The revision of the orbital tour in order to recover from the Huygens relay problem gave a bonus in the form of a non-targeted encounter with Enceladus on 17 February 2005 at a range of 1,264 kilometres. The imagery of the southern part of the trailing hemisphere at a resolution of about 100 metres per pixel revealed the smooth terrain to bear faults, fractures, folds and troughs with vertical relief of about 1 kilometre. Tracking by the Deep Space Network suggested that the rock/ice ratio of the interior was higher than expected, increasing the estimated density from the 1.0 to 1.3 g/cm3 of the Voyager era to 1.6 g/cm3. The Visual and Infrared Mapping Spectrometer showed the surface to be pure water-ice. As R.N. Clark of the US Geological Survey in Denver, Colorado, put it, ''The spectra look like laboratory fabricated water-ice.'' Ammonium compounds and carbon dioxide had been expected, but if present were only in trace amounts. The Ultraviolet Imaging Spectrograph determined the texture of the surface by exploiting the fact that water-ice appears dark at wavelengths shortward of 160 nanometres but brightens in the 160-190-nanometre range; the wavelength of this transition being dependent on the grain size. The results indicated coarsely grained ice near the fresh fractures and ridges, suggestive of fresh 'snow'. The magnetospheric and plasma science instruments analysed the space around Enceladus to investigate the putative association between the moon and the 'E' ring, which extends from the orbit of Mimas (at 3 planetary radii) out beyond the orbit of Dione (at 8 planetary radii) and has its greatest density in the vicinity of Enceladus's orbit.
Revolution 4 began with apoapsis at 44.3 planetary radii on 27 February, but there were no encounters with Titan this time.
When Cassini flew within 500 kilometres of Enceladus on 9 March to make the first targeted fly-by of the primary tour, it made a discovery - the moon possessed a tenuous envelope. During both close fly-bys, the Dual-Technique Magnetometer had observed Saturn's magnetic field being 'bent' around the moon by electric currents generated by the interaction of the magnetosphere with neutral atoms of gas. In addition, when neutrals were ionised by plasma they were 'picked up' by the magnetic field and produced oscillations at frequencies that enabled them to be identified as O + , OH+ and H2O+ ions, indicating the presence of water vapour. The gravity of Enceladus is so weak that its escape velocity is a mere 212 metres per second. Because gas would readily leak away, there would be a net motion away from the moon - making the envelope an exosphere rather than an atmosphere. The fact that the envelope persisted for the interval between the two fly-bys suggested that it was being replenished. The source could be outgassing from the interior through fractures in the icy crust, geysers or cryovolcanism. When Voyager revealed Enceladus to have undergone extensive resurfacing, the evident youth of the smooth terrain prompted speculation that it might still be active - and the presence of a gaseous envelope was the first evidence that this might be so. The Magnetospheric Imaging Instrument also detected an interaction between Saturn's magnetosphere and Enceladus that was consistent with a gaseous envelope. The High-Rate Detector of the Dust Analyser had recorded thousands of 'hits' on each fly-by, but as Thanasis Economou, a team member from the University of Chicago, noted, ''It will take a few more fly-bys to
A mosaic of Enceladus taken on 17 February 2005 of the intensely fractured trailing hemisphere viewing obliquely from about 50°S towards the equator.
distinguish whether this flux is originating from the 'E' ring or a source on Enceladus.'' On 21 April it was decided to lower the July fly-by from the planned altitude of 1,000 kilometres to 175 kilometres, to improve the sensitivity of the magnetospheric and plasma science instruments and directly sample the gaseous envelope.
Inbound on 7 March 2005, Cassini snapped the wispy streaks on Dione's trailing hemisphere from a range of 1.6 million kilometres. At periapsis, north of the ring plane several hours after the Enceladus fly-by on 9 March, it passed within 82,800 kilometres of Tethys, allowing high-resolution imaging. Revolution 5 began with apoapsis at 44.4 planetary radii on 19 March. The T4 fly-by on 31 March was at an altitude of 2,404 kilometres, and provided the best optical view to date of the areas lying north of Titan's equator that had been imaged by the radar during the Ta and T3 fly-bys, in particular the region to the east of Xanadu. On 29 March, after reviewing plans for the fly-bys of Tethys and Hyperion in September, it was decided to reduce the altitude at Tethys from 32,000 to 1,500 kilometres and at Hyperion
from 1,000 to 500 kilometres. Although this would affect revolutions 14 through 17 and cost about 8 metres per second, it would greatly improve the Tethys data. On 29 March Cassini flew by Epimetheus at 62,000 kilometres. An image taken the following day from 75,000 kilometres gave the best view yet of this irregularly shaped moonlet, showing a large number of 'softened' craters. The Visual and Infrared Mapping Spectrometer indicated its surface to be predominantly water-ice. Epimetheus and its larger partner, Janus, regularly exchange orbits. Their low densities suggested that they were loosely consolidated accretions rather than chips off larger bodies.246 Revolution 6 began with apoapsis at 38.0 planetary radii on 6 April. At periapsis on 14 April Cassini made a long-range inspection of Pan, and the next day turned its attention to Mimas and Calypso.
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