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.

Recent simulations have indicated that vinyl cyanide is the best candidate molecule for the formation of cell membranes/vesicle structures in Titan’s hydrocarbon-rich lakes and seas. Although the existence of vinyl cyanide (C₂H₃CN) on Titan was previously inferred using Cassini mass spectrometry, a definitive detection has been lacking until now. We report the first spectroscopic detection of vinyl cyanide in Titan’s atmosphere, obtained using archival data from the Atacama Large Millimeter/submillimeter Array (ALMA), collected from February to May 2014. We detect the three strongest rotational lines of C₂H₃CN in the frequency range of 230 to 232 GHz, each with >4σ confidence. Radiative transfer modeling suggests that most of the C₂H₃CN emission originates at altitudes of ≳200 km, in agreement with recent photochemical models. The vertical column densities implied by our best-fitting models lie in the range of 3.7 × 10¹³ to 1.4 × 10¹⁴ cm⁻². The corresponding production rate of vinyl cyanide and its saturation mole fraction imply the availability of sufficient dissolved material to form ~10⁷ cell membranes/cm³ in Titan’s sea Ligeia Mare.

We present sub-millimeter spectra of HCN isotopologues on Titan, derived from publicly available ALMA flux calibration observations of Titan taken in early 2014. We report the detection of a new HCN isotopologue on Titan, H¹³C¹⁵N, and confirm an earlier report of detection of DCN. We model high signal-to-noise observations of HCN, H¹³CN, HC¹⁵N, DCN, and H¹³C¹⁵N to derive abundances and infer the following isotopic ratios: ¹²C/¹³C = 89.8 ± 2.8, ¹⁴N/¹⁵N = 72.3 ± 2.2, D/H = (2.5 ± 0.2) × 10⁻⁴, and HCN/H¹³C¹⁵N = 5800 ± 270 (1σ errors). The carbon and nitrogen ratios are consistent with and improve on the precision of previous results, confirming a factor of ∼2.3 elevation in ¹⁴N/¹⁵N in HCN compared to N₂ and a lack of fractionation in ¹²C/¹³C from the protosolar value. This is the first published measurement of D/H in a nitrile species on Titan, and we find evidence for a factor of ∼2 deuterium enrichment in hydrogen cyanide compared to methane. The isotopic ratios we derive may be used as constraints for future models to better understand the fractionation processes occurring in Titan’s atmosphere.

Meridional brightness temperatures were measured on the surface of Titan during the 2004–2014 portion of the Cassini mission by the Composite Infrared Spectrometer. Temperatures mapped from pole to pole during five two-year periods show a marked seasonal dependence. The surface temperature near the south pole over this time decreased by 2 K from 91.7 ± 0.3 to 89.7 ± 0.5 K while at the north pole the temperature increased by 1 K from 90.7 ± 0.5 to 91.5 ± 0.2 K. The latitude of maximum temperature moved from 19 S to 16 N, tracking the sub-solar latitude. As the latitude changed, the maximum temperature remained constant at 93.65 ± 0.15 K. In 2010 our temperatures repeated the north–south symmetry seen by Voyager one Titan year earlier in 1980. Early in the mission, temperatures at all latitudes had agreed with GCM predictions, but by 2014 temperatures in the north were lower than modeled by 1 K. The temperature rise in the north may be delayed by cooling of sea surfaces and moist ground brought on by seasonal methane precipitation and evaporation.

We report the first detection on Titan of the small cyclic molecule cyclopropenylidene (c-C₃H₂) from high-sensitivity spectroscopic observations made with the Atacama Large Millimeter/submillimeter Array. Multiple lines of cyclopropenylidene were detected in two separate data sets: ∼251 GHz in 2016 (Band 6) and ∼352 GHz in 2017 (Band 7). Modeling of these emissions indicates abundances of 0.50 ± 0.14 ppb (2016) and 0.28 ± 0.08 (2017) for a 350 km step model, which may either signify a decrease in abundance, or a mean value of 0.33 ± 0.07 ppb. Inferred column abundances are (3–5) × 10¹² cm⁻² in 2016 and (1–2) × 10¹² cm⁻² in 2017, similar to photochemical model predictions. Previously the C₃H ion has been measured in Titan’s ionosphere by Cassini’s Ion and Neutral Mass Spectrometer (INMS), but the neutral (unprotonated) species has not been detected until now, and aromatic versus aliphatic structure could not be determined by the INMS. Our work therefore represents the first unambiguous detection of cyclopropenylidene, the second known cyclic molecule in Titan’s atmosphere along with benzene (C₆H₆) and the first time this molecule has been detected in a planetary atmosphere. We also searched for the N-heterocycle molecules pyridine and pyrimidine finding nondetections in both cases, and determining 2σ upper limits of 1.15 ppb (c-C₅H₅N) and 0.85 ppb (c-C₄H₄N₂) for uniform abundances above 300 km. These new results on cyclic molecules provide fresh constraints on photochemical pathways in Titan’s atmosphere, and will require new modeling and experimental work to fully understand the implications for complex molecule formation.

We report interferometric observations of carbon monoxide (CO) and its isotopologues in Titan’s atmosphere using the Atacama Large Millimeter/submillimeter Array (ALMA). The following transitions were detected: CO (J = 1–0, 2–1, 3–2, 6–5), ¹³CO (J = 2–1, 3–2, 6–5), C¹⁸O (J = 2–1, 3–2), and C¹⁷O (J = 3–2). Molecular abundances and the vertical atmospheric temperature profile were derived by modeling the observed emission line profiles using NEMESIS, a line-by-line radiative transfer code. We present the first spectroscopic detection of ¹⁷O in the outer solar system with C¹⁷O detected at >8σ confidence. The abundance of CO was determined to be 49.6 ± 1.8 ppm, assumed to be constant with altitude, with isotopic ratios ¹²C/¹³C = 89.9 ± 3.4, ¹⁶O/¹⁸O = 486 ± 22, and ¹⁶O/¹⁷O = 2917 ± 359. The measurements of ¹²C/¹³C and ¹⁶O/¹⁸O ratios are the most precise values obtained in Titan’s atmospheric CO to date. Our results are in good agreement with previous studies and suggest no significant deviations from standard terrestrial isotopic ratios.