Volcanism on Io
Io is the most volcanically active world in the solar system. Most of its surface has been sculpted by volcanic processes generated deep within its interior. Volcanic eruptions were first observed by the Voyager spacecraft in 1979, and have been witnessed in every flyby of the spacecraft Galileo, which is currently exploring the Jovian system. Galileo recently detected more than 100 erupting volcanoes, and scientists speculate that there may be as many as 300. In recent observations, some of the smaller, fainter volcanoes appear to turn off and on, changing from hot and glowing to cool and dim within a few weeks. Volcanic activity on Io is so relentless that there are no signs of impact craters on its surface, because they are rapidly filled in with volcanic material soon after they appear. Given the volcanic nature of Io, it is not surprising that place names on the moon are taken from various mythological associations with fire and volcanoes.
The most distinctive features on Io are its volcanic calderas, lava flows, and colorful deposits made by plume eruptions. Io also has vast regions of volcanic flood plains. Sulfur (S) and sulfur dioxide (SO2) are found everywhere on Io, as evidenced by its surface coloration of yellow, orange, red, and black. These colors represent the palette of sulfur at varying temperatures. The patchwork of white deposits is thought to be sulfur dioxide frost which has condensed out on Io's cold surface.
New information about the role of sulfur in Io's volcanism has been obtained by combining results from Galileo and Hubble Space Telescope observations. Sulfur gas (S2), which is composed of pairs of sulfur atoms, was recently detected above Io's volcano, Pele, by the Hubble Space Telescope. This compound is stable at the very high temperatures inside the volcano, but once it is ejected and lands on the cold surface, the sulfur atoms rearrange themselves into larger molecules of three or four atoms (S3 and S4). These varieties of sulfur are red in color, and they are the primary materials that make up the red debris ring surrounding the Pele plume.
The global extent of sulfur on Io caused considerable debate as to whether Io's volcanic features were produced by molten rock (silicate volcanism) or molten sulfur. The arguments favoring silicate volcanism were supported by the fact that the tall mountains and deep, steep-sided calderas on Io require a material of considerable strength to support them. The issue was finally resolved when Earth-based telescope detected temperatures at hot spots ranging from 1000 K to 1800 K. This is far too hot for sulfur to remain liquid, so silicate magma has to be involved in these high temperature eruptions. But that does not rule out the possibility that some of the lava flows on Io are composed primarily of sulfur. In fact, the distribution of sulfur on Io is still a subject of some debate. It may be that sulfur constitutes a relatively thin coating on Io's surface, or it could form relatively thick deposits in localized areas.
Photo Credits: NASA/JPL
Illustration Credit: NASA/JPL