Io, Galilean Satellite

Io, Galilean Satellite

All of travelers’ tales returned by Voyager, Carl Sagan’s concern the discoveries made on the innermost Galilean satellite, Io. (Frequently pronounced Eye-oh by Americans, because this is the preferred enunciation in the Oxford English Dictionary. But the British have no special wisdom here. The word is of Eastern Mediterranean origin and is pronounced throughout the rest of Europe correctly, as ee-oh). Before Voyager, scientists were aware of something strange about Io.

Scientists could resolve few features on its surface, but they knew it was red – extremely red, redder than Mars, perhaps the reddest object in the solar system. Over a period of years something seemed to be changing on it, in infrared light and perhaps in its radar reflection properties.

Scientists also knew that partially surrounding Jupiter in the orbital position of Io was a great doughnut-shaped tube of atoms, sulfur and sodium and potassium, material somehow lost from Io.

Would we be marveled if we discovered a new Univers? Elena (illustration by Elena)

When Voyager approached tis giant moon scientists found a strange multicolored surface unlike any other in the solar system. Io is near the asteroid belt. It must have been thoroughly pummeled throughout its history by falling boulders. Impact craters must have been made. Yet there were none to be seen. Accordingly, there had to be some process on Io that was extremely efficient in rubbing craters out or filling them in. The process could not be atmospheric, since Io’s atmosphere has mostly escaped to space because of its law gravity. It could not be running water; Io’s surface is far too cold. There were a few places that resembled the summits of volcanoes. But it was hard to be sure.

Linda Morabito, a member of the Voyager Navigation Team responsible for keeping Voyager precisely on its trajectory, was routinely ordering a computer to enhance an image of the edge of Io, to bring out the stars behind it. To her astonishment, she saw a bright plume standing off in the darkness from the satellite’s surface and soon determined that the plume was in exactly the position of one of the suspected volcanoes.

Voyager had discovered the first active volcano beyond the Earth. We know now of nine large volcanoes, spewing out gas and debris, and hundreds – perhaps thousands – of extinct volcanoes on Io. The debris, rolling and flowing down the sides of the volcanic mountains, arching in great jets over the polychrome landscape, is more than enough to cover the impact craters. We are looking at a fresh planetary landscape, a surface newly hatched. How Galileo and Huygens would have marveled.

Volcanoes of Io

The volcanoes of Io were predicted, before they were discovered, by Stanton Peale and his co-workers, who calculated the tides that would be raised in the solid interior of Io by the combined pulls of the nearby moon Europa and the giant planet Jupiter. They found that the rocks inside Io should have been melted, not by radioactivity but by tides; that much of the interior of Io should be liquid. It now seems likely that the volcanoes of Io are tapping an underground ocean of liquid sulfur, melted and concentrated near the surface. When solid sulfur in heated a little past the normal boiling point of water, to about 115C, it melts and changes color.

The higher the temperature, the deeper the color. If the molten sulfur is quickly cooled, it retains its color. The pattern of colors that we see on Io resembles closely what we would expect if rivers and torrents and sheets of molten sulfur were pouring out of the mouth of the volcanoes: black sulfur the hottest, near the top of the volcano; red and orange, including the rivers, nearby; and great plains covered by yellow sulfur at a greater remove. The surface of Io is changing on a time scale of months. Maps will have to be issued regularly, like weather reports on Earth. Those future explorers on Io will have to keep their wits about them.

The very thin and tenuous atmosphere of Io was found by Voyager to be composed mainly of sulfur dioxide. But this thin atmosphere can serve a useful purpose, because it may be just thick enough to protect the surface from the intense charged particles in the Jupiter radiation belt in which Io is embedded. At night the temperature drops so low that the sulfur dioxide should condense out as a kind of white frost; the charged particles would them immolate the surface, and it would probably be wise to spend the nights just slightly underground.

Jupiter will be a source of continuing provocation and excitement for the future human explorers of the Jovian moons. Image: © Elena

The great volcanic plumes of Io reach so high that they are close to injecting their atoms directly into the space around Jupiter. The volcanoes are the probable source of the great doughnut-shaped ring of atoms that surrounds Jupiter in the position of Io’s orbit. These atoms, gradually spiraling in toward Jupiter, should coat the inner moon Amalthea and may be responsible for its reddish coloration. It is even possible that the material outgassed from Io contributes, after many collisions and condensations, to the ring system of Jupiter.

A substantial human presence on Jupiter itself is much more difficult to imagine – although we suppose great balloon cities permanently floating in its atmosphere are a technological possibility for the remote future. As seen from the near sides of Io and Europa, that immense and variable world fills much of the sky, hanging aloft, never to rise or set, because almost every satellite in the solar system keeps a constant face to its planet, as the Mood does to the Earth.

Who knows, Io may look like this, but it's hardly possible. Photo by Elena