At the press briefing, Brad Smith made a spectacular announcement: “This morning I would like to add yet another important discovery to be claimed by this outstanding mission—that of a thin flat ring of particles surrounding Jupiter. Thus Jupiter now joins Saturn and Uranus to become the third planet of our solar system known to possess a planetary ring system and leaves Neptune as the only member of the group of giant planets without a known ring. The discovery of the ring was unexpected in that the current theory which treats long-term stability of planetary rings would not predict the existence of such a ring around Jupiter. The single Voyager camera image which recorded the ring was planned by the Voyager Imaging Team several years ago, not really with any great expectation of a positive result, but more for the purpose of providing a degree of completeness to Voyager’s survey of the entire Jupiter system. The observation, as planned, involved looking off to the right of the limb of Jupiter in the planet’s equatorial plane at the exact moment that the spacecraft would be crossing the equatorial plane. The image actually was taken at 16 hours and 52 minutes before encounter from a distance of about 1.2 million kilometers. Exposure time was 11.2 minutes. We weren’t certain of the exact moment that we would cross the equatorial plane, so we planned to open our shutter and leave it open as we went through.”
Since the Voyager 1 ring photograph was taken exactly edge-on, it was not possible to determine the width of the ring. In this artist’s conception, the ring is drawn as if it were ribbon-like, with very little width, quite unlike the broad flat rings of Saturn. Voyager 2 later showed this to have been a lucky guess. [P-21259]
The image showed six exposures of the ring, together with streaked trails of background stars. Smith reported that the thickness of the ring was less than 30 kilometers, and that it extended to a point 128 000 kilometers from the center of Jupiter, or 57 000 kilometers above the clouds. In response to this discovery, scientists were eagerly planning to alter the Voyager 2 encounter sequence to try to obtain additional information on the ring.
The newspapers carried stories of the ring discovery on Thursday, March 8. One story was seen by two University of Hawaii astronomers, Eric Becklin and Gareth Wynn-Williams, who were observing at Mauna Kea Observatory, at an altitude of 4200 meters on the Big Island of Hawaii. Within two days they had succeeded in detecting the rings by their reflected sunlight, at an infrared wavelength of 2.2 micrometers, providing a rapid confirmation of the Voyager discovery.
As the plasma measurements from the Io flyby were analyzed, an additional clue to the origin of sulfur and oxygen was revealed. Herb Bridge reported the detection of sulfur dioxide (SO₂), the simplest molecule composed of these two atoms.
The high-resolution tape-recorded pictures of Io baffled imaging scientists. A number of features looked like volcanic flows; together with the absence of impact craters, these features indicated a geologically active planet. A central point of discussion was the recent theoretical work on Io by Stanton Peale of the University of California and two NASA scientists, Pat Cassen and Ray Reynolds. These authors had just published a paper in the March 2 issue of Science showing how tidal heating from Jupiter’s gravity could melt the interior of Io. They wrote that “widespread and recurrent surface volcanism might occur.” It began to look as if the prediction had been correct.
Thursday, March 8.
The last Voyager 1 press briefing was held. Each speaker was allotted only a few minutes, prompting Larry Soderblom to preface his remarks by trying to explain how difficult it was to describe four new planets—the Galilean satellites—in the time allowed. “Torrence [Johnson] was sitting with me last night, puzzling. He said, ‘You know, Larry, it’s sort of like imagining we’d flown into the solar system the day before yesterday, and said, “There’s a thing we’ll call Mercury, and there’s the Moon, and there’s Earth, and there’s Mars. Now let’s explain them in ten minutes”.’” There was Callisto, with the highest density of craters of any Galilean satellite—the oldest of the Galilean surfaces—featuring a huge “bullseye” that is “the largest single contiguous feature seen so far in the solar system.” There was Ganymede, cratered, but also overrun with fault lines that looked, according to one person, like “tire tracks in the desert,” showing a surface that “had laterally slid—faulted and sheared and sheared again—twisted and torn apart.” There was Io, the most bizarre—the one that scientists had thought would be most lunarlike—showing a surface that had apparently been “cooked and steamed and fumed out leaving deposits all over the surface much like you might see around a fumarole at Yellowstone National Park. The fact that these things exhibit such youth makes it likely that the planet is still volcanically active.” There was Europa, with huge linear features unlike those of the other three Galileans—Europa the mystery satellite, waiting for the probing eyes of Voyager 2 to survey it in early July.
Ed Stone summed it all up: “I think we have had almost a decade’s worth of discovery in this two-week period, and I think that all of the people who have been talking to you feel the same saturation of new information which has occurred. And in fact, we will probably be studying it in great detail for at least five years.”