Titan's Atmosphere Research Articles

Brief communication: Lower-bound estimates for residence time of energy in the atmospheres of Venus, Mars and Titan

Abstract. The residence time of energy in a planetary atmosphere, τ, which was recently introduced and computed for the Earth's atmosphere (Osácar et al., 2020), is here extended to the atmospheres of Venus, Mars and Titan. τ is the timescale for the energy transport across the atmosphere. In the cases of Venus, Mars and Titan, these computations are lower bounds due to a lack of some energy data. If the analogy between τ and the solar Kelvin–Helmholtz scale is assumed, then τ would also be the time the atmosphere needs to return to equilibrium after a global thermal perturbation.

The interaction of deep convection with the general circulation in Titan's atmosphere. Part 1: Cloud resolving simulations

The deep convective cloud–environment feedback loop is likely important to Titan's global methane, energy, and momentum cycles, just as it is for Earth's global water, energy, and momentum budgets. General circulation models of Titan's atmosphere are unable to explicitly simulate deep convection and must instead parameterize the impact of this important subgrid-scale phenomenon on the model-resolved atmospheric state. The goal of this study is to better quantify through cloud resolving modeling the effects of deep convective methane storms on their environment and to feed that information forward to improve parameterizations in global models. Dozens of atmospheric profiles unstable with respect to deep moist convection are extracted from the global Titan Atmospheric Model (TAM) and used to initialize the cloud-resolving Titan Regional Atmospheric Modeling System (TRAMS). Mean profiles of heating/cooling and moistening/drying of the large-scale environment in TRAMS indicate that Titan's deep convection forces the environment in a manner analogous to Earth: Large-scale subsidence of the environmental air warms and dries the environment, but clouds can also moisten the environment through the detrainment and evaporation of condensate near cloud top. Relative humidity profiles and characteristic convective time scales are derived to guide the tuning of the deep convective parameterization implemented in TAM, as described in a companion paper. The triggering of convection, the dry convective mixing of the planetary boundary layer, and the entrainment of environmental air into rising air parcels are found to be critical to determining whether a deep convective cloud will form. Only profiles with relatively large convective available potential energy (CAPE) and well mixed planetary boundary layers with high relative humidity were found to produce storms. Environments with low-level thermal inversions and planetary boundary layers with low relative humidity or rapidly decreasing moisture with height failed to generate deep convection in TRAMS despite positive CAPE.

Experimental and Computational Studies on the Reactivity of Methanimine Radical Cation (H2CNH+•) and its Isomer Aminomethylene (HCNH2+•) With C2H2

Experimental and theoretical studies are presented on the reactivity of the radical cation isomers H2CNH+• (methanimine) and HCNH2+• (aminomethylene) with ethyne (C2H2). Selective isomer generation is performed via dissociative photoionization of suitable neutral precursors as well as via direct photoionization of methanimine. Reactive cross sections (in absolute scales) and product branching ratios are measured as a function of photon and collision energies. Differences between isomers’ reactivity are discussed in light of ab-initio calculations of reaction mechanisms. The major channels, for both isomers, are due to H atom elimination from covalently bound adducts to give [C3NH4]+. Theoretical calculations show that while for the reaction of HCNH2+• with acetylene any of the three lowest energy [C3NH4]+ isomers can form via barrierless and exothermic pathways, for the H2CNH+• reagent the only barrierless pathway is the one leading to the production of protonated vinyl cyanide (CH2CHCNH+), a prototypical branched nitrile species that has been proposed as a likely intermediate in star forming regions and in the atmosphere of Titan. The astrochemical implications of the results are briefly addressed.

On the formation of CN bonds in Titan's atmosphere-a unified reaction valley approach study.

In this work, we investigated the formation of protonated hydrogen cyanide HCNH+ and methylene amine cation CH[Formula: see text] (both identified in Titan's upper atmosphere) from three different pathways which stem from the interaction between CH4 and N+(3P). As a mechanistic tool, we used the Unified Reaction Valley Approach (URVA) complemented with the Local Mode Analysis (LMA) assessing the strength of the CN bonds formed in these reactions. Our URVA studies could provide a comprehensive overview on bond formation/cleavage processes relevant to the specific mechanism of eight reactions R1- R8 that occur across the three pathways. In addition, we could explain the formation of CH[Formula: see text] and the appearance of HCNH+ and CHNH[Formula: see text] along these paths. Although only smaller molecules are involved in these reactions including isomerization, hydrogen atom abstraction, and hydrogen molecule capture, we found a number of interesting features, such as roaming in reaction R3 or the primary interaction of H2 with the carbon atom in HCNH+ in reaction R8 followed by migration of one of the H2 hydrogen atoms to the nitrogen which is more cost effective than breaking the HH bond first; a feature often found in catalysis. In all cases, charge transfer between carbon and nitrogen could be identified as a driving force for the CN bond formation. As revealed by LMA, the CN bonds formed in reactions R1-R8 cover a broad bond strength range from very weak to very strong, with the CN bond in protonated hydrogen cyanide HCNH+ identified as the strongest of all molecules investigated in this work. Our study demonstrates the large potential of both URVA and LMA to shed new light into these extraterrestrial reactions to help better understand prebiotic processes as well as develop guidelines for future investigations involving areas of complex interstellar chemistry. In particular, the formation of CN bonds as a precursor to the extraterrestrial formation of amino acids will be the focus of future investigations. Formation of CN bonds in Titan's atmosphere visualized via the reaction path curvature.

The Reaction N(2D) + CH3CCH (Methylacetylene): A Combined Crossed Molecular Beams and Theoretical Investigation and Implications for the Atmosphere of Titan.

The reaction of excited nitrogen atoms N(2D) with CH3CCH (methylacetylene) was investigated 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 31.0 kJ/mol. Synergistic electronic structure calculations of the doublet potential energy surface (PES) were performed to assist the interpretation of the experimental results and characterize the overall reaction micromechanism. Theoretically, the reaction is found to proceed via a barrierless addition of N(2D) to the carbon–carbon triple bond of CH3CCH and an insertion of N(2D) into the CH bond of the methyl group, followed by the formation of cyclic and linear intermediates that can undergo H, CH3, and C2H elimination or isomerize to other intermediates before unimolecularly decaying to a variety of products. Kinetic calculations for addition and insertion mechanisms and statistical (Rice-Ramsperger-Kassel-Marcus) computations of product branching fractions (BFs) on the theoretical PES were performed at different values of total energy, including the one corresponding to the temperature (175 K) of Titan’s stratosphere and that of the CMB experiment. Up to 14 competing product channels were statistically predicted, with the main ones, at Ec = 31.0 kJ/mol, being the formation of CH2NH (methanimine) + C2H (ethylidyne) (BF = 0.41), c-C(N)CH + CH3 (BF = 0.32), CH2CHCN (acrylonitrile) + H (BF = 0.12), and c-CH2C(N)CH + H (BF = 0.04). Of the 14 possible channels, seven correspond to H displacement channels of different exothermicity, for a total H channel BF of ∼0.25 at Ec = 31.0 kJ/mol. Experimentally, dynamical information could only be obtained about the overall H channels. In particular, the experiment corroborates the formation of acrylonitrile + H, which is the most exothermic of all 14 reaction channels and is theoretically calculated to be the dominant H-forming channel (BF = 0.12). The products containing a novel C–N bond could be potential precursors to form other nitriles (C2N2, C3N) or more complex organic species containing N atoms in planetary atmospheres, such as those of Titan and Pluto. Overall, the results are expected to have a potentially significant impact on the understanding of the gas-phase chemistry of Titan’s atmosphere and the modeling of that atmosphere.

Electron scattering from molecules relevant to Titan's atmosphere

Present article reports calculation of elastic, momentum transfer, ionization, and total cross sections for electron scattering from C2N2, C4N2, C6N2, and HC5N molecules, relevant to Titan's atmosphere. The cross sections are predicted using spherical complex optical potential and complex potential ionization contribution method in the energy range from ionization threshold of the molecules to 5 keV. The cross section data reported here consider geometrical screening correction to avoid overestimation of cross section due to the overlap of charge density. A strong correlation is observed between Qionmax and (α/I)12 for C2N2, C4N2, and C6N2 molecular systems, which confirms the consistency of predicted ionization data. The cross sections reported here are important for mass spectrometry, especially in modelling Titan's atmosphere due the presence of these molecules along with abundant electrons in its environment. Most of the data are reported for the first time.

Global impacts from high-latitude storms on Titan.

One of the first large cloud systems ever observed on Titan was a stationary event at the southern pole that lasted almost two full Titan days. Its stationary nature and large extent are puzzling given that low-level winds should transport clouds eastward, pointing to a mechanism such as atmospheric waves propagating against the mean flow. We use a composite of 47 large convective events across 15 Titan years of simulations from the Titan Atmospheric Model to show that Rossby waves trigger polar convection-which halts the waves and produces stationary precipitation-and then communicate its impact globally. In the aftermath of the convection, forced waves undergo a complicated evolution, including cross-equatorial propagation and tropical-extratropical interaction. The resulting global impact from convection implies its detectability anywhere on Titan, both via surface measurements of pressure and temperature and through remote observation of the outgoing longwave radiation, which increases by ~0.5% globally.

Laser Desorption Mass Spectrometry at Saturn’s moon Titan

NASA's Dragonfly mission will send a rotorcraft lander to the surface of Titan to investigate the prebiotic chemistry, habitability, and potential presence of chemical biosignatures on Saturn’s largest moon. One of the key analytical instruments onboard is the Dragonfly Mass Spectrometer (DraMS) that will perform detailed chemical analyses of the Titan surface and atmosphere. The DraMS instrument is capable of multiple analytical modalities, including Laser Desorption Mass Spectrometry (LDMS), in which DraMS primarily seeks to characterize the low volatility/refractory organic content in acquired Titan surface samples. Several key design adaptations will be needed for DraMS to meet its LDMS measurement objectives at Titan as compared to previous development efforts for similar analysis at Mars. These adaptations include an altered sample collection and delivery approach, modifications to the LDMS ion source, and performance optimization for operations with Titan atmospheric gas.

Annular modes of variability in the atmospheres of Mars and Titan

Annular modes explain much of the internal variability of Earth’s atmosphere but have never been identified as influential on other planets. Using data assimilation datasets for Mars and a general circulation model for Titan, we demonstrate that annular modes are prominent in the atmospheres of both worlds, capturing a larger fraction of their respective variabilities than Earth’s. One mode describes latitudinal shifts of the jet on Mars, as on Earth, and vertical shifts of the jet on Titan. Another describes pulses of mid-latitude eddy kinetic energy on all three worlds, albeit with somewhat different characteristics. We demonstrate that this latter mode has predictive power for regional dust activity on Mars, revealing its usefulness for understanding Martian weather. The similarity of annular variability in dynamically diverse worlds suggests its ubiquity across the Solar System, potentially extending to exoplanets.

Modeling the formation of Menrva impact crater on Titan: Implications for habitability

Titan is unique in the solar system: it is an ocean world, an icy world, an organic world, and has a dense atmosphere. It is a geologically active world as well, with ongoing exogenic processes, such as rainfall, sediment transportation and deposition, erosion, and possible endogenic processes, such as tectonism and cryovolcanism. This combination of an organic and an ocean world makes Titan a prime target for astrobiological research, as biosignatures may be present in its surface, in impact melt deposits and in cryovolcanic flows, as well as in deep ice and water ocean underneath the outer ice shell. Impact craters are important sites in this context, as they may have allowed an exchange of materials between Titan's layers, in particular between the surface, composed of organic sediments over icy bedrock, and the subsurface ocean. It is also possible that impacts may have favored the advance of prebiotic chemical reactions themselves, by providing thermal energy that would allow these reactions to proceed. To investigate possible exchange pathways between the subsurface water ocean and the organic-rich surface, we modeled the formation of the largest crater on Titan, Menrva, with a diameter of ca. 425 km. The premise is that, given a large enough impact event, the resulting crater could breach into Titan's ice shell and reach the subsurface ocean, creating pathways connecting the surface and the ocean. Materials from the deep subsurface ocean, including salts and potential biosignatures of putative subsurface biota, could be transported to the surface. Likewise, atmospherically derived organic surface materials could be directly inserted into the ocean, where they could undergo aqueous hydrolysis to form potential astrobiological building blocks, such as amino acids. To study the formation of a Menrva-like impact crater, we staged numerical simulations using the iSALE-2D shock physics code. We varied assumed ice shell thickness from 50 to 125 km and assumed thermal structure over a range consistent with geophysical data. We analyze the implications and potential contributions of impact cratering as a process that can facilitate the exchange of surface organics with liquid water. Our findings indicate that melt and partial melt of ice took place in the central zone, reaching ca. 65 km depth and ca. 60 km away from the center of the structure. Furthermore, a volume of ca. 102 km3 of ocean water could be traced to depths as shallow as 10 km. These results highlight the potential for a significant exchange of materials from the surface (organics and ice) and the subsurface (water ocean), particularly in the crater's central area. Our studies suggest that large hypervelocity impacts are a viable and likely key mechanism to create pathways between the underground water ocean and Titan's organic-rich surface layer and atmosphere.

The interaction of deep convection with the general circulation in Titan’s atmosphere. Part 2: Impacts on the climate

The impact of methane convection on the circulation of Titan is investigated in the Titan Atmospheric Model (TAM), using a simplified Betts–Miller (SBM) moist convection parameterization scheme. We vary the reference relative humidity (RHSBM) and relaxation timescale of convection (τ) parameters of the SBM scheme. Titan’s atmosphere is mostly insensitive to changes in τ, but convective instability and precipitation are highly impacted by changes in RHSBM. Convection behavior changes from infrequent (<1 per Titan year), intense events at summer solstice that quickly encompass the entire globe at low RHSBM to near-continuous precipitation at the poles during summer at high RHSBM (85%). The intermediate regime (RHSBM=70%–80%) consists of frequent events (∼10 per Titan year) of moderate intensity that are limited in meridional extent to their respective hemisphere. Using results from the Titan Regional Atmospheric Modeling System (TRAMS) and observations, we tune the parameters of the SBM parameterization with optimum values of RH=80% and τ=28800 s. We present a simulated decadal climatology that qualitatively matches observations of Titan’s humidity and cloud activity and generally resembles previous results with TAM. Comparing this simulation to one without moist convection demonstrates that convection strengthens the meridional circulation, warms the mid-levels and cools the surface at the poles, and magnifies zonal-mean global moisture anomalies.

Electron impact scattering from pentane molecules and effect of isomerism on cross section

In the present article, electron scattering cross sections for n-pentane and its isomers, viz., isopentane (butane-2-methyl), and neopentane (propane-2-2-dimethyl) are reported. The presence of these molecules along with abundant energetic electrons in the atmosphere of Titan make it favorable for many scattering events. Hence, to study the physio-chemical processes taking place in this environment, the cross section data presented here are inevitable. In this work, the cross sections are calculated for a wide energy range. Elastic and momentum transfer cross sections are calculated using group additivity method implemented through spherical complex optical potential formalism, whereas the ionization cross sections are calculated employing the complex scattering potential ionization contribution method. Total cross section is the sum of elastic and ionization cross section. Present calculation includes geometric screening correction to alleviate the overestimation of cross section values arising due to the overlap of atomic electronic charge densities. A reasonable agreement was found between the present values and previous experimental and theoretical results, wherever available.

Absorption cross sections in the CH stretching region for propene broadened by helium and nitrogen

Propene infrared absorption cross-sections are important for quantitative analysis of atmospheric spectra. Propene has been observed in Titan's atmosphere and may be detected in the atmospheres of giant planets. Infrared absorption cross-sections of propene (C3H6) have been measured in the 2680–3220 cm−1 region by high-resolution Fourier transform spectroscopy. Samples of propene were held at 202, 232, 265, 295 K with helium and nitrogen broadening gas pressures of 10, 30, 100 Torr. The cross section files are available for download.

Prediction of aerodynamically-triggered condensation: Application to the Dragonfly rotorcraft in Titan's atmosphere

Expansion of the airflow over a wing or rotor causes adiabatic cooling, and in terrestrial aviation a localized condensation cloud can form in moist air. The cold methane-nitrogen atmosphere of Titan is, in relative terms, rather close to the condensation point and the possibility of analogous condensation on the NASA Dragonfly rotorcraft mission is examined using simple first-principles models and Computational Fluid Dynamics. It is found that nominal near-ground flight will not trigger condensation, but fog formation may be expected on rotor upper surfaces and in tip vortices in some extremes of the flight envelope, notably during high-altitude meteorological profiling flights. Evaluation of droplet re-evaporation and tip vortex decay timescales suggests, however, that obscuration of navigation sensors is not likely to occur from the vehicle body or rotor wake, even in cross-wind hover.

One dimension photochemical models in global mean conditions in question: Application to Titan

Observational data of Titan's atmosphere composition, gathered in particular by the Cassini mission, allow accessing latitudinal, longitudinal and temporal variations of the altitudinal profiles of several species. While 1D models are a powerful tool to gain insight into the chemistry occurring in Titan's atmosphere, they cannot capture the four-dimensional nature of these processes at the same time. However, due to the large extent of the atmosphere and the complexity of chemistry, global 2D and 3D models with the chemical complexity of 1D models do not exist yet. On the way of developing a 2D photochemical model of Titan, including the very complex chemical schemes used in 1D models and also taking latitudinal variations into account, we developed an ultraviolet radiative transfer model to compute the actinic flux in a 2D geometry. We found that photolysis rates calculated in classical plane-parallel models to derive the rates in mean conditions give results notably different from a model that calculates the mean rates with a 2D geometry. We demonstrate that the introduction of the 2D geometry affects significantly the density profiles on both neutrals and ions, especially on nitriles. As a consequence, we advocate using radiative transfer model in 2D geometry from now on to interpret the wide diversity of observational data about Titan's atmosphere (pending the emergence of 2D models with complex chemical schemes).

Aromatic Formation Promoted by Ion-Driven Radical Pathways in EUV Photochemical Experiments Simulating Titan's Atmospheric Chemistry.

In the atmosphere of Titan, Saturn's main satellite, molecular growth is initiated by 85.6 nm extreme ultraviolet (EUV) photons triggering a chemistry with charged and free-radical species. However, the respective contribution of these species to the complexification of matter is far from being known. This work presents a chemical analysis in order to contribute to a better understanding of aromatic formation pathways. A gas mixture of N2/CH4 (90/10%) within the closed SURFACAT reactor was irradiated at a relatively low pressure (0.1 mbar) and room temperature for 6 h by EUV photons (∼85.6 nm). The neutral molecules formed at the end of the irradiation were condensed in a cryogenic trap and analyzed by electron ionization mass spectrometry. An analysis of the dominant chemical pathways highlights the identification of benzene and toluene and underlies the importance of small ion and radical reactions. On the basis of the experimental results, a speculative mechanism based on sequential H-elimination/CH3-addition reactions is proposed for the growth of aromatics in Titan's atmosphere. Elementary reactions to be studied are given to instill future updates of photochemical models of Titan's atmosphere.

First-Principles Diffusivity Ratios for Atmospheric Isotope Fractionation on Mars and Titan.

Recent work used the kinetic theory of molecular gases, along with state-of-the-art intermolecular potentials, to calculate from first principles the diffusivity ratios necessary for modeling kinetic fractionation of water isotopes in air. Here, we extend that work to the Martian atmosphere, employing potential-energy surfaces for the interaction of water with carbon dioxide and with nitrogen. We also derive diffusivity ratios for methane isotopes in the atmosphere of Titan by using a high-quality potential for the methane-nitrogen pair. The Mars calculations cover 100 K to 400 K, while the Titan calculations cover 50 K to 200 K. Surprisingly, the simple hard-sphere theory that is inaccurate for Earth's atmosphere is in good agreement with the rigorous results for the diffusion of water isotopes in the Martian atmosphere. A modest disagreement with the hard-sphere results is observed for the diffusivity ratio of CH3D in the atmosphere of Titan. We present temperature-dependent correlations, as well as estimates of uncertainty, for the diffusivity ratios involving HDO, H2 17O, and H2 18O in the Martian atmosphere, and for CH3D and 13CH4 in the atmosphere of Titan, providing for the first time the necessary data to be able to model kinetic isotope fractionation in these environments.

UV irradiation of benzene in N2 matrix: A relevant study for titan's chemistry

We report in this contribution the first photochemical study of benzene trapped in solid nitrogen at low temperature (16 K) and irradiated with a mercury lamp (λ > 230 nm). We identify three benzene isomers (fulvene, Dewar benzene and benzvalene) and two radicals (phenyl and benzyne) as photo-dissociation products, along with acetylene derivatives and substituted benzene. We present for the first time the infrared spectrum of fulvene trapped in benzene ice, which is obtained after nitrogen sublimation. This solid-state characterization could be particularly useful for a better understanding of the photochemical fate of benzene ices recently detected in Titan's atmosphere.

Vibrational self-quenching rates of CO(v=1,2) measured via transient diode laser absorption spectroscopy

New laboratory measurements of the rate coefficients for the vibrational relaxation of CO(v=1) and CO(v=2) through collisions with CO have been performed. The experimental approach involved creating a temperature-jump in an equilibrium mixture of CO and bath gases by photolyzing a trace amount of O3 in the mixture with a pulsed UV laser. This shifted a small fraction of the CO population to excited vibrational states. Transient diode laser absorption spectroscopy was then used to observe the evolving vibrational state populations as a function of time. Rate coefficients were deduced from the change in collisional relaxation rate with quencher concentration. This work was motivated by the need for a robust non-LTE model to describe Titan's upper atmosphere. Because the gas collision frequencies are not sufficient to reach thermal equilibrium in a non-LTE environment, the distribution of states is not easily predicted. To properly model such a region, all contributing vibrational energy transfer mechanisms must be thoroughly characterized. Despite their importance, the uncertainties associated with several CO quenching rate parameters in the literature are large. In this work, the rate coefficient for CO(v=2) self-quenching was measured to be (2.7 ± 0.6) x 10−12 cm3 molecule−1 s−1and the overall rate coefficient for CO(v=1) self-quenching was measured to be (1.8 ± 0.3) x 10−12 cm3 molecule−1 s−1. Each represents a significant decrease in uncertainty compared with values being used in current Titan non-LTE models.

Isotope-specific reactions of acetonitrile (CH3CN) with trapped, translationally cold CCl.

The gas-phase reaction of CCl+ with acetonitrile (CH3CN) is studied using a linear Paul ion trap coupled to a time-of-flight mass spectrometer. This work builds on a previous study of the reaction of CCl+ with acetylene [K. J. Catani et al., J. Chem. Phys. 152, 234310 (2020)] and further explores the reactivity of CCl+ with organic neutral molecules. Both of the reactant species are relevant in observations and models of chemistry in the interstellar medium. Nitriles, in particular, are noted for their relevance in prebiotic chemistry and are found in the atmosphere of Titan, one of Saturn's moons. This work represents one of the first studied reactions of a halogenated carbocation with a nitrile and the first exploration of CCl+ with a nitrile. Reactant isotopologues are used to unambiguously assign ionic primary products from this reaction: HNCCl+ and C2H3 +. Branching ratios are measured, and both primary products are determined to be equally probable. Quantum chemical and statistical reaction rate theory calculations illuminate pertinent information for interpreting the reaction data, including reaction thermodynamics and a potential energy surface for the reaction, as well as rate constants and branching ratios for the observed products. In particular, the reaction products and potential energy surface stimulate questions regarding the strength and role of the nitrile functional group, which can be further explored with more reactions of this class.