Revolution 25 began with apoapsis at 68.3 planetary radii on 10 June 2006. On 1 July Cassini reached the half-way point in its 4-year primary tour. The T15 fly-by on 2 July was at an altitude of 1,906 kilometres. Revolution 26 began at 48.4 planetary radii on 12 July. Cassini was to begin the process of rotating the apoapsis of its orbit from down-Sun to up-Sun in order to switch from investigating the magnetotail to imaging Saturn in full sunlight. Rather than perform this rotation within the plane of the Saturnian system, it had been decided to do it by simultaneously shortening the orbit and using a series of high-latitude passes of Titan to increase the inclination of the orbit. The process would then be reversed by extending the apoapsis while returning to the plane of the system. This would give unprecedented opportunities to view Saturn and its magnetosphere, but it would be a slow evolution whose duration would depend on how closely Cassini was able to approach Titan. Despite concern regarding the minimum safe altitude, it had been decided to try a pass at 950 kilometres for the T16 fly-by on 22 July. If all went well, then by February 2007 a series of such encounters should have increased the inclination to the planned maximum of 59.8 degrees, and the process would be put into reverse. The initial fly-bys provided an opportunity to take high-resolution radar imagery of Titan's far northern latitudes. Just as the south polar region had been in continuous sunlight since Cassini's arrival in the system, the north polar region had been in continuous darkness. The T16 imaging track contained dozens of well-defined dark patches up to 1 kilometre in size, some that were tens of
Radar imagery of Titan taken on 30 April 2006 showed narrow sinuous radar-bright channels on the western part of the elevated Xanadu 'continent'.
When Cassini's radar imaged Titan's dark northern latitudes on 21 July 2006 it found the first convincing evidence of hydrocarbon lakes: centred at 92°W, 80°N (upper) and at 18°W, 78°N (lower). £
kilometres, and one that was approaching 100 kilometres. These were the darkest radar features yet seen, and dark usually signified either a smooth surface or a radar-absorbent material - and liquid hydrocarbon would be both. Furthermore, there were channels leading into or out from some patches with characteristics consistent with their having been created by flowing liquid.282 Evidently, Titan was, as J.I. Lunine had put it, ''a land of a thousand hydrocarbon lakes''.
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