Torrence Johnson of the Imaging Team commented on a photo of Io in which features resembling circular crater-type structures seemed to be visible. “Whatever they are, [those circular features] are approaching the size of the things that we would call basins if they were impact structures on other planets. We don’t really know whether they’re impact structures. They have some characteristics that look reminiscent of impact structures. They could be endogenic—volcanic in origin—or internally generated in some other way.” Johnson also showed another photograph of Io, this one taken against black sky, showing a “strikingly different face” looking, perhaps, like someone’s nightmare, glaring back at the intruder from Earth. One huge feature—a “bullseye” or “hoof print” on Io—appeared to be approximately 1000 kilometers long. No one had ever seen such a feature, and the imaging scientists could only speculate about its significance.
Small-scale structures in the jet streams of Jupiter’s north tropical zone reveal details of atmospheric circulation. The small dark oval near the right edge of the zone may offer a glimpse deep into Jupiter’s atmosphere. Between the regularly spaced dark ovals near the bottom of the frame are more small-scale features that are being studied for their roles in Jovian atmospheric activity. The blue-gray regions along the shear line between the equatorial zone and the north equatorial belt also appear to be windows into the deeper regions of the atmosphere. This photo was taken February 19 by Voyager 1 from a distance of 14 million kilometers. [P-21160C]
A few days earlier, someone had posted in the Imaging Team area a quote from a 1975 review paper on the Jovian satellites by David Morrison and Joseph Burns. The section on Io began, “Io is one of the most intriguing objects in the solar system.” This statement seemed more and more appropriate as Voyager images improved. Johnson referred to this day as equivalent to the “late 1960s” in our study of the Jovian satellites. “We can see much more clearly than ever before, but still not clearly enough to provide understanding of what we are seeing.”
Sunday, March 4.
(Range to Jupiter, 1.2 million kilometers). At 4:37 a.m., the near encounter phase began: Voyager 1 was almost there! In the press room, someone taped a fortune cookie message to the Voyager TV monitor: “There is a prospect of a thrilling time ahead for you.”
Pulled by the powerful gravity of Jupiter, the spacecraft was now on a curved path through the inner Jovian system. At 2 p.m. PST, it crossed the orbit of Ganymede, and later in the afternoon it passed within less than 2 million kilometers of Europa, providing Voyager 1’s closest look at this satellite. During the afternoon and evening, a number of views were obtained of Amalthea, the small inner satellite, at a range of less than 500 000 kilometers. At 8 p.m. the orbit of Europa was crossed, and increasing attention was drawn to the coming encounter with Io. At about 7 p.m. a full-frame color sequence of Io was received with a resolution of 16 kilometers. During the night, as Imaging Team members scratched their heads trying to prepare a press release caption to interpret the peculiar structures seen, the JPL Image Processing Lab rushed to prepare a color version for release the next day.
The Great Red Spot became more and more spectacular as Voyager 1 approached, with each day revealing new and intricate detail in the clouds. This view was obtained on March 1 at a distance of 5 million kilometers; the smallest features that can be made out are about 100 kilometers across. To the west of the Great Red Spot is a region of great turbulence, and to the south is one of the three white ovals. [P-21182C]
