Revealing the Secrets of Our Solar System's Most Tormented Moon
Compared to Earth's other volcanoes, Sicily's Mt. Etna is rarely quiet. When Dr. Rosaly Lopes went there to conduct field research in 1979, a crater only a mile away exploded. Several people were killed.
"I really learned to respect volcanoes," Lopes said.
But even Mt. Etna's earthshaking eruptions pale when compared to Io, a tormented moon orbiting Jupiter that is covered with erupting volcanoes. The violent eruptions - caused by the constant tug of Jupiter's powerful gravity - keeps Io's surface in a constant state of change as molten material spews out onto its surface. It is unlike any world in our solar system.
Lopes was just getting started in volcanology in 1979 when theVoyager spacecraft revealed for the first time Io's volcanic nature. Her timing was perfect. NASA was planning an ambitious mission called Galileo to orbit Jupiter for the first time. She started planning the Io observations with an instrument called a near-infrared mapping spectrometer (NIMS).
Galileo has since completed its mission and Lopes is currently working on the Cassini mission to Saturn. But Io's volcanoes are never far from her mind.
Q: How volcanic is Io?
A: We consider Io the most volcanic body in the solar system because its volcanoes put out the most heat. We have found more than 150 volcanoes on Io, but Earth has more than 600 active volcanoes, so it's not the number that make Io the most volcanic. It's the heat output. Io is only about one-third as big as Earth, but it puts out about twice the energy. One of Io's volcanoes, Loki, is more powerful than all of Earth's volcanoes combined.
Q: Are Io's volcanoes like Earth's volcanoes?
A: Yes and no. The types of eruptions we have observed on Io are similar to types of eruptions on Earth -- lava flows, calderas, fire fountains like in Hawaii -- but there are some very different aspects. One is that lava on Io is much hotter than any lava that flows on Earth today. Billions of years ago Earth had lava that hot. Another difference is that the calderas, the volcanic craters, on Io are much larger than on Earth. Lava flows are much larger, too. [An Io volcano named] Amirani has a lava flow 300 kilometers [190 miles] long, and that's much longer than any on Earth. Globally, Io erupts more than 100 times as much lava per year as Earth, including Earth's undersea eruptions.
Q: Where could you go on Earth that might look like Io?
A: The big island of Hawaii has the Kilauea volcano that has been active for about two decades. Yellowstone is a large caldera that has many areas with brightly colored sulfur. Stromboli [in Italy] has been active for at least 2,000 years. Some very old lava flows on Earth, such as some in South Africa, are a composition called komatiite, which we think is the composition of Io's lavas. Probably the most similar place on Earth just in terms of the great amount of volcanic activity is under the ocean at the mid-ocean ridge.
Q: What makes Io so volcanic?
A: Although both Earth and Io have active volcanism, the way the volcanism happens is quite different. On Earth, volcanism is tied to plate tectonics, and we don't believe Io has plate tectonics. Io is in a tug of war between Jupiter and Europa and Ganymede, two of the other large moons of Jupiter, and that is what heats it up. If Io weren't in its very peculiar orbit around Jupiter, it wouldn't have active volcanism. It would have cooled off a long time ago.
Q: Some people say Io looks like a pizza. What are all those bright colors?
A: We think the bright colors are due to sulfur in various forms but that the very dark colors are due to active lavas. Every place we see high temperatures, if we look at the surface we see dark materials. That would be the olives on the pizza. The reds are deposits from the plumes of volcanoes. With time, the reds become yellow because of changing to a different form of sulfur. We're still quite puzzled by what some of the very small green areas are. We joke and call them golf courses. They may be areas rich in sulfur but contaminated by another material. Another possibility is they are very olivine- rich lava. Olivine is a green mineral.
Q: What is there about Io that is still a mystery to you?
A: There are other questions that still remain after Galileo. What is the composition of the lavas? We are using their temperature to say what is the most likely composition based on comparison with lavas on Earth, but we don't really know whether lavas on Io are something entirely different. We won't know that until we can go back with more refined instruments or maybe someday go there and bring back some samples. When we study volcanoes on Earth, one of the first things we do is collect samples of lava and take them back to the lab, but for Io, that's a long way in the future.
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