Seasonal variations in Titan’s middle atmosphere during the northern spring derived from Cassini/CIRS observations - Archive ouverte HAL Access content directly
Journal Articles Icarus Year : 2015

Seasonal variations in Titan’s middle atmosphere during the northern spring derived from Cassini/CIRS observations

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Abstract

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 C2H2, C2H6 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.
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hal-01102741 , version 1 (13-01-2015)

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Sandrine Vinatier, Bruno Bézard, Sébastien Lebonnois, Nick A. Teanby, Richard K. Achterberg, et al.. Seasonal variations in Titan’s middle atmosphere during the northern spring derived from Cassini/CIRS observations. Icarus, 2015, 250, pp.95-115. ⟨10.1016/j.icarus.2014.11.019⟩. ⟨hal-01102741⟩
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