Energy for the Io Volcanoes
Clearly, something extraordinary is happening to Io to generate the observed level of volcanism. The primary heat source for the interiors of the terrestrial planets is the decay of the long-lived radioactive elements thorium and uranium. But Io would have to be supplied with a hundred times its quota of these elements to explain the observed activity.
A way out of this difficulty was provided by a theoretical investigation carried out by Stanton Peale of the University of California at Santa Barbara and Pat Cassen and Ray Reynolds at the NASA Ames Research Laboratory. Working in the months before the first Voyager flyby, they calculated that the tidal effects of Jupiter on Io could generate large-scale heating of the satellite. Io is about the same distance from Jupiter as the Moon from Earth, but the much greater mass of Jupiter raises enormous tides in its satellite. These tides distort its shape, but no other effect would be present if Io remained at a constant distance from Jupiter. What Peale, Cassen, and Reynolds realized was that the distance of Io from Jupiter varies as the result of small gravitational perturbations from the other Galilean satellites. Therefore the tidal distortions also vary, in effect squeezing and unsqueezing Io each orbit. Such flexing pumps energy into the interior of Io in the form of heat; theorists calculated that the heat supplied could be as high as 10¹³ watts. They predicted, in a paper published just three days before the Voyager flyby of Io, that “widespread and recurrent surface volcanism might occur,” and that “consequences of a largely molten interior may be evident in pictures of Io’s surface returned by Voyager.”
Voyager 2 obtained beautiful views of the volcanic eruptions during its ten-hour Io volcano watch on July 9. On the edge of the crescent image are the volcanoes Amirani (P₅) and below it Maui (P₆), each sending up fountains about 100 kilometers above the surface. The blue color is probably the result of sunlight scattered by tiny particles of sulfur dioxide snow condensing in the erupting plume. [P-21780]
| VOLCANIC ERUPTIONS ON IO | ||||
|---|---|---|---|---|
| Plume Number | Name | Location (latitude/longitude) | Height During Voyager 1 Flyby (kilometers) | Activity During Voyager 2 Flyby |
| 1 | Pele | -20°/255° | 280 | ceased |
| 2 | Loki | +20°/300° | 100 | increased |
| 3 | Prometheus | -5°/155° | 70 | increased |
| 4 | Volund | +20°/175° | 95 | no data |
| 5 | Amirani | +25°/120° | 80 | similar |
| 6 | Maui | +20°/120° | 80 | similar |
| 7 | Marduk | -25°/210° | 120 | similar |
| 8 | Masubi | -40°/ 50° | 70 | similar |
Between the two encounters, the volcanic eruption at Loki (P₂) changed character. The single plume emanating from the western end of an apparent dark fissure seen in March was joined by a second fountain of similar size about 100 kilometers to the east. The plume also increased in height, from about 120 kilometers in the Voyager 1 image (left) to 175 kilometers in the Voyager 2 image (right). [260-662A and B]
The detailed structure near the volcano Loki is like nothing seen elsewhere on Io. [260-642B]
When this Voyager 1 picture was taken, the main eruptive activity (P₂) came from the lower left of the dark linear feature (perhaps a rift) in the center. Below it is the “lava lake,” a U-shaped dark area about 200 kilometers across. In this specially processed image, detail can be seen in the dark surface of this feature, possibly due to “icebergs” of solid sulfur in a liquid sulfur lake.
(Bottom) The IRIS on Voyager 1 found this “lava lake” to be the hottest region on Io, with a temperature about 150° C higher than that of the surrounding area.
One model for the structure of Io indicates that an ocean of liquid sulfur with a solid sulfur crust covers most of the satellite. Heat escapes from the interior in the form of lava, which erupts beneath the sulfur ocean. Secondary eruptions in the sulfur ocean heat liquid sulfur dioxide, which is mixed with solid sulfur in the crust. The rapid expansion of sulfur dioxide gas then produces the great eruptive plumes, which consist of a mixture of solid sulfur, sulfur dioxide gas, and sulfur dioxide snow.
Solid sulfur + solid SO₂ Solid sulfur + liquid SO₂ Molten sulfur Solid silicate
The Voyager observations appear to confirm the theoretical calculations. The tidal heat source has presumably been acting since Io was formed more than 4 billion years ago. With a totally molten interior and continuing large-scale volcanism, Io has had an opportunity to thoroughly sort out its composition. In the process it would have lost all the volatile gases such as water and carbon dioxide, explaining why Io now has no appreciable atmosphere in spite of the outpouring of material from the interior. In addition, most of the sulfur from the interior could have risen to the surface, where it would be constantly recycled through volcanic activity.
The presence of large amounts of sulfur on the surface may help explain the extraordinary nature of the Io volcanoes. One model considered for the satellite postulates that it is covered by a sea of liquid sulfur several kilometers deep, with a crust of solid sulfur and, below the surface, liquid sulfur dioxide. Calculations by Sue Kieffer of the U.S. Geological Survey and others indicate that the expansion of the sulfur dioxide in such a model can explain the observed eruption velocities of up to a kilometer per second.
The volcanic plumes on Io appear to be made primarily of sulfur and sulfur dioxide. Both are molten as they emerge from the vent, but they quickly cool as the plume rises 100 or more kilometers into the near vacuum of space. Unlike terrestrial volcanoes, there is almost no gas in the plumes. It requires about half an hour for the fine particles of solidified sulfur and sulfur dioxide snow to fall back to the surface, where they form the colorful rings that mark the major eruptive sites.
Almost all the roughly 100 000 tons of material erupted each second by the Io volcanoes snows back to the surface. But apparently a part—perhaps 10 tons per second—escapes from Io and is captured by the Jovian magnetosphere. Another part contributes to an ionosphere—a tenuous atmosphere of electrons and ions—that surrounds Io. The injection of several tons of particles each second into the magnetosphere has dramatic consequences that can be seen even from Earth.
Direct evidence of an atmosphere on Io was obtained during the Voyager 1 flyby by the IRIS. In the region near the volcano Loki and its associated “lava lake,” infrared spectra clearly showed the signature of sulfur dioxide gas. It is not known whether this gas was a temporary feature associated with the eruption of Loki or if it might be present on Io more generally. Other evidence, however, points to the sulfur dioxide atmosphere as a transient feature. A small amount of the sulfur dioxide escapes and is broken apart by sunlight to provide the oxygen and sulfur ions observed in the Io torus.
Sulfur dioxide gas cloud Plume -235° F Hot surface areas (45° F)