[Illustration: 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]] [Illustration: 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.