Stories from Io

Io, Jupiter's third largest moon, is the most exuberantly volcanic world in our solar system, containing more than 400 active volcanoes and lava lakes hundreds of kilometers across. Plumes of molten sulphur ejected from these volcanoes can climb as high as 500 km above Io’s surface, before raining down and hardening into sulphurous frost. This extreme vulcanism is a result of the friction generated within the moon's interior as it is pulled between Jupiter and its companion moons, Europa, Ganymede and Callisto. As Jupiter turns on its axis (a giant spinning once every 9h 56m), its enormous magnetosphere strips Io of around 1,000 kilograms of material every second. This material forms a cloud of intense radiation along the path of its orbit called a plasma torus. Some of the ions are pulled into Jupiter's atmosphere along lines of magnetic force, where they create auroras and flashes of lightning.


Jupiter's moon Io. NASA ID: PIA00583.

World of Fire and Thunder

Io's 244 named features take their names from myths and stories associated with fire and thunder, as well as Dante Alighieri’s Inferno (c. 1308), a medieval poem about a journey through hell. Nine of Io's names derive from Norse myth. Fiorgynn Fluctus (flow terrain) is a river of lava 414.07 km long named after the god Fjörgynn, the father of Óðinn's wife, Frigg. Four are attached to volcanoes (active eruptive centres), including Volund (427.26km diameter, 29.33 N, 171.75 W), named for the master smith Völundr whose story is related in the eddic poem Völundarkvíða, and Thor (240.3km diameter, 39.2 N, 133.12 W) for the Norse god of thunder. 

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Surt (45.37 N, 337.21 W).

Firey Surtr

The volcano Surt takes its name from Surtr in Norse myth, a giant who will wield a flaming sword against the gods at Ragnarök, the series of events that will lead to the world’s destruction and renewal. According to the eddic poem Völuspá (The Seeress's Prophecy), Surtr will come from the south, slaying the god Freyr and consuming the world in flames. Snorri Sturluson tells us that Surtr rules the firey realm of Múspell, and will lead its inhabitants in battle at Ragnarök.

Surt comes from the south with branches-ruin,
the slaughter-gods' sun glances from his sword;
rocky cliffs clash together and the troll-women are abroad,
heroes tread the hell-road and the sky splits apart.

Völuspá (The Seeress's Prophecy), verse 50

This same Surt lends his name to Iceland's newest volcanic island. In November 1963, the sea off the coast of the Icelandic Westman Islands began to boil. A volcano had begun to form on the seabed, 130m below sea level, and a new volcanic island broke the waves within the week. In 1966, this new island was named Surtsey (Icelandic 'Surt's island'), befitting its fiery origin. The Surt on Io was discovered and named in 1979, when the moon was newly imaged by the Voyager 1 space craft. 


Thor (240.3km diameter, 39.2 N, 133.12 W).


Thor is a volcano situated on the hemisphere of Io locked facing away from Jupiter, and takes its name from the Norse god of thunder. It is responsible for one of the largest volcanic plumes ever detected in our solar system. On 6th August 2001, Galileo measured dust particles as high as 500 km above the moon's surface, and was able to observe changes to its surrounding terrain over the course of two flybys in that year.

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The Eyrarland statue (c. 1000). The National Museum of Iceland, Þjms. 10880.

Þórr is renowned for his incredible strength and bad temper, and swings his devastating hammer Mjölnir in his battles with giants. In the eddic poem Þrymskviða (Thrym's Poem), the god wakes to find his hammer stolen, and in possession of the giant Thrym. The giant tells Þórr that he will only return Mjölnir if he brings him the goddess Freyja to be his bride. Freyja angrily refuses, and Þórr and Loki have no choice but to disguise themselves as a bride and her maid and retrieve the hammer themselves. The Eyrarland figure, a bronze figure 6.7 cm tall, may depict Þórr grasping his hammer in both hands.

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Loki Patera (226.57km diameter, 13.01 N, 308.79 W) is the largest volcanic depression on Jupiter's moon Io. NASA ID: PIA00320.

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Loki (18.41 N, 302.58 W) rising above Loki Patera (226.57km diameter, 13.01 N, 308.79 W).

Loki and Logi

Loki lends his name to two features on Io, the volcano Loki and Loki Patera (an irregular or complex crater, plural paterae). Siding sometimes with the gods and sometimes with the giants, Loki on occasion seems keen to hasten the coming of Ragnarök and its consuming fires. His apetite for destruction is shown in an entertaining story told by Snorri Sturluson in his Edda (and apparently known also to the Danish historian Saxo Grammaticus). Loki and Þórr, the latter eager to test his strength against the giants, travel eastwards into a strange land. When they come to the court of the giant king Útgarða-Loki, the companions are challenged to various contests of strength. Loki is challenged to an eating competition, and is brought to face his challenger across a long table, piled high with food. Loki eats furiously, quickly reaching the table’s middle and leaving white bones picked clean in his wake. When he looks up, however, he sees that his opponent has not only consumed the meat, but the bones and table too. At the end of their games, the king reveals that Loki's impossible adversary was Logi (Old Norse 'flame'), all-consuming fire itself. The king nevertheless marvels at the companions' fearsome abilities, before he and the mysterious castle disappear. Loki Patera, black against the surrounding sulphur yellow, is an enormous lava lake. The whiter regions on its slopes are probably sulphurous frosts.

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Sigurd Patera (52.87 km diameter, -5.77 N, 98.05 W).

Sigurðr the Dragon Slayer

A further two paterae take their names from Norse myth. Balder Patera (36.95 km diameter, 11.46 N, 156.16 W) is named for Baldr, son of Óðinn and the brightest of the gods. Sigurd Patera takes its name from the legendary hero Sigurðr Fáfnísbani, slayer of the dragon Fafnir. In the eddic poem Fáfnismál (The Lay of Fáfnir), Sigurðr slays the dragon and roasts its heart over a fire. Checking to see if it is done, he burns his finger and hastily puts it in his mouth. When the dragon’s blood touches his tongue, he gains the remarkable ability to understand the language of birds.

Sigurðr took Fáfnir’s heart and roasted it on a spit. And when he thought that it was done, and the juice was dripping out of the heart, he prodded it with his finger to see if it was done. He burnt himself and stuck his finger in his mouth. And when Fáfnir’s heart-blood came on his tongue, he understood the speech of birds. 

Fáfnismál (Fáfnir's Sayings).


Sigurðr vanquishes the dragon Fáfnir by hiding in a hollow in the ground and driving his sword into its stomach when it slides over. This historiated initial may depict the slaying, the loop of the Þ forming the hollow in which Sigurðr waits. Reykjavík, Stofnun Árna Magnússonar, GKS 3269 a 4to, f. 15v.

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Bulicame Regio (514.24 km diameter, 34.79 N, 189.94 W).

Capaneus the Blasphemer

Three of Io's names are derived from the Inferno, the first part of Dante Alighieri’s poetic journey through the realms of the afterlife. Bulicame Regio takes its name from the bulicame ('boiling streams') that run through the seventh circle of hell, where the souls of the violent are tormented (Inferno XII.117,127-28). There Dante and his guide, the poet Virgil, see tyrants submerged up to their necks in boiling streams. Bulicame was also the name of a real place in Dante's Italy, the sulphurous hot springs on the road between Rome and Florence (Inferno XIV.79-81). A regio (region) is an area that is typically lighter than its surroundings, likely to be plains of sulphurous frost. 

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Capaneus Mensa on Io (288.08 km diameter, -16.82 N, 121.4 W) is a large plateau around 9.2-9.5 km high, higher than Mount Everest.

In hell's seventh circle Dante and Virgil encounter the giant Capaneus the Blasphemer, who lends his name to a large prominence on Io’s eastern hemisphere. In the Inferno (XIV. 43-72), the giant, whom Dante borrows from Statius’s epic poem the Thebaid, is condemned to lying on his back atop burning sands, while fire rains down on him from the sky. He is being punished for the worst kind of violence, blasphemy (violence directed towards God), having dared Jupiter to strike him down as he assailed the city walls of Thebes. Jupiter, taking a thunderbolt forged on Mongibello Mons (also named on Io, 214.57 km diameter, 22.67 N, 67.05 W, a mountain 8.6 km high), obliges. If you were able to stand on Io's Capaneus Mensa, you would see enormous Jupiter, in angry shades of red and orange, looking back down at you.

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Capaneus the Blasphemer, by William Blake (1824-1827). Held by the National Gallery of Victoria, Melbourne, Australia.