Channels On Titan

The T6 fly-by on 22 August 2005 at an altitude of 3,669 kilometres was the first close encounter since 16 April. This time the Composite Infrared Spectrometer was given priority from 11 hours prior to closest approach through to 19 hours afterwards. At closest approach, the first high-resolution imagery of the latitudes to the south of Xanadu was secured. On 30 June the Titan Atmospheric Model Working Group had discussed the results of the T5 fly-by at the compromise altitude of 1,026 kilometres, and on 6 July the Navigation Team discussed options for the T7 fly-by in September. In view of variations in the state of the atmosphere noted to date it was difficult to predict the density with any confidence, but when worst-case modelling established that the nominal altitude of 1,025 kilometres might not be safe the Working Group decided on 12 July to raise T7 by 50 kilometres. Revolution 14 began with apoapsis at 37.8 planetary radii on 28 August. However, a software fault during the T7 fly-by

A radar image of Titan taken on 7 September 2005 showing a 'contact' between the light and dark areas suggestive of a shoreline.

on 7 September denied the spacecraft one 'side' of its solid-state memory, and the other overflowed 12 minutes prior to closest approach and caused science activities to be curtailed 8 minutes into the radar imaging sequence. Nevertheless, the imagery that was returned was welcome because it showed the contact between a bright area and a very dark and evidently smooth terrain. The pattern of embayments, and the fact that channels on the bright area ended at the contact, suggested that this was a shoreline.264 While there was still no firm evidence of liquid anywhere on the surface, the radar imagery from the Ta and T3 encounters showed several types of channels. Although some were clearly drainage networks for rainfall runoff, others, including some in the T7 imagery, were long and deep, had few tributaries, and ran for great distances.

A deeply incised channel in a radar image taken on 7 September 2005.

Gary Parker, a geologist at the University of Illinois at Urbana-Champaign, had made a study of terrestrial rivers. He argued that if river morphodynamics were truly understood at the physical level, then relations that give reasonable results on Earth should describe how flowing liquid methane on Titan transports disaggregated icy sediment.265 Such variables are expressed as dimensionless relations developed from dimensioned parameters. "There are only three parameters that differ significantly on Earth and Titan," said Parker. "First is the acceleration due to gravity - on Titan this is about one-seventh the value on

Earth. Second is the viscosity of the flowing liquid - the viscosity of liquid methane on Titan is about one-fifth that of water on Earth. Third is the submerged specific gravity of the sediment - the value on Titan is about two-thirds that on Earth." Terrestrial rivers are characterised by dimensionless relations for the threshold of motion, the threshold of significant suspension, and the 'bank-full' hydraulic geometry. ''This means that for the same discharge of liquid methane as to water, the channel characteristics on Titan should be remarkably similar to those on Earth. However, because of the smaller acceleration due to gravity, channel slopes on Titan should be wider, deeper and less steep than those on Earth.'' But there was a caveat, ''The interaction of sunlight with a hydrocarbon-rich atmosphere could precipitate sticky compounds that would give streams on Titan a degree of cohesion that would make them behave differently." On the other hand, as noted earlier, it was likely that that haze particulates 'hardened' during their long, slow descent through the atmosphere.

Cruising

On 5 September 2005 Cassini flew within 52,000 kilometres of Pandora, the outer shepherd of the 'F' ring, showing it to be irregularly shaped, about 80 kilometres in size, with craters that were smoothed over by some loose material. Revolution 15 began with apoapsis at 41.5 planetary radii on 14 September. Approaching periapsis on 23 September, Cassini was able to image Calypso at a range of 91,000 kilometres with the moonlet, 24 kilometres in size, spanning about 35 pixels.

Pandora imaged on 5 September 2005 (left) and Calypso on 23 September 2005.

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