Roughly 40 years ago, the Voyager 1 spacecraft sailed past one of Jupiter’s large moons, and it revealed something amazing: The rocky moon, called Io, is a volcanic champion, featuring the first erupting volcanoes seen anywhere other than Earth. Some of its hundreds of fiery craters are many times more expansive than our largest cities. Io’s powerful eruptions can produce plumes of epic proportions, sometimes reaching heights of 300 miles.

Now, scientists pouring over five years’ worth of images taken from the top of a Hawaiian volcano have unveiled the most detailed atlas yet of this unusual moon. Their work, recently published in The Astronomical Journal, reminds us how far we’ve come since Voyager delivered those first grainy images of plumes on Io.

The research also confirms that Io is even stranger and more difficult to explain than anyone thought. Volcanoes don’t seem to be in the right places, the brightest eruptions seem largely confined to just one hemisphere, and Loki Patera—a 8,100-square-mile depression filled with lava—steadfastly refuses to play by anyone’s rules. (Explore the many intriguing moons of the solar system in our interactive atlas.)

While the new data are full of these and other mysteries, they are also a “gift to the planetary science community,” says study coauthor Ashley Davies, a volcanologist at NASA’s Jet Propulsion Laboratory. After all, the profuse but geochemically primitive eruptions on Io are similar to those that once took place on Earth, so looking at Io is a way of understanding large-volume eruptions that happened here within the last 500 million years or so.

“It’s a window into Earth’s past,” Davies says.

Brian T. Jacobs, Katie Armstrong, Matthew W. Chwastyk, NG Staff. Sources: Gazetteer of Planetary Nomenclature, NASA Galileo Orbiter, NASA/JPL

Going with the flow

Unlike Earth’s erupting peaks and vents, Io’s volcanism isn’t powered by trapped heat left over from its formation or by the decay of naturally occurring radioactive compounds in its rocks. Instead, it’s a result of its strange orbital shenanigans.

Io is one of the four so-called Galilean moons of Jupiter, along with Europa, Ganymede, and Callisto. These larger moons, discovered by Galileo Galilei in 1610, are all visible from Earth with a small telescope. As it turns out, for every single orbit Io makes around the gas giant, Europa orbits twice and Ganymede orbits four times. This pattern of orbital resonance means that Io’s orbit is forced to be more elliptical than it otherwise would be, and the resulting gravitational tugs among the moons make Io’s solid surface ebb and flow by as much as 330 feet.

Combined with Jupiter’s gravitational pull, this orbital ballet generates a huge amount of frictional heating inside Io that ultimately produces a lot of magma, something that a 1979 paper predicted even before anyone saw volcanic plumes on its surface.

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Thanks to its weird volcanism, Io is a world of freakish extremes. Its ephemeral, unstable atmosphere is too thin to trap any heat, so Io’s average surface temperature is -202°F. Conversely, some of its lava flows may reach 3,000°F, hotter than anything we see on Earth today.

Even though Io is roughly 4.5 billion years old, its copious lava production means that its surface is no more than a couple million years old, adds Alfred McEwen, a planetary geologist at the University of Arizona. Its volcanoes often take the form of cauldrons depressed into the crust, known as paterae. Sometimes they contain their lava flows, sometimes they spill out, and sometimes explosions propel material skyward.

The trickster god

With so much volcanic complexity to unravel, study leader Katherine de Kleer, a planetary scientist at the California Institute of Technology, turned to the Keck and Gemini observatories atop Mauna Kea in Hawaii, hoping to see how Io’s behavior changed over time in fine detail. Due to so many people wanting to use these telescopes, she sometimes viewed Io for as little as 20 minutes at a time, but doing this over five years allowed her to build up a considerable dataset.

As ever, Loki Patera stole the show. Io’s most persistently powerful volcano, Loki Patera is responsible for a staggering 10 percent of the moon’s total heat output. Combining old data with the new observations appears to show the volcano brightening and fading every 460 or 480 days, which de Kleer says tracks with repeated variations in Io’s ellipse-shaped orbit.

More data is needed to verify this pattern, and right now it’s hard to see if less powerful Ionian volcanoes also follow a similar cycle. McEwen says it’s a reasonable assumption, since the brightness of the south polar plumes on the ice moon Enceladus also varies with its orbit around Saturn.

But every time somebody writes a paper saying they think they know what’s going on at Loki Patera, its subsequent behavior contradicts that paper, de Kleer cautions.

Just over a year ago, a team led by Julie Rathbun, a senior scientist at the Planetary Science Institute, used Loki Patera’s apparent brightening and dimming cycle to predict that the next eruption would begin in May 2018, and they were right. They then predicted an eruption should occur in September of this year, and they were instead treated to a colossal paroxysm in early July, one which, against all expectations, ended just a few days later.

“We need to stop naming features after trickster gods!” Rathbun says.

It’s thought that Loki Patera’s brightening and dimming could be influenced by the lava lake effectively regenerating. When parts of the lava lake there cool, they sink beneath the surface, and this might trigger a progressive, sweeping wave pattern seen at the surface. Unfortunately, it’s quite difficult to get satisfactory answers by comparing Loki to modern lava lakes on Earth, as the patera is far too big for the terrestrial mechanisms to simply scale up, Davies says.

Burning questions

The data also seem to confirm another Ionian mystery: Bright eruptions dominate on the trailing hemisphere of Io, the back side of the moon in terms of its orbital motion around Jupiter. No one can really explain this volcanic bias right now, with de Kleer calling it “entirely enigmatic at this point.”

In addition, the volcanoes themselves don’t appear to sit where any model says they should be. Depending on which part of Io’s interior is being tidally heated, models predict that you should see more volcanoes either near the poles or near the equator. Observations, however, show that the real positions of Io’s volcanoes don’t match up with any major heating model.

There’s also debate as to what the hellish subsurface looks like. The Galileo spacecraft’s close flybys in 1990s and early 2000s delivered some clues, with a few experts suggesting that a ginormous underground ocean of magma lurks beneath the surface. But it’s also possible Io could just have pockets of magma rather than one giant ocean, or even a fluid-filled sponge layer instead.

Io also experiences “outburst” eruptions that are so powerful, one blast essentially doubles Io’s brightness. Despite seeing three of them over just two weeks in 2013, de Kleer didn’t see anything like them for the next five years. “That’s weird,” she says. “Where are they?”

New perspectives

While it may seem as if the new study delivers more questions than it answers, the work highlights the many ways a better grasp on Io can help unravel mysteries on Earth and other geologically active bodies.

“The history of volcanology is that you look at ancient deposits and you’re puzzled,” McEwen says. “Then you see it erupt, and then you go, Ah-ha, now I understand.” In a similar vein, looking at eruptions on Io could help piece together the story of ancient eruptions back home. These include the Siberian Traps, where mighty, prolonged eruptions likely triggered the worst mass extinction in Earth’s history.

And comprehending the effects of Io’s heat source may shine a light into the depths of watery worlds like Enceladus. There, similar tidal friction may have helped conjure up a geothermally influenced liquid ocean underneath that moon’s icy shell–one that could be suitable for life.

“Io really changed our perspective on the future of all exploration and what we might expect to find,” says Linda Morabito, who was the first to spot Io’s volcanism while working at NASA’s Jet Propulsion Laboratory in 1979. No wonder, then, that McEwen and his colleagues hope to send a spacecraft screaming around Io—the perfect close-up compliment to the new study’s pioneering ground-based observations of the lighthouse of the solar system.