The term catena (Latin ‘chain’, plural catenae) describes a pitted line of craters formed when a comet or meteorite breaks up on collision with a planet's surface. On Callisto, there are eight catenae, each of which takes its name from the rivers listed by Grímnir in the eddic poem Grímnismál (Sayings of Grímnir).
Eikþyrnir is the hart’s name, who stands on Father of Hosts’ hall
and browses on Lærad’s branches;
and from his antlers there's dripping into Hvergelmir,
from thence all waters make their way:
Síð and Víð, Sækin and Eikin,
Svöl and Gunnþró, Fjörm and Fimbulþul,
Rín and Rennandi, Gipul and Göpul,
Gömul ok Geirvimul, they flow round the gods' treasure hoard,
Þyn and Vín, Þöll and Höll, Gráð and Gunnþorin.
Grimnismál (Sayings of Grimnir), verses 26-27
The shadowy Grímnir (‘mask’, or ‘hood’) is one of the guides of Óðinn, a Norse god of wisdom, war and poetry, who throughout the Sayings of Grímnir reveals his deep mastery over various kinds of mythological knowledge. He tells us about Óðinn’s hall Valhöll, the hall of the slain, and the mighty stag Eikþyrnir (Old Norse 'Oak-thorny’) who picks at the grass on its roof.
Eikþyrnir's horns glisten with dew, which falls down in drips into a spring at its feet called Hvergelmir (Old Norse ‘bubbling spring’). From this pool, all the world's rivers flow. According to Snorri Sturluson, the waters of Hvergelmir writhed with snakes, and the terrible dragon Níðhöggr (‘Hostile-Hewer’) stirred in its depths.
On Callisto, these pitted lines of craters may extend to 100s of kilometers in length, and reach through a world teeming with names taken from eddic poems including Grimnismál, Rígsþula (The List of Ríg), and Hyndluljóð (The Song of Hyndla).
Gipul Catena, located in Callisto's north polar region, is the longest of them, extending over 620 km. It is an enormous structure, made up of craters whose individual diameters can reach up to 40 km.
These rivers, which are called Élivágar, when they had got so far from their source that the poisonous flow accompanying them began to harden like the cinder that comes from a fire, it turned to ice; and when this ice came to a halt and stopped flowing, the vapour that was rising from the poison froze on the top and congealed into rime, and this rime grew, layer upon layer, right across Ginungagap.
Snorri Sturluson, Gylfaginning (c. 1220)
The rivers flowing from Hvergelmir were sometimes called the Élivágar (‘Mighty Waves’). These rivers coursed through the early cosmos, steaming with noxious vapours and spitting poisonous droplets. According to Vafþrúðnismál (The Sayings of Vafþrúðnir), these torrents froze over into terrible shapes, the poisonous spray from these rivers hardening into the ancestors of the giants.
The hver in Hvergelmir means hot spring in Icelandic. When medieval Icelanders thought about the mythical spring in Grímnismál they might have had their own distinctive landscape in mind. The grotesque shapes formed by mineral deposits around Icelandic hotsprings resemble the strange shapes that we are told freeze over Hvergelmir, becoming the giants.