2004–2017” (2019, ApJS, 244, 14) Conor A. Nixon , Todd M. Ansty, Nicholas A. Lombardo , Gordon L. Bjoraker , Richard K. Achterberg , Andrew M. Annex , Malena Rice , Paul N. Romani, Donald E. Jennings, Robert E. Samuelson, Carrie M. Anderson, Athena Coustenis , Bruno Bézard , Sandrine Vinatier , Emmanuel Lellouch , Regis Courtin, Nicholas A. Teanby , Valeria Cottini , and F. Michael Flasar 1 Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; conor.a.nixon@nasa.gov 2 Department of Space Science, Cornell University, Ithaca, NY 14853, USA 3 Center for Space Science and Technology, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD, USA 4 Department of Astronomy, University of Maryland College Park, College Park, MD, USA 5 Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA 6 Department of Astronomy, Yale University, New Haven, CT 06511, USA 7 Detector Systems Branch, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 8 Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 9 LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, F-92195 Meudon, France 10 School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Received 2021 January 16; published 2021 December 10

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 utilized aerosol extinction coefficient inferred from Cassini/CIRS spectra in the far and mid infrared region to derive the extinction cross-section near an altitude of 190 km at 15 degrees S (from far-IR) and 20 degrees S (from mid-IR). By comparing the extinction cross section that are derived from observations with theoretical calculations for a fractal aggregate of 3000 monomers, each having a radius of 0.05 pm, and a fractal dimension of 2, we are able to constrain the refractive index of Titan's aerosol between 70 and 1500 cm(-1) (143 and 6.7 mu m). As the real and imaginary parts of the refractive index are related by the Kramers-Kronig equation, we apply an iterative process to determine the optical constants in the thermal infrared. The resulting spectral dependence of the imaginary index displays several spectral signatures, some of which are also seen for some Titan's aerosol analogues (tholins) produced in laboratory experiments. We find that Titan's aerosols are less absorbent than tholins in the thermal infrared. The most prominent emission bands observed in the mid-infrared are due to C-H bending vibrations in methyl and methylene groups. It appears that Titan's aerosols predominantly display vibrations implying carbon and hydrogen atoms and perhaps marginally nitrogen. In the mid infrared, all the aerosol spectral signatures are observed at three additional latitudes (56 degrees S, 5 degrees N and 30 degrees N) and in the 193-274 km altitude range, which implies that Titan's aerosols exhibit the same chemical composition in all investigated latitude and altitude regions. (c) 2012 Elsevier Inc. All rights reserved.

Vertical distributions and spectral characteristics of Titan's photochemical aerosol and stratospheric ices are determined between 20 and 560 cm(-1) (500-18 mu m) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15 degrees N, 15 degrees S, and 58 degrees S. where accurate temperature profiles can be independently determined.In addition, estimates of aerosol and ice abundances at 62 degrees N relative to those at 15 degrees S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are similar to 3 times more abundant at 62 degrees N than at 15 degrees S. Generally, nitrile ice clouds (probably HCN and HC3N), as inferred from a composite emission feature at similar to 160 cm(-1), appear to be located over a narrow altitude range in the stratosphere centered at similar to 90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58 degrees S.There is some evidence of a second ice cloud layer at similar to 60 km altitude at 58 degrees S associated with an emission feature at similar to 80 cm(-1). We speculate that the identify of this cloud may be due to C2H6 ice, which in the vapor phase is the most abundant hydrocarbon (next to CH4) in the stratosphere of Titan.Unlike the highly restricted range of altitudes (50-100 km) associated with organic condensate clouds, Titan's photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15 degrees N and 58 degrees S latitude. The ratio of aerosol-to-gas scale heights range from 1.3-2.4 at about 160 km to 1.1-1.4 at 300 km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at similar to 140 cm-1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics.Finally, volume extinction coefficients N chi(E) derived from 15 degrees S CIRS data at a wavelength of lambda = 62.5 mu m are compared with those derived from the 10 degrees S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 mu m. This comparison yields volume extinction coefficient ratios N chi(E)(1.583 mu m)/N chi(E)(62.5 mu m) of roughly 70 and 20, respectively, for Titan's aerosol and stratospheric ices. The inferred particle cross-section ratios chi(E)(1.583 mu m)/chi(E)(62.5 mu m) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles. Published by Elsevier Inc.

We have analyzed the continuum emission of limb spectra acquired by the Cassini/CIRS infrared spectrometer in order to derive information on haze extinction in the 3–0.02mbar range (∼150–350km). We focused on the 600–1420cm−1 spectral range and studied nine different limb observations acquired during the Cassini nominal mission at 55°S, 20°S, 5°N, 30°N, 40°N, 45°N, 55°N, 70°N and 80°N. By means of an inversion algorithm solving the radiative transfer equation, we derived the vertical profiles of haze extinction coefficients from 17 spectral ranges of 20-cm−1 wide at each of the nine latitudes. At a given latitude, all extinction vertical profiles retrieved from various spectral intervals between 600 and 1120 cm−1 display similar vertical slopes implying similar spectral characteristics of the material at all altitudes. We calculated a mean vertical extinction profile for each latitude and derived the ratio of the haze scale height (Hhaze) to the pressure scale height (Hgas) as a function of altitude. We inferred Hhaze/Hgas values varying from 0.8 to 2.4. The aerosol scale height varies with altitude and also with latitude. Overall, the haze extinction does not show strong latitudinal variations but, at 1mbar, an increase by a factor of 1.5 is observed at the north pole compared to high southern latitudes. The vertical optical depths at 0.5 and 1.7mbar increase from 55°S to 5°N, remain constant between 5°N and 30°N and display little variation at higher latitudes, except the presence of a slight local maximum at 45°N. The spectral dependence of the haze vertical optical depth is uniform with latitude and displays three main spectral features centered at 630cm−1, 745cm−1 and 1390cm−1, the latter showing a wide tail extending down to ∼1000cm−1. From 600 to 750cm−1, the optical depth increases by a factor of 3 in contrast with the absorbance of laboratory tholins, which is generally constant. We derived the mass mixing ratio profiles of haze at the nine latitudes. Below the 0.4-mbar level all mass mixing ratio profiles increase with height. Above this pressure level, the profiles at 40°N, 45°N, 55°N, at the edge of the polar vortex, display a decrease-with-height whereas the other profiles increase. The global increase with height of the haze mass mixing ratio suggest a source at high altitudes and a sink at low altitudes. An enrichment of haze is observed at 0.1mbar around the equator, which could be due to a more efficient photochemistry because of the strongest insolation there or an accumulation of haze due to a balance between sedimentation and upward vertical drag.

We have analyzed data recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft during the Titan flybys T0–T10 (July 2004–January 2006). The spectra characterize various regions on Titan from 70° S to 70° N with a variety of emission angles. We study the molecular signatures observed in the mid-infrared CIRS detector arrays (FP3 and FP4, covering roughly the 600–1500 cm−1 spectral range with apodized resolutions of 2.54 or 0.53 cm−1). The composite spectrum shows several molecular signatures: hydrocarbons, nitriles and CO2. A firm detection of benzene (C6H6) is provided by CIRS at levels of about 3.5×10−9 around 70° N. We have used temperature profiles retrieved from the inversion of the emission observed in the methane ν4 band at 1304 cm−1 and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. No longitudinal variations were found for these gases. Little or no change is observed generally in their abundances from the south to the equator. On the other hand, meridional variations retrieved for these trace constituents from the equator to the North ranged from almost zero (no or very little meridional variations) for C2H2, C2H6, C3H8, C2H4 and CO2 to a significant enhancement at high northern (early winter) latitudes for HCN, HC3N, C4H2, C3H4 and C6H6. For the more important increases in the northern latitudes, the transition occurs roughly between 30 and 50 degrees north latitude, depending on the molecule. Note however that the very high-northern latitude results from tours TB–T10 bear large uncertainties due to few available data and problems with latitude smearing effects. The observed variations are consistent with some, but not all, of the predictions from dynamical-photochemical models. Constraints are set on the vertical distribution of C2H2, found to be compatible with 2-D equatorial predictions by global circulation models. The D/H ratio in the methane on Titan has been determined from the CH3D band at 1156 cm−1 and found to be 1.17−0.28+0.23×10−4. Implications of this deuterium enrichment, with respect to the protosolar abundance on the origin of Titan, are discussed. We compare our results with values retrieved by Voyager IRIS observations taken in 1980, as well as with more recent (1997) disk-averaged Infrared Space Observatory (ISO) results and with the latest Cassini–Huygens inferences from other instruments in an attempt to better comprehend the physical phenomena on Titan.

A strong, broad spectral emission feature at 85° N latitude centered at 221 cm−1 remains unidentified after candidate ices of H2O and pure crystalline CH3CH2CN are unambiguously ruled out. A much shallower weak emission feature starts at 160 cm−1 and blends into the strong feature at ∼190 cm−1. This feature is consistent with one formed by an HCN ice cloud composed of ⩽5 μm radius particles that resides in the lower stratosphere somewhere below an altitude of 160 km. Titan's stratospheric aerosol appears to have a spectral emission feature at about 148 cm−1. The aerosol abundance at 85° N is about a factor 2.2 greater than at 55° S.