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kilometre-per-pixel resolution. The theoreticians promptly set to work developing models for how this moon might have come to be so extensively resurfaced. Its similarity in size to Mimas suggested to some researchers that the cause had to be exogenic, perhaps tidal heating derived from the gravitational stresses of orbital resonances with other satellites, several of which also displayed features suggestive of activity, although to a lesser extent. However, E.M. Shoemaker countered that the resurfacing may have been driven by heat liberated by early massive impacts which melted large areas of the icy surface, transforming them into smooth plains on which crevasses formed as the water froze. Other researchers cited the paucity of craters as evidence that the resurfacing occurred 'recently'. What was certain, however, was that Enceladus did not fit the stereotype of an ancient icy moon. An hour and a half later, it was Dione's turn. The fly-by geometry provided a view of the Saturn-facing hemisphere from a minimum range of 162,000 kilometres. The wispy streaks on the trailing hemisphere were mostly beyond the limb, but there was a paucity of craters on the section of the associated dark patch that was visible. Much of the rest of the moon is heavily cratered but there are degraded irregular valleys suggestive of troughs, and relatively smooth plains on the leading hemisphere. The Rhea fly-by was three and a half hours after the closest approach to Saturn. For a sequence of frames for a mosaic of the moon's northern hemisphere, the computer rotated the spacecraft during the shuttering action in order to compensate for the high relative velocity of the 72,000-kilometre encounter that would otherwise have smeared the image.77 The imagery of the polar region, with a resolution of a few kilometres per pixel, showed a mixture of shallow craters having subdued shapes suggestive of great age and others whose sharp rims looked 'fresh'. Part of Rhea's surface is so saturated with craters that it is reminiscent of the surface of the planet Mercury.

At 21:00 Voyager 1 emerged from behind the 'A' ring. Forty-five minutes later, with its trajectory bent back by the slingshot, it re-crossed the ring plane just short of Rhea's orbit. Earlier in the year, when the ring system had been edge-on as viewed from Earth, William Baum at the Lowell Observatory utilised a CCD camera to establish that the exceedingly tenuous 'E' ring is a 90,000-kilometre-wide band that spans Enceladus's orbit.78 The fact that the brightest part of the ring lies close to the moon prompted speculation that the material is derived from it.79 The resurfaced area is so sparsely cratered that the process might still be active, and the ring might be ice crystals spewed into space from cryovolcanic geysers. As the spacecraft re-crossed the ring plane, an image looking 'back' showed these fine particles forward-scattering sunlight. The spacecraft was once again able to view the illuminated face of the ring system. Now, however, by virtue of its out-of-plane vantage point, its view of the system was more 'open' than before. A close examination of this imagery revealed that the system actually comprises many hundreds - perhaps even 1,000 - ringlets. Indeed, in the early count for a press conference R.J. Terrile stopped counting at 300 ringlets after he grew bored. This tremendous complexity could not be explained in terms of resonances with the moons orbiting beyond.

The ring system was rather different in appearance from beyond Saturn's orbit.

As Voyager 1 re-crossed the ring-plane just short of Rhea's orbit, it aimed its camera back and took this time-exposure which (for the first time) documented the 'E' ring forward-scattering sunlight. This broad but tenuous feature is centred on the orbit of Enceladus, and may well be composed of ice crystals from the moon. The streaks are stars trailed during the exposure.

As Voyager 1 re-crossed the ring-plane just short of Rhea's orbit, it aimed its camera back and took this time-exposure which (for the first time) documented the 'E' ring forward-scattering sunlight. This broad but tenuous feature is centred on the orbit of Enceladus, and may well be composed of ice crystals from the moon. The streaks are stars trailed during the exposure.

The spokes which had previously appeared dark on the bright 'B' ring were now brighter than the ring material. Clearly, if the spokes were particles suspended away from the ring plane by electrical forces, then the fact that they did not reflect sunlight efficiently meant that when they were viewed from up-Sun they were visible only by the shadows they cast on the ring, whereas because they forward-scattered sunlight efficiently they were directly visible from a down-Sun vantage point.80

About 13 hours after Saturn, Voyager 1 crossed Hyperion's orbit, but the moon was 877,000 kilometres away and was presenting its non-illuminated hemisphere. Nevertheless, it was obviously an irregularly shaped body. At mid-afternoon on 13 November, the spacecraft switched to its 'post-encounter' sequence. Over the next day, as it closed to within 2.5 million kilometres of Iapetus, it snapped pictures of the non-illuminated trailing hemisphere. Although this, the most enigmatic of moons, was not well placed for study, the geometry offered an oblique view over the north pole of a cluster of large craters situated on the intrinsically brighter terrain on the illuminated leading hemisphere.

As to Saturn itself, the data from the radio occultation as Voyager 1 crossed over the limb, and the remote sensing of its disk by the IRIS, gave a vertical profile of the semi-transparent outer envelope down to the 1 bar level, at which the temperature is 133K. By the tropopause, at the 0.07-bar level, the temperature had fallen to 93K.

Raumsonden Auf Saturn
Voyager 1 revealed Saturn's ring system to be incredible complex, with considerably more structure than suspected by even the most eagle-eyed of telescopic observers. Notice the ringlets within Cassini's Division and the arc of the thin 'F' ring beyond the cusp of the 'A' ring.

There is an inversion in the stratosphere due to methane absorbing sunlight, with the temperature increasing to 143K by the 0.001-bar level. Because Saturn's magnetic field is considerably weaker than Jupiter's, the solar wind is able to enter the polar cusps to induce auroral activity in the ionosphere.81 There is also strong ultraviolet emission in the thermosphere on the sunlit hemisphere. Orbiting farther from the Sun, Saturn's outer envelope is about 15°C colder than Jupiter's, so the ammonia-crystal layer spans a 100-kilometre range from just below the 1-bar level up to the tropopause. The cloud coverage is virtually continuous, but the infrared radiation is readily able to leak out if a short-lived hole opens.

This image of Saturn's rings was taken by Voyager 1 as it withdrew, from a range of 1.6 million kilometres. From this vantage point, the 'spokes' on the 'B' ring appear bright when forward-scattering sunlight. Notice the 'F' ring's clumpy strands, and that the planet is visible through the translucent rings.

Two Voyager 1 views of Iapetus. The early view (left) from 3.2 million kilometres is of the Saturn-facing hemisphere. The large circular feature is at longitude 330 degrees west. The illumination is actually face-on, and the dichotomy shows the boundary of the intrinsically dark Cassini Regio, on the leading hemisphere. The second image is looking over the north pole (which is just beyond and between the two large craters) onto the leading hemisphere and the winding dichotomy is a combination of Cassini Regio and the actual terminator. North is towards the top in each case.

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