Cassini Imaging of Titan’s North Polar Cloud With the Visual and Infrared Mapping Spectrometer

Details Cassini Imaging of Titan’s North Polar Cloud With the Visual and Infrared Mapping Spectrometer Cassini Imaging of Titan’s North Polar Cloud With the Visual and Infrared Mapping Spectrometer

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p51g1195l&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P51G-1195

Physics

[0343] Atmospheric Composition And Structure / Planetary Atmospheres, [3311] Atmospheric Processes / Clouds And Aerosols, [6281] Planetary Sciences: Solar System Objects / Titan, [6297] Planetary Sciences: Solar System Objects / Instruments And Techniques

Scientific paper

We report on the Visual and Infrared Mapping Spectrometer (VIMS) observations of a giant cloud over the north pole of Titan. Griffith et al. [Science, 2006] described the first evidence of a north polar feature having the form of an ubiquitous bright band at 51° to 68°N in VIMS images acquired in December 2004, August and September 2005. Rodriguez et al. [Nature, 2009] systematically detect spectral signatures of clouds with VIMS at latitudes higher than 50°N between July 2004 and December 2007, with no attempt to show the structure of each individual cloud. The first good opportunity to observe the fully illuminated north pole, that we described here, occurred on December 28, 2006. The north cloud was then continuously monitored in various geometries during 3 years after its first discovery. This cloud shows much less signal at 5-µm than southern and tropical clouds which are thought to be composed of liquid/solid methane. This indicates a lower backscattering at 5-µm. It is consistent with clouds composed of micron-sized particles made of solid ethane. A radiative transfer model in spherical geometry (SPDISORT) shows that it is found at an altitude between 30 and 40 km. This observation confirms the IPSL Titan global circulation model of Rannou et al. [Science 2006] that predicted the formation of large polar ethane clouds due to the downwelling of atmospheric streams exactly at the same latitudes. The limits of the observed northern cloud between 50-60°N corresponds to the limit between filled and dry lakes as observed by the RADAR of Cassini. The cloud cover appears less widespread in the last observations, which could indicate that it is progressively vanishing. This is also in agreement with the predictions of the IPSL general circulation model as we approach the equinox in August 2009. Dedicated observations by the Cassini spacecraft during the extended mission possibly up to 2017 should allow the observation of the forthcoming seasonal circulation turnover, with possibly the complete disappearance of this type of cloud that caps the north pole.

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