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Provides information on Titan, the only moon in the Solar system to have a thick atmosphere. Observations on Titan when the Voyager I spacecraft approached Saturn in 1980; Implication of the radio occultation soundings on the mean molecular weight of Titan's atmosphere; Important component of Titan's atmosphere.

Highlights • We fit the spectrum propene in Titan's stratosphere for the first time. • Propene is present in Titan's stratosphere at 3–11 ppbv within 60° of the equator. • Equatorward of 60°, propene is most abundant within 20° of the equator. • Using new line data we propose a 3- σ upper-limit of for allene of 2.5 ppbv. Abstract Of the C 3 H x hydrocarbons, propane (C 3 H 8) and propyne (methylacetylene, CH 3 C 2 H) were first detected in Titan's atmosphere during the Voyager 1 flyby in 1980. Propene (propylene, C 3 H 6) was first detected in 2013 with data from the Composite InfraRed Spectrometer (CIRS) instrument on Cassini. We present the first measured abundance profiles of propene on Titan from radiative transfer modeling, and compare our measurements to predictions derived from several photochemical models. Near the equator, propene is observed to have a peak abundance of 10 ppbv at a pressure of 0.2 mbar. Several photochemical models predict the amount at this pressure to be in the range 0.3–1 ppbv and also show a local minimum near 0.2 mbar which we do not see in our measurements. We also see that propene follows a different latitudinal trend than the other C 3 molecules. While propane and propyne concentrate near the winter pole, transported via a global convective cell, propene is most abundant above the equator. We retrieve vertical abundances profiles between 125 km and 375 km for these gases for latitude averages between 60°S–20°S, 20°S–20°N, and 20°N–60°N over two time periods, 2004 through 2009 representing Titan's atmosphere before the 2009 equinox, and 2012 through 2015 representing time after the equinox. Additionally, using newly corrected line data, we determined an updated upper limit for allene (propadiene, CH 2 CCH 2 , the isomer of propyne). We claim a 3- σ upper limit mixing ratio of 2.5 × 10 − 9 within 30° of the equator. The measurements we present will further constrain photochemical models by refining reaction rates and the transport of these gases throughout Titan's atmosphere. [ABSTRACT FROM AUTHOR]

We analyzed spectra acquired at the limb of Titan in the 2006–2013 period by the Cassini/Composite Infrared Spectrometer (CIRS) in order to monitor the seasonal evolution of the thermal, gas composition and aerosol spatial distributions. We are primarily interested here in the seasonal changes after the northern spring equinox and interpret our results in term of global circulation seasonal changes. Data cover the 600–1500 cm −1 spectral range at a resolution of 0.5 or 15.5 cm −1 and probe the 150–500 km vertical range with a vertical resolution of about 30 km. Retrievals of the limb spectra acquired at 15.5 cm −1 resolution allowed us to derive eight global maps of temperature, aerosols and C 2 H 2 , C 2 H 6 and HCN molecular mixing ratios between July 2009 and May 2013. In order to have a better understanding of the global changes taking place after the northern spring equinox, we analyzed 0.5 cm −1 resolution limb spectra to infer the mixing ratio profiles of 10 molecules for some latitudes. These profiles are compared with CIRS observations performed during the northern winter. Our observations are compatible with the coexistence of two circulation cells upwelling at mid-latitudes and downwelling at both poles from at last January 2010 to at least June 2010. One year later, in June 2011, there are indications that the global circulation had reversed compared to the winter situation, with a single pole-to-pole cell upwelling at the north pole and downwelling at the south pole. Our observations show that in December 2011, this new pole-to-pole cell has settled with a downward velocity of 4.4 mm/s at 450 km above the south pole. Therefore, in about two years after the equinox, the global circulation observed during the northern winter has totally reversed, which is in agreement with the predictions of general circulation models. We observe a sudden unexpected temperature decrease above the south pole in February 2012, which is probably related to the strong enhancement of molecular gas in this region, acting as radiative coolers. In July and November 2012, we observe a detached haze layer located around 320–330 km, which is comparable to the altitude of the detached haze layer observed by the Cassini Imaging Science Subsystem (ISS) in the UV. [ABSTRACT FROM AUTHOR]

Abstract: Observations made by the Imaging Science Subsystem (ISS), Visible and Infrared Mapping Spectrometer (VIMS) and the long-wavelength Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft reveal that the large, long-lived cyclonic vortex at Saturn''s south pole has a 4200-km-diameter cloud-free nearly circular region. This region has a 4 K warm core extending from the troposphere into the stratosphere, concentric cloud walls extending 20–70 km above the internal clouds, and numerous external clouds whose anticyclonic vorticity suggests a convective origin. The rotation speeds of the vortex reach . The Saturn polar vortex has features in common with terrestrial hurricanes and with the Venus polar vortex. Neptune and other giant planets may also have strong polar vortices. [Copyright &y& Elsevier]