More Enceladus Discoveries

Revolution 10 began with apoapsis at 40.7 planetary radii on 17 June 2005. There were non-targeted fly-bys of Titan on 22 June at a range of 920,703 kilometres and on 26 June at 874,627 kilometres. Revolution 11 began on 5 July. While there were no encounters with Titan this time, Cassini returned to Enceladus on 14 July, with its point of closest approach over the southern hemisphere providing the first view of the south pole, recently discovered to be marked by arcuate features dubbed 'tiger stripes'.255 During the fly-by, the Dust Analyser was to determine the composition of the particles in the vicinity of the moon; the Magnetospheric Imaging Instrument was to investigate how these particles interacted with the electrons and ions in Saturn's magnetosphere; the Ion and Neutral Mass Spectrometer was to sample the neutral atoms and positive ions in the immediate vicinity of the moon; and the Ultraviolet Imaging Spectrograph was to monitor an occultation of gamma Orionis (Bellatrix) on the approach in order to measure the density of the gaseous envelope along that line of sight. A similar observation in February had given no result, most likely because the star observed at that time, lambda Scorpii, was rather faint in the ultraviolet. The Visual and Infrared Mapping Spectrometer was to characterise any non-ice materials on the surface, and the Composite Infrared Spectrometer was to produce thermal maps of both the sunlit and darkened hemispheres. The geometry was specified prior to the discovery of the 'tiger stripes', but they were well observed. The imagery revealed that the south polar region had a complex history. Remarkably, there were icy boulders up to 100 metres in size littering an area that was notable for lacking the finely grained frost that was otherwise ubiquitous. In addition, this area contained the largest exposures of coarsely grained ice fractures to be found anywhere on the moon. Parallel ridges and valleys appeared to define a

Approaching Enceladus on 14 July 2005 Cassini imaged the anti-Saturn hemisphere centred at 42°S, showing the long 'tiger stripes' at the south pole (left). An image from an altitude of 208 kilometres taken just before closest approach, and smeared due to the relative motion, showed the south polar terrain to be highly fractured and strewn with blocks of ice between 10 and 100 metres in size.

Approaching Enceladus on 14 July 2005 Cassini imaged the anti-Saturn hemisphere centred at 42°S, showing the long 'tiger stripes' at the south pole (left). An image from an altitude of 208 kilometres taken just before closest approach, and smeared due to the relative motion, showed the south polar terrain to be highly fractured and strewn with blocks of ice between 10 and 100 metres in size.

boundary around the south polar terrain which, judging from the absence of impact craters, was extremely young.256,257

The existence of the gaseous envelope was confirmed. On this occasion, Cassini penetrated the electrically conducting region and the Dual-Technique Magnetometer found the envelope to be asymmetric.258 ''It's a strange atmosphere," said team leader M.K. Dougherty of the University of London. ''It seems to be concentrated at the south pole, and the best way to match our observations is with almost a cometary jet coming off the south pole.'' The Dust Analyser saw a peak in the number of fine particles coming off the surface when Cassini was at an altitude of 460 kilometres, about 1 minute prior to closest approach - which was 172 kilometres above the coordinates 23°S, 325°W. The Ion and Neutral Mass Spectrometer measured a large peak in the water vapour density about 35 seconds prior to closest approach, which corresponded to reaching the trajectory's most southerly latitude at an altitude of 270 kilometres. It was inferred from these results that Cassini had passed through the fringe of a cloud of vapour concentrated over the south pole. The results from the Plasma Spectrometer and the Ultraviolet Imaging Spectrograph were consistent with this interpretation. The variation of vapour density with altitude indicated that there was a localised source on the surface. The persistence of the cloud over an extended period in this weak gravity field indicated continuous replenishment. In fact, the process that issued water vapour and the process that ejected particulates were aspects of the same phenomenon, which was indeed similar to the venting of a comet. The Dust Analyser established that the 'E' ring comprised water-ice particles in the size range 0.3 to 2 micrometres.259,260 Terrestrial studies had shown that whereas the

As Cassini closed in on Enceladus on 14 July 2005 it took images to be assembled into a high-resolution mosaic, featuring the 'tiger stripes' at the south pole.

material in the 'F' and 'G' rings had a power-law distribution that was characteristic of debris from impacts, the material in the 'E' ring was of a uniform size.261,262 It had long been thought that Enceladus was responsible for the 'E' ring, and this was now seen to be the case. But where was the source of this material? The Composite Infrared Spectrometer provided the vital clue. Enceladus has such a high albedo that it reflects most of the incident sunlight, making its surface the coldest of the Saturnian satellites. The temperature at the subsolar point was expected never to exceed 80K. Even though the south pole was in continuous sunlight, the oblique angle of illumination suggested that it should be significantly colder. This fly-by gave the instrument its first view of the south pole, and it took data at wavelengths between 9 and 16.6 microns. The team were amazed to discover the south pole to be the warmest place on the moon. Although the average across the polar area was just 85K, in the 'tiger stripes' it was 145K.263 As J.R. Spencer, a member of the team at the Southwest Research Institute in Boulder, Colorado, pointed out, ''This is as astonishing as if we had flown by Earth and found Antarctica to be warmer than the Sahara.'' Evidently, the south polar region was being warmed by heat escaping from the interior. ''And so the plot thickens!'' observed C.C. Porco. ''The fact that Enceladus is so alive, and yet Mimas, the moon next door and roughly the same size, is so dead, is really testing our understanding of the internal workings of planetary satellites. But we're not complaining; we like it this way!''

Cruising

On 27 June 2005 Cassini imaged Tethys at a range of 500,000 kilometres, catching the giant crater Odysseus on the terminator. On 14 July it documented Rhea from a distance of 250,000 kilometres, and provided a new perspective on Epimetheus from 88,000 kilometres. Revolution 12 began with apoapsis at 40.8 planetary radii on 24 July, in the hiatus from 20 to 27 July for solar conjunction during which Cassini's activities were restricted to magnetospheric and plasma science observations and the data was stored for later replay. On 1 August it imaged the south polar region of Dione from 270,000 kilometres, Rhea at 255,000 kilometres, and caught Pan orbiting in the Encke Division. A non-targeted fly-by of Mimas on 2 August at 62,700 kilometres gave the best view of this moon, and showed there to be a landslide in Herschel crater. Non-targeted encounters with Titan occurred on 2 August at 939,574 kilometres and 6 August at 837,216 kilometres. Revolution 13 began at apoapsis at 40.7 planetary radii on 11 August, and the rotation of Hyperion was documented on 16-17 August from a range of 600,000 kilometres.

On the fly-by of Enceladus on 14 July 2005 the Composite Infrared Spectrometer revealed the south polar region to be anomalously warm.
Neptune Moon

Imagery of Mimas on 2 August 2005 gave the best view yet of this battered moon. A sequence on 16-17 August 2005 showed the chaotic rotation of Hyperion (top).

Epimetheus on 14 July 2005 (upper left); Tethys on 27 June 2005 featuring the giant crater Odysseus (upper middle); the moonlet Pan in the Encke Division on 1 August (upper right); and the south polar region of Dione on 1 August 2005 (lower pair).

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