At the regular 11 a.m. press briefing, Brad Smith glowed. “We’re all recovering from what I would call the most exciting, the most fascinating, what may ultimately prove to be the most scientifically rewarding mission in the unmanned space program. The Io pictures this morning were truly spectacular and the atmosphere up in the imaging area was punctuated by whoops of joy or amazement or both.” The new color photo of Io taken the night before was released, showing strange surface features in tones of yellow, orange, and white. The image defied description; the Imaging Team used terms like “grotesque,” “diseased,” “gross,” “bizarre.” Smith introduced the picture with the comment, “Io looks better than a lot of pizzas I’ve seen.” Larry Soderblom added, “Well, you may recall [that we] told you yesterday that when we flew by we’d figure all this out. I hope you didn’t believe it.”
One thing was certain: There were no impact craters on Io. Unless the satellites of Jupiter had somehow been shielded from the meteoric impacts that cratered objects such as the Moon, Mars, and Mercury, the absence of craters must indicate the presence of erosion or of internal processes that destroy or cover up craters. Io did not look like a dead planet. Imaging Team member Hal Masursky, looking at the “pizza” picture, estimated that the surface of Io must be no more than 100 million years old—that is, some agent must have erased impact craters during the last 100 million years. This interpretation depended on how often cratering impacts occur on Io. No one could be sure that there had been any interplanetary debris in the Jovian system to impact the surfaces of the satellites. Perhaps none of them would be cratered. The forthcoming flybys of Ganymede and Callisto would soon provide this information.
The giant volcanic feature Pele, about 1000 kilometers across, mystified Voyager scientists when this picture of Io was taken on March 5 from a distance of about 400 000 kilometers. The brilliant colors, the strange shapes of the surface deposits, and the absence of impact craters all testified that Io was unlike any world previously encountered in the exploration of the solar system. [P-21226C]
As Voyager 1 approached Io, the images of the surface became more and more spectacular. On the morning of March 5, at a range of 130 000 kilometers, this picture was taken centered near longitude 320° and latitude 10°S. The width of the picture is about 1000 kilometers (the distance from the Mexican border to the northern edge of California). There are no impact craters, signifying a geologically young surface, and the dark center with radiating red flows indicates recent volcanic activity of some sort. [P-21277C]
The close flyby of Ganymede took place at 6:53 p.m., at a range of 115 000 kilometers. During the preceding four hours, photos revealed a surface covered with impact craters. Watching these photos and supplying commentary for the television listeners, David Morrison remarked, “While I’m delighted to see craters, it’s just the opposite of what I would have expected. I was telling everyone a few days ago that I thought Io would have plenty of craters and that Ganymede, because of the ice surface, simply would not be able to hold large craters over geological time. So this is fascinating and this is confusing—both what has happened on Io to erase the craters and why Ganymede’s surface is strong enough to preserve them.”
Just before closest approach, at 6:35 p.m., the ultraviolet instrument watched as a bright star passed behind Ganymede and reemerged ten minutes later. No dimming that could be attributed to an atmosphere was seen; when the data were analyzed later, scientists set an upper limit for any gas on this satellite at one-billionth of the atmospheric pressure at the surface of the Earth.
As encounter day drew to a close, celebrations took place all over JPL. For many, however, the excitement was tempered by exhaustion. After 48 hours of intense activity, sleep was imperative for some. But the close approach to Callisto was still to come, as was an examination of the data already received.
