Keywords

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.

The dayside ionosphere of the Saturnian satellite Titan is generated mainly from photoionization of N₂ and CH₄. We compare model-derived suprathermal electron intensities with spectra measured by the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) in Titan's sunlit ionosphere (altitudes of 970–1250 km) focusing on the T40, T41, T42, and T48 Titan flybys by the Cassini spacecraft. The model accounts only for photoelectrons and associated secondary electrons, with a main input being the impinging solar EUV spectra as measured by the Thermosphere Ionosphere Mesosphere Energy and Dynamics/Solar EUV Experiment and extrapolated to Saturn. Associated electron-impact electron production rates have been derived from ambient number densities of N₂ and CH₄ (measured by the Ion Neutral Mass Spectrometer/Closed Source Neutral mode) and related energy-dependent electron-impact ionization cross sections. When integrating up to electron energies of 60 eV, covering the bulk of the photoelectrons, the model-based values exceed the observationally based values typically by factors of ∼3 ± 1. This finding is possibly related to current difficulties in accurately reproducing the observed electron number densities in Titan's dayside ionosphere. We compare the utilized dayside CAPS/ELS spectra with ones measured in Titan's nightside ionosphere during the T55–T59 flybys. The investigated nightside locations were associated with higher fluxes of high-energy (>100 eV) electrons than the dayside locations. As expected, for similar neutral number densities, electrons with energies <60 eV give a higher relative contribution to the total electron-impact ionization rates on the dayside (due to the contribution from photoelectrons) than on the nightside.

HCN is an important constituent in Titan's upper atmosphere, serving as the main coolant in the local energy budget. In this study, we derive the HCN abundance at the altitude range of 960–1400 km, combining the Ion-Neutral Mass Spectrometer data acquired during a large number of Cassini flybys with Titan. Typically, the HCN abundance declines modestly with increasing altitude and flattens to a near constant level above 1200 km. The data reveal a tendency for dayside depletion of HCN, which is clearly visible below 1000 km but weakens with increasing altitude. Despite the absence of convincing anti-correlation between HCN volume mixing ratio and neutral temperature, we argue that the variability in HCN abundance makes an important contribution to the large temperature variability observed in Titan's upper atmosphere.

Titan's thermospheric photochemistry is primarily driven by solar radiation. Similarly to other planetary atmospheres, such as Mars', Titan's atmospheric structure is also directly affected by variations in the solar extreme-UV/UV output in response to the 11-year-long solar cycle. Here, we investigate the influence of nitrogen on the vertical production, loss, and abundance profiles of hydrocarbons as a function of the solar cycle. Our results show that changes in the atmospheric nitrogen atomic density (primarily in its ground state N(⁴S)) as a result of photon flux variations have important implications for the production of several minor hydrocarbons. The solar minimum enhancement of CH₃, C₂H₆, and C₃H₈, despite the lower CH₄ photodissociation rates compared with solar maximum conditions, is explained by the role of N(⁴S). N(⁴S) indirectly controls the altitude of termolecular versus bimolecular chemical regimes through its relationship with CH₃. When in higher abundance during solar maximum at lower altitudes, N(⁴S) increases the importance of bimolecular CH₃ + N(⁴S) reactions producing HCN and H₂CN. The subsequent remarkable CH₃ loss and decrease in the CH₃ abundance at lower altitudes during solar maximum affects the overall hydrocarbon chemistry.

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 $\pm$ 1.8 ppm, assumed to be constant with altitude, with isotopic ratios ¹²C/¹³C = 89.9 $\pm$ 3.4, ¹⁶O/¹⁸O = 486 $\pm$ 22, and ¹⁶O/¹⁷O = 2917 $\pm$ 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.

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.

Titan's atmosphere is composed mainly of molecular nitrogen, methane being the principal trace gas. From the analysis of 8 solar occultations measured by the Extreme Ultraviolet channel of the Ultraviolet Imaging Spectrograph (UVIS) on board Cassini, we derived vertical profiles of N₂ in the range 1100–1600 km and vertical profiles of CH₄ in the range 850–1300 km. The correction of instrument effects and observational effects applied to the data are described. We present CH₄ mole fractions, and average temperatures for the upper atmosphere obtained from the N₂ profiles. The occultations correspond to different times and locations, and an analysis of variability of density and temperature is presented. The temperatures were analyzed as a function of geographical and temporal variables, without finding a clear correlation with any of them, although a trend of decreasing temperature toward the north pole was observed. The globally averaged temperature obtained is (150 ± 1) K. We compared our results from solar occultations with those derived from other UVIS observations, as well as studies performed with other instruments. The observational data we present confirm the atmospheric variability previously observed, add new information to the global picture of Titan's upper atmosphere composition, variability, and dynamics, and provide new constraints to photochemical models.

A condensate cloud on Titan identified by its 220 cm⁻¹ far-infrared signature continues to undergo seasonal changes at both the north and south poles. In the north, the cloud, which extends from 55 N to the pole, has been gradually decreasing in emission intensity since the beginning of the Cassini mission with a half-life of 3.8 years. The cloud in the south did not appear until 2012 but its intensity has increased rapidly, doubling every year. The shape of the cloud at the south pole is very different from that in the north. Mapping in 2013 December showed that the condensate emission was confined to a ring with a maximum at 80 S. The ring was centered 4° from Titan's pole. The pattern of emission from stratospheric trace gases like nitriles and complex hydrocarbons (mapped in 2014 January) was also offset by 4°, but had a central peak at the pole and a secondary maximum in a ring at about 70 S with a minimum at 80 S. The shape of the gas emission distribution can be explained by abundances that are high at the atmospheric pole and diminish toward the equator, combined with correspondingly increasing temperatures. We discuss possible causes for the condensate ring. The present rapid build up of the condensate cloud at the south pole is likely to transition to a gradual decline from 2015 to 2016.

Motivated by the recent detection of propene (C₃H₆) in the atmosphere of Titan, we use a one-dimensional Titan photochemical model with an updated eddy diffusion profile to systematically study the vertical profiles of the stable species in the C₃-hydrocarbon family. We find that the stratospheric volume mixing ratio of propene (C₃H₆) peaks at 150 km with a value of 5 × 10⁻⁹, which is in good agreement with recent observations by the Composite Infrared Spectrometer on the Cassini spacecraft. Another important species that is currently missing from the hydrocarbon family in Titan's stratosphere is allene (CH₂CCH₂), an isomer of methylacetylene (CH₃C₂H). We predict that its mixing ratio in the stratosphere is about 10⁻⁹, which is on the margin of the detection limit. CH₂CCH₂ and CH₃C₂H equilibrate at a constant ratio in the stratosphere by hydrogen-exchanging reactions. Thus, by precisely measuring the ratio of CH₂CCH₂ to CH₃C₂H, the abundance of atomic hydrogen in the atmosphere can be inferred. No direct yield for the production of cyclopropane (c-C₃H₆) is available. From the discharge experiments of Navarro-González & Ramírez, the abundance of cyclopropane is estimated to be 100 times less than that of C₃H₆.

The significant variations in both measured and modeled densities of minor species in Titan's atmosphere call for the evaluation of possible influencing factors in photochemical modeling. The effect of nitrogen photoabsorption cross section selection on the modeled vertical profiles of minor species is analyzed here, with particular focus on C₂H₆ and HCN. Our results show a clear impact of cross sections used on all neutral and ion species studied. Affected species include neutrals and ions that are not primary photochemical products, including species that do not even contain nitrogen. The results indicate that photochemical models that employ low-resolution cross sections may significantly miscalculate the vertical profiles of minor species. Such differences are expected to have important implications for Titan's overall atmospheric structure and chemistry.