A few days before Cassini reached apoapsis at 59.3 planetary radii on 1 February it performed a main engine burn of 120.1 seconds for a delta-V of 18.68 metres per second to put it on course for revolution 3 of the nominal tour, with some revisions to the timings: on the T18-5 plan, the T3 fly-by of Titan was to have been at an altitude
A prediction of the areal coverage of the down-facing imager on the Huygens probe during its descent superimposed on an image of the intended landing site, together with (top) a contextual view taken on 26 October 2004.
of 1,194 kilometres, but when the tour was revised to overcome the Huygens relay issue this was lowered to 955 kilometres, then the change to prevent Iapetus from perturbing the probe had the side-effect of raising it to 1,577 kilometres. As the revised geometry ruled out the Earth occultation in which the attenuation of the spacecraft's radio signal was to have been used to profile the atmosphere in the near-equatorial zone - the only such opportunity on the primary tour - it was decided to reassign this observing time to the radar. The T3 fly-by on 15 February 2005 was to provide coordinated (although not simultaneous) optical and radar imaging observations. For 2 hours around the time of closest approach, the radar was to image several areas, including a part of Xanadu, this being the first location to be imaged by radar that was unambiguously bright in the near-infrared. The radar was also to take altimetry to determine whether the extreme flatness measured on the Ta fly-by was typical. The optical imaging provided full-disk mosaics, a 5-by-5-frame mosaic of the western part of Xanadu and the dark area where the Huygens probe landed, plus two areas in support of the radar. The radar imaging track ran essentially parallel to that from the Ta fly-by, but offset to the south. It included an 80-kilometre circular feature that was clearly an impact crater surrounded by a blanket of ejecta.234 However, without direct altimetry it was not possible to know whether it had been reduced to a palimpsest.235 In addition, there was a 440-kilometre-diameter feature that
16°W, 11 °N, that was clearly an impact crater surrounded by a blanket of ejecta. It was later named Sinlap.
could be a multiple-ringed basin that, in view of its size, was nicknamed Circus Maximus; it was later named Menrva by the International Astronomical Union. A radar-bright channel system on the adjacent terrain ran across its southwestern 'rim'. To the west were groups of dark linear features spaced 2-3 kilometres apart that ran for hundreds of kilometres. These 'cat scratches' gave the impression of being dunes of windblown material.
By this point, Cassini had provided one distant look at the south polar region and three close inspections of the equatorial region, with two radar imaging tracks which together covered less than 3 per cent of the surface. The first step in photogeology is to objectively classify surface units by their appearance. The radar imagery to-date indicated a variety of units, including: (1) a homogeneous unit that made up 50-60 per cent of the Ta swath and 20 per cent of the T3 swath; (2) the radar-bright Xanadu unit; (3) features unambiguously caused by impact; (4) sinuous features; (5) a radar-bright mottled unit with a gradational boundary; (6) a radar-bright lineated unit; and (7) a radar-bright unit that had lobate, sheet and digitate characteris-tics.236,237,238,239,240 The application of interpretations to surface units is subjective and therefore open to dispute. Nevertheless, it was clear that (as in the case of Earth) the surface of Titan had been shaped by tectonism, erosion, winds and (seemingly) volcanism - although in Titan's case, cryovolcanism.
Also on the T3 fly-by, the Composite Infrared Spectrometer made full-hemisphere thermal maps to study the circulation of the upper atmosphere. The stratopause was at an altitude of 310 kilometres, where the temperature was 186K. The stratosphere was coldest in the northern hemisphere, which was in continuous darkness, with the zonal winds reaching 160 metres per second. At mid-to-high northern latitudes the wind had inhibited mixing and produced an isolated vortex around the pole. Being cold, the stratosphere in this area was sinking, drawing down and concentrating the organics created in the upper haze.241 As regards the mid-latitude clouds seen in the southern hemisphere, where solar heating was greatest and the rising air flow of the pole-to-pole circulation system was located, C.A. Griffith, now at the University of Arizona at Tucson, suggested that the clouds occurred along a given line of latitude because they were the result of the global circulation system.242 This case was based on the fact that 40°S coincided with the abrupt cut-off in the layer of diffuse particles that surrounded the south polar region, and which might well cause a change in circulation at this latitude. A general circulation model by Pascal Rannou at the
University of Paris in France took into account the haze cap at the summer pole and indicated that the thick haze would inhibit surface heating at high latitudes and cause the updrafts and convergence driven by heating of the surface to occur at temperate latitudes.243 In effect, therefore, the mid-latitude clouds on Titan would correspond to the distinct bands of cloud that form near Earth's equator. On Titan, such updrafts would cause the formation of clouds, the tops of which would be drawn out for several thousand kilometres by the strong zonal wind. ''It is no coincidence that Titan's south polar cap of smog extends from the pole to 40°S; exactly where the methane cloud bands appear,'' argued Griffith. However, H.G. Roe proposed that the mid-latitude clouds were the result of cryovolcanism.244 Of 24 clouds observed over a 2-year period using telescopes fitted with adaptive optics, all were at low altitude (10 to 35 kilometres) and most were clustered near 350°W. A likely mechanism for geographic control was the episodic release of methane by the surface. One way to produce a cloud was to impart heat and induce convection, with the methane carried by the rising airflow being chilled and forming droplets. The other way was to increase the humidity by releasing methane into the atmosphere, which suggested the presence of a major site of cryovolcanic activity. As the clouds produced by this site were drawn out by the west-to-east wind, that latitudinal band would become humidified, making it easier for sites situated downwind to produce clouds. Roe ventured that this was ''the first strong evidence for currently active methane release from the surface''. As later observations by Cassini would reveal, there is a range of tall mountains in this vicinity, suggesting the clouds formed as the rising airflow condensed methane.
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