In addition to its giant volcanic plumes or fountains, Io possesses other indications of current volcanic activity. One of these takes the form of intermittent blue-white patches that may be caused by gas venting from the interior. In this pair of photographs, the same region of the surface is shown about six hours apart. On the right, there is an arcuate bright gas cloud; on the left the same region is black. It is believed that the venting gas is sulfur dioxide, and that the condensation of this gas produces fine particles of “snow” that look blue. [260-508]
Some of the most dramatic changes in the surface of Io between March and July took place in the vicinity of the volcano Loki, at longitude 310° and latitude 15°N. On the left is a Voyager 1 view; on the right, one from Voyager 2. The “lava lake” associated with Loki has become less distinct, apparently as a result of deposits that fell on the northern part of the dark U-shaped feature. Perhaps the surface had also cooled between these photos. In the upper left center, a new dark volcanic caldera with bright spots near it and a large, faint bright ring had appeared by July, although it was not active at the time Voyager 2 flew by. [260-687AC]
No new eruptions were seen by Voyager 2; between them, the two spacecraft effectively surveyed the whole surface of Io for plumes down to 40 kilometers height. Interestingly, the smallest plume seen was 70 kilometers high; there appeared to be a real absence of smaller eruptions.
From the number and size of the observed eruptions, it is possible to calculate the resurfacing rate for Io due to these plumes. The result is that each plume is erupting about 10 000 tons of material per second, or more than 100 billion tons per year. This quantity corresponds to about 10 meters of deposition over the whole surface in a million years. When additional note is taken of surface flows, the deposition rate could easily be ten times higher, or 100 meters per million years, in agreement with the rate estimated from the absence of impact craters.
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]