Titan's Atmosphere Research Articles

Nonadiabatic quantum dynamics explores non-monotonic photodissociation branching of N2 into the N(4S) + N(2D) and N(4S) + N(2P) product channels.

Vacuum ultraviolet (VUV) photodissociation of N2 molecules is a source of reactive N atoms in the interstellar medium. In the energy range of VUV optical excitation of N2, the N-N triple bond cleavage leads to three types of atoms: ground-state N(4S) and excited-state N(2P) and N(2D). The latter is the highest reactive and it is believed to be the primary participant in reactions with hydrocarbons in Titan's atmosphere. Experimental studies have observed a non-monotonic energy dependence and non-statistical character of the photodissociation of N2. This implies different dissociation pathways and final atomic products for different wavelength regions in the sunlight spectrum. We here apply ab initio quantum chemical and nonadiabatic quantum dynamical techniques to follow the path of an electronic state from the excitation of a particular singlet 1Σ+u and 1Πu vibronic level of N2 to its dissociation into different atomic products. We simulate dynamics for two isotopomers of the nitrogen molecule, 14N2 and 14N15N for which experimental data on the branching are available. Our computations capture the non-monotonic energy dependence of the photodissociation branching ratios in the energy range 108 000-116 000 cm-1. Tracing the quantum dynamics in a bunch of electronic states enables us to identify the key components that determine the efficacy of singlet to triplet population transfer and therefore predissociation lifetimes and branching ratios for different energy regions.

A JWST Survey for Planetary Mass Brown Dwarfs in IC 348* *Based on observations made with the NASA/ESA/CSA James Webb Space Telescope and the Gaia mission.

We have obtained images of the center of the star-forming cluster IC 348 with the James Webb Space Telescope and have identified brown dwarf candidates based on their photometry and point-like flux profiles. Low-resolution spectroscopy has been performed on four promising candidates, three of which have molecular absorption bands that indicate late spectral types. Among those late-type objects, the brightest is similar to known young L dwarfs while the other two show the so-called 3.4 μm feature that has been previously observed in the diffuse interstellar medium and in the atmospheres of Saturn and Titan, which has been attributed to an unidentified aliphatic hydrocarbon. Those two objects also exhibit features between 1.1 and 2.6 μm that we identify as the overtone and combination bands for that hydrocarbon. After accounting for the hydrocarbon bands, the remaining spectral features are consistent with youth and inconsistent with field dwarfs. Based on the low extinctions of those objects and the strengths of the overtone and combination bands, we conclude that the hydrocarbon resides in their atmospheres rather than in foreground material. Thus, our detections of the 3.4 μm feature are the first in atmospheres outside of the solar system. The presence of this hydrocarbon is not predicted by any atmospheric models of young brown dwarfs. Based on its luminosity and evolutionary models, the faintest new member of IC 348 has an estimated mass of 3–4 M Jup, making it a strong contender for the least massive free-floating brown dwarf that has been directly imaged to date.

The Heat and Momentum Budgets of Titan's Middle Atmosphere

AbstractThe thermal and dynamical structure of Titan's middle atmosphere (the stratosphere and mesosphere) has been observed to evolve over seasonal timescales. Measurements from the Composite Infrared Spectrometer on the Cassini spacecraft indicated the presence of a westerly jet with the strongest winds exceeding 200 m s−1 near the autumn and spring poles. The strength of the winds varied substantially with latitude and altitude, and weakened throughout most of winter. The strong winds also served to trap short‐lived trace molecules near the winter pole, leading to a chemically enriched environment. Here, to better understand the evolution of the middle atmosphere jet, we quantify the heat and zonal momentum budgets in Titan's stratosphere and mesosphere over the course of a Titan year using a three‐dimensional general circulation model. We confirm that the dominant heating balance is between the net radiative and adiabatic heating rates, and also show that the convergence of sensible heat by the atmospheric flow is important at low stratospheric altitudes above the winter pole. We show that the polar jet is maintained by the convergence of zonal momentum by the mean meridional flow, while the low‐latitude winds are maintained by an up‐gradient transport of momentum by eddies that occur on time scales of less than one Titan day. The heat and momentum budgets we determine here will be useful in constraining the factors controlling the evolution of Titan's middle atmosphere over the coming decades.

Spectroscopic characterization and photochemistry of HC3N− and CH3C3N−: implications for ion chemistry in Titan's atmosphere

ABSTRACT Molecular ions are key intermediates in the build-up of chemical complexity in interstellar clouds. Among the more than 300 interstellar molecules, only eight negative ions, i.e. C2n+1N− (n = 0–3) and HC2n− (n = 2–5), have been astronomically observed. Understanding the formation mechanism of these ions under the interstellar conditions is essential for astrochemical modelling and establishing the astrochemical networks. Cyanopolyynes including the parent molecule HC3N are carbon-chain molecules that have been observed in a variety of astronomical objects such as the Titan's atmosphere. Herein, two cyanoacetylene anions HC3N‒ and CH3C3N‒ were generated in solid Ne matrix at 3 K and characterized with matrix-isolation infrared spectroscopy, as aided by isotopic substitutions and the ab initio calculations at the UCCSD(T)-F12a/cc-pVTZ-F12 level using second-order vibrational perturbation theory. Upon red-light irradiation at 625 nm, both ions undergo electron detachment by reformation of the neutral species. Importantly, the concomitant dehydrogenation of HC3N‒ has also been observed in the matrix, providing new insight into the intriguing mechanism for the formation of C3N‒ in the upper atmosphere of Titan.

Experimentally-measured rate parameters for vibrational quenching of CO by N2

Carbon monoxide is a trace species in Titan's atmosphere that is not consistently found in local thermodynamic equilibrium at all altitudes. Understanding how CO exchanges energy with atmospheric collision partners is therefore crucial for accurate atmospheric modelling. Particularly significant could be vibrational relaxation of CO by nitrogen due to the high collision frequency in Titan's N2-based atmosphere. Some CO(v)-N2 rate coefficients have never been experimentally measured while others were measured decades ago in low-precision experiments. In this work, we present precise experimental measurements of the rate coefficients for the quenching of CO(v = 1,2) by N2. To perform the experiment, an equilibrium mixture of CO, N2, O3, and bath gases was subjected to a temperature-jump, creating a small amount of vibrationally-excited CO. Transient diode laser absorption spectroscopy was then used to observe the evolving vibrational state populations as a function of time. The rate coefficients of interest were found by measuring quenching rate as a function of quencher concentrations. The rate coefficient for quenching of CO(v = 1) by N2 was measured to be k = (1.3 ± 0.2) x 10−14 cm3 molecule−1 s−1 and the rate coefficient for quenching of CO(v = 2) by N2 was measured to be k = (2.1 ± 0.5) x 10−13 cm3 molecule−1 s−1 at 315 K.

Infrared Spectra and Vapor Pressures of Crystalline C2N2, with Comparisons to Crystalline HCN

In a continuation of our work on nitriles, we have examined cyanogen (C2N2) as a crystalline solid, comparing the results to our recent work on crystalline hydrogen cyanide (HCN). A density and refractive index for C2N2 ice were measured and used to prepare solid samples from which infrared (IR) spectra, band strengths, and optical constants were measured. The vapor pressures (sublimation pressures) of both C2N2 and HCN ices were determined with a quartz-crystal microbalance at temperatures relevant to Titan's atmosphere and much lower than those in the literature. Comparisons of the newly measured low-temperature vapor pressures to values extrapolated from higher temperatures revealed differences on the order of 45 and 130%. The enthalpies of sublimation of C2N2 and HCN in the 120 K region were measured and found to be similar to those at higher temperatures. Comparisons were made to previous spectroscopic work, but hindered in the case of IR intensities by the lack of published details.

A computational characterization of the reaction mechanisms for the reactions N(2D) + CH3CN and HC3N and implications for the nitrogen-rich organic chemistry of Titan

The reactions of atomic nitrogen in its first electronically excited state, N(2D), with acetonitrile (CH3CN) and cyanoacetylene (HC3N) have been investigated by performing electronic structure calculations of the doublet potential energy surfaces and RRKM estimates of the product branching fractions. According to our results, the insertion of N(2D) into one of the sigma CH bonds of acetonitrile leading to the formation of cyanomethanimine (also known as iminoacetonitrile) and the N(2D) addition to the CC triple bond of cyanoacetylene leading to the formation of dicyanocarbene are the main reaction pathways under the conditions typical of the upper atmosphere of Titan, the massive moon of Saturn. Other molecular products originating from other reaction pathways only give a minor contribution.Implications for the atmospheric chemistry of Titan, as well as implications in prebiotic chemistry and in the chemistry of the interstellar medium, are also noted.

Perspectives and Recent Progresses on the Simulation of the Entry into the Atmospheres of the Outer Ice Giants

The relatively recent decision of NASA and ESA to plan new missions to the so-called Ice Giants, namely Uranus and Neptune, has prompted a resurgence of interest in the experimental analysis of the aero-heating environment that probes entering such atmospheres would experience. In the present study, arc-jet facilities, previously used to simulate space flight in the atmospheres of Earth, Mars, and Titan, are considered as a relevant basis for the implementation of a more complex framework adequately accounting for the atmospheric features of the Ice Giants. It is shown that the key to the successful realization of such an endeavor is a new operating mode for the plasma torch (relying on a nitrogen–hydrogen mixture) together with the inclusion of a new gas control unit, a new mixing chamber to generate relevant gas mixtures (mimicking to a sufficient extent the Ice Giants atmosphere) and a new thermo-chemical model of the overall flow process. The outcomes of some initial tests are presented to demonstrate the adequacy and performances of the implemented approach with respect to typical entry conditions related to these two planets.

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.

Shock Synthesis of Organic Molecules by Meteoroids in the Atmosphere of Titan

Thermochemical modeling and shock-tube experiments show that shocks applied to N2/CH4 gas mixtures can synthesize organic molecules. Sufficiently large, hypersonic meteoroids entering the atmosphere of Saturn’s moon Titan should therefore drive organic chemistry. To do so meteoroids must be sufficiently large compared to the atmospheric mean free path at a given altitude to generate shocks, and deposit enough energy per path length to produce temperatures high enough to excite and dissociate the relevant molecules. The Cassini spacecraft imaged multiple meteoroid impacts on Saturn’s rings, allowing for the first time an empirical estimate to be made of the flux and size–frequency distributions of meteoroids in the millimeter-to-meter size range. We combine these results with an atmospheric entry model and thermochemical and experimental shock production efficiencies for N2/CH4 atmospheres and calculate the shock production rates for HCN, C2H2, and C2H4 as well as the resulting H2 generation. We find that meteoroids may be producing these molecules at as much as ∼1% the production rate of photochemistry driven by UV photons, and may be depositing more energy than magnetospheric ions and 90–100 nm UV photons. Moreover, these meteoroids produce these organic molecules hundreds of kilometers lower in Titan’s atmosphere than the relevant UV photons and magnetospheric ions penetrate, with peak production occurring between 200 and 500 km altitudes, i.e., at the observed haze layer. Meteoroid-driven shock generation of molecules may therefore be crucial to understanding Titan’s atmospheric chemistry.

The study of electronic kinetics of molecular nitrogen in the Titan’s middle atmosphere during the precipitation of cosmic rays

The kinetics of the \({{{\text{A}}}^{3}}\Sigma _{u}^{ + },\) B3Πg, W3Δu, \({{{\text{B}}}^{{{\text{'}}3}}}\Sigma _{u}^{ - },\) and C3Πu triplet states of molecular nitrogen at the heights of the middle atmosphere of Titan during action of cosmic rays into the atmosphere has been studied. The calculations consider the intramolecular and intermolecular electron energy transfer during inelastic collisions of electronically excited molecular nitrogen with N2, CH4, and CO molecules. The interaction of electronically excited N2 molecules with molecules of acetylene C2H2 and ethylene C2H4 in the middle atmosphere of Titan at altitudes of 50–250 km has been studied. For the first time, the dominance of reactions with metastable molecular nitrogen N2(\({{{\text{A}}}^{3}}\Sigma _{u}^{ + }\)) in the formation of C2H and C2H3 radicals at these heights has been shown.

Studying the Electronic Kinetics of Molecular Nitrogen in the Middle Atmosphere of Titan during the Precipitation of Cosmic Rays

Studying the Electronic Kinetics of Molecular Nitrogen in the Middle Atmosphere of Titan during the Precipitation of Cosmic Rays

Rapid Aggregation and Dissolution of Organic Aerosols in Liquid Methane on Titan

AbstractComplex organic aerosols in the upper atmosphere of Saturn's moon Titan reach the troposphere and surface, where a methane (CH4)‐based hydrological cycle occur. Previous studies have assumed no interactions between organic aerosols and liquid CH4, although the dissolution of low‐molecular‐weight photochemical products in liquid CH4 has been considered. Here we report experimental results of soaking a laboratory analog (so‐called tholin) of Titan's organic aerosols in liquid CH4 at 93–98 K for several hours and then evaporating the liquid, simulating wet–dry cycling on Titan. After wet–dry cycling, residual tholin particles form aggregates through cementation. Solid evaporitic deposits formed by evaporation of interacted liquid contain nitrogen‐bearing aromatics, suggesting selective dissolution of aromatics. Our results suggest that organic aerosols or high‐molecular‐weight compounds adsorbed on them partly dissolve in liquid CH4 on Titan, even during short‐term wetting events, promoting the growth of aerosols to dune particles via aggregation and providing aromatics to evaporites.

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.

Reaction Kinetics of CN + Toluene and Its Implication on the Production of Aromatic Nitriles in the Taurus Molecular Cloud and Titan’s Atmosphere

Reactions between cyano radicals and aromatic hydrocarbons are believed to be important pathways for the formation of aromatic nitriles in the interstellar medium (ISM) including those identified in the Taurus molecular cloud (TMC-1). Aromatic nitriles might participate in the formation of polycyclic aromatic nitrogen-containing hydrocarbons (PANHs) in Titan's atmosphere. Here, ab initio kinetic simulations reveal a high efficiency of ∼10−10 cm3 s−1 and the competition of the different products of the CN + toluene reaction at 30–1800 K and 10−7–100 atm. In the star-forming region of the TMC-1 environment, the product yields of benzonitrile and tolunitriles for CN reacting with toluene are approximately 17% and 83%, respectively. Detections of the main products, tolunitriles, can serve as proxies for the undetected toluene in the ISM due to their much larger dipole moments. Competition between bimolecular and unimolecular products is extremely intense in the warmer and denser PANH-forming region of Titan's stratosphere. Computational results show that the fractions of tolunitriles, adducts, and benzonitrile are 19%–68%, 15%–64%, and 17%, respectively, at 150–200 K and 0.0001–0.001 atm (Titan's stratosphere). Then, benzonitrile and tolunitriles may contribute to the formation of PANHs by consecutive C2H additions. The kinetic information of aromatic nitriles for the CN + toluene reaction calculated here helps to explain the formation mechanism of polycyclic aromatic hydrocarbons or PANHs under different interstellar environments and constrains corresponding astrochemical models.

Laboratory generation of hazes in Titan's upper atmosphere using ECR plasma

Laboratory generation of hazes in Titan's upper atmosphere using ECR plasma

Experimental determination of interaction between the radiation fields of Dragonfly's MMRTG and Titan's environment II: Gamma induced atmospheric conductivity

The apparatus described in part I of this investigation was filled with various blends of ultra high purity nitrogen and a mixture of 95% nitrogen/5% methane, simulating Titan's atmosphere with up to 50% relative methane humidity, as well as an Earth atmosphere for comparison, with a source of gamma rays suspended therein. Readings were taken from a plate immersed in the atmosphere at the top of the chamber connected to a conductivity probe, with the source at various distances from the plate. Gamma-induced conductivity was not found to be significant at a distance from the source equivalent to that on the surface of the hull segment on Dragonfly containing the radioisotope generator when gamma ray energies and field geometry were corrected for, however ion buildup in the experiment chamber indicated the possibility of similar buildup within the hull segment itself. The effect of methane in the atmosphere was statistically indeterminate, reflecting earlier simulation results, however molecular ions were shown more likely to be the predominant carriers of charge than the free electrons measured in the simulation.

Infrared spectra and optical constants of astronomical ices: V. cyclopropane and ethylene oxide

Infrared (IR) spectra of solid cyclopropane (c-C3H6) and ethylene oxide (c-C2H4O), two cyclic aliphatic compounds, have been recorded in their amorphous and crystalline states. The spectra have been combined with measurements of density and refractive index (λ = 670 nm) for each form of each solid to compute IR intensities (i.e., absorption coefficients and band strengths) and optical constants for use in laboratory experiments and astronomical observations. These are the first such measurements of absolute IR intensities and optical constants for each solid at temperatures relevant to the outer Solar System and the interstellar medium, with all results coming from a single laboratory. Two literature band strengths for ethylene oxide's strongest IR feature, one value a calculation and the other a gas-phase measurement, need to be increased by ∼25% to match the solid-phase value reported here. Suggestions are made for applications and future work related to the chemistry of icy bodies in the Solar System and beyond, including Pluto's surface, Titan's atmosphere, and the formation of cometary C3.

Reactions O(3P, 1D) + HCCCN(X1Σ+) (Cyanoacetylene): Crossed-Beam and Theoretical Studies and Implications for the Chemistry of Extraterrestrial Environments.

Cyanoacetylene (HCCCN), the first member of the cyanopolyyne family (HCnN, where n = 3, 5, 7, ...), is of particular interest in astrochemistry being ubiquitous in space (molecular clouds, solar-type protostars, protoplanetary disks, circumstellar envelopes, and external galaxies) and also relatively abundant. It is also abundant in the upper atmosphere of Titan and comets. Since oxygen is the third most abundant element in space, after hydrogen and helium, the reaction O + HCCCN can be of relevance in the chemistry of extraterrestrial environments. Despite that, scarce information exists not only on the reactions of oxygen atoms with cyanoacetylene but with nitriles in general. Here, we report on a combined experimental and theoretical investigation of the reactions of cyanoacetylene with both ground 3P and excited 1D atomic oxygen and provide detailed information on the primary reaction products, their branching fractions (BFs), and the overall reaction mechanisms. More specifically, the reactions of O(3P, 1D) with HCCCN(X1Σ+) have been investigated under single-collision conditions by the crossed molecular beams scattering method with mass spectrometric detection and time-of-flight analysis at the collision energy, Ec, of 31.1 kJ/mol. From product angular and time-of-flight distributions, we have identified the primary reaction products and determined their branching fractions (BFs). Theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) were performed to assist the interpretation of the experimental results and clarify the reaction mechanism. Adiabatic statistical calculations of product BFs for the decomposition of the main triplet and singlet intermediates have also been carried out. Merging together the experimental and theoretical results, we conclude that the O(3P) reaction is characterized by a minor adiabatic channel leading to OCCCN (cyanoketyl) + H (experimental BF = 0.10 ± 0.05), while the dominant channel (BF = 0.90 ± 0.05) occurs via intersystem crossing to the underlying singlet PES and leads to formation of 1HCCN (cyanomethylene) + CO. The O(1D) reaction is characterized by the same two channels, with the relative CO/H yield being slightly larger. Considering the recorded reactive signal and the calculated entrance barrier, we estimate that the rate coefficient for reaction O(3P) + HC3N at 300 K is in the 10-12 cm3 molec-1 s-1 range. Our results are expected to be useful to improve astrochemical and photochemical models. In addition, they are also relevant in combustion chemistry, because the thermal decomposition of pyrrolic and pyridinic structures present in fuel-bound nitrogen generates many nitrogen-bearing compounds, including cyanoacetylene.

An experimental and theoretical investigation of the N(2D) + C6H6 (benzene) reaction with implications for the photochemical models of Titan.

We report on a combined experimental and theoretical investigation of the N(2D) + C6H6 (benzene) reaction, which is of relevance in the aromatic chemistry of the atmosphere of Titan. Experimentally, the reaction was studied (i) under single-collision conditions by the crossed molecular beams (CMB) scattering method with mass spectrometric detection and time-of-flight analysis at the collision energy (Ec) of 31.8 kJ mol-1 to determine the primary products, their branching fractions (BFs), and the reaction micromechanism, and (ii) in a continuous supersonic flow reactor to determine the rate constant as a function of temperature from 50 K to 296 K. Theoretically, electronic structure calculations of the doublet C6H6N potential energy surface (PES) were performed to assist the interpretation of the experimental results and characterize the overall reaction mechanism. The reaction is found to proceed via barrierless addition of N(2D) to the aromatic ring of C6H6, followed by formation of several cyclic (five-, six-, and seven-membered ring) and linear isomeric C6H6N intermediates that can undergo unimolecular decomposition to bimolecular products. Statistical estimates of product BFs on the theoretical PES were carried out under the conditions of the CMB experiments and at the temperatures relevant for Titan's atmosphere. In all conditions the ring-contraction channel leading to C5H5 (cyclopentadienyl) + HCN is dominant, while minor contributions come from the channels leading to o-C6H5N (o-N-cycloheptatriene radical) + H, C4H4N (pyrrolyl) + C2H2 (acetylene), C5H5CN (cyano-cyclopentadiene) + H, and p-C6H5N + H. Rate constants (which are close to the gas kinetic limit at all temperatures, with the recommended value of 2.19 ± 0.30 × 10-10 cm3 s-1 over the 50-296 K range) and BFs have been used in a photochemical model of Titan's atmosphere to simulate the effect of the title reaction on the species abundances as a function of the altitude.