A probe sweeps through space. Roughly 4.5 million kilometers (2.6 million miles) away, you sit and watch images of another world appear. You notice a mottled surface, and on its horizon, jetting an incredible 260km (162mi) above its surface, a plume.

This is the first volcano ever seen erupting outside your planet.

This is a world where volcanic plumes are sulfur dioxide snow, and are so large they can be seen from Earth orbit by the Hubble Space Telescope, and from Earth-based telescopes as outbursts of infrared. Tectonics are driven by tides rather than internal heat; volcanoes vent ultramafic lavas hotter than anything seen on Earth in billions of years. 425 volcanic centers, 70 of them currently active, rework the surface at a remarkable rate of 1 centimeter (over a third of an inch) per year. This world is 25 times as volcanically active as Earth, bumping us to second place for geologic activity, but is barely larger than our own Moon. It's the only other planet in the solar system we know of that has active volcanoes. It claims the prize for longest lava flows. And if folks before the 20th century had known that, they might have named it Vulcan. Galileo called it Medician Planet I, then Jupiter I when he realized it was a moon. Scientists in the 19th century named it Io.

Isn't she a beauty?

She was a bit of a surprise, but even before Voyager snapped that famous image of the first volcano caught erupting outside Earth, scientists suspected they'd find active volcanoes there. Io, you see, is caught between Jupiter's enormous bulk and two other substantial moons, Europa and Ganymede. We've got tides up to 18 meters (60 feet) high, caused by the tug of our one moon: Io's tides are more than 100 meters (328 feet) high - and mind you, that's 100 meters of rock. It takes a tremendous amount of force to move that much solid stuff, and the solid stuff heats up to remarkable temperatures. It's how a world far tinier than ours can sustain eruptions Earth hasn't been hot enough to experience in billions of years.

This is a world that's undeniably alien, and yet eerily familiar. Visit Hawaii's volcanoes, and you can get a sense of what's going on with Io. Yes, her explosive eruptions are driven more by sulfur dioxide than water and carbon dioxide like ours are, and her lavas average a scorching 1,600°C (2,900°F) compared to our chillier lavas (1200-1300°C/2200-2400°F). Yes, outside of the searing-hot areas of active volcanism, the surface is a frigid -150°C (-300°F). And, of course, things go higher in that thin atmosphere and low gravity - Old Faithful, for instance, would jet a whopping 37 kilometers (23 miles) high there. But outside of all that, the volcanoes aren't so different. They erupt lavas composed of chemicals we're familiar with here: sulfur dioxide, silicon, oxygen, iron, magnesium, potassium, sodium, calcium. They erupt in patterns that are quite familiar.

For instance, we can watch a curtain of fire emerge from a fissure at Tvashtar Catena:

We can see lava tubes, complete with skylights:

We can see calderas, some complete with lava lakes:

We've mapped lava fields, volcanic domes, pyroclastic deposits, mountains and plains - oh, and did I mention, we've done a geologic map of the place? That's right. A whole geologic map of a whole other world, created by the USGS.

That map showed us another fascinating fact about Io: there aren't any impact craters on it. Every other body in the solar system has got one, but Io doesn't. Its volcanoes busily erase every single one. Amazing, am I right?


There's more to be said about this beautiful, brilliant moon. Far more. And I will say it someday, after I've had a chance to read up a bit further (and after we've finished with Mount St. Helens, which we have not - not by a long shot). Consider this a mere introduction, and a promise, and a reminder that geology happens in far more places than we typically consider. The Earth sciences can be quite unearthly.

Not to mention, heartbreakingly beautiful.


ASU News: "Geologic map of Jupiter’s moon Io details an otherworldly volcanic surface." Last accessed 9/7/2012.

How Volcanoes Work: "Volcanism on Io." Last accessed 9/7/2012.

NASA Science: "Io's Alien Volcanoes." Last accessed 9/7/2012.

NASA Solar System Exploration: "Volcanism on Io." Last accessed 9/7/2012.

USGS Hawaiian Volcano Observatory: "An Eye on Io's Volcanism." Last accessed 9/7/2012.

USGS: "Geologic Map of Io." Last accessed 9/7/2012.