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From 2004 to 2017, the Cassini spacecraft orbited Saturn, completing 127 close flybys of its largest moon, Titan. Cassini's Composite Infrared Spectrometer (CIRS), one of 12 instruments carried on board, profiled Titan in the thermal infrared (7–1000 μm) throughout the entire 13 yr mission. CIRS observed on both targeted encounters (flybys) and more distant opportunities, collecting 8.4 million spectra from 837 individual Titan observations over 3633 hr. Observations of multiple types were made throughout the mission, building up a vast mosaic picture of Titan's atmospheric state across spatial and temporal domains. This paper provides a guide to these observations, describing each type and chronicling its occurrences and global-seasonal coverage. The purpose is to provide a resource for future users of the CIRS data set, as well as those seeking to put existing CIRS publications into the overall context of the mission, and to facilitate future intercomparison of CIRS results with those of other Cassini instruments and ground-based observations.

We investigate the thermal equation of state, bulk modulus, thermal expansion coefficient, and heat capacity of MH-III (CH₄ filled-ice Ih), needed for the study of CH₄ transport and outgassing for the case of Titan and super-Titans. We employ density functional theory and ab initio molecular dynamics simulations in the generalized-gradient approximation with a van der Waals functional. We examine the temperature range 300–500 K and pressures between 2 and 7 GPa. We find that in this P-T range MH-III is less dense than liquid water. There is uncertainty in the normalized moment of inertia (MOI) of Titan; it is estimated to be in the range of 0.33–0.34. If Titan's MOI is 0.34, MH-III is not stable at present in Titan's interior, yielding an easier path for the outgassing of CH₄. However, for an MOI of 0.33, MH-III is thermodynamically stable at the bottom of an ice-rock internal layer capable of storing CH₄. For rock mass fractions $\lessapprox 0.2$ upwelling melt is likely hot enough to dissociate MH-III along its path. For super-Titans considering a mixture of MH-III and ice VII, melt is always positively buoyant if the H₂O:CH₄ mole fraction is >5.5. Our thermal evolution model shows that MH-III may be present today in Titan's core, confined to a thin (≈10 km) outer shell. We find that the heat capacity of MH-III is higher than measured values for pure water ice, larger than heat capacity often adopted for ice-rock mixtures with implications for internal heating.

The Cassini mission performed 127 targeted flybys of Titan during its 13 yr mission to Saturn, culminating in the Grand Finale between 2017 April and September. Here we demonstrate the use of the Atacama Large Millimeter/submillimeter Array (ALMA) to continue Cassini's legacy for chemical and climatological studies of Titan's atmosphere. Whole-hemisphere, interferometric spectral maps of HCN, HNC, HC₃N, CH₃CN, C₂H₃CN, C₂H₅CN, and C₃H₈ were obtained using ALMA in 2017 May at moderate (≈02, or ≈1300 km) spatial resolution, revealing the effects of seasonally variable chemistry and dynamics on the distribution of each species. The ALMA submillimeter observations of HCN and HC₃N are consistent with Cassini infrared data on these species, obtained in the same month. Chemical/dynamical lifetimes of a few years are inferred for C₂H₃CN and C₂H₅CN, in reasonably close agreement with the latest chemical models incorporating the sticking of C₂H₅CN to stratospheric aerosol particles. ALMA radial limb flux profiles provide column density information as a function of altitude, revealing maximum abundances in the thermosphere (above 600 km) for HCN, HNC, HC₃N, and C₂H₅CN. This constitutes the first detailed measurement of the spatial distribution of HNC, which is found to be confined predominantly to altitudes above 730 ± 60 km. The HNC emission map shows an east–west hemispheric asymmetry of 13% ± 3%. These results are consistent with very rapid production (and loss) of HNC in Titan's uppermost atmosphere, making this molecule an effective probe of short-timescale (diurnal) ionospheric processes.

The upper atmosphere of Titan is highly variable as characterized by the variations of the thermospheric and exospheric temperatures from in situ measurements by Cassini at different Titan encounters. A related question has to do with the escape flux of CH₄ that might also change with the complex plasma environment and ionospheric conditions. In this study, the atmospheric density profiles obtained by the INMS experiment on Cassini are examined in the context of a bi-Maxwellian approximation proposed by Jiang et al. The results are compared to the escape fluxes generated by magnetospheric and pickup ion sputtering and ionospheric processes. It is found that a CH₄ flux at a level of the order of 10²³–10²⁵ CH₄ s⁻¹ could be maintained during the quiet condition. But episodic events with the corresponding CH₄ escape rate reaching as large as 10²⁷ s⁻¹ might be possible. Such a time variability could be indicative of a global change of Titan's atmospheric temperature at relatively short timescales.

By the close of the Cassini mission in 2017 the Composite Infrared Spectrometer had recorded surface brightness temperatures on Titan for 13 yr (almost half a Titan year). We mapped temperatures in latitude from pole to pole in seven time segments from northern mid-winter to northern summer solstice. At the beginning of the mission the warmest temperatures were centered at 13 S where they peaked at 93.9 K. Temperatures fell off by about 4 K toward the north pole and 2 K toward the south pole. As the seasons progressed the warmest temperatures shifted northward, tracking the subsolar point, and at northern summer solstice were centered at 24 N. While moving north the peak temperature decreased by about 1 K, reaching 92.8 K at solstice. At solstice the fall-off toward the north and south poles were 1 K and 3 K, respectively. Thus the temperature range was the same 2 K at the two poles. Our observed surface temperatures agree with recent general circulation model results that take account of methane hydrology and imply that hemispherical differences in Titan's topography may play a role in the north–south asymmetry on Titan.

The Cassini Composite Infrared Spectrometer (CIRS) observed thermal emission in the far- and mid-infrared (from 10 to 1500 cm⁻¹), enabling spatiotemporal studies of ethane on Titan across the span of the Cassini mission from 2004 through 2017. Many previous measurements of ethane on Titan have relied on modeling the molecule's mid-infrared ν₁₂ band, centered on 822 cm⁻¹. Other bands of ethane at shorter and longer wavelengths were seen, but have not been modeled to measure ethane abundance. Spectral line lists of the far-infrared ν₄ torsional band at 289 cm⁻¹ and the mid-infrared ν₈ band centered at 1468 cm⁻¹ have recently been studied in the laboratory. We model CIRS observations of each of these bands (along with the ν₁₂ band) separately and compare the retrieved mixing ratios from each spectral region. Nadir observations of the ν₄ band probe the low stratosphere below 100 km. Our equatorial measurements at 289 cm⁻¹ show an abundance of (1.0 ± 0.4) × 10⁻⁵ at 88 km from 2007 to 2017. This mixing ratio is consistent with measurements at higher altitudes, in contrast to the depletion that many photochemical models predict. Measurements from the ν₁₂ and ν₈ bands are comparable to each other, with the ν₁₂ band probing an altitude range that extends deeper in the atmosphere. We suggest that future studies of planetary atmospheres may observe the ν₈ band, enabling shorter wavelength studies of ethane. There may also be an advantage to observing both the ethane ν₈ band and nearby methane ν₄ band in the same spectral window.

The Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) instrument on board Cassini revealed an unexpected abundance of negative ions above 950 km in Titan's ionosphere. In situ measurements indicated the presence of negatively charged particles with mass-over-charge ratios up to 13,800 u/q. At present, only a handful of anions have been characterized by photochemical models, consisting mainly of C_nH⁻ carbon chain and C_n−1N⁻ cyano compounds (n = 2–6); their formation occurring essentially through proton abstraction from their parent neutral molecules. However, numerous other species have yet to be detected and identified. Considering the efficient anion growth leading to compounds of thousands of u/q, it is necessary to better characterize the first light species. Here, we present new negative ion measurements with masses up to 200 u/q obtained in an N₂:CH₄ dusty plasma discharge reproducing analogous conditions to Titan's ionosphere. We perform a comparison with high-altitude CAPS-ELS measurements near the top of Titan's ionosphere from the T18 encounter. The main observed peaks are in agreement with the observations. However, a number of other species (e.g., CNN⁻, CHNN⁻) previously not considered suggests an abundance of N-bearing compounds, containing two or three nitrogen atoms, consistent with certain adjacent doubly bonded nitrogen atoms found in tholins. These results suggest that an N-rich incorporation into tholins may follow mechanisms including anion chemistry, further highlighting the important role of negative ions in Titan's aerosol growth.

The issue of CH₄ escape on Titan is still under debate, and a range of escape rates from 10²⁴ to 10²⁷ s⁻¹ has been reported in previous studies. One effective way of solving the CH₄ escape dilemma is to investigate the morphology of the CH₄ torus around Saturn, which varies with both the total CH₄ escape rate on Titan and the CH₄ energy distribution near its exobase. Such a torus is modeled via a test particle Monte Carlo approach in this study for a variety of CH₄ escaping scenarios characterized by different energy distributions near the exobase. The model calculations indicate that the extension of the CH₄ torus depends critically on the population of the high-energy tail of the CH₄ energy distribution. The model also predicts several distinctive cavities in CH₄ density related to mean motion resonances between Titan and the torus particles.

We calculate the illumination conditions at Titan's surface using the Monte Carlo radiative transfer model SRTC++, motivated by the proposed Dragonfly Titan lander. We find significant surface illumination during twilight after sunset, with the twilight flux maximized near $1.0\,\mu {\rm{m}}$ wavelength. Out to 30° past Titan's terminator, the twilight illumination exceeds that of Earth's Moon at full phase in visible red wavelengths ($0.65\,\mu {\rm{m}}$). Imaging at night should be quite effective for stationary surface landers if they use long integration times, though it would be less effective for platforms floating on Titan's seas. Titan sunsets should be underwhelming events at visible wavelengths, with the Sun fading out while still well above the horizon and overall illumination diminishing slowly as the Sun falls below the horizon. Shadows below the lander should receive illumination from diffusely scattered light low in the sky near Titan's horizon. The total near-horizon illumination maximizes when the Sun is highest in the sky owing to the intensity of multiple scattering.

We present a new computer program, SRTC++, to solve spatial problems associated with explorations of Saturn's moon Titan. The program implements a three-dimensional structure well-suited to addressing shortcomings arising from plane-parallel radiative transfer approaches. SRTC++'s design uses parallel processing in an object-oriented, compiled computer language (C++) leading to a flexible and fast architecture. We validate SRTC++ using analytical results, semianalytical radiative transfer expressions, and an existing Titan plane-parallel model. SRTC++ complements existing approaches, addressing spatial problems like near-limb and near-terminator geometries, non-Lambertian surface phase functions (including specular reflections), and surface albedo nonuniformity.