(The following exciting narrative of the discovery of Io's volcanoes by the JPL Voyager team is taken from "Voyage To Jupiter," David Morrison and Jane Samz, NASA SP-439, U. S. Government Printing Office, 1980, Washington, D.C., p. 86.)



Thursday, March 8. The last Voyager I 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 bizzare‹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."

Over the next day or so the press packed up and went home. The TV monitors showed the spacecraft's parting glance at a crescent Jupiter, only hinting at the vastness of space Voyager I would travel until its encounter with Saturn in the autumn of 1980. Maybe now there would be a relative calm that would allow the scientists to begin analyzing that "decade's worth of data." But things were not to be calm just yet.

Fire and Brimstone

At about 5 a.m. on March 8, Voyager had taken a historic picture. Looking back at a crescent Io from a distance of 4.5 million kilometers, the camera had been used to obtain a long-exposure view for the spacecraft navigation team‹one that showed the satellite against the field of background stars. During the day, Linda Morabito, an optical navigation engineer, began to work with this picture on her computer-controlled image display. She noted what appeared to be a crescent cloud, extending beyond the edge of Io. But Io has no atmosphere, so a cloud rising hundreds of kilometers above the surface did not seem to make sense.

The next day, working with her colleagues, Morabito eliminated all possibilities for the new feature on Io except the obvious‹a cloud. If it were a cloud, it must be the result of an ongoing volcanic eruption of incredible violence. The picture was shown to members of the imaging team, who agreed with the identification. But it was Friday and Brad Smith and Larry Soderblom, along with most of the other team members, had left for the weekend to try to get some rest. The picture would have to wait two more days.

Meanwhile, new information about Jupiter was released to the public. A long-exposure (three minutes and twelve seconds) image of the dark side of the planet, taken with the wideangle camera while in the shadow of the planet, caught Jupiter showing off some Jovian "fireworks." A long, broad, white streak across the picture was a visible aurora, the largest aurora ever seen‹almost 29 000 kilometers long. In addition, nineteen smaller bright splotches, looking insignificant by comparison, were in reality "superbolts" of lightning. Since huge electrical discharges such as lightning can, under the right circumstances, power chemical reactions that form complex organic molecules, the discovery of lightning on Jupiter could have profound implications. Was "lightning inspired" organic synthesis going on in Jupiter's atmosphere? No one knew.

Returning to JPL on Sunday night, Brad Smith got his first look at the Morabito picture of the volcanic cloud. Early Monday morning, other Imaging Team scientists saw it. As soon as the JPL computers were operating, Joseph Veverka and Robert Strom began working with the two interactive TV terminals to look for evidence in other pictures of ongoing eruptions. Faint clouds or plumes would not show up in normally processed pictures, but could be brought out easily with the computer-controlled displays. By midmorning, several additional volcanic plumes had been found.

Meanwhile, on March 11, John Pearl ot the IRIS team had independently drawn the conclusion that volcanic activity must be taking place on Io. He and Rudy Hanel found evidence of strongly enhanced thermal emission from parts of the satellite. The most prominent was a source nearly 200° C hotter than its surroundings. On March 12, Pearl brought his new results to the imaging team, and sure enough the hot spot was located near one of the volcanic plumes! A month later, continuing analysis of IRIS spectra yielded identification of sulfur dioxide (SO2) gas over this same erupting volcano. At last a source had been located for the enigmatic sulfur and oxygen ions in the magnetosphere.

The volcanoes provided a thread with which to weave together the disparate data on Io. A few months earlier there had been a report of a sudden brightening of Io in the infrared; now it seemed plausible that thermal emission from an eruption was the source. The Voyager ultraviolet experimenters had been worrying over the source of the intense sulfur emissions they had seen and had been disturbed by the changes in the gas clouds around Io since the Pioneer 10 and 11 flybys; now a variable source for these gas clouds was identified. In addition, the craterfree surface and bizarre features seen in the Voyager images could be recognized as the product of violent explosive eruptions on Io. It appeared that Pearl, Cassen, and Reynolds had found, in their theoretical calculations, the key to the most geologically active body ever encountered in the solar system.

News of the discovery was released to the press on Monday, March 12. During the next few days, a total of eight gigantic eruptions were located in the Voyager pictures of Io. Within a few weeks, scientists all over the world were thinking with renewed energy about this incredible satellite.

With four new planet-sized satellites now photographed, there was a sudden requirement for maps and for names to be assigned to the newly discovered features. The maps were produced from Voyager images at the Astrogeology Branch of the U.S. Geological Survey at Flagstaff, Arizona. The names, proposed by a group of scientists headed by Voyager imaging team members, Tobias Owen and Hal Masursky, were given official approval by the International Astronomical Union in August. For a time, a dual nomenclature persisted for the erupting volcanoes on Io. The eruption plumes were given numbers, Pl, P2, etc., while the volcanic features were given names taken from the mythology of fire and volcano legends. Thus the "hoof print" of Io was called Pele, for the Hawaiian volcano goddess, and the 280 kilometer-high plume associated with it was called Pl. By the time of the Voyager 2 encounter, scientists had prepared maps on which to plot their new discoveries.

While the Voyager scientists fanned out across the world to share their findings with colleagues, attention at JPL turned to Voyager 2. In response to the discoveries of the first encounter, changes were required in the sequencing of scientific observations for July. Voyager 2, still troubled by a faulty receiver, might require more coddling from the spacecraft team than had its sister spacecraft, now safely on the way to Saturn.