Dr. Soderblom also announced that Voyager 2 images had detected another giant ring structure on Callisto, bringing the total to three, and there were probably more. This particular ring feature was estimated to be about 1500 kilometers across. It was also noted that although Callisto generally seemed to be saturated with shoulder-to-shoulder craters, the crater density near the ring structures seemed to be lower.
Saturday was a fairly quiet time for the scientists but not for the spacecraft or the spacecraft team. “We blocked out about 7½ hours,” explained Michael Devirian, Ground Data Systems Development, Integration, and Test Director, “in which we could send it a set of commands and re-send it if necessary to make sure all close-encounter commands were received by Voyager 2 until all the commands got through. The whole thing went perfectly the first time.” So everything was “go” for close encounter. The near encounter phase began at 6:36 p.m. PDT.
A new face of Ganymede was revealed to Voyager 2. This image was taken July 7 from a distance of 1.2 million kilometers and clearly shows the large dark area Regio Galileo, as well as much of the lighter grooved terrain discovered by Voyager 1. The bright spots are impact craters. This image also shows what appear to be polar caps, extending down to about latitude 45° in both the northern and southern hemispheres. [260-670]
Sunday, July 8.
(Range to Jupiter, 1.5 million kilometers). At 2:30 a.m. the first long-exposure sequence of ring pictures was taken, and at 3:00 a.m. the intensive period of the Callisto encounter began. Eighty high-resolution images were obtained of the satellite, centered around closest approach (215 000 kilometers) at 6:13 a.m. Incoming photos showed some features that looked like double-walled craters, but no more giant ring structures were seen. It appeared that there was an asymmetry in the distribution of large impact features over Callisto’s surface. “Callisto may turn out to be the most heavily cratered body in the solar system,” Torrence Johnson remarked. Garry Hunt was to add later on, “There’s just not room for another crater on that body—it’s totally full.”
At the press conference, Brad Smith confirmed the earlier finding that Io’s volcano Pele was quite dead—at least for now. Although P₄ had not yet been looked at, all other volcanoes discovered by Voyager 1 were still active, but no new plumes had been found. However, new ultraviolet images of P₂ (Loki) suggested that the eruption had increased in size. (In a later report, the imaging team announced that P₂ had increased in height to 175 kilometers and had changed to a two-column plume.) The new photographs of Jupiter’s ring showed it to be quite narrow and ribbonlike, Dr. Smith announced. The artist’s drawing (released during the Voyager 1 encounter), intended to show the outer edge of the ring, turned out to be a good representation of the actual ring, Dr. Smith said.
There seemed to be less high-speed sulfur and oxygen inside Jupiter’s magnetosphere than there had been during the Voyager 1 encounter, George Gloeckler announced. Voyager 2’s low energy charged particle instrument was finding substantial amounts of carbon, silicon, magnesium, and other elements of solar origin, but the Io-associated elements were almost depleted. The ultraviolet instrument had found as much glowing sulfur in Io’s torus as before, but less of it seemed to be raised to energies high enough to leave the torus and be detected elsewhere in the magnetosphere.
There were other indications of Jupiter’s changing weather. In a Voyager report Sunday evening, Garry Hunt remarked, “One very exciting observation came the other day which caused major excitement down in the imaging area. We actually saw a white cloud starting to intrude across a dark barge [large brownish oval-shaped feature in Jupiter’s northern hemisphere]. Atmospheric scientists get very excited by that because this is showing us how the colors layer themselves up—that white cloud is clearly above the dark brown. We’re desperately trying to understand the relationship of colors on Jupiter.”
