Close-ups of Io reveal a wide variety of volcanic phenomena. This Voyager 1 view of an equatorial region near longitude 300° shows several large surface flows that originate in volcanic craters or calderas. At the right edge is a light flow about 250 kilometers long. Another dark, lobate flow with bright edges is just left of center, with an exceedingly dark caldera to its left. [260-468A]
The discovery of the ongoing eruptions on Io, made shortly after the Voyager 1 flyby, did much to clarify the confused evidence pouring in concerning the apparent youth of Io’s surface. Here, under the very eyes of Voyager, eruptions were taking place on a scale that dwarfed anything ever seen before. The discovery picture alone, taken from a distance of 4 million kilometers, showed two eruptions (Pele and Loki), each of which was much larger than the most violent volcanic eruption ever recorded on Earth.
Voyager 1 found eight giant eruptions, with fountains or plumes rising to heights of between 70 and 280 kilometers. To reach these altitudes, the material must have been ejected from the vents at speeds of between 300 and 1000 meters per second, several times greater than the highest ejection velocities from terrestrial volcanoes. Although widely spaced in longitude, these volcanoes were concentrated toward the equator; seven of the eight were at latitudes between +30° and -30°, and the eighth at -44°.
When Voyager 2 arrived four months later, it was able to reobserve seven of the eight volcanoes. (To be identified reliably, the volcanic plumes must be silhouetted against dark space at the edge of the disk.) Six of these were still erupting; one, Pele, the largest plume seen by Voyager 1, had ceased activity. The plume associated with Loki had also changed markedly, increasing in height from 100 to 210 kilometers in visible light. (All the plumes appear larger when viewed in the ultraviolet.) Loki had developed a more complex structure; in March it appeared to originate near the south end of a 250-kilometer-long dark feature, but in July there was a double plume, with activity at both ends of the dark feature.
Differences in surface elevation can clearly be seen in a few of the Io close-ups from Voyager 1. This remarkable picture is of the center of the great volcano Pele, at latitude 15°S and longitude 224°. A low mountain with flow features can be seen. In the background, there are several large irregular depressions with flat floors that appear to be the result of collapse. The diffuse dark features in the center are probably the ejecta plumes being erupted from the Pele vent. [P-21220B/W]
At the highest resolution obtained by the Voyager cameras, Io revealed some landscapes that looked familiar to terrestrial geologists. This picture, taken by Voyager 1 at a range of only 31 000 kilometers, shows a region about the size of the state of Maryland at a resolution of 300 meters. Clearly seen is a volcano not too different from some of those on the Earth or Mars. At the center is an irregular composite crater or caldera about 50 kilometers in diameter with dark flows radiating from its rim. The style of volcanism illustrated here is quite different from the explosive plumes or fountains with their associated rings of bright material deposited on the surface. This volcano is located at about longitude 330°, latitude 70°S. [260-502]
