Large brown ovals in the northern hemisphere of Jupiter are apparently regions in which an opening in the upper, ammonia clouds reveals darker regions below. This oval, about the same length as the diameter of the Earth, was at latitude 15°N. Features of this sort are not rare on Jupiter and have an average lifetime of one to two years. Above the feature is the pale orange north temperate belt, bounded on the south by the high-speed north temperate current, with winds of 120 meters per second. The range to Jupiter at the time this photograph was obtained on March 2 was 4 million kilometers, with the smallest resolvable features being 75 kilometers across. [P-21194C]
The best Voyager 1 photos of Europa were obtained on March 4 from a distance of about 2 million kilometers. This view of the hemisphere centered at about 300° longitude has a resolution of about 40 kilometers. Most of the surface is bland and highly reflective, being composed almost entirely of water-ice. No craters resulting from meteoric impacts can be seen. The most striking visual features, which set Europa off from the other satellites, are the dark streaks, as much as several thousand kilometers long, that cross the surface. Just barely visible to Voyager 1, these streaks were dramatically apparent to Voyager 2 in its closer flyby in July. [P-21208C]
Once again, tracking station problems interrupted the smooth flow of data from the spacecraft. Failure at Madrid to reestablish the correct receiver frequency after a spacecraft maneuver resulted in the loss of 53 minutes of irreplaceable data; an additional eleven minutes were lost an hour later. As the first problem was being corrected, the tracking antenna became misaligned, resulting in a noisy data return. Meanwhile, only a few of the watchers at JPL mourned the lost data, so exciting were the other new results that kept pouring in.
“For the highlights of this morning Dr. Soderblom will come up and show some beautiful satellite pictures,” Brad Smith announced at the daily press briefing. Larry Soderblom’s excitement was hard to contain. “Today is probably going to turn out to be one of the most memorable days in our exploration program. For the planetary geologists it’s truly Christmas Eve. We see tonight the beginning of the exploration of four new worlds. We’re racing through time and space at an incredible rate—the rate at which we are learning things is awe-inspiring in itself.” Callisto was still too far away to see well “but the things we’re seeing on the closer three satellites have really got us charged in anticipation.” Ganymede—“What can I say? Loops and swirls and incredible patches that are difficult now to hazard a guess about.” Io—an eerie-looking red, orange, and yellow world—“this one we’ve got all figured out. [Laughter from the press and applause.] It is covered with thin candy shells of anything from sulfates and sulfur and salts to all kinds of strange things.”
The low energy charged particle instrument had discovered high-speed sulfur in the outer Jovian magnetosphere, ten times as far from the planet as the sulfur torus around Io. The high-speed sulfur, which whizzed by Voyager 1 at 8000 kilometers per second, was first detected as the spacecraft crossed the magnetopause and entered the magnetosphere. Apparently this sulfur had picked up speed as it moved outward from Io’s torus, but the mechanism for this acceleration was not known. Roby Vogt reported that the cosmic ray instrument was detecting two distinct groups of atoms closer in to the planet. One group was apparently of solar composition (derived from the solar wind), and the other showed enhanced oxygen as well as sulfur. The plasma instrument had also begun to measure sulfur in the same form (S III) as that detected by the UVS in the Io torus. Where, the scientists asked, could the oxygen and sulfur be coming from? Could Io be the source of these atoms?
At one point, as questions from the press became too detailed, asking for “instant analyses” of the data, Dr. Smith was prompted to make the following statement: “I want to say something about what’s going on right now in the imaging area because we get a lot of pointed questions. We have a remarkably good system that is getting extremely good photographs. Not only are the cameras working well, but Jupiter and the satellites are cooperating by showing a lot of truly remarkable detail. And it may sound unprofessional, but a lot of the people up in the Imaging Team area are just standing around with their mouths hanging open watching the pictures come in, and you don’t like to tear yourself away to go and start looking at numbers on a printout. We will do that, but in the meantime we’re just caught up in the excitement of what’s going on.”
While the press briefing was underway, between 11:38 and 11:50 a.m., a special experiment was being tried on board the Voyager. As the spacecraft passed through the plane of the equator of Jupiter, it aimed its narrow-angle camera to a point in space halfway between the cloud tops and the orbit of Amalthea and took a single, eleven-minute exposure. The purpose: to search for a possible faint ring around Jupiter, which, if present, could best be seen by sighting along the plane of the equator. The image appeared briefly on the TV monitors, clearly showing something—a strange band of light—streaking across the center of the frame. Up in the imaging area, analyses began at once to determine if the strange band were really the sought-for ring, but it would not be until March 7 that the identification would be confirmed and the discovery announced.
The “bullseye” of Io was first photographed at a range of 2.8 million kilometers on March 3, and this color version was released on March 4. At the time, Imaging Team scientists wrote that “The large heart-shaped feature with a dark spot near its center could be Io’s equivalent of an impact basin such as Mare Orientale on the Moon. Its outer dimensions are about 800 by 1000 kilometers. Subsequent high-resolution coverage should reveal whether the small dark spots are impact craters, or perhaps something more exotic such as volcanoes. The reddish color of Io has been attributed to sulfur in the salts which are believed by some to make up the surface of Io.” It would be another week before this feature, later named “Pele” for the Hawaiian volcano goddess, would be recognized as an active, erupting volcano. [P-21187C]