Perhaps the most spectacular of all the Voyager photos of Io is this mosaic obtained by Voyager 1 on March 5 at a range of 400 000 kilometers. A great variety of color and albedo is seen on the surface, now thought to be the result of surface deposits of various forms of sulfur and sulfur dioxide. The two great volcanoes Pele and Loki (upper left) are prominent. [260-464]
In place of impact craters, the surface of Io has a great many volcanic centers, which generally take the form of black spots a few tens of kilometers across. In a few cases, high-resolution pictures show the characteristic shapes associated with volcanic calderas on Earth and Mars, and, if the other volcanic centers are similar, about 5 percent of the entire surface of Io is occupied by calderas. These are extremely black, reflecting less than 5 percent of the sunlight; often they are surrounded by irregular, diffuse halos nearly as black as the central spot. The calderas seem more like the Valles caldera in New Mexico, which is associated with vents that produced large quantities of ash, than with those of Hawaiian-type shield volcanic mountains.
There is evidence in many of the Voyager photos of extensive surface flows on Io. These originate in dark volcanic centers and either spread to fan shapes, typically 100 kilometers across, or else snake out in long, twisting tentacles. Some of the flows are lighter than the background and some are darker. Most are red or orange in color, often outlined by fringes of contrasting albedo.
The equatorial regions of Io are quite flat, with no vertical relief greater than about 1 kilometer high; indeed, many of the volcanic centers do not appear to correspond to mountains or domes at all. There are, however, a number of long, curvilinear cliffs or scarps and narrow, straight-walled valleys a few hundred meters deep. These appear to be places in which the crust has broken under tension, somewhat similar to terrestrial faults and the valleys called graben. A few rugged mountains of uncertain origin are visible in low Sun elevation pictures.
Near the poles of Io the terrain is more irregular. There are few volcanic centers, but more mountains, some with heights of several kilometers. In addition, there are regions that appear to be made of stacked layers of material. These so-called layered terrains are revealed when erosion cuts into them, exposing the layers along the cliff or scarp. The largest such plateau or mesa has an area of about 100 000 square kilometers. The scarps sometimes intersect each other, suggesting a complex history of deposition, faulting, and erosion. Voyager geologists believe that these scarps may be areas in which the release of liquid sulfur or sulfur dioxide has undercut cliffs, analogous to internal sapping by groundwater at similar scarps on Earth.
Perhaps the most distinctive surface features on Io are the circular or oval albedo markings that surround the great volcanoes. The first of these to be seen was the 300-kilometer-wide white donut of Prometheus, on the equator at longitude 150°. Much more spectacular is the hoofprint of Pele, about 700 by 1000 kilometers. These symmetric rings mark the locations of the kinds of eruptions that generate large fountains or plumes, and may be produced by condensible sulfur or sulfur dioxide raining down from the volcanic fountain. At least one new ring appeared during the four months between the Voyager encounters, centered at longitude 330°, latitude +20°, but by the time Voyager 2 photographed this area, no plume remained active.
During the Voyager 1 flyby, temperature scans of the surface of Io were made with the infrared interferometer spectrometer (IRIS). A number of localized warm regions were found, the most dramatic being just south of the volcano Loki. Here the images showed a strange, U-shaped black feature about 200 kilometers across. The IRIS team interpreted its data to indicate a temperature for the black feature of 17° C (or room temperature), in contrast to the surrounding surface at -146° C. Perhaps the dark feature was some sort of lava lake, either of molten rock or molten sulfur. The melting point of sulfur is 112° C. If there were a scum of solidifying sulfur on top of the “lake,” this interpretation might well be the correct one.
The brilliant reds and yellows of the surface of Io immediately suggest the presence of sulfur. When heated to different temperatures and suddenly cooled, sulfur can assume many colors, ranging from black through various shades of red to its normal light-yellow appearance.
Even before Voyager, laboratory studies had shown that sulfur matches the overall properties of the spectrum of Io, including the low albedo in the ultraviolet and the high reflectivity throughout the infrared. Contemporary with the Voyager flybys, additional telescopic observations and laboratory studies by Fraser Fanale at JPL and Dale Cruikshank at the University of Hawaii identified another component on Io, sulfur dioxide. Sulfur dioxide is an acrid gas released from terrestrial volcanoes, where it combines with water in the Earth’s atmosphere to produce sulfuric acid. At the temperature of the surface of Io, sulfur dioxide is a white solid. Researchers guessed that the extensive bright white areas in the Voyager pictures of Io might be covered with sulfur dioxide frost or snow. The presence of this material on Io was confirmed when the infrared IRIS instrument obtained a spectrum of sulfur dioxide gas over the erupting volcano Loki during the Voyager 1 encounter.