Realms of the Gods
When Voyager 1 returned the first images of Callisto to Earth in 1979, planetary scientists saw the largest multi-ring basin in the solar system. Basins are unfathomably large craters, their diameters exceeding around 290 km, which have been formed by giant impacts. These structures are usually characterised by a massive basin at their centre, encircled by concentric mountain chains. The names assigned to these structures on Callisto derive from the names of the realms of the gods in northern mythologies. Three of these – Asgard, Utgard, and Valhalla – are divine realms in the Old Norse mythic universe, while another – Adlinda – derives from Inuit religion.
‘There gold-bright Valhall
extends out widely;
there Óðinn chooses every day
those dead in combat.’
Grimnismál (Sayings of Grimnir), verse 8
Valhalla is a structure of planetary proportions. With a diameter of around 3000 km, it comprises a bright circular impact crater at its centrer, measuring 360 km across, encircled by concentric ranges of broken mountains extending out 1,900 km. It takes its name from Óðinn’s great hall, Valhöll. Óðinn, Norse god of war, wisdom, and poetry, sows discord among humans so that he may select the best of those who die in battle for his Einherjar, warriors who will fight alongside the Norse gods at Ragnarök. His Valkyries, ‘choosers of the slain’, watch over battles, and decide which side will emerge victorious. Those they choose, however, may not always be the strongest or most worthy. In the eddic poem Lokasenna (Loki's Quarrel), Loki accuses Óðinn of betraying the best warriors in battle so that they can join him sooner in Valhöll (verse 22). Valhalla's enormous size is in proportion with its description in Grimnismál, where Grimnir tells us that Valhöll had 540 doors, each large enough enough for 960 warriors to march through (verse 23).
'The land is sacred which I see lying
near the Æsir and the elves'.
Grimnismál (Sayings of Grimnir), verse 4
Asgard is smaller than Valhalla, but with a diameter of around 1400 km, remains a striking presence on Callisto's landscape. It takes its name from the Norse Ásgarðr, home of the Norse divinities called the Æsir. At its middle stands Yggdrasill, the world tree, its branches touching the sky and its roots penetrating deep into the earth. Oðinn’s hall, Valhöll, stands in Ásgarðr at the foot of the world tree.
Then they saw a castle standing on some open ground and had to bend their heads back to touch their spines before they could see up over it.
Snorri Sturluson, Gylfaginning (c. 1220)
Utgard is the smallest of Callisto’s multi-ring basins, located within the northern part of Asgard. It takes its name from Útgarðr, an enormous castle in the land of the giants, home to King Útgarða-Loki. In this entertaining story, related in Snorri Sturluson's Edda (and apparently known also to the Danish historian Saxo Grammaticus), Þórr and Loki travel eastwards and across the sea into a strange land. When they find themselves at the court of the giant King Útgarða-Loki, Þórr and his companions are challenged to contests of strength and ability – a race, a wrestling match, an eating competition – and are soundly beaten by the king’s courtiers. Unknown to the gods, however, Útgarðr is wreathed in illusion. At the end of their games, the king reveals his courtiers' true identities. Þórr's challenger in wrestling was not an old lady, as she appeared, but old age itself, while Loki's challenger in the eating competition was the personification of all-consuming fire. Though the gods were beaten, King Útgarða-Loki marvels at their fearsome abilities. In awe, he and the mysterious castle of Útgarðr disappear.
The Adlinda structure takes its name from a realm of the gods not in Norse myth, but from Inuit religion. In Inuit religion, the Adlivun or Idliragijenget (‘those who live beneath us’) are the souls of the departed who, in some traditions, rest at the bottom of the ocean before journeying to their final resting place on the moon (Qudlivun). Adlivun, which is usually described as a frozen realm at the bottom of the sea, is ruled over by Sedna, goddess of the sea and marine animals, who prepares souls for the next stage of their journey.
Both Adlivun and Qudlivun have been assigned places in our solar system. Quidlivun Cavus is the name provisionally assigned to a small depression on Pluto, discovered by New Horizons in 2015. Since 2004, Sedna has been the name of a dwarf planet in the outer reaches of the solar system, beyond even Pluto's orbit. At its most distant, Sedna lies 900 times more distant than the Earth from the sun, completing a single circuit of the sun once in every 11,400 years. From this cold and lonely world, the sun would scarcely be bigger than any other star in the sky.