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.
HIGHLIGHTS OF THE VOYAGER 1 SCIENTIFIC FINDINGS[2]
Atmosphere
Uniform wind speeds for cloud features with widely different size scales, suggesting that mass motion and not wave motion is being observed.
Rapid formation and spreading of bright cloud material, perhaps the result of disturbances that trigger convective activity.
A pattern of east-west winds in the polar regions, previously thought to have been dominated by convective upwelling and downwelling.
Anticyclonic motion of material associated with the Great Red Spot, with a rotational period of about six days.
Interactions of smaller spots with the Great Red Spot and with each other.
Auroral emissions in the polar regions, both in the ultraviolet (which were not present during the 1973 Pioneer encounter) and in the visible.
Cloud-top lightning bolts, similar to terrestrial superbolts, and meteoritic fireballs.
A temperature inversion layer in the stratosphere and a temperature of 160 K at the level at which the atmospheric pressure is 0.01 bar.
Very strong ultraviolet emission from the disk, indicating a thermospheric temperature of more than 1000 K.
A hot (1100 K) upper ionosphere on the dayside that was not observed by Pioneer 10, suggesting there may be large temporal or spatial changes.
An atmospheric composition with volume fraction of helium of 0.11 ± 0.03.
A substantially colder atmosphere above the Great Red Spot than in the surrounding regions.
Satellites and Rings
At least eight currently active volcanoes on Io, probably the result of tidal heating of the interior of the satellite, with plumes extending up to 250 kilometers above the surface.
A large hot spot on Io near the volcano Loki that is about 150 K warmer than the surrounding surface.
Numerous intersecting, linear features on Europa, possibly due to crustal cracking.
Two distinct types of terrain, cratered and grooved, on Ganymede, suggesting that the entire ice-rich crust was once under tension.
An ancient, heavily cratered crust on Callisto, with vestigial rings of enormous impact basins since erased by flow of the ice-laden crust.
The elliptical shape of Amalthea (270 × 160 kilometers).
A faint ring of material about Jupiter, with an outer edge of 128 000 kilometers from the center of the planet.
Magnetosphere
An electrical current of more than a million amperes flowing in the magnetic flux tube linking Jupiter and Io.
Very strong ultraviolet emissions from ionized sulfur and oxygen in the Io plasma torus, indicating a hot (hundred thousand degree) plasma that evidently was not present at the time of Pioneer 10 encounter.
Plasma electron densities exceeding 4500 per cubic centimeter in some regions of the Io plasma torus.
A cold, corotating plasma inside 6 RJ with ions of sulfur, oxygen, and sulfur dioxide.
High-energy trapped particles inside 6 RJ with significantly enhanced abundances of oxygen, sodium, and sulfur.
Hot plasma near the magnetopause predominantly composed of protons, oxygen, and sulfur.
Jovian radio emission at kilometer wavelengths, which may be generated by plasma oscillations in the Io plasma torus.
Corotating plasma flows unexpectedly far from Jupiter in the dayside outer magnetosphere.
Evidence suggesting a transition from closed magnetic field lines to a Jovian magnetotail at about 25 RJ from Jupiter.
Whistler emission interpreted as lightning whistlers from the Jovian atmosphere.
[1]Adapted from a paper by E. C. Stone and A. L. Lane in the Voyager 1 Thirty-Day Report.
[2]Adapted from a summary prepared by E. C. Stone and A. L. Lane for the Voyager 1 Thirty-Day Report.
The dramatic discovery of active volcanoes on Io was made by Linda Morabito and her colleagues from this navigation picture, taken March 8 at a range from Io of 4.5 million kilometers. On the bright edge, the immense plume of volcanic ash from Pele (P₁) rises nearly 300 kilometers above the surface. At the terminator, the border between day and night on Io, a second smaller cloud from the volcano Loki (P₂) catches the sunlight. These two eruptions—captured on this single discovery photograph—are much larger than the largest terrestrial volcanic eruption known. [P-21306 B/W]
Once the existence of giant volcanic eruptions on Io was recognized, a reexamination of the Voyager 1 encounter pictures revealed many more plumes. These two views of Prometheus (P₃) were found by Joseph Veverka and Robert Strom on March 12 when they reproduced earlier pictures.
The plume is silhouetted against the black space, although it is also possible to see dark “feet” where the falling material reaches the surface. [P-21295]
The complex jets of material are clearly seen as dark streaks against the light background of the surface of Io. The plume itself rose more than 100 kilometers above Io’s surface. [P-21294]
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 wide-angle 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.
Linda Morabito shows the discovery photo of the volcanic eruptions on Io. [P-21718]
The dark side of Jupiter revealed many surprising phenomena to Voyager 1. A wide-angle view, taken on March 5, led to the discovery of a double auroral arc at north-polar latitudes and numerous flashes of lightning illuminating the clouds during this 3-minute, 12-second exposure, taken at a range of half a million kilometers. [260-460]
Meanwhile, on March 11, John Pearl of 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 Peale, 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, P₁, 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 P₁. 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.
The particles of the rings of Jupiter are stronger reflectors of red light than of blue, as can be seen in this view, assembled from two images taken in orange and violet light. Since the images were registered on the rings, not the planet, the bright bands of colors along the edge of Jupiter are an artifact of misalignment. [P-21779]