The first close-up views of Europa were both exciting and perplexing to Voyager scientists. The best Voyager 1 resolution had been only about 30 kilometers, but the Voyager 2 trajectory permitted a much closer flyby. These pictures, taken on July 9 at a range of 240 000 kilometers, have a resolution of about 5 kilometers. The bright icy crust of Europa is covered with a spectacular series of dark streaks, giving the satellite a cracked appearance. In a few cases, narrower light lines run down the centers of the dark streaks, which are typically a few tens of kilometers in width. Very few, if any, impact craters are visible on Europa. [P-21760C and P-21764C]
Monday, July 9.
(Range to Jupiter at encounter, 722 000 kilometers). Encounter day! And not just one encounter, but a whole sequence: Ganymede, Europa, Amalthea, Jupiter, and Io. By early Sunday evening, a wealth of new data on Ganymede was pouring in. Not only was this a side of the satellite not seen before, but Voyager 2 would pass closer to Ganymede than had Voyager 1. Encounter took place at 1:06 a.m., at a range of 62 000 kilometers. Between 9:00 p.m. Sunday night and 1:30 a.m. Monday morning a total of 217 photos, plus infrared and ultraviolet spectra, were scheduled. Sixty-nine photos were sent back in real time; others were recorded for playback later.
As the Ganymede pictures appeared on the TV screens, they revealed a world of tremendous variety. Some regions were heavily cratered: “Ganymede looks like Mercury or the highlands of the moon,” one Voyager scientist remarked. Other parts of the surface, however, showed very different features: long, parallel mountain ridges that looked like grooves made with a giant’s rake; narrow, segmented lines; white ejecta blankets from impacts that looked like a dazzling, snow-covered landscape. Some of the pictures suggested cracking and slipping of Ganymede’s crust, while others showed what appeared to be remnants of ancient terrain unaffected by subsequent intense geologic activity. Many of the highest resolution frames were not seen at this time; they were recorded on the spacecraft for playback later.
Starting at about 8 a.m., Earth began receiving the first closeup views of Europa. Europa “could be the most exciting satellite in the whole Jovian system,” said Larry Soderblom, “because it’s sort of the transition body between the solid silicate body, Io, and the ice balls, Ganymede and Callisto.” The icy crust looked as though it “had been ruptured all over—as though it was in pieces—just as though it had been broken in place and left there.” At 11:43 a.m., closest approach took place at a range of 206 000 kilometers. By this time the scientists were dazzled by what they had seen; some were calling Europa the most bizarre of all the Galilean satellites. In the Imaging Team viewing area, David Morrison compared Europa’s surface to “a cracked egg,” and Gene Shoemaker said, “It looks like sea ice to me.” When someone commented that the canal-like streaks were reminiscent of Mars, Torrence Johnson replied, “It looks like some pictures of Mars I’ve seen, but only on the walls of Lowell Observatory.” Another quipped, “Where is Percival Lowell, now that we need him!”
There were to be two press conferences: one to present spacecraft and scientific results and one to celebrate the second successful flyby and to talk of new goals—Saturn, and perhaps Uranus.
At the first conference, Ed Stone began by discussing the radiation Voyager was experiencing. One of Jupiter’s surprises was that the radiation environment was greater than had been anticipated, and this caused problems with the radio receiver. The receiver frequency was shifting “more rapidly than we had anticipated,” said Ray Heacock, “and we have not been able to keep an uplink continuously with the spacecraft.” The solution was to keep sending up commands at different frequencies until a frequency the spacecraft would accept was found. Just how bad were the radiation levels? Ed Stone commented, “The penetrating radiation at a given distance is more intense now at this distance than it was when Voyager 1 flew by.” From a preliminary analysis it seemed that, overall, Voyager 2 would still be subjected to lower radiation levels than Voyager 1 had been, but to higher levels than had been expected. In addition, Voyager 2’s radio receiver was much more sensitive. The higher than expected radiation intensity also led Voyager scientists to have the ultraviolet spectrometer shut off, since that instrument was also quite sensitive to the radiation.
The fourth member of the Galilean satellites had finally been seen, and Larry Soderblom happily introduced Europa. “Well, some few months ago, before the Voyager 1 encounter, we thought we had some idea of what planets were like—at least the planets in the inner solar system: Mars, Mercury, the Moon, the Earth. And we’ve discovered many times over in the last couple of months how narrow our vision really was. Included in the Jovian collection of satellites are the oldest (Callisto), the youngest (Io), the darkest (Amalthea), the brightest (Europa), the reddest (Amalthea and Io), the whitest (Europa), the most active (Io), and the least active (Callisto). Today we found the flattest (Europa).”
In spite of the appearance of a cracked or broken surface, Europa showed no topography at all. Toward the sunset line, where the low angle of illumination should reveal even low relief, “the surface disappears—as if it were the surface of a billiard ball.” It seemed clear that Europa has much less relief than the other two icy satellites, Ganymede and Callisto. But why can’t Europa’s surface support relief? Perhaps Europa has a thick ice mantle—on the order of 100 kilometers. If Europa is affected by tidal heating, then such an ice mantle might be “sort of soft and slushy” rather than rigid as are the crusts of Ganymede and Callisto. “The fact that the surface of Europa cannot support relief of any substantial amount suggests that the surface must be soft.” But, Dr. Soderblom added, there does not appear to be much lateral motion or rotation causing the surface markings—they don’t seem to be offset—rather, “it’s as if Europa had been cracked, broken, by some process which crushed it like an eggshell and just left the pieces sitting there. Expansion and contraction of ice and water are a good way to crunch up the surface.”