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By Emily Lakdawalla

Io, Still a Mystery Moon

Sep. 7, 2006 | 11:53 PDT | 18:53 UTC

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by Rosaly Lopes

The Galileo spacecraft may be long gone, observing time for Io may be limited, but this very strange moon still holds our interest. However, there has been a hiatus on the acquisition of new spacecraft data. The last Io observations made by Galileo were obtained in early 2002, while the eagerly anticipated New Horizons observations during its Jupiter fly-by will not happen until February of next year. In the meantime, there has been new analysis and interpretations of Galileo data and new telescopic data, both ground-based and from the Hubble Space Telescope. Papers continue to be published (Alex Blackwell picked up on a couple in the For Members forum), and a book summarizing our knowledge after the Galileo mission is in press. Forgive me for advertising my own book, but Iophiles everywhere will want to see this one, as its 12 chapters are written by top Io researchers, summarizing what we know about Io from its formation and interior to its atmosphere and torus. "Io After Galileo" will be published by Springer-Praxis in early 2007. Unfortunately, research-level books tend to have small print runs and therefore be expensive. Get your Library to buy a copy!

Click to enlarge >

Jupiter's Moon Io
Swirls of brightly colored sulfur-rich surface deposits make volcanically active Io one of the most colorful places in the solar system. Scale: 2005.00 meters per pixel. Created: 12 December 1995. Credit: NASA/JPL/DLR

Being chief editor of this book was an excellent way to gain insight into how much we don't know about Io. From the inside out, Io keeps its mysteries. Galileo data showed that Io has a large core, but its composition is not known. There are now better constraints on the heat flow, but more accurate measurements over time are needed to understand how tidal heating works in the Jovian system. We know that Io has very tall mountains, but their origin is uncertain, as is the thickness of the lithosphere that supports them. Temperature measurements of Io's active volcanoes from Galileo suggested very high temperatures -- higher than basaltic lavas can be -- but the composition of Io's lavas is still unknown. We think the lavas are silicate lavas (similar to those on Earth). Sulfur volcanism was suggested by Carl Sagan and others from Voyager data, but we are still not sure if sulfur volcanism exist on Io at all. A lot needs to be learned about how volcanic eruptions work on Io, from lava lakes to plumes, and these may give us important clues about how volcanic eruptions work in other environments, including Earth. Io's atmosphere and torus have their own mysteries. We do not know if the atmosphere is mainly supplied by volcanic venting or sublimation of Sun-warmed sulfur dioxide frost. And we still don't understand how variations in volcanic activity controls the supply of plasma to the torus. There is much more we don't know, but what we do know we tried to include in our book.

Click to enlarge >

Eruption at Tvashtar Catena, Io
The Galileo spacecraft caught Io in the act of an active volcanic eruption on February 22, 2000. Tvashtar Catena is a chain of calderas, collapse pits formed by volcanic eruptions. The active site of the eruption is visible on the left edge of the image, where infrared imaging sees the glow of a hot lava flow more than 60 kilometers (40 miles) long. This picture is about 250 kilometers (about 155 miles) across. North is toward the top and illumination from the Sun is from the west (left). Credit: NASA / JPL

Will New Horizons help us understand Io better? Yes, but the brief fly-by cannot answer all these questions. I am sure that New Horizon's instruments, though designed for distant Pluto, will return invaluable data and show us what Io looks like years after Galileo. Will surface changes be evident? Will we find new plumes and hot spots? I'm betting we will.

Io's activity is so intense and variable that the way to understand this very strange world is to have constant monitoring with a dedicated mission that can last months if not years. This is extremely hard to do, because of the very high radiation environment around Io (a human would get 4,000 times the lethal dose in one day). This environment is not healthy to spacecraft either. Will we have a dedicated Io mission one day? I'm sure the answer is yes, but it won't be anytime soon. If I am lucky, they will bring me out of retirement for it. In the meantime, I hope that we catch glimpses of Io as spacecraft bound elsewhere fly by, and that another mission to the Jupiter system spends some of its precious resources giving us new views of this most strange world.