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 photo was taken by Voyager 1 looking back 2.6 million miles (4.5 million km) at Io, three days after its historic encounter. This is the same image in which Linda A. Morabito, a Jet Propulsion Laboratory engineer, discovered the first extraterrestrial volcanic eruption (the bright curved volcanic cloud on the limb). Image courtesy NASA/JPL.

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.

This global view of Jupiter's moon, Io, was obtained during the tenth orbit of Jupiter by NASA's Galileo spacecraft on 19 September 1997 at a range of more than 500,000 km (310,000 miles). Io (which is slightly larger than Earth's moon) is the most volcanically active body in the solar system. Image courtesy NASA/JPL/University of Arizona.

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 beautiful image of the crescents of volcanic Io and more sedate Europa was snapped by New Horizons' color Multispectral Visual Imaging Camera (MVIC) at 10:34 UT on March 2, 2007, about two days after New Horizons made its closest approach to Jupiter. Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

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:

An active fissure eruption at TvashtarCatena, a chain of giant calderas, was recorded by the Galileo Orbiter on November 25, 1999. The eruption has generated a "curtain of fire" that is more than 30 km long and 1.5 km high. Lava flows emanate from the fissure source. Image courtesy NASA/How Volcanoes Work.

We can see lava tubes, complete with skylights:

The summit caldera of Culann Patera appears to be the source of a lava tube that feeds the large, dark green flow to the west. The axis of the western part of the flow is marked by what appears to skylights lying above the tube's continuation. These features are typical of basalt lavas found on Earth. Image courtesy NASA.

We can see calderas, some complete with lava lakes:

An active volcanic eruption on Jupiter's moon Io was captured in this image taken by NASA's Galileo spacecraft. Tvashtar Catena, a chain of giant volcanic calderas centered at 60 degrees north, 120 degrees west, was the location of an energetic eruption caught in action in November 1999. A dark, "L"-shaped lava flow to the left of the center in this more recent image marks the location of the November eruption. White and orange areas on the left side of the picture show newly erupted hot lava, seen in this false color image because of infrared emission. The two small bright spots are sites where molten rock is exposed to the surface at the toes of lava flows. The larger orange and yellow ribbon is a cooling lava flow that is more than more than 60 kilometers (37 miles) long. Dark, diffuse deposits surrounding the active lava flows were not there during the November 1999 flyby of Io. Image courtesy NASA Planetary Photojournal.

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.

Geologic Map of Io, produced by the USGS and Arizona State University, incorporating information collected by Voyager and Galileo. Four distinct image products were created by the USGS and were used in this mapping project: (1) a global mosaic of SSI color images; (2) a global mosaic of the best resolution Galileo SSI monochrome images; (3) a global mosaic of the best quality and highest resolution Voyager Imaging Science Subsystem (ISS) and Galileo SSI monochrome images; and (4) a merged product combining Galileo color information with the higher resolution combined monochrome mosaic. Image courtesy 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.

In this image by Voyager we see Jupiter's moon Io with active volcanoes. Image courtesy NASA/JPL.


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.