Hot Spot On Titan

The T5 fly-by on 16 April 2005 at an altitude of 1,026 kilometres was the first 'low pass' of the tour, although for safety it was made a little higher than the originally planned 950 kilometres. In addition to offering improved data on any intrinsic magnetic field and the moon's interaction with its environment, this fly-by was to enable the Ion and Neutral Mass Spectrometer to gather more data to study the composition of the thermosphere and ionosphere and the thermal structure at various local times and latitudes in ambient magnetospheric conditions. It found a wide variety of complex hydrocarbons and carbon-nitrogen compounds (i.e. nitriles) with masses ranging up to as many as seven carbon atoms; methane, the simplest hydrocarbon, has one. The amount of ethane and octane was surprising, as it had been thought that in such a cold environment large molecules would sink soon after formation. The altitude selected for this fly-by marked a compromise designed to gather data to assist in determining the lowest safe altitude, and the data was fed into the Titan Atmosphere Model. The geometry provided an opportunity to re-examine a number of features on the Saturn-facing hemisphere, this time with coordinated observations in the visual and near-infrared. In particular, on the Tb and T3 fly-bys Cassini had seen a bright arcuate feature 550 kilometres wide at 85°W, 30°S. Owing to its shape and orientation, it was nicknamed 'the smile'. The International Astronomical Union later named it Hotei Arcus. On the T4 and T5 fly-bys the Visual and Infrared Mapping Spectrometer saw a bright feature in this vicinity in the 5-micron band. Combining the observations showed that the infrared feature was bounded to the south by the arc. Since this wavelength tends to represent thermal emission and this was the brightest point on the moon, it prompted speculation that this might be a 'hot spot' marking a recent impact or cryovolcanic eruption. Another possibility was a 'stationary' cloud of methane released by such a terrain feature, although its spectrum was different to other clouds the instrument had seen. But when microwave radiometry established there to be no temperature difference between the putative 'hot spot' and its surroundings, this ruled out both active cryovolcanism and a mountainous feature - in the latter case, due to the fact that in the troposphere temperature decreases with increasing altitude, and the summit of a mountain would be colder than the surrounding terrain.247 When a search

An image of Titan taken on 16 April 2005 in the wavelength range 1.7-5.0 microns revealed a '5-micron hot spot' (lower left). A full-disk image taken on 10 December 2004 showed a bright arcuate feature located southeast of Xanadu (upper left) that was nicknamed 'the smile'. A projection of the near-infrared (lower middle) and visual (lower right) views revealed that the 'hot spot' was contained within this arc. On the 16 April 2005 fly-by Cassini sampled the uppermost atmosphere and found a rich mix of complex hydrocarbons (upper right). Note also the two streaky clouds in the full-disk image at 38°S.

An image of Titan taken on 16 April 2005 in the wavelength range 1.7-5.0 microns revealed a '5-micron hot spot' (lower left). A full-disk image taken on 10 December 2004 showed a bright arcuate feature located southeast of Xanadu (upper left) that was nicknamed 'the smile'. A projection of the near-infrared (lower middle) and visual (lower right) views revealed that the 'hot spot' was contained within this arc. On the 16 April 2005 fly-by Cassini sampled the uppermost atmosphere and found a rich mix of complex hydrocarbons (upper right). Note also the two streaky clouds in the full-disk image at 38°S.

of the Keck Observatory archives showed that this feature had been present for at least 3 years, this argued against a stationary cloud. ''If this is a cloud,'' said J.W. Barnes of the University of Arizona, ''it would have to be a persistent ground fog.'' The 5-micron feature would therefore seem to be a surface marking of some as yet unidentified type; perhaps a contamination of the ice.248

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