. The mixing ratios of propane dimers inferred, with a maximum abundance of 10 cm−3 near 100 km, is found to be under the detection limit of the Composite Infrared Spectrometer of the Cassini spacecraft. Based on our results, homogeneous nucleation of the most abundant species appears not to be relevant for the growth of aerosols. Future studies should focus on homogeneous nucleation of polar molecules or alternatively on heterogeneous processes, which are usually more efficient.

Multiphase-equilibria analysis: Application in modeling the atmospheric and lacustrine chemical systems of Saturn's moon Titan

Titan's lower atmosphere has condensed phases in equilibrium with it. It is well known that there is a liquid phase in lakes and liquid condensates in the lower troposphere that are part of the liquid cycle in Titan's system. This work analyzes further the phase equilibria using CRYOCHEM equation of state by accounting for solid phases that can exist at the extremely cold conditions on Titan, as well as the possibility of a second liquid phase in the deep lakes, all within the ternary system nitrogen-methane-ethane. Titan's fluid exhibits rich variation of phases that allow us to show multiphase equilibria in full action. The multiphase equilibria covered in this work are two-phase (vapor-liquid, liquid-liquid, solid-liquid, solid-vapor, solid-solid), three-phase (vapor-liquid-liquid, vapor-liquid-solid, vapor-solid-solid, liquid-liquid-solid), and four-phase equilibria (solid-liquid-liquid-vapor); issues associated with the unphysical phases in the calculations are also discussed. The results of multiphase-equilibria analysis describe new understandings about Titan's surface, troposphere, and subsurface lake liquids, as described in the conclusions.

The photochemical fractionation of nitrogen isotopologues in Titan’s atmosphere

Nitrogen isotopologues could give in principle valuable constraints on the formation and evolution of Titan’s atmosphere and its interior over geological time. For this purpose, we developed the first photochemical model dedicated to the study of the fractionation of several nitrogen isotopologues. Emphasis has been placed on several nitriles: HCN, CH3CN, HC3N, C2H3CN, C2H5CN. We show that the HCN/HC15N and HC3N/HC315N ratios are very sensitive to the production of magnetospheric electrons. So, these compounds can serve as probes to study the putative evolution with time of the production of magnetospheric electrons throughout the atmosphere. We also show that the CH3CN/CH3C15N and C2H5CN/C2H5C15N ratios are highly sensitive to cosmic rays. So, they can serve as probes to estimate their effect in the lower atmosphere of Titan (100–300 km). Detection of new isotopologues (particularly CH3C15N) could give strong constraints to photochemical models and could improve our understanding of the main physical and chemical processes at work in Titan’s atmosphere.

Infrared characterisation of acetonitrile and propionitrile aerosols under Titan's atmospheric conditions.

Pure, crystalline acetonitrile (CH3CN) and propionitrile (CH3CH2CN) particles were formed in a collisional cooling cell allowing for infrared (IR) signatures to be compiled from 50 to 5000 cm-1. The cell temperature and pressure conditions were controlled to simulate Titan's lower atmosphere (80-130 K and 1-100 mbar), allowing for the comparison of laboratory data to the spectra obtained from the Cassini-Huygens mission. The far-IR features confirmed the morphology of CH3CN aerosols as the metastable β-phase (monoclinic) ice, however, a specific crystalline phase for CH3CH2CN could not be verified. Mie theory and the literature complex refractive indices enabled of the experimental spectra to be modelled. The procedure yielded size distributions for CH3CN (55-140 nm) and CH3CH2CN (140-160 nm) particles. Effective kinetic profiles, tracing the evolution of aerosol band intensities, showed that condensation of CH3CH2CN proceeded at twice the rate of CH3CN aerosols. In addition, the rate of CH3CH2CN aerosol depletion via lateral diffusion of the particles from the interrogation volume was approximately 50% faster than that of CH3CN. The far-IR spectra recorded for both nitrile aerosols did not display absorption profiles that could be attributed to the unassigned 220 cm-1 feature, which has been observed to fluctuate seasonally in the spectra obtained from Titan's atmosphere.

The effect of like-charge attraction on aerosol growth in the atmosphere of Titan

The formation of aerosols in the atmosphere of Titan is based extensively on ion-neutral chemistry and physical condensation processes. Herein it is shown that the formation of aerosols may also occur through an alternative pathway that involves the physical aggregation of negatively charged particles, which are known to be abundant in the satellite’s atmosphere. It is shown that, given the right circumstances, like-charged particles with a dielectric constant characteristic of nitrated hydrocarbons have sufficient kinetic energy to overcome any repulsive electrostatic barrier that separates them and can subsequently experience an attractive interaction at very short separation. Aerosol growth can then unfold through a charge scavenging process, whereby nitrated aggregates preferentially grow by assimilating smaller like-charged particles. Since hydrocarbon aerosols have much lower dielectric constants, it is shown that a similar mechanism involving hydrocarbon particles will not be as efficient. As a consequence of this proposed growth mechanism, it is suggested that the lower atmosphere of Titan will be enriched in nitrogen-containing aerosols.

Infrared Spectroscopy and Phase Behavior of n-Butane Aerosols and Thin Films at Cryogenic Temperatures

Spectroscopic studies of two phase transitions of solid n-butane aerosol droplets performed under conditions representative of those in the lower atmosphere of Titan are presented. Pure n-butane aerosols and mixed ensembles of n-butane/acetylene, n-butane/carbon dioxide and n-butane/water aerosols were generated in a bath gas cooling cell at 78 K and their phase transition dynamics monitored using infrared extinction spectroscopy. For pure n-butane aerosols the volume and surface nucleation constants were found to range from JV = 10(12) -10(14) cm(-3) s(-1) and JS = 10(13) -10(15) cm(-2) s(-1), respectively, for the first observed transition, and JV = 10(9) -10(11) cm(-3) s(-1) and JS = 10(11) -10(13) cm(-2) s(-1) for the second observed transition. The phases of the n-butane aerosols were determined by comparing their spectroscopic signatures with spectra collected from thin films of liquid and solid n-butane. The first observed transition was from an amorphous-annealed phase into the metastable crystalline phase II of n-butane. The second transition was from the metastable crystalline phase II into the crystalline phase III. The effect of the presence of a second aerosol substance (acetylene, carbon dioxide or water) was examined; while this accelerated the first phase transition, it did not directly influence the rate of the second phase transition. The kinetic studies might be important for the understanding of cloud formation on Titan, while the spectral data provided, which include the first reported spectrum of liquid n-butane close to the melting point, are expected to be of use for remote sensing applications.

Simulating Titan's Atmosphere

Planetary Science Saturn's largest moon, Titan, has a thick atmosphere dominated by nitrogen (N2) and methane (CH4); the photochemistry of these two gases in the upper part of the atmosphere, initiated by solar wind particles and solar ultraviolet radiation, produces organics, including polymeric compounds called tholins that are thought to give Titan its distinctive orange-yellow color. Laboratory work by Gudipati et al. shows that tholin formation can continue at low altitudes, down to 200 km from the surface and below, through longer-wavelength photochemistry in condensed organic aerosols. They synthetized dicyanoacetylene (C4N2), a compound that has been detected on Titan and can be used as a model system for other larger unsaturated condensing compounds, cooled it to 100 K, a temperature relevant to Titan's lower atmosphere, and irradiated it with laser-produced light at 532, 355, and 266 nm. Solar light at wavelengths longer than 200 nm is expected to penetrate through Titan's atmosphere and make it to low altitudes without much attenuation. In the simulations, irradiation of C4N2 ice with light at wavelengths as long as 355 nm induced the formation of tholins. These organic compounds, which do not occur naturally on Earth, can thus be produced on Titan by different photochemical processes at different atmospheric altitudes. ![Figure][1] CREDIT: NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE Nat. Comm. 4 , 1648 (2013). [1]: pending:yes

Titan's Methane Weather

Conditions in Titan's troposphere are near the triple point of methane, the second most abundant component of its atmosphere. Our understanding of Titan's lower atmosphere has shifted considerably in the past decade. Ground-based observations, Hubble Space Telescope images, and data returned from the Cassini and Huygens spacecraft show that Titan's troposphere hosts a methane-based meteorology in direct analogy to the water-based meteorology of Earth. What once was thought to be a quiescent place, lacking in clouds or localized weather and changing only subtly on long seasonal timescales, is now understood to be a dynamic system with significant weather events regularly occurring against the backdrop of dramatic seasonal changes. Although the observational record of Titan's weather covers only a third of its 30-year seasonal cycle, Titan's atmospheric processes appear to be more closely analogous to those of Earth than to those of any other object in our solar system.

The abundance of H2 in Titan's troposphere from the Cassini CIRS investigation

We derived the mole fraction of H2 in the troposphere of Titan from an analysis of the H2–N2 dimer S0(0) rotational transition seen near 355cm−1 in spectra recorded by the Composite Infra-Red Spectrometer (CIRS) on the Cassini spacecraft between March 2006 and February 2007. We find a globally-averaged value of (9.6±2.4)×10−4, consistent with the earlier determination based on the Voyager IRIS dataset, and with the value obtained from the Gas Chromatograph Mass Spectrometer (GCMS) experiment on the Huygens probe, but a factor of ∼4 lower than that measured by the Ion and Neutral Mass Spectrometer (INMS) in the 1025–1475km altitude range. Interestingly, the latitudinal distribution of H2 appears to be non-uniform, with a mole fraction above 50°N in excess of 30–70% (±40%) relatively to the globally-averaged value. Such a non-uniform distribution is unexpected since H2 is thought to be chemically inert in the lower atmosphere of Titan. It is proposed that the observed enhancement in the north polar region is caused by downwelling occurring during the winter season in the polar vortex. This interpretation is dependent on the existence of a vertical gradient of the H2 concentration in the stratosphere, as suggested by the differences between the INMS values and those measured by GCMS and CIRS, although such a gradient is not consistent with the present understanding of dynamics and chemistry in Titan's atmosphere.

RAPID ASSOCIATION REACTIONS AT LOW PRESSURE: IMPACT ON THE FORMATION OF HYDROCARBONS ON TITAN

Photochemical models of Titan's atmosphere predict that three-body association reactions are the main production route for several major hydrocarbons. The kinetic rate constants of these reactions strongly depend on density and are therefore only important in Titan's lower atmosphere. However, radiative association reactions do not depend on pressure. The possible existence of large rates at low density suggests that association reactions could significantly affect the chemistry of Titan's upper atmosphere and better constraints for them are required. The kinetic parameters of these reactions are extremely difficult to constrain by experimental measurements as the low pressure of Titan's upper atmosphere cannot be reproduced in the laboratory. However, in the recent years, theoretical calculations of kinetics parameters have become more and more reliable. We therefore calculated several radical-radical and radical-molecule association reaction rates using transition state theory. The calculations indicate that association reactions are fast even at low pressure for adducts having as few as four C atoms. These drastic changes have however only moderate consequences for Titan's composition. Locally, mole fractions can vary by as much as one order of magnitude but the column-integrated production and condensation rates of hydrocarbons change only by a factor of a few. We discuss the impact of these results for the organic chemistry. It would be very interesting to check the impact of these new rate constants on other environments, such as giant and extrasolar planets as well as the interstellar medium.

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.

Composition of Titan's lower atmosphere and simple surface volatiles as measured by the Cassini‐Huygens probe gas chromatograph mass spectrometer experiment

The Cassini‐Huygens probe gas chromatograph mass spectrometer (GCMS) determined the composition of the Titan atmosphere from ∼140 km altitude to the surface. After landing, it returned composition data of gases evaporated from the surface. Height profiles of molecular nitrogen (N2), methane (CH4), and molecular hydrogen (H2) were determined. Traces were detected on the surface of evaporating methane, ethane (C2H6), acetylene (C2H2), cyanogen (C2N2), and carbon dioxide (CO2). The methane data showed evidence that methane precipitation occurred recently. The methane mole fraction was (1.48 ± 0.09) × 10−2 in the lower stratosphere (139.8–75.5 km) and (5.65 ± 0.18) × 10−2 near the surface (6.7 km to the surface). The molecular hydrogen mole fraction was (1.01 ± 0.16) × 10−3 in the atmosphere and (9.90 ± 0.17) × 10−4 on the surface. Isotope ratios were 167.7 ± 0.6 for 14N/15N in molecular nitrogen, 91.1 ± 1.4 for 12C/13C in methane, and (1.35 ± 0.30) × 10−4 for D/H in molecular hydrogen. The mole fractions of 36Ar and radiogenic 40Ar are (2.1 ± 0.8) × 10−7 and (3.39 ± 0.12) × 10−5, respectively. 22Ne has been tentatively identified at a mole fraction of (2.8 ± 2.1) × 10−7. Krypton and xenon were below the detection threshold of 1 × 10−8 mole fraction. Science data were not retrieved from the gas chromatograph subsystem as the abundance of the organic trace gases in the atmosphere and on the ground did not reach the detection threshold. Results previously published from the GCMS experiment are superseded by this publication.

METHANE GAS STABILIZES SUPERCOOLED ETHANE DROPLETS IN TITAN'S CLOUDS

Strong evidence for ethane clouds in various regions of Titan's atmosphere has recently been found. Ethane is usually assumed to exist as ice particles in these clouds, although the possible role of liquid and supercooled liquid ethane droplets has been recognized. Here, we report on infrared spectroscopic measurements of ethane aerosols performed in the laboratory under conditions mimicking Titan's lower atmosphere. The results clearly show that liquid ethane droplets are significantly stabilized by methane gas which is ubiquitous in Titan's nitrogen atmosphere—a phenomenon that does not have a counterpart for water droplets in Earth's atmosphere. Our data imply that supercooled ethane droplets are much more abundant in Titan's clouds than previously anticipated. Possibly, these liquid droplets are even more important for cloud processes and the formation of lakes than ethane ice particles.

A gas management system for an ultra long duration Titan blimp

This paper presents analyses, designs and experimental results for the gas management system of a hydrogen-filled blimp capable of flying in the lower atmosphere of Titan for a period of 1 year or more. The engineering strategy has two basic elements: first, to minimize leakage rates from the blimp envelope and ballonets; and second, to provide auxiliary subsystems to mitigate the life-limiting effects resulting from those leaks. Leak minimization is achieved through use of cryogenically compatible balloon materials and adhesives, and selection of ballonet geometries that minimize pinhole generation via folding and material fatigue. Hydrogen loss to the environment through leaks in the blimp envelope is compensated by producing new hydrogen through chemical processing of atmospheric methane. Nitrogen leaked into the blimp from the ballonets is removed by a carbon absorption system and periodically vented to the atmosphere. Data is presented on the measured leak rate from a full scale (13 m long) prototype blimp envelope and on the performance of a low mass, low power prototype device that generates hydrogen from methane. These results are factored in to an overall system design that quantifies the mass and power requirements for a minimum 1 year operational lifetime.

The coupling of winds, aerosols and chemistry in Titan's atmosphere

The atmosphere of Titan is a complex system, where thermal structure, radiative transfer, dynamics, microphysics and photochemistry are strongly coupled together. The global climate model developed over the past 15 years at the Pierre-Simon Laplace Institute has been exploring these different couplings, and has demonstrated how they can help to interpret the observed atmospheric structure of Titan's lower atmosphere (mainly in the stratosphere and troposphere). This review discusses these interactions, and our current understanding of their role in the context of this model, but also of other available works. The recent Cassini results, and the importance of the production mechanisms for Titan's haze, have put forward the need to explore the mesosphere and the couplings between upper and lower atmosphere, as well as the current limits of available models.

The Huygens Surface Science Package sound speed measurements and the methane content of Titan's atmosphere

The Huygens probe descended through Titan's atmosphere in January 2005. On board was the Surface Science Package (SSP), a set of nine sensors, which included a speed-of-sound sensor. We present a detailed description of the SSP speed of sound measurements and report constraints on the methane content in Titan's lower atmosphere based on these measurements. After extensive instrument calibration and subsequent Bayesian analysis of the data, the most likely result derived from our measurements in Titan's lower atmosphere is a methane fraction of approximately 2% at 10 km, increasing to 3.5% at lower altitudes. These estimates are based on a binary composition. Our data show that any large scale variation of methane within the lower 11 km of Titan's atmosphere is unlikely. Within experimental and theoretical uncertainties, our estimates are lower than, but compatible with earlier estimates obtained from the mass spectrometry experiment.