The Composition and Chemistry of Titan’s Atmosphere

Conor A Nixon

doi:10.1021/acsearthspacechem.2c00041

. 2024 Feb 29;8(3):406–456. doi: 10.1021/acsearthspacechem.2c00041

The Composition and Chemistry of Titan’s Atmosphere

Conor A Nixon ^1,^*

PMCID: PMC10961852 PMID: 38533193

Abstract

graphic file with name sp2c00041_0046.jpg

In this review I summarize the current state of knowledge about the composition of Titan’s atmosphere and our current understanding of the suggested chemistry that leads to that observed composition. I begin with our present knowledge of the atmospheric composition, garnered from a variety of measurements including Cassini–Huygens, the Atacama Large Millimeter/submillimeter Array, and other ground- and space-based telescopes. This review focuses on the typical vertical profiles of gases at low latitudes rather than global and temporal variations. The main body of the review presents a chemical description of how complex molecules are believed to arise from simpler species, considering all known “stable” molecules—those that have been uniquely identified in the neutral atmosphere. The last section of the review is devoted to the gaps in our present knowledge of Titan’s chemical composition and how further work may fill those gaps.

Keywords: Titan, Astrochemistry, Astrobiology, Atmospheres, Photochemistry

Introduction

Titan, Saturn’s largest moon, was first observed in 1655 by the Dutch astronomer Christiaan Huygens. This important discovery that Saturn, in addition to Jupiter, had its own satellite helped to consolidate the Copernican worldview: that the Earth was no longer to be considered the center of the Solar System but rather one of several planets orbiting the Sun and possessing natural satellites of their own. In his excitement at this new finding, Huygens would have little suspected what would transpire 350 years later: that a machine devised and launched into the heavens by humanity and bearing his name would traverse an unimaginable void and then land softly on his new world, finding it stranger and more alien than even the machine’s designers had anticipated.¹

Over the 13 years from 2004 to 2017, the Cassini–Huygens mission^2,3 was able to significantly reveal Titan, both to our eyes and to our minds. Between the successful landing of the ESA-built Huygens probe carrying six scientific suites in January 2005 and the 127 flybys of the NASA-built Cassini spacecraft with its own 12 science instruments, our knowledge of Titan now is vastly greater than before the mission arrived. So, in our privileged position of hindsight, what do we now know about Titan, almost four centuries since its discovery?

First and foremost, it is a moon with a dense atmosphere (Figure 1), the only such body known in our Solar System. Also, this atmosphere, composed primarily of molecular nitrogen and methane, is a largely anoxic environment, with little oxygen to cause the termination of complex organic reactions. The result is a chemical wonderland, with a breathtaking array of complex organic molecules, of which we presently have only the most rudimentary understanding. Figure 2 shows a schematic overview of the presumed chemistry that occurs in Titan’s upper atmosphere, where the “raw ingredients” of its photochemical reactions, N₂ and CH₄, are broken apart and recombined into successively larger molecules and finally haze particles.

Atmosphere of Titan seen by *Cassini*’s Imaging Science Subsystem (ISS). The dayside image (December 16, 2011) shows Titan’s ubiquitous golden haze, opaque to visible light. A darker haze collar is seen around the north pole, along with fainter haze hoods over both the north and south poles, thought to be created by the interaction between chemical and dynamical processes. The nightside image (PIA14924, June 6, 2012, range: 216,000 km) clearly shows the detached atmospheric haze surrounding the entire limb and an elevated stratospheric condensate cloud over the south pole, thought to be composed at least partly of HCN. Image credit: NASA/JPL-Caltech/Space Science Institute/CICLOPS, with reprocessing by Kevin M. Gill.

Simplified portrayal of Titan’s atmospheric photochemistry, showing the presumed progression from simpler molecules (CH₄, N₂) to more complex molecules and eventually haze particles that can sediment out from the atmosphere. Image adapted from ESA graphic.

Titan is a world that is today both tantalizingly more known and more unknown than ever before. Our direct investigations of its atmosphere by the Huygens lander and Cassini spacecraft have both increased our understanding of Titan enormously and also multiplied our questions. Significant outstanding questions include:

Why is Titan the only moon in the Solar System with a significant atmosphere, and how did it come to be in its present state?
Is the atmosphere today in a steady state, or is it growing or shrinking or changing in some way?
To what degree has the atmosphere interacted with and shaped the surface and subsurface?
What degree of chemical complexity is reached in Titan’s atmosphere, and are precursor biomolecules among the products?
Are there other moons with atmospheres similar to Titan’s elsewhere in the galaxy?

Achieving a better understanding of Titan’s atmosphere and its chemistry is important both for the sake of Titan science itself and because of its potential to inform us about other environments. This includes the present-day Earth, since Titan and Earth are the only objects in the Solar System today to have a hydrological cycle of evaporation, condensation, and precipitation and associated rivers, lakes, and seas.⁴⁻⁷ Like Earth, Titan also experiences seasons, due to orbiting close to Saturn’s equatorial plane, which is tilted ∼27° to the ecliptic. Titan therefore experiences summer and winter in each hemisphere, seasons that last ∼7.4× longer than on Earth, with transitional equinox periods of equal daylight at all latitudes.

We also note the relevance to the early Earth, which likely had a much more chemically reducing atmosphere in its distant past,⁸⁻¹³ before the Great Oxidation Event.¹⁴⁻¹⁶ Finally we can surmise the likely relevance to exoplanets, which vastly outnumber the planets in our own Solar System and more likely than not include Titan-like bodies somewhere in our galaxy.¹⁷⁻¹⁹

In this review, I attempt to lay out a simple picture of the known characteristics of Titan’s atmosphere, with a focus on the composition and chemistry of the dense lower atmosphere. By necessity, this review will not cover, except in passing, many related areas: the origin of the atmosphere and possible replenishment mechanisms by internal or external sources; isotopic composition and time evolution of isotopic ratios; winds and dynamics; condensates and meteorology; and the chemical composition of large particulates (haze particles). All of these topics have been covered extensively in reviews and chapters elsewhere,^7,20,21 and in two books written about the results of the Cassini–Huygens mission.^22,23

In presenting an overview and summary focusing on the chemistry and composition of the neutral atmosphere, it is hoped that I will do sufficient justice to this one area to make this review a useful primer for undergraduate or graduate students or others new to the field, to quickly gain a basic understanding of Titan’s bulk atmospheric composition and why it is that way—at least at the present era.

The review is organized as follows: I first review basic knowledge about Titan’s atmospheric temperature structure and gas composition. The main section of the review contains an exposition on the chemistry of the 24 known molecules in the neutral lower atmosphere. This is followed by a detailed discussion of future research directions in Titan atmospheric composition studies, followed by a Summary and Conclusions.

Atmospheric Composition and Structure

Atmospheric Composition

Titan’s atmosphere is largely composed of two gases: N₂ and CH₄. The vertical profile of methane comes from measurements by instruments on the Cassini–Huygens space mission, primarily the Huygens Gas Chromatograph and Mass Spectrometer (GCMS)²⁴ from 0–146 km, the Cassini Visual and Infrared Mapping Spectrometer (VIMS)²⁵ (50–850 km), the Cassini Ultraviolet Imaging Spectrometer (UVIS)²⁶ (400–1650 km), and the Cassini Ion and Neutral Mass Spectrometer (INMS)²⁷ (900–1500 km). Their results have been reported in publications from the mission.²⁸⁻³¹

Aside from noble gases (³⁶Ar, ⁴⁰Ar, and ²²Ne),^31,32 22 molecular species other than N₂ and CH₄ have been definitively detected in Titan’s atmosphere at the time of writing (see Figure 3): 10 hydrocarbons (C₂H₂, C₂H₄, C₂H₆, c-C₃H₂, CH₂CCH₂, CH₃CCH, C₃H₆, C₃H₈, C₄H₂, and c-C₆H₆), eight cyanides¹ (HCN, HNC, HC₃N, C₂N₂, CH₃CN, C₂H₃CN, C₂H₅CN, and CH₃C₃N), three oxygen-bearing species (CO, CO₂, and H₂O), and H₂. These gases were originally detected by a variety of astronomical and remote sensing techniques from the ground and space.

Molecules detected in Titan’s neutral atmosphere sorted by number of atoms and composition. Hydrocarbons are the most abundant and complex species type, followed by nitriles. No complex oxygen-bearing molecules, including organics, have been detected on Titan to date.

All of the major types of hydrocarbons have been detected (alkanes, alkenes, alkynes, a carbene, and an aromatic ring). However, major chemical families of nitrogen-bearing molecules (including amines, imines, azines, and N-heterocyclic rings) and oxygen-bearing molecules (such as aldehydes, ketones, alcohols, and ethers) are possible ingredients of the atmosphere but remain undetected—a subject we will return to in a later section.

Oxygen has yet to be detected on Titan in an organic molecule such as methanol (CH₃OH) or formaldehyde (H₂CO), being found so far only in the simple inorganic molecules CO, CO₂, and H₂O. This limits the presently confirmed scope of astrobiological molecules (i.e., those with the elements CHON in a variety of functional groups), at least in the atmosphere. At the surface and in the subsurface, where hydrocarbons are thought to be readily hydrolyzed as seen in laboratory experiments,³³⁻³⁵ the astrobiological potential may be much greater.³⁶⁻³⁸

The reaction pathways that lead between these molecules have been compiled into computational models of the atmospheric chemistry, which have largely been successful at replicating the observed gas abundances. Models predating the Cassini–Huygens mission³⁹⁻⁴⁷ primarily focused on replicating the observed neutral gas abundances as measured by Voyager(48) and the Infrared Space Observatory (ISO).⁴⁹ However, some models were also developed for the ionosphere.⁵⁰⁻⁵⁴ During the Cassini–Huygens mission and since, new information collected by the spacecraft, especially from direct sampling of the ionosphere,^28,55−78 has prompted many new and revised models of Titan’s atmosphere.⁷⁹⁻¹⁰³

At the opposite end of the size scale, molecular growth by covalent bonding and agglomeration results in macromolecular haze particles,^{11,61,104−112} composed of thousands to millions of individual atoms.¹¹³ As these particles reach a size of ∼1 μm, they begin to sediment (or form the nuclei for condensate growth) and are removed from the atmosphere,^114−117 apparently forming vast dune fields on the surface.^118−120

Atmospheric Temperature Structure

Titan’s atmospheric temperature structure (Figure 4) is a result of the competing heating and cooling processes that take precedence at different altitudes. In the dense lower atmosphere, convection driven by surface heating leads to a vertically decreasing temperature profile, as warm air rises and adiabatically cools. A temperature minimum is reached at ∼45 km, the tropopause, as confirmed by direct measurements¹²¹ from the Huygens Atmospheric Structure Experiment¹²² (HASI) and occultations by the Cassini Radio Science Subsystem¹²³ (RSS).^124−126

Above the tropopause, heating by absorbed solar energy, primarily by atmospheric haze particles, causes temperatures to rise again in the stratosphere.¹²⁹ Stratospheric temperatures have been measured by HASI,¹²¹ the Cassini Composite Infrared Spectrometer¹³⁰ (CIRS),^131,132 and the Atacama Large Millimeter/submillimeter Array (ALMA)^128,133,134 as well as through radio occultations for the lower stratosphere.^124−126 At around 250–400 km, a temperature maximum, the stratopause, is reached at ∼180 K.¹³⁵ The exact altitude (pressure) and temperature of the stratopause vary with both latitude and season^127,136,137 over the course of Titan’s long year (29.46 Earth years), being higher and warmer (by ∼20 K) over the winter pole. This somewhat counterintuitive result can be understood as resulting from adiabatic compression of air in the descending branch of the global stratospheric Hadley cell.

Temperatures fall throughout the next layer, the mesosphere, as haze becomes thin, and radiative cooling by gases such as HCN and C₂H₂ becomes increasingly important.^129,135 Titan’s mesopause is reached at ∼600 km,¹²⁸ above which altitude temperatures rise again. This is primarily due to methane UV absorption,¹³⁸ blocking of outgoing IR radiation by C₂H₆, and far-infrared HCN rotational lines.¹²⁹ This is the thermosphere, a region where gas collisions are rare and molecules must wait to spontaneously emit a photon to lose energy.

The temperature structure of the upper atmosphere is highly variable.⁵² Thermal oscillations of significant amplitude were inferred by HASI¹²¹ above 500 km, while in situ measurements of electron temperature by the Cassini Radio and Plasma Wave Spectrometer (RPWS)¹³⁹ and density and composition by INMS have shown significant time variability on diurnal^66,140,141 and longer time scales depending on the level of solar activity¹⁴² as well as the position of Titan within Saturn’s magnetosphere.^143−146 More recently ALMA measurements are now able to probe the thermal structure of the upper atmosphere as well, providing the ability to monitor secular changes over time.^128,147

Above the four layers of the bound atmosphere is the exosphere, beginning at the exobase (∼1500 km¹⁴⁸), a region where gases can freely escape to space. These five regions mirror the temperature structure of the Earth’s atmosphere, but with a substantially larger scale height (approximately ×5) due to the lower surface gravity. Overlapping the upper thermosphere is the ionosphere (z > ∼1000 km), defined as the region where “significant numbers of free thermal (<1 eV) electrons and ions are present”.¹⁴⁹

It is important to note that the vertical profiles of temperature, minor gas abundances, and haze density all vary with both latitude and season. Titan—like Earth and other planets with atmospheres—exhibits one or more convection cells in the middle atmosphere. Near the equinoxes, air rises at midlatitudes and flows to both poles, where it descends and then returns equatorward.^150−154 However, close to the solstices the circulation more closely resembles a single cell with flow from the summer to winter hemisphere. These cells act to redistribute thermal energy, trace gases, and hazes in both altitude and latitude. This topic has been the subject of extensive measurements and modeling in the literature (see, e.g., refs (127) and (155−161) and references therein). In this review I will not further discuss latitudinal or longitudinal variations in atmospheric structure and will focus only on the vertical chemistry variations typical of mean conditions at low latitudes.

Gas Vertical Profiles

Figure 5 shows typical vertical profiles of the 24 known molecular species at low latitudes, compiled from a combination of ground- and space-based measurements, and some photochemical model profiles constrained by observations. These include two pairs of structural isomers: HCN/HNC and CH₃CCH/CH₂CCH₂.

Typical vertical profiles of gases in Titan’s atmosphere at low latitudes. Sources: N₂, CH₄, H₂—Niemann et al. (2010);³¹ CO—Serigano et al. (2016);¹³³ C₂H₂, HCN—Teanby et al. (2007);¹⁶² C₂H₆, CH₃CCH, C₃H₈, C₄H₂—Vinatier et al. (2007);¹⁶³ HC₃N—Marten et al. (2002);¹⁶⁴ H₂O, C₂N₂—Loison et al. (2015);⁹⁸ c-C₃H₂—Nixon et al. (2020);¹⁶⁵ C₂H₃CN—Palmer et al. (2017);¹⁶⁶ C₂H₅CN—Cordiner et al. (2015);¹⁶⁷ CH₃C₃N—Thelen et al. (2020);¹⁶⁸ CH₃CN—2015 profile from Thelen et al. (2019)¹⁶⁹ with extensions to the troposphere from Marten et al. (2002)¹⁶⁴ and the mesosphere from Loison et al. (2015);⁹⁸ C₃H₆—Lombardo et al. (2019);¹⁷⁰ CH₂CCH₂—Lombardo et al. (2019);¹⁷¹ HNC—combination of Lellouch et al. (2019)¹²⁸ and Dobrijevic et al. (2016);¹⁰¹ C₂H₄, c-C₆H₆, CO₂—photochemical models from Vuitton et al. (2019).¹⁰³

Some important trends can be noted. Methane is an unusual outlier, with a greater abundance in the troposphere (z ≤ 45 km) than above. This is due to the “cold trap” effect, where it reaches saturation as the tropospheric temperature drops with altitude, and therefore, its mixing ratio is reduced as it forms clouds at ∼15–30 km. The gases N₂ and CO are well-mixed, having approximately uniform profiles throughout the atmosphere due to long photochemical lifetimes. Two other gases, H₂ and C₂H₄, also do not condense at the “cold trap”, the coldest part of the atmosphere around the tropopause at 45 km (∼70 K). The profile of H₂ is shown here as constant at the 0.1% value typical of the lower stratosphere, since measurements of its vertical profile remain uncertain.

All other gas species show profiles that typically decrease downward from the upper atmosphere because they have a source due to photochemistry at high altitudes and then become diluted as they are mixed downward into the denser part of the atmosphere. In many cases the actual measured profiles are still rudimentary, constrained by only a few data points and with even less knowledge of meridional and temporal variations. Some gases may exhibit increases again toward the stratosphere due to either secondary production peaks (e.g., due to cosmic ray deposition) or redistribution by atmospheric circulation.

Atmospheric Photochemical Processes

In this section I present a brief overview of the types of reactions that occur in Titan’s atmosphere as a prelude to the discussion of the chemistry of individual molecules in the following section (see also Table 5 of Vuitton et al.¹⁰³ and the description therein). Different chemical processes become important at different levels of the atmosphere due to the altitude variation of temperature, density, and penetration depths of charged particles and photons that affect the reactions. For example, Saturn magnetospheric electrons are stopped high up in the ionosphere, while solar photons penetrate to varying depths depending on wavelength.¹⁰³ High-energy cosmic rays are deep-penetrating rays in the atmosphere, peaking in energy deposition at around 100–150 km altitude.¹⁰³ The various processes, depicted in Figure 6, are now examined in more detail.

Principal processes and reaction types relevant to the chemistry in Titan’s atmosphere. Plus (+) and minus (−) superscripts indicate ions, while A and A′ are molecules that have rearranged into a different structure without change of composition.

Photodissociation and Electron Impact Dissociation

Dissociation (breakup or fragmentation) of a molecule occurs when a sudden influx of energy breaks molecular bonds. In Titan’s atmosphere this happens readily in the upper atmosphere (z > ∼700 km) due to both energetic UV solar photons (hν) and impact of fast-moving electrons (e^–) trapped in Saturn’s magnetic field^172,173 (Figure 2).

Dissociation often leads to neutral (uncharged) molecular fragments, which may be in either the ground state or excited states, e.g.:^174−176

where ³CH₂ is ground-state methylene and ¹CH₂ is an excited state. Note that other possible fragmentation products are possible—the examples given above are only one possibility in each case. Also note that for simplicity, electronic states of molecules are usually omitted in this paper unless they are required to distinguish between two otherwise identical reagents that have significantly different properties.

The molecular fragments are often radicals, i.e., highly reactive atomic or molecular species that have unpaired electrons, such as H, CH, and N. These radicals are quick to react with other radicals or with neutral species.

A secondary peak of dissociation is expected to occur in the deep atmosphere at ∼100–150 km^177−180 due to extremely high energy cosmic rays (hν).

Ionization

Instead of breaking up (dissociation), a molecule may instead become ionized (positively or negatively charged), typically by losing an electron to become a cation, e.g.:^29,181,182

However, dissociative ionization may also occur, e.g.:^183,184

Ionization of neutrals may also occur through dissociative electron attachment, e.g.:^185,186

For larger molecules, radiative electron attachment is also important:^93,187

Ion Reactions

Dissociative recombination is the process whereby a positive ion reunites with an electron and in the process breaks apart. An example in Titan’s atmosphere is^188,189

Radiative association is a reaction whereby two species combine, shedding excess energy via a photon. These reactions typically occur only in rarefied environments (p < 10^–5 mbar¹⁹⁰), where a metastable intermediary complex has time to form and then stabilize by emission of a photon. Despite being impossible to observe in laboratory conditions due to the long lifetimes of the intermediate states,^191,192 such reactions are thought to occur in interstellar clouds^193−196 as well as in Titan’s upper atmosphere. Radiative ion–neutral association reactions on Titan may include:^103,197

In charge transfer reactions,^103,198 at least two products result:

Ions may also react with each other, leading to neutral products, although Vuitton et al.¹⁰³ argued that positive–negative ion recombination rates are too low to compete with ion–neutral reaction pathways.

Radical Reactions

Radicals (molecules with an unpaired electron) react with other radical and nonradical species in multiple ways. Radicals may react with each other in association reactions:⁴⁷

Radicals may attack neutral molecules, for example in the case of hydrogen abstraction, which is a major loss process for methane:^{41,103,199,200}

Other examples include substitutions of terminal atoms or groups, typically at carbon–carbon double or triple bonds:^201−203

and insertions, which allow heavier molecules to be built from simpler ones:^204−206

Three-body association reactions occur when two reactants meet to form a metastable intermediate complex that is then stabilized by collision with a third, nonreacting body that carries away energy, allowing the metastable complex to stabilize. The two steps are

where the asterisk is used to denote an excited state. In this paper I typically simplify such reactions to a single step:

Such reactions are of critical importance to the formation of many hydrocarbons, especially alkanes, e.g.:^197,207,208

Note that three-body reactions are limited to Titan’s dense lower atmosphere, where there is a sufficiently high collision rate to allow the collisional stabilization to occur.⁹⁴

Radiative association reactions may also occur between radicals. Vuitton et al.⁹⁴ studied the effect of radiative associations on Titan’s chemistry and proposed that reactions such as

and

may occur in Titan’s atmosphere.

Other Reactions

Molecules may also reorganize their structure to become more stable, such as in collisional isomerization. Two important known instances of this on Titan are the conversion of propadiene (allene) to propyne by H atoms,^41,99 with a barrier of 65.1 kcal/mol:²⁰⁹

and conversion of HNC to HCN¹⁰⁰ with a barrier of 30.2 kcal/mol:²¹⁰

Polymerization is the process where multiple similar unsaturated hydrocarbons join together form linear chains:

Polymerization is thought to be one of the principal mechanisms leading to the formation of larger polyyne compounds, which may be a significant component of Titan’s haze particles.^{80,104,106,108,116,211−218}

Chemistry of the Neutral Atmosphere

In this review, I focus on the 24 molecules detected in Titan’s dense neutral atmosphere (Figure 7). Many other neutral species have been inferred from ion and neutral mass spectroscopy in Titan’s upper atmosphere^{28,58,61,65,219,220} through photochemical models^{79−88,90−103} and via laboratory experiments (see the review by Cable et al.¹¹¹). The choice to limit the discussion to the chemistry and composition of the 24 definitively identified molecules of the neutral atmosphere was made for several reasons: (i) this set of molecules includes the most easily detectable and likely the most abundant molecules of the atmosphere, which therefore provide a good overview of bulk chemistry and composition; (ii) other molecules inferred only through single-stage mass spectrometry do not have robust structural identifications, since this technique cannot typically distinguish between isomers having the same chemical formula occurring at the level of complexity of three carbon atoms and beyond; (iii) to allow for a more detailed discussion of the molecules that have been unambiguously detected; and (iv) because the composition of the neutral lower atmosphere is the most important chemical inventory for consideration of other processes such as condensation and meteorology, sedimentation to the surface, and astrobiology at Titan’s surface and interior.

Molecules detected in Titan’s neutral atmosphere, along with the mass of the most abundant isotopologue in Da.

Note on Chemical Names

The topic of chemical nomenclature remains eternally problematic. For example, the relatively simple compound C₂H₃CN has been commonly referred to as “vinyl cyanide” in most 20th century astronomical literature, though preference has shifted more recently toward the simpler single-word name “acrylonitrile” (also with “methyl cyanide” to “acetonitrile”). In fact, multiple valid names for C₂H₃CN exist, including “2-propenenitrile”, “cyanoethene/cyanoethylene”, and “propenenitrile”; however, it must be noted that the preferred official (IUPAC) name is actually the rather cumbersome “prop-2-enenitrile”.

IUPAC molecular nomenclature certainly has its place. However, for the purposes of discourse in planetary atmospheric chemistry, dominated by small molecules, the formulaic names for molecules can be not only inconvenient but also an actual obstacle to reading and digesting information. Therefore, in this work I have followed a naming convention based on a combination of modernity modified in places for greater simplicity: (a) single-word names are generally preferred over multiple-word names (e.g., “propyne” over the older standard “methyl acetylene”); (b) numbers in names of small molecules are avoided except where necessary, and (c) it is accepted that some names are close enough to be interchangeable (e.g., there is little confusion engendered by using either “ethylene” or “ethene”, “propylene” or “propene”, etc.).

While I run the risk of offending practicing chemists, I hope that the terminology is consistent enough for readers to understand what molecule is being referred to and convenient enough for simplified writing in typical planetary science usage.

Hydrocarbons and Hydrogen

Hydrocarbons are molecules formed from atoms of only hydrogen and carbon. Due to the fourfold valency of carbon, many bonding configurations are possible. The most common families of aliphatic (acyclic) hydrocarbons include (i) the alkanes, where carbon is saturated, having four single bonds, and the two unsaturated types: (ii) alkenes, featuring carbon–carbon double bonds, and (iii) alkynes with carbon–carbon triple bonds. Once there are three or more carbon atoms, mixed types become possible (see Figure 8).

Hydrocarbon molecules detected on Titan.

Cyclic hydrocarbons occur where there are one or more closed rings of carbon atoms (at least three are needed to make a ring, or cycle). Cyclic molecules that have delocalized π bonding electrons are known as “aromatic” (small aromatics are typically volatile at room temperature), although not all rings are aromatic. For example the unsaturated six-carbon ring benzene (c-C₆H₆) is a well-known aromatic, whereas its saturated cousin cyclohexane (c-C₆H₁₂) is a nonaromatic cycle. Carbenes are reactive molecules where carbon has two unbonded but paired electrons, such as methylene (CH₂).

Hydrocarbon molecules are created through the breakup and recombination of fragments of methane. A prime example is the formation of ethane from two methyl radicals:^221,222

Note that in the process, significant amounts of molecular hydrogen will form from the photodissociated hydrogen atoms:²²³

leading to the trace amounts of hydrogen (∼0.1%) found in Titan’s lower atmosphere. Note that this termolecular reaction is an important process leading to the creation of H₂ in the interstellar medium (ISM).

Significant loss of hydrogen to space is thought to occur,^{40,148,224−228} preventing methane from being recycled, as occurs on the giant planets, leading to gradual depletion of methane in Titan’s atmosphere in the absence of outgassing or other replenishment.^41,229−232

A network diagram showing the principal neutral pathways for hydrocarbon molecule formation is shown in Figure 9. Hereafter follows a high-level description of the key chemistry for each of the hydrocarbons and hydrogen leading to their relative abundances in the neutral atmosphere.

Hydrocarbon chemical reagent and reaction networks showing key pathways. Photolysis pathways are shown in purple dashed lines, three-body reactions in red, and other reactions in blue. As the number of carbon atoms increases, the number of possible species multiplies.

Hydrogen

Detection

Molecular hydrogen (H₂) was was tentatively detected by Trafton²³³ using the 107 inch telescope at the McDonald Observatory via absorption in the S(0) and S(1) quadrupole lines of the 3–0 band at ∼0.82 μm. Hydrogen was clearly detected in the far-infrared by the Voyager 1 Infrared Interferometer Spectrometer (IRIS)²³⁴ and confirmed by Cassini CIRS.²³⁵ The H₂ volume mixing ratio (VMR) was measured directly by the Cassini INMS instrument in the ionosphere at 0.4%²⁸ and in the lower stratosphere and troposphere by Huygens GCMS at 0.1%.³¹ The fact that hydrogen, presumed to be produced in the upper atmosphere by photolysis of methane,^40,41 was measured to have a decreasing abundance downward, has proved difficult to replicate in models. Models have required a sink for H₂ at the surface,^135,225,237 which has even been suggested as possibly biological in origin.²³⁸ Important reactions for hydrogen are shown in Figure 10.²³⁹

Production

Molecular hydrogen can be produced in ion-phase reactions, such as²⁴⁰

and in neutral-phase radical reactions, including^103,241

Loss

H₂ may be lost to photolysis, yielding two H atoms, although Vuitton et al.¹⁰³ have argued that this is a relatively small source of H in Titan’s atmosphere due to shielding of H₂ by CH₄ and N₂, with most H production coming from photolysis of methane.

H₂ may also be lost due to ion reactions, e.g.,^240,242,243

and radical reactions:^244−246

Future Work

Measurement and modeling work is still required to confirm our understanding of the vertical H₂ profile^21,135,225 and determine whether a solution to the vertical gradient lies in instrumental errors²²⁸ or unknown processes in the atmosphere.

Methane

Profile

Methane (CH₄) was the first molecule to be positively identified in Titan’s atmosphere, via visible and near-IR absorptions seen by Kuiper²⁴⁷ using the 82 inch reflector at McDonald Observatory. We now know that methane is the basic ingredient enabling all of Titan’s complex organic (i.e., carbon) chemistry, allowing reactions to proceed up to the creation of haze particles. Some key reactions for methane are shown in Figure 11.^248,249

Methane production and loss pathways. Reactions numbered and shown in bold correspond to discussion in the text.

Nevertheless, methane may be a gradually depleting resource since it is not permanently recycled, unless it is replenished by an as yet unidentified mechanism.^{41,231,232,250,251} Speculative mechanisms include crustal destabilization leading to outgassing from methane clathrates,²⁵² outgassing from cryovolcanism,^253−255 and displacement from near-surface clathrate materials by condensed ethane,²⁵⁶ yet observational evidence for these processes remains inconclusive at best.

Methane’s vertical profile can be divided into three zones: (i) a tropospheric zone where the fractional abundance gradually decreases from 5.5% at the surface to a minimum at the tropopause of around 1.4%, due to reaching saturation at decreasing VMRs as the temperature decreases toward the tropopause (“cold trap”); (ii) a relatively constant amount of 1.4% in the stratosphere, mesosphere, and thermosphere; and (iii) a gradually increasing mixing fraction above an altitude of 800–850 km (the methane homopause) in the increasingly collisionless regime, due to the differing scale heights of different molecules.¹³⁵ It is presently uncertain whether methane has any variation with latitude on Titan, although a variation from ∼1.0% to 1.5% in the lower stratosphere has been reported²⁵⁷ based on infrared measurements by Cassini CIRS.²⁵⁷

Loss

The photolysis of methane in the upper atmosphere leads to the formation of radicals, including methyl (CH₃), methylidene (CH), and the carbene methylene (¹CH₂ or ³CH₂),¹⁰³ which undergo a chain of reactions to form all the hydrocarbons found in Titan’s atmosphere. Note that as much as 75% of methane photolysis above 700 km is due to the solar Lyman-α line at 121.6 nm.²⁵⁸

The fate of most methane is ultimately to form ethane via the addition of two methyl radicals (reaction 31) or via the creation of ethyl radical,

leading to permanent loss of methane. Methyl radicals are produced either directly by primary photolysis or by reaction of methane with radicals:^205,259

Note that the reactions of the ethynyl and vinyl radicals (among others) with methane are catalytic destruction processes since the acetylene and ethylene generated are easily photolyzed back to their radical forms, where they can continue to destroy methane molecules. This process may be repeated hundreds of times before the radical catalysts are themselves lost to form higher hydrocarbons. This is the main source of methane depletion in Titan’s atmosphere.¹⁰³

Methane fragments participate in ion chemistry. They first are ionized by charge transfer:²⁴³

Then they build up C₂H_x ions, e.g.:²⁴⁰

Future Directions

Although the chemistry of methane is perhaps one of the best-understood for any molecule on Titan, the most pressing questions remain the nature of its origin and possible replenishment.²¹

Acetylene

Acetylene was the third molecule to be identified in Titan’s atmosphere,²⁶⁰ following the detection of methane and ethane, when Gillett observed its 13 μm band in mid-infrared spectroscopy with 2 and 4 m telescopes at Kitt Peak Observatory. Important reactions for acetylene are shown in Figure 12.^{103,258,261,262}

Production

Acetylene may be produced from photolysis products of methane either directly, i.e.,²⁶³

or indirectly via C₂H₄ and other species from photolysis²⁶⁴ (see Figure 13):

Dissociative electron recombination is another production pathway, e.g.,^265,266

Loss

In the ionosphere, photolysis of acetylene and electron transfer to N₂⁺ produce C₂H₂⁺,²⁴³ which reacts with neutrals to build heavier ions,²⁴⁰ e.g.,

which may (dissociatively) recombine with e^– to form neutral C₃H_x species.^267,268 Likewise, proton transfer to neutral acetylene leads to C₂H₃⁺, which can also form C₃ species:²⁴⁰

Finally, neutral C₂H₂ in the ionosphere may combine with other ions to build heavier species,²⁴⁰ e.g.,

In the neutral atmosphere, acetylene is lost by reaction with methylene to form propargyl:^269−271

Acetylene absorbs photons at longer wavelengths (∼230 nm⁷⁹) than methane, so its photolysis continues into the stratosphere (see Figure 12). This produces ethynyl (C₂H) and carbyne (C₂), which are potent means of methane depletion via hydrogen abstraction:²²³

The acetylene produced by this reaction is recycled back to ethynyl by photolysis, and thereby each acetylene/ethynyl may cause the loss of hundreds of methane molecules before being lost itself to another reaction pathway, such as²⁷²

Catalytic destruction of methane in this way is the principal means of methane depletion in Titan’s atmosphere.^41,79,103 In the lower atmosphere (z < 500 km), the dominant loss process for acetylene is conversion to ethylene by a two-step reaction with atomic hydrogen:^94,103

Future Directions

Acetylene has been detected on Titan’s surface^32,273 and is likely to be present in the northern lakes and seas.^274,275 Future investigation by the mass spectrometer (DrAMS) instrument of Dragonfly(276) will further refine the surface and near-surface abundance.

C₂H₂ was one of the first molecules investigated to form a cocrystal^277,278 at Titan surface temperatures, an organized cocondensate of two or more chemical species. The validity of multiple cocrystal types has since been established, but further laboratory work is required to determine the full parameter space of possible crystalline types.

Ethylene

Ethylene (C₂H₄) was discovered by infrared spectroscopy at the same time as acetylene.²⁶⁰ The vertical profile of ethylene exhibited a surprising trend to decrease in abundance upward in the lower stratosphere early in the Cassini mission,^163,279 although this faded at later seasons.^127,137

Ethylene is a crucial two-carbon neutral molecule that provides a stepping stone from methane to higher hydrocarbons (Figure 13).^280,281 Ethylene is remarkable in being one of the few molecules (along with CO, H₂, and N₂) that does not condense at the tropopause and therefore persists in significant quantities into the troposphere.

Production

Ethylene may be produced in the ionosphere by dissociative recombination of heavier ions with an electron:^267,282,283

In the neutral atmosphere, ethylene is largely formed through reactions between methane and its derived radicals or between radicals:^{79,258,263,269,270,284}

At lower altitudes, production through the C₂H₅ intermediate is also important:²²³

Loss

In the ionosphere, the ethylenium ion—produced from ethylene photoionization—can be lost in various reactions with neutrals:²⁴⁰

Also, ethylene can be lost during the formation of heavier ions:²⁴⁰

Photolysis of ethylene leads to acetylene (reactions 50 and 51). Insertion/addition reactions onto ethylene by CH, for example, can lead to higher hydrocarbons:^204,284,285

Below 500 km, loss via H addition becomes important:⁹⁴

The CN radical can also substitute onto ethylene to create vinyl cyanide (also known as acrylonitrile or propenitrile):^98,202,203

Future Directions

Ethylene is the simplest alkene, the family of hydrocarbons having a C=C double bond. Photolysis or other breaking of the alkene C=C bond leads to radicals which rapidly react, leading to formation of polymers. The role of polymer formation in Titan’s atmosphere is incompletely understood but is likely to be an important process in the formation of haze particles.

Ethane

Ethane was the second molecule to be discovered in Titan’s atmosphere, via its strong ν₉ band at 12 μm²⁸⁶ seen by Gillett with the 60 inch telescope on Mount Lemmon. Ethane forms one of the primary trace gases in Titan’s atmosphere, with concentrations greater than 1 ppm in the stratosphere,^48,287,288 and is the primary sink for methane loss.^41,47 Due to this observation, a global deep ethane ocean was originally predicted^289,290 but later proved not to be the case.^291,292

The liquid hydrocarbon bodies eventually detected on Titan’s surface²⁹³ have a measured ethane content that varies between different seas and is generally less than the methane fraction,^294,295 except for the southern lake Ontario Lacus.^296,297 Ethane may form cocrystals in Titan lakes with other organics, such as benzene.²⁹⁸ Ethane has also been implicated in displacing methane from clathrate hydrate, allowing for a partial resupply mechanism of methane to the atmosphere,²⁵⁶ which is otherwise continuously lost by chemistry.^41,299

Production

Ethane is primarily produced by the addition of two methyl radicals (reaction 31) but also reforms from the ethyl radical:³⁰⁰

Ethane is also formed in ion reactions, as shown in Figure 14.^301−303

Loss

Ethane can be lost through photolysis back to two CH₃ radicals or to stable molecules such as ethylene and acetylene with loss of hydrogen (see Figure 14). Ethane can also be attacked by reactive radicals such as methylidene, methylene, and ethynyl (from acetylene photolysis):^{79,204,223,284}

Ethane may form heavier ions through reactions such as²⁴⁰

Future Directions

Large amounts of ethane are thought to condense in Titan’s lower stratosphere and form a significant fraction of Titan’s lakes and seas.^274,304 Ethane was implicated in the formation of a vast north polar cloud seen during northern winter in 2005 by Cassini VIMS,³⁰⁵ although other interpretations have suggested that this cloud is condensed methane.³⁰⁶ While the basic chemistry of ethane is well-understood, an improved understanding of its condensation—especially cocondensation with other gases—will be crucial to a more accurate interpretation of Titan’s meteorology.

Cyclopropenylidene

Cyclopropenylidene (c-C₃H₂) is the first carbene (a molecule having two unbonded, self-paired valence electrons from a carbon atom) and the second cyclic molecule to be found in Titan’s atmosphere (after benzene). c-C₃H₂ was detected using millimeter-wavelength astronomy with ALMA,¹⁶⁵ the third molecule whose first detection on Titan was achieved with this telescope.

Production

In the upper atmosphere, production of a precursor, the cyclopropenyl cation (c-C₃H₃⁺) is thought to proceed by¹⁰³

c-C₃H₃⁺ then recombines with e^– to produce c-C₃H₂³⁰⁷ and H.

Another possible pathway involves photolysis of propargyl:

In the neutral atmosphere, the dominant production pathways may include CH addition to acetylene^308,309 (see Figure 15):^310,311

Below 600 km, the following reaction can occur:³¹²

c-C₃H₂ can also result from collisional isomerization from its isomers propynylidene (t-C₃H₂) and propadienylidene (l-C₃H₂):³¹²

Loss

Once ionized, the c-C₃H₂⁺ ion may abstract hydrogen from neutrals to form the c-C₃H₃⁺ ion:^240,313

100

c-C₃H₃⁺ in turn may dissociatively recombine with an electron, returning to c-C₃H₂, or splinter into smaller acyclic fragments:^267,314

101

102

103

104

In the neutral atmosphere above 600 km, c-C₃H₂ photodissociates to form c-C₃H, l-C₃H, and C₃. It is also lost by reaction with CH₃ to form acylic species:³¹²

105

Below 600 km, these pathways continue to be significant, but there is additional c-C₃H₂ loss via³¹²

106

107

Future Work

Chemical pathways leading to and from c-C₃H₂ in Titan’s atmosphere remain to be explored, especially given the multiple possible structures for C₃H₂, including propynylidene (HC₃H, t-C₃H₂), propadienylidene (H₂CCC, i-C₃H₂), cyclopropyne (CCHCH), and propenediylidene (HCCHC).^310,311 Experimental and theoretical work on reaction pathways, rates, and branching ratios during photolysis will greatly help to clarify the production, loss, and stability of cyclopropenylidene.

Several isomers of C₃H₂ have been detected in space, including H₂CCC,³¹⁵ which should prompt further astronomical observations to determine if these isomers also exist in Titan’s upper atmosphere. Intriguingly, the CHCCH form has been shown to dimerize to form p-benzyne,³⁰⁸ as discussed in a later section.

Propyne

The propyne (CH₃CCH, methyl acetylene) isomer of C₃H₄ was first detected in the infrared by Voyager IRIS³¹⁶ via long-wavelength infrared emission bands at 328 and 633 cm^–1.³¹⁷

Production

Propyne and its symmetric isomer propadiene (CH₂CCH₂, allene) are produced by addition of CH into ethylene (reaction 79; see Figure 16)^318−320 but also through H addition to propargyl,³²¹

108

or through dissociation of C₃H₆ (see Figure 18):³²²

109

Propyne production and loss pathways. Reactions numbered and shown in bold correspond to discussion in the text.

Propene production and loss pathways. Reactions numbered and shown in bold correspond to discussion in the text.

In the ionosphere, the C₃H₅⁺ ion is a precursor to C₃H₄ and is produced via the following reactions:²¹⁹

110

111

It then forms C₃H₄ by proton transfer:^323,324

112

113

Loss

In the ionosphere, propyne is lost through ion reactions such as²⁴⁰

114

115

116

Propyne may also be lost by photolysis in the upper atmosphere³²⁵ and to three-body reactions:¹⁰³

117

118

Future Work

Many reactions forming or depleting C₃H₄ have uncertain branching ratios between CH₃CCH and its isomer CH₂CCH₂. Further work is needed to improve knowledge of these quantities. Collisional interconversion between the two isomers may be mediated by atomic hydrogen,⁹⁹ so accurate measurement of both isomers may be a way to provide a constraint on the abundance of otherwise short-lived and difficult-to-measure H atom.

Propadiene

Propadiene (CH₂CCH₂) is a less abundant and less thermodynamically stable isomer of C₃H₄, which is more abundant in Titan’s atmosphere in the form of propyne. Propadiene was detected in Titan’s atmosphere using high-resolution ground-based spectroscopy at NASA’s Infrared Telescope Facility (IRTF) with the Texas Echelon Cross Echelle Spectrograph (TEXES) instrument via its ν₁₀ band at ∼845 cm^–1.³²⁶

Production

Like propyne (CH₃CCH), propadiene is produced in the upper atmosphere by CH addition to ethylene (reaction 79; see Figure 17)^{319,327−329} and by H addition to C₃H₃:³²¹

119

Propadiene production and loss pathways. Reactions numbered and shown in bold correspond to discussion in the text.

Lower in the atmosphere, where propene is more plentiful, it can be photodissociated to produce propadiene (see Figure 18):³³⁰

120

Ion formation pathways of CH₂CCH₂ are less certain but may follow similar channels as propyne, with branching ratios that are currently uncertain.

Loss

Propadiene is lost to direct photolysis:³¹⁹

121

It is also lost to ion reactions such as¹⁰³

122

or by collision with H to form propyne:¹⁰³

123

Future Directions

As with propyne, the branching ratios of reactions implicated in the formation of C₃H₄ isomers remain uncertain, so further experimental and theoretical work is required. Accurate measurement of both propyne and propadiene is a possible means to indirectly infer the abundance of atomic hydrogen⁹⁹ in the lower atmosphere, in the absence of direct in situ measurements.

Propene

Propene (C₃H₆, propylene) was first detected using data from Cassini CIRS via its ν₁₉ band emission near 11 μm.^171,331

Production

Ion reactions lead to creation of propene from C₃H₅⁺, e.g.:²⁴⁰

124

125

Propene is predicted to be produced in the upper atmosphere by both H addition to C₃H₅ (62%) and CH insertion into ethane (38%):⁹¹

126

127

and also lower in the atmosphere by a termolecular reaction:¹⁰³

128

Loss

Propene is lost through both photodissociation (Figure 18^{322,330,332−335}) and ion reactions, e.g.,²⁴⁰

129

130

In the lower atmosphere, propene is predicted to be lost through a termolecular reaction with H atom addition:¹⁰³

131

Future Directions

Propene, as an alkene, may also undergo polymerization to form polypropylene, a notable and widespread plastic used on Earth. Most likely polyynes in Titan’s atmosphere are not pure polymers of a single repeated monomer type (ethylene, propylene, etc.) but rather an assorted mixture of many types, with the lighter, more abundant alkenes more heavily represented than larger, heavier units. Further research into polymerization of mixed monomers will yield insights into the formation of Titan’s haze.

Propane

Propane (C₃H₈) was detected contemporaneously with propyne (CH₃CCH) by Voyager’s IRIS instrument³¹⁷ via an infrared band at 748 cm^–1 and subsequently confirmed by ground-based observations³³⁶ and with Cassini CIRS.^163,287,337

Production

A significant pathway for the production of propane is by addition of CH₃ to C₂H₅ (Figure 19):^338,339

132

It is also formed to a lesser extent by the association reaction^103,340

133

Loss

Propane is primarily lost in the upper atmosphere by photolysis to propene, but it participates in other reactions as shown in Figure 19. Propane also undergoes H abstraction by ethynyl to recycle acetylene:³⁴¹

134

However, the fate of C₃H₇ is largely to react with H to reform propane.¹⁰³

Future Directions

While the chemistry of propane remains relatively well-known, its role in cloud formation and lake composition on Titan remains to be fully explored. Quantum-mechanical analysis of propane’s 23 infrared-active bands³³⁷ remains incomplete, preventing accurate modeling at high resolution for these bands. However, in recent years the pseudolinelist technique has proved useful for providing practical absorption coefficients across a wide bandwidth for calculation at medium resolution.³⁴²

Diacetylene

The presence of diacetylene (C₄H₂, butadiyne) was inferred from infrared spectroscopy of Titan’s atmosphere with Voyager’s IRIS instrument³⁴³ via emission bands at 220 and 628 cm^–1. At present, it remains the only C₄ hydrocarbon species confirmed in Titan’s atmosphere (although it should be noted that the nitrile CH₃C₃N, detected with ALMA,¹⁶⁸ also has four carbon atoms).

Production

Diacetylene can be produced by the aforementioned reaction of the ethynyl radical with acetylene (reaction 63) or by stepwise addition to acetylene:^103,284,344

135

136

137

Loss

Diacetylene may undergo ionization to C₄H₂⁺ (Figure 20)^345,346 and subsequent loss to processes such as²⁴⁰

138

139

while neutral C₄H₂ may be lost through ion reactions, including^243,347

140

141

Diacetylene production and loss pathways. Reactions numbered and shown in bold correspond to discussion in the text.

Insertion of ethynyl is a way to lengthen the polyyne chain from diacetylene to triacetylene:³⁴⁸

142

Future Directions

To date, the triacetylene molecule (C₆H₂) has remained elusive on Titan, despite its detection in space at high relative abundances compared to C₄H₂.³⁴⁹ Detection of triacetylene would help to clarify the importance of the C₆H radical, which contributes to the depletion of methane in photochemical models along with the smaller related radicals C₂H and C₄H, as well as the efficacy of polyyne formation in general.

Benzene

Benzene (c-C₆H₆) (Figure 21)^{102,350−353} was the second new species detection on Titan made by the Infrared Space Observatory (ISO) in 2003,⁴⁹ via its strong hydrogen bending mode at 674 cm^–1. Benzene was the first cyclic (closed-ring) molecule to be detected on Titan and remains the only confirmed aromatic molecule (molecules with delocalized π bonding electrons). The detection of benzene is highly significant since it provides a measurement of the basic six-membered ring from which larger, multiring molecules can be formed,^354,355 building toward macromolecular haze particles (see discussion later in this review).

Production

In the upper atmosphere (800–950 km), a significant pathway for creation of benzene is dissociative recombination (DR) of the phenylium ion (C₆H₇⁺). Phenylium is created through the reactions^92,102

143

144

and then forms benzene through the reaction³⁵⁶

145

A second source roughly equal in importance is thought to be formation followed by dissociative recombination of the C₇H₇⁺ ion (benzylium or tropylium; Figure 22):¹⁰²

146

147

148

Potential benzene precursor molecules (top) and products (bottom). Molecule images: Wikimedia Commons.

Yet a third ion channel is the DR of C₈H₁₁⁺ with e^–, leading to benzene plus other hydrocarbon fragments.¹⁰²

Radical chemistry also leads to to benzene, such as C₂ addition to 1,3-butadiene proposed to occur in the ISM:³⁵⁷

149

An alternate pathway,

150

also leads to benzene, but in a highly excited state where it mostly dissociates to C₆H₅ + H.¹⁰² Recently, a new pathway via a smaller five-membered-ring radical (cyclopentadienyl, Figure 22) has been proposed by Kaiser et al.:³⁵⁵

151

However, this has yet to be added to photochemical models to assess its relative importance.

At higher pressures lower in the atmosphere, the three-body reaction combining two propargyl radicals becomes the dominant pathway for creation of benzene:^79,80,92,102

152

Loss

Benzene is lost through ionization to the phenylium ion (C₆H₅⁺) and through photolysis to form phenyl (C₆H₅).^92,358 The phenyl radical then either reforms benzene via⁹⁴

153

or reacts with other radicals and neutral species, leading to molecules such as toluene (C₆H₅CH₃), styrene (C₆H₅C₂H), and benzonitrile (C₆H₅CN).³⁵⁹

Future Directions

Benzene is a highly significant molecule as the precursor to larger, multiring molecules. Further studies of its creation and loss mechanisms, especially pathways to larger molecules (Figure 22),³⁵⁴ are important future directions.

Nitrogen Compounds

Nitrogen compounds are formed by chemical combination of dissociation products from initial N₂ and CH₄ and have formulas C_xH_yN_z. All of the eight known heteroatomic nitrogen compounds are cyanides, wherein nitrogen is bonded to carbon by a triple bond (−C≡N) and therefore have the formula C_xH_y(CN)_z. These are HCN, HNC, CH₃CN, C₂H₃CN, C₂H₅CN, HC₃N, CH₃C₃N, and C₂N₂ (see Figure 23). Other than the light molecules HCN and HNC, the remaining molecules are nitriles (organic cyanides).

Nitrogen-bearing molecules detected on Titan.

Hydrogen isocyanide (HNC) is less stable than hydrogen cyanide (HCN) and is converted exothermically to HCN as it descends in the atmosphere. This leads to a predicted steep decrease in abundance with increasing pressure³⁶⁰ and its present nondetection at lower altitudes.

In the lower atmosphere, nitrogen has always been found to date to be triple-bonded in the terminal position of a molecule: other types of species (amines, imines, etc.) have not yet been detected. We will return to the topic of what additional nitrogen compounds may be waiting to be discovered in a later section. The chemistry of known N-bearing molecules in the neutral atmosphere is now summarized.

Nitrogen

A major but unobserved constituent in Titan’s atmosphere was necessitated by the observed collisional broadening of methane spectral lines:³⁶¹ this was hypothesized to be molecular nitrogen,³⁶² which would be invisible at visible and longer wavelengths. The first conclusive observations of nitrogen were by Voyager 1’s UVS instrument, which detected dayside airglow at 96 and 98 nm and longer-wavelength absorptions with occultation measurements.^363,364 Measurements of nitrogen were greatly extended by Cassini’s UVIS instrument.^26,365−368

Production

The origin of nitrogen in Titan’s atmosphere has been long debated and is not the subject of this review. In brief, two major theories exist: enclathratization of N₂ gas in the protosolar nebula³⁶⁹ and accretion in the form of NH₃ ice followed by later photodissociation to eventually form N₂ through a reaction cascade:³⁷⁰

154

155

156

157

The latter scenario is currently favored due to the low temperatures in the subnebula required to capture molecular nitrogen directly. Variations on the theory include impact conversion of either NH₃ or ammonium sulfate ((NH₄)₂SO₄) to N₂.^371,372

N₂ can also be recycled by recombination or proton transfer of one its ions:^240,243,373

158

159

or through recycling of one of its radicals:^103,374

160

161

Loss

Molecular nitrogen is dissociated and/or ionized by short-wavelength solar radiation at λ < 127 nm,³⁷⁵ Saturn magnetosphere electrons,³⁷⁶ and galactic cosmic rays (GCRs)^177−180 (Figure 24).¹⁷³

Nitrogen ions react with abundant neutrals, including CH₄ and H₂:^243,377

162

163

164

It can recycle to N₂ through reaction with hydrogen, methane, and other hydrocarbons, e.g.,²⁴³

165

166

167

168

However, molecular nitrogen in the un-ionized state has very low reactivity, which in part contributes to its great abundance and significant longevity in the atmosphere.

Isotopes

Since ¹⁴N₂ and ¹⁴N¹⁵N have significantly different UV cross sections,³⁷⁸ it is important to correctly account for both isotopes and the wavelength variation of the solar spectrum to arrive at correct dissociation rates. Self-shielding by the more abundant ¹⁴N¹⁴N is thought to reduce photolysis rates relative to the less abundant, less shielded ¹⁴N¹⁵N, causing a lower ¹⁴N/¹⁵N ratio in nitrogen atoms than in the original molecules. Since significant amounts of atomic nitrogen go on to form nitriles, this skew toward increased production of ¹⁵N may explain the lower ¹⁴N/¹⁵N ratio in nitriles than in N₂ itself.^378−380

Future Directions

The dissociation and reaction pathways for N₂ and its daughter ions and radicals remain one of the better known areas of Titan chemistry. However, gaps remain, in particular whether nitrogen exists in chemicals such as amines and imines or if it is incorporated into heterocyclic ring molecules. This is further discussed in a later section.

Hydrogen Cyanide

Hydrogen cyanide was first detected by Voyager 1 IRIS³⁸¹ through its strong infrared emission at 712 cm^–1 and later at submillimeter wavelengths from ground-based observatories.^{164,382−384} Although it is a relatively simple molecule that has been included in photochemical models for more than four decades, gaps in our knowledge of HCN formation may still exist, and new pathways have been identified recently.³⁸⁵

Production

HCN is primarily produced in the upper atmosphere by the reaction of methane and nitrogen dissociation products (Figure 25):³⁸⁶

169

170

171

It also may be reformed from its ion by ion–molecule reactions, e.g.,²⁴⁰

172

Hydrogen cyanide production and loss pathways. Reactions numbered and shown in bold correspond to discussion in the text.

In the lower atmosphere, reactions with CN radicals become important:^{91,202,203,387}

173

174

175

along with photodissociation of C₂H₃CN (Figure 30)³⁸⁸ and reaction of other nitriles with H:^91,103

176

177

Loss

At high altitudes (z ≥ 1000 km) HCN is primarily destroyed by reaction with N(²D):^91,103

178

HCN may also be lost in a two-step process, beginning with proton transfer from a lower-proton-affinity molecule, e.g.,¹⁰³

179

followed by dissociative recombination:³⁷⁴

180

Lower in the atmosphere, radical reactions and photolysis become important:⁹¹

181

182

As noted by previous authors, the C≡N triple bond is extremely stable, and therefore, the CN unit tends to persist when HCN is photolyzed, being incorporated into heavier nitriles, e.g.:

183

184

Also, at low altitudes (z < 650 km),¹⁰³ H addition can lead to formation of the methylene amidogen radical:

185

Future Directions

Although well-studied for decades, recent work^385,389 has identified new pathways for the formation of HCN in planetary atmospheres for which reaction rates are currently unknown. Theoretical predictions now exist, but experimental confirmation is needed.

HCN has been shown to form cocrystals with hydrocarbons at Titan-relevant temperatures,³⁹⁰ the study of which will be important for understanding the solids and liquids on the surface. HCN, along with HC₃N, has also been implicated in the formation of C₄N₂ in grain-surface chemical reactions,³⁹¹ which requires further study to elucidate reaction rates and whether this process is sufficient to explain observed ice spectral properties.³⁹²

Finally, HCN has been implicated in processes of astrobiological importance. A well-known example is its proposed ability to directly form the nucleobase adenine (C₅H₅N₅)^393−396 from the rearrangement (oligomerization) of five HCN molecules. Although the importance of this reaction for the seeding of life on the early Earth has been disputed,^397,398 it may be more prevalent on Titan, where HCN occurs in greater abundance.^399,400 HCN may also have the potential to polymerize into polyimines, structures that may catalyze astrobiologically important reactions.⁴⁰¹ The astrobiological potential of HCN therefore remains under continued investigation.^{385,402−404}

Hydrogen Isocyanide

Hydrogen isocyanide, a higher-energy isomer of hydrogen cyanide,⁴⁰⁵ was discovered on Titan using the Herschel space observatory by its submillimeter transition at 544 GHz⁴⁰⁶ and subsequently measured by ALMA as well.^128,360 HNC is readily interconverted to the more stable HCN (releasing 14.4 ± 1.0 kcal/mol)⁴⁰⁷ and therefore is predicted to have a steeply diminishing mixing ratio profile with altitude.^98,103

Production

HNC is produced by the same neutral reactions as HCN:

186

187

where the relative productions are estimated at 1300 km.⁹⁰ At 1000 km, reaction 186 becomes dominant. Note that there are two important production pathways for H₂CN:

188

189

with reaction 188 dominating in the thermosphere and reaction 189 becoming important in the mesosphere and below.⁹⁰ Ion pathways may also be similar (see Figure 26),³⁸⁶ although branching ratios are in most cases more uncertain than for HCN, e.g., through dissociative recombination of HCNH⁺:⁴⁰⁸

190

HNC may also be produced as a photodissociation product of C₂H₃CN³⁸⁸ in the upper atmosphere, and a further production peak may occur due to cosmic ray chemistry at 100–150 km.⁹⁸

Loss

At high altitudes (∼1300 km) the principal loss channels for HNC are⁹⁰

191

192

while at lower altitudes collisional isomerization to the lower-energy HCN becomes important and is dominant by 600 km:⁹⁰

193

194

Future Directions

HNC/HCN is now one of two isomer pairs known in Titan’s atmosphere (the other being C₃H₄). Studies of the branching ratios and reaction rates leading to and from isomer pairs/triples, etc., are of importance because the less stable isomer(s) may follow different reaction pathways compared to the more abundant molecule(s). Therefore, for a complete understanding of Titan’s atmospheric chemistry, all isomers must be included in models. Studies of the vertical ratio between HCN/HNC and CH₃CCH/CH₂CCH₂ may also provide useful information on the abundance of atomic H, as collisions with H can cause conversion between the isomers.

Acetonitrile

Acetonitrile was first detected on Titan in the early 1990s by millimeter wavelength astronomy,⁴⁰⁹ followed 10 years later by the first measurement of its vertical profile¹⁶⁴ using the 30 m telescope at IRAM. CH₃CN was the first Titan molecule to be first detected at millimeter wavelengths, an astronomical technique that was to yield many other discoveries later with ALMA.

Production

Acetonitrile is produced in the upper atmosphere by the reaction of N radicals with ethylene:⁴¹⁰

195

196

and by the termolecular reaction of H with cyanomethyl (CH₂CN) in a chain that begins with acrylonitrile (C₂H₃CN):^98,103

197

198

199

Loss

The major loss mechanism for acetonitrile is proton transfer from another ion to form CH₃CNH⁺, e.g.,⁴¹¹

200

201

followed by dissociative recombination:^412,413

202

203

and in the lower atmosphere by photolysis^414,415 (Figure 27).^98,416,417

Acetonitrile production and loss pathways. Reactions numbered and shown in bold correspond to discussion in the text.

Future Directions

Acetonitrile, like many other simple molecules, has been implicated in formation of a cocrystal with acetylene,⁴¹⁸ providing an interesting avenue for further investigation of its solid-phase properties, with possible implications for cloud particle growth.

Cyanoacetylene

Cyanoacetylene (HC₃N, propynenitrile) was first detected in Titan’s atmosphere by Voyager IRIS in the infrared³⁴³ at 500 and 663 cm^–1, following a prediction by Capone et al.⁴¹⁹ Cyanoacetylene, like diacetylene and cyanogen, was found in 1980 to be greatly enhanced over Titan’s northern (winter) pole, which was interpreted as evidence of a global stratospheric circulation cell. Gases such as HC₃N with relatively short photochemical lifetimes (compared to a Titan year) have volume mixing profiles with steep vertical gradients at most latitudes, decreasing in a downward direction as the gases become depleted and diluted. However, the presence of a strong downward motion from the mesosphere (∼500 km) causes enrichment in trace species to show up much lower down in the lower stratosphere (∼100 km).

Production

Cyanoacetylene is produced above 1000 km by the reaction of acetylene with CN radical from photolysis of HCN (see Figure 28):^{98,202,203,420}

204

and to a lesser extent by photodissociation of acrylonitrile (see Figure 30).

Loss

As with other nitriles, the principal loss pathway for cyanoacetylene in the upper atmosphere is proton transfer to form HC₃NH⁺, e.g.,²⁴⁰

205

followed by dissociative recombination to break up the molecule:⁴¹³

206

On the other hand, photolysis is not a significant loss channel, since C₃N is thought to rapidly recycle back to HC₃N through reaction with methane:⁹⁸

207

While HC₃N does react without a barrier with radicals such as CN and C₂H, the main loss channel in the neutral atmosphere is thought to be successive hydrogen addition:⁹⁸

208

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210

Future Directions

In interstellar space (e.g., molecular clouds such as TMC-1), cyanopolyynes of the form HC_xN have been detected with x = 1, 3, 5, 7, 9, 11.^421−423 HC₅N has been sought but not yet detected in Titan’s neutral atmosphere. Detection of this molecule may provide some clues as to the relative abundances of cyanopolyynes versus N-heterocycles.

Cyanogen

Cyanogen (C₂N₂), like cyanoacetylene, was first detected in Titan’s atmosphere by Voyager IRIS in the infrared³⁴³ at 233 cm^–1.

Production

Cyanogen is thought to be produced mainly by addition of CN to HNC,

211

and through the radical–radical reaction⁹⁸

212

via the intermediate adduct NCHCN. Neither of these reactions is expected to have an entrance barrier,^98,424,425 while the reaction of CN with HCN is inefficient due to the low rate constant.^426−428

Loss

Cyanogen is lost by photodissociation (Figure 29)⁴²⁹ and by H addition (with an entrance barrier of ∼14–30 kJ/mol):⁹⁸

213

214

Cyanogen production and loss pathways. Reactions numbered and shown in bold correspond to discussion in the text.

Future Directions

A larger cousin to cyanogen, dicyanoacetylene (C₄N₂), is likely to exist in Titan’s atmosphere, and detection of its ice has been proposed to explain a feature seen in Voyager IRIS and Cassini CIRS spectra at 478 cm^–1,^392,430,431 although a lack of detection of the corresponding gas emission at 471 cm^–1 has remained puzzling. Anderson et al.³⁹¹ proposed a possible explanation by way of ice grain surface chemistry combining HCN and HC₃N, but further laboratory and perhaps in situ experimental measurements are required to verify this hypothesis. For the time being, C₂N₂ remains the only dicyanide molecule known in Titan’s atmosphere.

Acrylonitrile

Acrylonitrile (C₂H₃CN) was the second molecule to be discovered on Titan at millimeter wavelengths using ALMA,¹⁶⁶ following the detection of propionitrile,¹⁶⁷ which is discussed in the next section.

Production

Acrylonitrile (see Figure 30)^{98,416,432,433} is produced above 800 km by substitution of the CN radical onto ethylene:^202,203

215

Below 800 km, acrylonitrile may be produced by the following termolecular reaction chain:^98,103

216

217

Loss

In a similar manner to HCN, HC₃N, and other nitriles, C₂H₃CN is lost in the ionosphere by the two-step process of proton transfer, e.g.,^434,435

218

219

followed by dissociative electron recombination:⁴³⁶

220

221

In the lower atmosphere it may be lost to photodissociation (which tends to recycle acrylonitrile) or by H addition:⁹⁴

222

Future Directions

Several small nitrile molecules, which tend to exhibit polar properties, have been investigated in a theoretical study for the potential to self-organize into spherical vesicles or membranes in nonpolar liquids (e.g., Titan lakes and seas of methane–ethane–nitrogen). These calculations showed that acrylonitrile was the best candidate for forming so-called “azotosomes”,⁴³⁷ which, if experimentally confirmed, could be significant for astrobiology as vesicles (containers) for self-replicating organisms. However, at this time experimental verification of azotosomes is still lacking, while a later study has questioned the ability of these structures to form.⁴³⁸

Propionitrile

Propionitrile (C₂H₅CN) was the second molecule to be originally detected using submillimeter astronomy and the first molecule to be detected with ALMA.¹⁶⁷

Production

Propionitrile has been posited to be produced above 900 km¹⁰³ by the following association reactions (see Figure 31):^98,439

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224

Propionitrile production and loss pathways. Reactions numbered and shown in bold correspond to discussion in the text.

Another proposed route to propionoitrile formation is^82,440

225

In the middle atmosphere (400–900 km),¹⁰³ successive rounds of hydrogen addition to acrylonitrile via termolecular reactions can produce propionitrile:

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The following termolecular reaction can also occur:⁹⁸

228

Loss

As with other nitriles, the first step in loss of this nitrile in the ionosphere is proton transfer, forming C₂H₅CNH⁺:

229

This is followed by either dissociative electron recombination,⁴⁴¹

230

or ion–neutral reactions such as³²⁴

231

Future Directions

Propionitrile has been asserted to condense in pure crystalline form in Titan’s atmosphere⁴⁴² on the basis of an unexplained feature in Titan’s far-infrared spectrum. This has been questioned based on vapor pressure of the gaseous form,^443,444 although it is possible that a cocondensed ice containing C₂H₅CN along with other gases may replicate the unexplained “haystack” emission.⁴⁴⁵ Further work on spectroscopy will be required to determine if this is a unique solution or if other possibilities exist.

Cyanopropyne

Cyanopropyne (CH₃C₃N) was the fourth molecule to be discovered by ALMA spectroscopy of Titan at millimeter wavelengths,¹⁶⁸ following previous detection in the ISM.⁴⁴⁶

Production

Production pathways for cyanopropyne (see Figure 32) are more uncertain than for many other molecules due to the size and complexity of the molecule, allowing for more numerous reaction pathways, and multiple isomers of C₄H₃N. Pathways involving radicals include the following: (i) CN substitution onto propyne or butadiene:^447,448

232

233

(ii) C₂N attack on ethylene:⁴⁴⁹

234

(iii) C₂N attack on acetylene via a three-step process with three-body reactions:^98,449,450

235

236

237

Loss

Cyanopropyne is thought to be lost through either photolysis or protonation, e.g.,¹⁰³

238

followed by dissociative electron recombination:¹⁰³

239

240

Future Directions

C₄H₃N has at least three stable isomers that have been detected in space.^451,452 Besides the currently detected isomer, cyanopropyne (CH₃C₃N, also called butynenitrile or methylcyanoacetylene), there are also cyanoallene (CH₂C₂HCN)⁴⁵³ and propargyl cyanide (HC₃H₂CN),⁴⁵⁴ both being first detected in the Taurus Molecular Cloud (TMC-1) at radio wavelengths. These provide good targets for detection on Titan, and their measurement would help to constrain photochemical pathways and models. Further more exotic arrangements of the same atoms may also exist and remain to be detected.

Oxygen Compounds

The oxygen chemistry of Titan’s atmosphere is apparently straightforward, with few species involved (only CO, CO₂, and H₂O are presently observed; see Figure 33), but it has proven remarkably difficult to replicate in models. Early work showed difficulty in producing sufficient CO from an external flux of water (OH),^44,46 which was originally presumed to come from meteoritic and cometary materials. The discovery of the Enceladus plumes,^455−457 the connection to Saturn’s E-ring (or Enceladus torus), and the subsequent finding of both OH and O⁺ entering Titan’s upper atmosphere⁵⁷ apparently from Enceladus, provided an abundant and unambiguous source of oxygen. Subsequent work by Hörst et al.⁸¹ showed that the combination flux of O⁺ could finally explain the abundance of CO. In the most recent work, Vuitton et al.¹⁰³ showed that OH alone is sufficient to produce the CO via previously unrecognized reaction intermediates.

Oxygen-bearing molecules detected on Titan.

Water

Water was first detected in Titan’s atmosphere through infrared spectroscopy with ISO⁴⁵⁸ through detection of emission lines at 39.4 and 43.9 μm, which was subsequently confirmed with Cassini CIRS.^459,460

Production

Water (Figure 34)^461−463 is thought to mainly be derived by the recombination of OH infalling at the top of the atmosphere, primarily sourced from dissociated Enceladus water, with methane and its dissociation products:

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242

Loss

Water is lost to photolysis throughout the atmosphere, reforming hydroxyl (OH). A large fraction of this OH reacts with CH₃ to reform water (see reaction 241). However, OH participates in several other reactions. Above 1000 km, it reacts with N(⁴S) to form NO:^97,374

243

In the middle atmosphere it reacts with CO to form CO₂:⁹⁷

244

Water also reacts directly with excited-state nitrogen atoms above 900 km:^97,464

245

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247

Any remaining unreacted water is ultimately lost by condensation in the lower stratosphere.

Future Directions

Due to its low vapor pressure, water remains difficult to measure in Titan’s atmosphere. Currently there are large uncertainties in its vertical profile,^459,460 and its latitudinal distribution remains unknown. Further work to better constrain these distributions may help to elucidate the relative importance of meteoritic versus Enceladus sources.⁹⁷

Carbon Monoxide

Carbon monoxide (Figure 35)⁴⁶⁵ was first detected on Titan by near-IR spectroscopy, showing an absorbance of CO at 1.6 μm,⁴⁶⁶ and the detection was soon confirmed at radio wavelengths.³⁸² Estimates of its abundance fluctuated throughout the years following its discovery,⁴⁶⁷ and the fact that these measurements were often sensitive to different altitudes led to the suggestion that the vertical profile was nonuniform.^382,468 Subsequent measurements with high-sensitivity telescope arrays at Owens Valley and Mauna Kea, however, showed evidence for a uniform profile, converging on a mixing ratio of ∼50 ppb.^469,470

Recently, high-sensitivity observations with ALMA have narrowed the experimental error to a range of 50 ± 2 ppm,¹³³ making it the fourth most abundant species in Titan’s atmosphere after N₂, CH₄, and H₂. As a triple-bonded molecule, CO once produced is resistant to both photolysis and chemical reaction as well having no loss through condensation, and hence, all evidence available at present points to a uniform vertical profile.

Production

In the model of Vuitton et al.¹⁰³ CO is mostly produced via formation of formaldehyde and subsequent photolysis:^471−473

248

249

250

(Formaldehyde may also be created by the reaction of OH with ³CH₂, C₂H₄, etc.).

Alternatively, H₂CO may undergo a two-step process to form CO:^223,474

251

252

(also OH + ³CH₂, OH + C₂H₄, etc.). Note that formaldehyde has yet to be detected in Titan’s atmosphere.

Lesser routes to CO production may be through reaction of atomic oxygen (deposited to the top of the atmosphere from Enceladus) with methane fragments, e.g.,⁸¹

253

254

Loss

CO is primarily lost slowly through reaction with OH to form CO₂:⁴⁷⁵

255

CO₂ is in turn lost through condensation.

Future Directions

Laboratory chemistry simulations of Titan’s atmosphere have shown that CO may react with methane and nitrogen when sufficiently stimulated, forming amino acids and even nucleobases.⁴⁷⁶ This provides an exciting possibility of astrobiology that now requires remote and in situ measurements to confirm.

Carbon Dioxide

Carbon dioxide was one of seven new gas species detected Voyager IRIS^477,478 and subsequently by Cassini CIRS.^279,479 Unlike shorter-lived chemical chemical species (e.g., HC₃N, C₄H₂), CO₂ exhibits little variation with latitude in the lower stratosphere, lacking a polar enhancement.

Production

CO₂ is thought to be mostly produced from CO + OH, as shown in the previous section (reaction 255). See also Figure 36.^103,465,480

Loss

The principal loss pathway of CO₂ is through photolysis:^97,481−485

256

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CO₂ is also lost through condensation in the lower stratosphere.

Future Directions

CO₂ remains the most significant atmospheric oxygen repository after CO, but it is shorter-lived and more reactive. It exhibits surprisingly little spatial and seasonal variation in Titan’s atmosphere,^127,486 which requires further astronomical monitoring to confirm.

Growth of Large Particles

As hydrocarbon molecules grow to ever-larger sizes, they may take several forms: long chains, fused rings (polycyclic aromatic hydrocarbons (PAHs)), or rings connected by bonds (polyphenyls) (see Figure 37). Nitrogen incorporation is also likely, for example, in the form of polycyclic aromatic nitrogen heterocycles (PANHs). It is thought that eventually larger molecules clump together due to electrostatic forces to form fractal aggegrates,^{212,487−489} i.e., the Titan haze particles which form the well-known golden haze at visible wavelengths. These in turn become the nuclei for stratospheric hydrocarbon ice particles or tropospheric methane raindrops^114−117 and fall to the surface, where they form Titan’s dune fields.¹¹⁸

Cyclic or “ring” molecules. Naphthalene: two fused carbon rings. Quinoline: two fused carbon rings with one nitrogen substitution (blue). Anthracene: three fused carbon rings. Biphenyl: two bonded but not fused carbon rings. Image credit: individual PAH graphics from Wikimedia Commons.

The presence of PAHs on Titan has been studied as far back as the 1990s⁴⁹⁰ in laboratory experiments. However, despite these predictions, detection of specific PAHs has remained elusive. The closest we have come so far to identification of a unique PAH in Titan’s atmosphere was the sighting of a peak at m/z 178 in the CAPS spectrum by Waite et al.,⁶¹ along with another peak at twice the mass(m/z 356). These were tentatively identified as due to anthracene and its dimer (Figure 38), although nonaromatic structures could not be ruled out. Note that the dimer itself may have formed by wall reactions in the instrument, but this would not rule out the fact that anthracene had entered the instrument. Likewise, no polyphenyls or N-heterocycles have been uniquely identified.

Production

The formation of PAHs by addition to benzene rings has been a topic of debate for considerable time. Recently, Kaiser and Hansen³⁵⁴ categorized possible pathways into five principal mechanisms:

hydrogen abstraction–C₂H₂ addition (HACA)⁴⁹¹
hydrogen abstraction–vinylacetylene addition (HAVA)
phenyl addition–dehydrocyclization (PAC)
radical–radical reactions (RRR)
methylidene addition–cyclization aromatization (MACA)

Each of these offers potential pathways to larger molecules. In brief, the HAVA mechanism, in which vinylacetylene (C₄H₄) adds to aromatic rings (such as benzene) in a barrierless reaction, is thought to be the principal mechanism by which additional six-membered rings are added to existing rings at low temperatures, such as in planetary atmospheres. The other mechanisms offer alternate routes to addition of five- and six-sided rings, predominantly at high temperatures of thousands of kelvins (HAVA, PAC, RRR), although MACA may operate at low temperatures to form indene. The reader is directed to the perspective by Kaiser and Hansen³⁵⁴ for a full description, which is beyond the scope of this review.

No discussion of aerosol particle growth would be complete without mention of negative ions. The discovery of large negatively charged ions at high altitudes by Cassini’s CAPS instrument was one of the major surprises about Titan’s atmosphere early in the mission.^{60,61,68,69,492,493} Small negative ions may be formed by several processes, including dissociative electron attachment, e.g.,

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259

and radiative attachment to a radical species already formed through photochemistry:¹⁰³

260

261

262

263

Once H^– is produced, it leads to the creation of some larger negative ions through proton abstraction:¹⁰³

264

265

Successively larger aerosol particles are produced through a variety of ion–neutral and ion–ion reactions.^494−496 The largest charged particles tend to be predominantly negative ions, due to their higher mobility.

Loss

Since aromatic rings, once formed, are very stable, with many possibilities to disperse absorbed energy internally, their principal loss channels will be either (a) to form radicals and then larger molecules, (b) to agglomerate, or (c) to condense.

Future Directions

Much work is still required to further elucidate the growth of large particles, especially the relative importance of ion versus neutral chemistry at different altitudes. See also the later section on the topic of PAHs.

Gaps in Our Knowledge

In this section we consider where the gaps are in our current knowledge and understanding of Titan’s atmospheric composition and chemistry and how these gaps might be addressed in the near future through combinations of astronomy, laboratory, and theoretical work.

Aliphatic Species

Hydrocarbons

At present, C₄H₂ remains the only C₄ hydrocarbon species definitively detected in the neutral atmosphere, while no aliphatic C_n species with n ≥ 5 have been detected (benzene, c-C₆H₆, being detected as a ring).

Some example reactions producing C₄H_x species that are thought to occur include the following:^{79,300,497,498}

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Species with four or more carbon atoms show increased possibilities for structural isomerization, as illustrated in Figure 39. This results in both increased challenges for observational detection and new and interesting chemical possibilities such as branched-chain molecules.

Structural isomers of butene (C₄H₈) and butane (C₄H₁₀), showing that branched chains become possible when four or more carbon atoms are present. Not all possible isomers are shown—additional cyclic forms are also possible (e.g., cyclobutane, C₄H₈).

Upper limits for n-butane and i-butane (C₄H₁₀) have been calculated as 5 × 10^–7 and 4 × 10^–8, respectively, from CIRS spectra at 200–250 km, 30° N–30° S.⁴⁹⁹

Future Work

Astronomical detection of species such as C₄H_x, x = 4, 6, 8, 10 (i.e., butynes and butenes) and even C₅H_x would greatly help to improve constraints on photochemical models, which are currently lacking in data to model for this regime. Building our knowledge of aliphatic species such as C₄ and C₅ molecules may provide a better understanding of the pathways to benzene or other cyclic species.

In parallel, photochemical models that currently treat multiple isomers under single formulas such as C₄H₈ and C₄H₁₀ must continue to expand treatment of separate isomers. To date this has been sparse and primarily for a few specific cases for which isomeric data exist: especially HCN/HNC and CH₂CCH₂/CH₃CCH, which are considered separately in current models.^90,91,99,103 A good reason for lack of inclusion of separate isomers is lack of knowledge of branching ratios and isomer-specific reaction rates. Monte Carlo simulations have proved useful at showing which reaction rate uncertainties have the biggest effects on the uncertainties in the solution.^87,88 These studies thereby provide important prioritization of where resources such as laboratory time and theoretical effort (e.g., TST calculations) can best be spent to most rapidly improve our knowledge.

Nitrogen Species

All of the nine detected nitrogen compounds in Titan’s neutral atmosphere, formed from dissociation products of N₂ and CH₄, are triple-bonded. These include N₂ itself and eight known cyanides, which include nitrogen in a terminal −C≡N functional group.

Nitriles (cyanides) appear to be stable and plentiful in Titan’s atmosphere, and further examples are sure to be found. In 1985, in the wake of the Voyager IRIS discoveries, Cerceau et al.⁵⁰⁰ studied the infrared spectra of seven undetected nitriles to facilitate further new detections, and 35 years later, four of these seven species had been detected on Titan (CH₃CN, C₂H₃CN, C₂H₅CN, and CH₃C₃N), although ironically all these detections were made through submillimeter wave astronomy, not infrared spectroscopy.^{164,166−168} Upper limits for the three remaining undetected species plus one other were calculated by Coustenis et al.:⁵⁰¹

crotononitrile (CH₃(CH)₂CN)	2.5 × 10^–7
butanenitrile (CH₃(CH₂)₂CN)	5.0 × 10^–7
isobutyronitrile ((CH₃)₂CHCN)	2.0 × 10^–7
cyanocyclopropane (Δ−CN)	1.5 × 10^–7

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Nitrogen has yet to be detected with other types of bonding (i.e., where it is not in a triple bond), such as in amines, imines (beyond HNC), azines, and nitrogen heterocycles (see Figure 40). A simple example is ammonia (NH₃), which was tentatively inferred from Cassini’s mass spectra at high altitudes at mixing fractions of ∼3–4 × 10^–5 but has not yet been uniquely detected due to the barometric degeneracy problem of unit-resolution mass spectroscopy.^219,502

Examples of detected and nondetected nitrogen molecular families in Titan’s atmosphere.

The major channel for ammonia production is thought to be via NH₂ + H₂CN, as follows:¹⁰³

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Ammonia is mostly lost through photodissociation.

Upper limits for some molecules in the lower atmosphere have been estimated, including 1.3 ppb (3σ) for NH₃ at 107 km, 25° S⁵⁰³ and 0.35 ppb (3σ) for methanimine (CH₂NH) in the stratosphere.⁵⁰⁴

Future Work

No molecules having both oxygen and nitrogen (e.g., HNO) and more complex amino acids have been detected. Detection of such functional groups and molecules, which are important for biological activity on Earth, is a key area of future research. Several nitriles have been detected only through submillimeter astronomical techniques, as previously mentioned, but not using infrared techniques, despite several decades of attempts.^{49,500,501,503} In fact, laboratory line lists with intensities do not exist for many of the nitriles sought in the infrared, except CH₃CN,^505,506 hampering the search and implying a need for new laboratory work to obtain low-temperature spectra.

Oxygen Species

Few oxygen compounds have been detected in Titan’s atmosphere: only CO, CO₂, and H₂O to date. Diatomic molecular oxygen, O₂, which is readily found in the atmospheres of the inner planets, is absent, allowing organic chemistry to proceed to great complexity. The oxygen compounds detected appear to be attributable to an external source of oxygen (both O and OH), most likely originating from Enceladus.^57,81 Prior to the discovery of the Enceladus plumes, a meteoritic source was favored,⁵⁰⁷ and some meteoritic contribution may still be present.⁹⁷

Other than CO, which is present at a relatively high abundance (∼50 ppm¹³³), oxygen is a minor although potentially important ingredient of Titan’s atmosphere. This is because many molecules of biological importance require oxygen.⁵⁰⁸ As early as the 1980s it had already been demonstrated that hydrolysis of Titan tholins (H_xC_yN_z) added oxygen to form amino acids of biological relevance,⁵⁰⁹ which was subsequently confirmed in many similar experiments. More recently, Hörst et al.⁴⁷⁶ showed that even in the gas phase, amino acids may be synthesized in a Titan-like atmosphere when CO is added to mixtures of CH₄ and N₂ in radiofrequency discharge experiments.

Photochemical models⁹⁷ predict that trace amounts of molecules such as HNO, HNCO, H₂CO, and CH₃OH should be present in the atmosphere and perhaps detectable at high altitudes by observatories such as ALMA. To date, few published studies have attempted to directly identify further oxygen species. The Cassini INMS team published upper limits for methanol (CH₃OH) and acetaldehyde (CH₃CHO) of 30 and 10 ppb, respectively, in the ionosphere at 1100 km.²¹⁹ In the stratosphere, upper limits have been determined for methanol and formaldehyde of 6 and 2 ppb, respectively, at 107 km, 25° S⁵⁰³ from infrared spectroscopy with Cassini CIRS.

Future Work

Vibrational (IR) and/or rotational (submillimeter) line lists exist for most small oxygen compounds currently undetected in Titan’s atmosphere (see Figure 41), making astronomical searches viable.^510−512

Examples of detected and nondetected oxygen molecular families in Titan’s atmosphere.

Cyclic Molecules

Single Hydrocarbon Rings

Two small cyclic molecules have been definitively detected in Titan’s neutral atmosphere: cyclopropenylidene (c-C₃H₂)¹⁶⁵ and benzene (c-C₆H₆).⁴⁹ Other small cyclic molecules likely to exist include the saturated cycloalkanes—cyclopropane (c-C₃H₆), cyclobutane (c-C₄H₈), and larger—and possibly cycloalkenes such as cyclopropene (c-C₃H₄), cyclobutene (c-C₄H₆), cyclobutadiene (c-C₄H₄), and others. Substituted rings are also possible: cyanocyclopropane (c-C₃H₅CN).

In the ISM, several single-ring molecules have been detected. These include the small three-carbon rings cyclopropenylidene (c-C₃H₂)³⁰⁷ and its related radical c-C₃H⁵¹³ and a substituted species, ethynyl cyclopropenylidene (c-C₃HCCH).⁵¹⁴ The five-sided ring cyclopentadiene (c-C₅H₆) and the six-sided rings benzyne (o-C₆H₄) and benzene (c-C₆H₆) have also been detected.^349,514,515

Future Work

Work is needed on all fronts to advance our understanding of the formation pathways, stability, and prevalence of small cyclic molecules in Titan’s atmosphere (see Figure 42). These include astronomical observations, laboratory work, and photochemical modeling. In the 2030s, we anticipate direct in situ measurements of such molecules by the Dragonfly probe’s Dragonfly Mass Spectrometer (DraMS) instrument, which unlike Cassini INMS will have the ability to definitively identify molecular structure through a combination of tandem mass spectrometry (MS/MS) and gas chromatography–mass spectrometry (GCMS).⁵¹⁶

Examples of detected and nondetected monocyclic hydrocarbons in Titan’s atmosphere.

Multiring Hydrocarbons: PAHs and Polyphenyls

PAHs are multiring molecules composed of carbon and hydrogen that exhibit aromatic character, that is to say, they have delocalized π bonding electrons. Example include naphthalene (two fused six-membered rings), indene (one five-sided and one six-sided ring), anthracene and phenanthrene (three six-membered rings), and larger examples (see Figure 43). Benzene is not considered to be a PAH since it has only one ring.

Benzene and polycyclic aromatic hydrocarbon molecules (PAHs). Image credit: individual PAH graphics from Wikimedia Commons.

PAHs have long been suspected to exist in interstellar space⁵¹⁷ and have been implicated as culprits responsible for the so-called diffuse interstellar bands (DIBs)⁵¹⁸ (for a review, see ref (519)). To date, only a single nonfunctionalized PAH has been uniquely identified in space (indene, C₉H₈⁵¹⁴), although a greater number of CN-substituted single and double rings (cyano-PAHs) have been identified, assisted by their strong rotational lines due to the cyanide group—see the review by McCarthy and McGuire.⁵²⁰

Near-IR emission at 3.28 μm has also been seen in Titan’s dayside spectrum by Cassini VIMS.⁵²¹ In a model by López-Puertas et al.,⁵²² this emission was attributed to a combination of PAHs with 9–96 carbons (up to 11 fused rings) using laboratory cross sections as measured in the Ames PAH database,⁵²³ However, unique identification of individual PAHs was not possible.

Laboratory experimental work on haze formation by UV photolysis has shown that the inclusion of benzene in initial reagent mixtures along with N₂ and CH₄ leads to the formation of significantly larger molecules than when a N₂/CH₄ mixture is used.⁵²⁴ More recently, naphthalene has also been used as a starting reagent in lab tholin experiments⁵²⁵ showing similar results. It should be noted that on Titan both benzene and naphthalene would presumably first have to form from methane, so their use in lab experiments may be considered an acceleration of a natural process that could in principle start from a pure N₂/CH₄ mixture and arrive at the same result over long time periods.

Multiring organic molecules are not constrained to form as fused-ring PAHs such as naphthalene but may instead form as polyphenyls (see Figure 37). It has been argued that a significant amount of carbon rings in Titan’s atmosphere may be in the form of polyphenyls rather than fused rings.⁵²⁶ Titan’s aerosols are likely to be a mixture of fused and unfused rings, forming monomers and then fractal aggregates.⁵²⁷

Future Work

Future laboratory and eventually in situ experimental work is required to determine the relative importance of fused versus nonfused rings.

Fullerenes

Fullerenes are carbon allotropes formed of rings of five to seven atoms in closed or partially closed mesh structures. These can include buckminsterfullerenes (“buckyballs”), such as the spherical (C₆₀) and ellipsoidal (C₇₀) molecules,^528,529 and also sheets (graphene) and cylinders (carbon nanotubes) (see Figure 44). Despite their large size and apparent complexity, buckyballs have been detected in space both as neutrals⁵³⁰ and ions⁵³¹ and in meteorites.⁵³² Fullerenes have been hypothesized to exist in Titan’s atmosphere,⁵³³ although a recent attempt to detect them in Spitzer data proved unsuccessful.⁵³⁴

Future Work

Greater sensitivity with observatories such as the James Webb Space Telescope (JWST)⁵³⁵ may enable more sensitive searches for fullerenes in Titan’s atmosphere. In addition, little lab work has been done at present to determine what effect the presence of fullerenes could have on Titan’s atmospheric chemistry, aerosol formation, and surface geology.

PANHs

Nitrogen heterocycles and PANHs are similar to PAHs but with nitrogen incorporation into the ring structure (see Figure 45).¹⁶⁵ Nitrogen heterocycles have been sought unsuccessfully in interstellar space, with upper limits for molecules such as pyridine and quinoline derived.⁵³⁶ Mass spectroscopy of Titan’s atmosphere with Cassini INMS has identified peaks at masses that could correspond to N-heterocycles, such as C₅H₅NH⁺ (could be protonated pyridine) at mass 80 and C₄H₄N₂H⁺ (possibly protonated pyrimidine) at mass 81.²¹⁹ However, aliphatic variants are possible, making the PANH ion identification uncertain.

Recently, Nixon et al.¹⁶⁵ made the first astronomical search for pyridine and pyrimidine in Titan’s atmosphere using ALMA and derived upper limits on their disk-averaged (global) abundances: pyridine (c-C₅H₅N) at 1.15 ppb (2σ) above 300 km¹⁶⁵ and similarly pyrimidine (c-C₄H₄N₂) at 0.85 ppb (2σ) also above 300 km.

Laboratory work has examined how tholin (Titan haze analog) formation in UV photolysis experiments is affected by the inclusion of N-heterocycles such as pyridine and quinoline.^537,538 Changes in the spectrum show similarities to features in Titan’s haze spectrum,⁵³⁷ and the structures formed show a mixture of polymeric and random copolymeric structures.⁵²⁵

Future Work

In the future, more sensitive astronomical searches may be undertaken, for example using ALMA and JWST. Photochemical models at present do not include detailed descriptions of N-heterocycle formation and growth, which must be included in future generations to adequately model processes leading to haze formation.

Sulfur and Phosphorus Chemistry

To date, no compounds of sulfur or phosphorus have been found in Titan’s atmosphere. Nixon et al.⁵³⁹ made the first quantitative assessment of upper limits for the most simple reduced species, PH₃ and H₂S, of 1 and 300 ppb, respectively, in the stratosphere (∼250 km). Phosphine gas has even been mooted as a biosignature gas in planetary atmospheres,⁵⁴⁰ although this conclusion has been debated.⁵⁴¹ A sensitive search for CS was made by Teanby et al.⁵⁰⁴ with ALMA, yielding an upper limit of either 7.4 ppt (uniform profile above 100 km) or 25.6 ppt (uniform profile above 200 km).

The first (and so far only) photochemical model to include sulfur chemistry was published by Hickson et al.⁵⁴² and predicts that CS and H₂CS should be the most abundant sulfur-bearing species in the upper atmosphere, transitioning to C₃S, H₂S, and CH₃SH in the lower atmosphere. However, in the absence of constraints, predictions remain highly uncertain.

In principle, the O/S ratio should allow further constraint of the source of Titan’s oxygen flux, since the O/S ratio is predicted to be some 1000× less for a cometary source (O/S ∼ 100)⁵⁴³ than an Enceladus source (O/S ∼ 10⁵).⁵⁴⁴ Detection of sulfur in Titan’s atmosphere may also provide evidence for cryovolcanic activity.²⁵³

Both phosphorus and sulfur are among the six most essential elements for biochemistry on Earth (the so-called CHNOPS elements). With four of the six already detected on Titan, it is therefore of considerable interest to seek the remaining two, to further assess Titan’s potential for astrobiology. Recent reports that all six CHNOPS elements have now been detected in the plume material of Enceladus^544,545 make it feasible that some trace amounts of P and S arrive at the top of Titan’s atmosphere, as is the case apparently with O.⁵⁷

Future Work

Future work is required on both the direct detection of P- and S-containing substances by both astronomical and in situ techniques and also in laboratory work, computer photochemical modeling, and clarification of reaction pathways and rates, especially at low temperatures.

Radicals

For completeness, we include consideration of radical species—atoms and molecules with unpaired free electrons—even though these are highly reactive and unlikely to be found in significant numbers or significantly deep in Titan’s neutral atmosphere. Radicals include fragments of CH₄, such as CH (methylidene), CH₂ (methylene), and CH₃ (methyl), as well as ground-state and excited nitrogen atoms formed from breakup of N₂, such as as N(²D) and N(⁴S).

Many radicals (CN, OH, etc.) have been observed in the ISM,⁴⁵¹ including at least one cyclic radical (c-C₃H),⁵¹³ and also in comets⁵⁴⁶ and tenuous satellite exospheres.⁵⁴⁷ However, to date only the methyl radical has been detected by astronomical techniques in a planetary atmosphere.^548,549

Future Work

ALMA observations are particularly sensitive to Titan’s upper atmosphere and can sense HCN at altitudes of up to 1200 km.^128,159 Future investigations with ALMA may allow the detection of radicals with dipoles, and more sensitive infrared observatories such as JWST may prove effective at detecting radicals such as methyl in the infrared.⁵³⁵

Conclusions

The organic-rich atmosphere of Titan constitutes the most complex atmospheric chemical network known outside of Earth. This provides a unique natural laboratory for understanding the synthesis of organic compounds, processes that may have been important early in the history of the Solar System^550,551 and may have seeded the origins of life on Earth.^552,553

Therefore, it is of substantial scientific importance to better understand these processes and chemical results. This field of inquiry brings together astronomers, laboratory chemists, theoretical chemists, and atmospheric modelers whose combined approaches are needed to unravel the entire picture. Substantial progress has been made, especially stimulated by the recent wealth of data from the Cassini–Huygens mission^2,3 and the selection of the Dragonfly mission²⁷⁶ that will arrive in the 2030s.

Astronomy

Titan is a distant object, and gathering robust information is difficult in the absence of spacecraft: astronomy is currently the only means to gather data about Titan directly. Currently active ground- and space-based observatories such ALMA, IRTF, and JWST are providing continuity of data collection since the end of Cassini–Huygens regarding seasonal changes in Titan’s atmosphere^147,159 and new information on the chemistry, composition, and isotopic ratios.^{133,165,166,168,170,171,326,554,555}

Laboratory Studies

While data collection through astronomy and remote sensing continues, a robust campaign of laboratory experiments and theoretical work is continuing in parallel to understand the origins and chemical evolution of the atmosphere and interaction with the surface and subsurface. These diverse inquiries include spectroscopy of gases,^{342,499,556−564} measurement of reaction rates,^{243,348,355,359,410,565−569} and experimental work on cocrystals,^{277,278,298,418,570,571} ices,^572−583 and hazes (tholins).^{110,111,584−593}

Modeling

The chemistry of Titan’s atmosphere cannot proceed without disturbing the medium in which it takes place: the minor gases and haze generated have a substantial effect on the heating and cooling of the atmosphere,⁵⁹⁴ and in turn, changes to the thermal structure of the atmosphere lead to dynamical motions, including vertical eddy mixing and meridional transport. As gases are transported, they enter atmospheric regions that may have greater or lesser photon flux from the Sun, have fewer or greater opportunities to interact with electrons, and encounter differing densities of radicals and other reagents. Therefore, decoupling chemistry from dynamics is not possible. Combining the current generation of global circulation models (GCMs)^{155,595−599} and photochemical models^{99,100,102,103,600} to create 2D and 3D coupled chemical-GCMs is a challenging but important task that must occupy the next generation of modelers.

Future Missions

The Dragonfly mission, estimated to land on Titan in ∼2034, will provide a wealth of new data about Titan’s surface and atmospheric boundary layer. However, Dragonfly will only investigate the low latitudes, including dunes and the crater Selk. In the future, an orbiter, as envisaged by several published studies,^601−603 would provide the important benefit of a truly global picture, including potentially complete global mapping of the atmosphere and surface at uniform resolution, with time-domain information to search for changes occurring. Other elements, such as a balloon, airplane, and/or floating lake probe, could provide valuable in situ information about other environs.^604−606

There is no doubt that Titan still offers many challenges to our understanding that will provide fertile areas of study for future generations of scientists^21,607 and offer rewards through important insights into the chemical evolution of the Solar System and origins of life in the universe.

Acknowledgments

Funding for this work was through NASA’s Astrobiology Program. The author thanks the Astrobiology Program Manager, Dr. Mary Voytek (NASA HQ), and the Principal Investigator of the CAN-8 Project “Habitability of Hydrocarbon Worlds: Titan and Beyond”, Dr. Rosaly Lopes (JPL/Caltech), for their support of this work. Thanks is also due to Nicholas Lombardo and Alexander Thelen for providing text files of retrieved gas profiles from prior publications. The author sincerely thanks two anonymous reviewers and the guest editors of the ACS Earth Space Chem. special edition on astrochemistry, Martin Cordiner and Christopher Bennett, for their thoughtful comments and feedback that helped to improve the manuscript and the journal editor, Eric Herbst, and staff for their assistance in the review and publication process. Last but not least, the author is very grateful to a cadre of early-career students and postdocs who proofread parts of the final submitted version of the manuscript: Brandon Coy (University of Chicago), Nicholas Kutsop (Cornell University), Paige Leeseberg (University of Iowa/SURA), Siobhan Light (University of Maryland/SURA), Nicholas Lombardo (Yale University), Edward Molter (University of California Berkeley), Jonathon Nosowitz (Catholic University), and Alexander Thelen (Caltech). Any remaining errors or inaccuracies are the responsibility of the author.

Glossary

Acronyms and Abbreviations

ALMA: Atacama Large Millimeter/submillimeter Array
CAPS: Cassini Plasma Spectrometer
CDA: Cosmic Dust Analyzer (Cassini instrument)
CIRS: Composite Infrared Spectrometer (Cassini instrument)
DR: dissociative recombination
DSMC: direct simulation Monte Carlo
ESA: European Space Agency
GCM: global circulation model
GCMS: Gas Chromatograph/Mass Spectrometer (Huygens instrument)
HASI: Huygens Atmospheric Structure Instrument
INMS: Ion and Neutral Mass Spectrometer (Cassini instrument)
IRAM: Institut de Radioastronomie Millimetrique
IRIS: Infrared Interferometer Spectrometer (Voyager instrument)
IRTF: Infrared Telescope Facility
ISM: interstellar medium
ISO: Infrared Space Observatory
IUPAC: International Union of Pure and Applied Chemistry
JWST: James Webb Space Telescope
LTE: local thermodynamic equilibrium
NASA: National Aeronautics and Space Administration
PAH: polyaromatic hydrocarbon
PANH: polyaromatic nitrogen heterocycle
RPWS: Radio and Plasma Wave Spectrometer (Cassini instrument)
RSS: Radio Science Subsystem (Cassini instrument)
SNR: signal to noise ratio
TEXES: Texas Echelon Cross Echelle Spectrograph
TMC: Taurus Molecular Cloud
TST: transition state theory
UVIS: Ultraviolet Imaging Spectrometer (Cassini instrument)
VIMS: Visible and Infrared Mapping Spectrometer (Cassini instrument)
VMR: volume mixing ratio

The author declares no competing financial interest.

This paper was originally published ASAP on February 29, 2024. The TOC graphic and Figure 2 were revised, and the corrected version reposted on March 11, 2024.

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PERMALINK

The Composition and Chemistry of Titan’s Atmosphere

Conor A Nixon

Abstract

Introduction

Figure 1.

Figure 2.

Atmospheric Composition and Structure

Atmospheric Composition

Figure 3.

Atmospheric Temperature Structure

Figure 4.

Gas Vertical Profiles

Figure 5.

Atmospheric Photochemical Processes

Figure 6.

Photodissociation and Electron Impact Dissociation

Ionization

Ion Reactions

Radical Reactions

Other Reactions

Chemistry of the Neutral Atmosphere

Figure 7.

Note on Chemical Names

Hydrocarbons and Hydrogen

Figure 8.

Figure 9.

Hydrogen

Detection

Figure 10.

Production

Loss

Future Work

Methane

Profile

Figure 11.

Loss

Future Directions

Acetylene

Figure 12.

Production

Figure 13.

Loss

Future Directions

Ethylene

Production

Loss

Future Directions

Ethane

Production

Figure 14.

Loss

Future Directions

Cyclopropenylidene

Production

Figure 15.

Loss

Future Work

Propyne

Production

Figure 16.

Figure 18.

Loss

Future Work

Propadiene

Production

Figure 17.

Loss

Future Directions

Propene

Production

Loss

Future Directions

Propane

Production

Figure 19.

Loss

Future Directions

Diacetylene

Production