On 2 July 2004, some 30 hours into the capture orbit, designated revolution '0', Cassini passed Titan at a range of 339,120 kilometres. Although non-targeted, this fly-by, referred to as T0, provided a welcome 'first look' prior to the close encounter scheduled for October. At closest approach, the frame of the narrow angle camera was tighter than the moon's disk, and because the spacecraft was south of the plane of the Saturnian system it offered a view of the illuminated south polar region. The atmospheric haze proved to be so efficient at scattering light that it degraded the resolution from the ideal 2 kilometres per pixel to a more modest 10 kilometres per pixel. At an impromptu press conference the next morning, D.L. Matson of JPL delightedly announced, ''Although the images appear bland and hard to interpret, we ri
Views taken over an interval of several hours on 2 July 2004 showing the evolution of clouds in Titan's south polar region.
have seen Titan's surface with unprecedented clarity.'' E.P. Turtle of the University of Arizona at Tucson noted, ''There are linear features, circular features, curvilinear features - this suggests geological activity, but we don't know how to interpret them yet.'' Cassini documented the motions of methane clouds over a period of 8 hours. Images taken by different filters indicated that the clouds tops were at an altitude of 30 kilometres, in the upper troposphere. All but one of the clouds were located near the 80°S latitude line; the exception being in the temperate zone at latitude 38°S.
Although the Visual and Infrared Mapping Spectrometer took a spectrum for each pixel, the atmosphere had to be 'subtracted' before the composition of the surface could be studied, and this analysis was impaired by the fact that so few characteristic absorption features fell within the narrow spectral windows at which the atmosphere was transparent. Reporting the preliminary study, K.H. Baines, a team member at JPL, said, ''We see dark areas of relatively pure ice, and brighter areas with a much higher amount of non-ice materials - such as simple hydrocarbons. This is different from what we expected.'' In fact, it was precisely the opposite of what had been expected, since it suggested that the dark areas were not hydrocarbon lakes. But further analysis cast doubt on this interpretation, prompting team leader R.H. Brown of the University of Arizona at Tucson to retract the claim that the dark areas were water-ice, and to suggest instead that the bright and dark areas had similar bulk compositions and were simply mantled by different materials. One surprise was that Titan glowed both day and night, powered by the emissions from atmospheric methane and carbon monoxide. A solar-driven fluorescence by methane in the upper atmosphere had been expected, but the night-time glow was a surprise.
The most important observations from the T0 encounter provided temperature and composition data required to validate the model of Titan's atmosphere used to plan the entry of the Huygens probe. On 14 July JPL hosted the Huygens Probe Mission Risk Review, with the objective of assessing the risks and risk-mitigation measures for the probe's mission.
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