A polar projection of Voyager 2's route through the Jovian system.
Three months later, Voyager 2 made its approach. When it sensed the bow shock on 2 July 1979, it became evident that the solar wind had abated slightly, and the magnetosphere had re-inflated.20 The spacecraft's computer had been programmed to follow-up its predecessor's discoveries. This time, most of the satellites were encountered on the way in.21 First was Callisto on 8 July at 215,000 kilometres, then Ganymede on 9 July at 62,000 kilometres. Fortunately, the hemispheres which had previously been in darkness were now illuminated, so these two satellites were well documented. Callisto was found to be virtually saturated with craters, and there was no indication of any endogenic resurfacing activity. Callisto's ancient surface still bears the scars of its accretion.
Ganymede is not only the largest of the Jovian moons, it is the largest satellite in the Solar System. In fact, being somewhat larger than the planet Mercury, it is really a small planet that is orbiting around Jupiter. In 1849, W.R. Dawes in England made a study of Ganymede and concluded that its most prominent surface feature was a bright 'polar spot'. Further observations were made by E.E. Barnard, Percival Lowell and E.M. Antoniadi. In 1951 E.J. Reese combined their observations as a single map. B.F Lyot produced maps of all four of satellites, but these were not published until 1953, after his death. In terms of the distribution of albedo features, Lyot's Ganymede map was in fair agreement with Reese's. The maps published by Audouin Dollfus in 1961 were generally considered to be the best. Between them the Voyagers documented about 80 per cent of the surface, revealing two main types of terrain. The dark cratered terrain comprised several large circular features and a large number of small polygonal blocks. The largest feature, appropriately named Galileo Regio, was an oval that spanned one-third of the anti-Jovian hemisphere. While this had been seen by telescopic observers, in general the distribution of the
The sharp terminator in this Voyager 2 view of Europa as a crescent implied that its surface is exceedingly flat. This vantage point documented the 'fractured terrain' on the anti-Jovian hemisphere.
dark features was not well represented on any of the maps. The rest of the surface was a brighter 'sulci' terrain which, in places, seemed to have fractured the darker features in a way that implied that the moon had undergone significant internal thermal processing at some time in its history, resulting in the extrusion of icy fluids:
Next was a view of Europa's anti-Jovian hemisphere from 204,000 kilometres. The generally reflective surface was revealed to be darkly 'mottled' and criss-crossed by linear features which, although only a few kilometres wide, nevertheless extended for thousands of kilometres. The exceptionally 'sharp' terminator line meant that the range of elevation from pole to pole was only a few hundred metres. The discovery that Europa was as smooth as a cue-ball suggested that it had once been covered by an ocean that had frozen. The paucity of impact craters implied that this icy shell had formed 'recently'. Could there be a deep ocean of liquid water beneath the shell -one with more water than all the Earth's oceans combined? It was a remarkable prospect. For Voyager 2, Io was inconveniently located on the far side of its orbit, but the long-range imagery showed that most of its volcanoes were still active, and a new eruption was now underway.
Voyager 2's closest point of approach to Jupiter on 9 July was just outside the orbit of Europa. Upon crossing the equatorial plane the next day, it looked back to investigate the ring system and in so doing detected three new moonlets nearby. The brightest part of the ring system lay between 51,000 and 57,000 kilometres above the cloud tops. There was a tenuous 'halo' closer in that extended above and below the equatorial plane. Beyond was another tenuous zone which formed a thick disk, dubbed the 'gossamer' ring. In fact, the rings and moonlets are related. The moonlets Metis and Adrastea, whose orbits are only several thousand kilometres apart, define the outer boundary of the main part of the system. Amalthea orbits about half way out through the gossamer ring and Thebe defines its periphery, at a planetocentric range of 3.11 radii. The suggestion that the rings might be composed of dust blasted off the moonlets by the energetic impacts of micrometeoroids that had been drawn in and accelerated by Jupiter's mighty gravitational field was later confirmed by the Galileo spacecraft.
The particles and fields instruments found that each of the Galilean moons had a 'wake' extending as much as 200,000 kilometres ahead of it as the rapidly rotating inner magnetosphere swept past them. As a result of this 'magnetospheric wind', their trailing hemispheres are strongly irradiated with charged particles, an effect that might chemically alter the surface materials. Another dynamic derives from the fact that the axis of the planet's magnetic field is inclined at 9.5 degrees to its spin axis, so the moons in the equatorial plane endure cyclic polarity reversals as the magnetosphere rotates. At about 25 radii on the down-Sun side of the magnetosphere, the field lines switch from 'closed' to 'open', and the magnetosphere is drawn downstream by the solar wind. It was speculated that this magnetotail might be so lengthy as to extend beyond the orbit of Saturn.
In retrospect, the most significant insight gleaned from the Jovian fly-bys by the Voyagers was that despite being in a frozen realm far from the Sun, the objects that Galileo, the discoverer of the major satellites, had described as ''marvellous things'', were indeed marvellous in a way that we had not predicted, and each was a miniature world with its own highly distinctive geological history.22
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