Norse group

The Norse group comprises the retrograde irregular satellites of Saturn. Although they are not a single dynamically related group, all of Saturn's retrograde moons are collectively classed as the Norse group. They are unlikely to have a common origin and their orbital parameters are widely dispersed; more probably they are composed of a number of dynamical clusters with more homogeneous orbital and physical parameters. As of January 2026, there are 197 known members, making it by far the largest group of Saturn's confirmed moons. The Norse group is dominated by moons that are smaller in size, which could be indicative of a recent catastrophic collision event within the population.^[1]

The International Astronomical Union (IAU) reserves names taken from Norse mythology (mostly giants) for the retrograde satellites of Saturn. The exception is Phoebe, whose name is taken from Greek mythology, which was discovered and named long before the others. Only 31 of the moons have names at present.

The discovery of 17 new satellites in this group was announced in October 2019. A team led by Scott S. Sheppard using the Subaru Telescope at Mauna Kea discovered 20 new moons, each about 5 kilometres (3 miles) in diameter. 17 of these fit into the Norse group, one of which was the most distantly orbiting satellite of Saturn known. A public naming contest for the satellites was announced, restricted to names from Norse mythology.^[2] Ten of the satellites received official names in August 2022.^[3]

It is generally considered improbable for all of the Norse group to have originated from a single collision, instead, the members likely have multiple separate origins. Their orbital elements are diverse and distributed; semi-major axes range between 11 and 28 million km, inclinations between 136° and 178° and eccentricities between 0.02 and 0.91. The Saturnian irregular moon population has a steep size distribution, meaning it has a large proportion of small moons relative to larger ones. It has been suggested there was a recent catastrophic collision event that could explain the large number of small moons.^[4] The cause of the steep size distribution was later identified with a portion of the retrograde population, proposed as the "Mundilfari subgroup".^[1]

The role of Phoebe, as the largest irregular moon of Saturn, is significant. While it is unlikely for the entirety of the retrograde moons to be the collisional family of Phoebe,^[4] it is accepted that Phoebe should be involved in the plurality of collisions among the irregulars due to its large diameter.^[5] There are different hypotheses on what it implies for a potential collisional family. Some argue that members of the group would be later collisionally removed by Phoebe due to sharing similar orbital elements,^[5] while others assert the high number of collision events involving it means some collisional family likely exists.^[4]

Several attempts have been made over the years to divide the retrograde satellites of Saturn, now known as the Norse group, into subcategories based on the satellites' orbital characteristics, mainly their semi-major axes or inclinations, and sometimes their eccentricities.

In their 2001 article reporting on the discovery of the first irregular moons around Saturn since that of Phoebe, Gladman et al. separated by inclination four of the five newly discovered retrograde satellites (later named Ymir, Thrymr, Mundilfari, and Suttungr) in a group with Phoebe called the "Phoebe group". They tentatively placed the last moon (Skathi) on its own due to its differing inclination,^[6] though Mundilfari's inclination also differed enough from the rest of the group to be questionable.^[1] A later study disputed Ymir's membership, determining that its colour and semi-major axis made it incompatible with having originated from Phoebe in a collision. They did not attempt to validate the membership of the other three moons.^[7]

In 2008, Nicholson et al. split the retrograde satellites into three groups, but made no attempt to justify their potential shared dynamic origins, simply roughly sorting them by inclination alone. Each group was centred on a moon and their inclination. They did mention the convention of naming all retrograde satellites after figures in Norse mythology, but did not treat them as a group of their own. Their groups were the Phoebe group with an inclination around 175° (Phoebe, Suttungr, Thrymr, Ymir, and Fornjot), the Mundilfari group with an inclination at 168° (Mundilfari, S/2004 S 7, Aegir, S/2004 S 12, S/2004 S 13, Hati, Fenrir, and S/2004 S 17), and the Skathi group with an inclination at 153° (Skathi, Narvi, Farbauti, Bergelmir, and Bestla). They did not provide exhaustive lists of all members in their groups, and commented that nine newly discovered satellites at the time all fell into their inclination groupings but did not specify which moons went into which groups.^[8]

Also in 2008, Turrini et al. pointed out that members from a potential Phoebe family would likely be collisionally removed by the moon itself, which argues against the existence of the family. As half of all potential collisions between irregular moons would involve Phoebe due to its large size, this produces a "sweeping effect" that they presumed was probably the cause of a lack of known moons with semi-major axes around Phoebe's. They proposed that it was not related to any of the other known irregular moons, noting that none of the other satellites had compatible colours. Using an algorithm called the Hierarchical Clustering Method, they found several retrograde "families", which did not cover all 27 of the known retrograde satellites at the time. A group was qualified as a viable family if it passed the condition that the members were close enough in their orbital elements to potentially have a shared collisional origin with realistic dispersion velocities (≤200 m/s). Six of these were found to be acceptably realistic. The six groups were:^[5]

• Bergelmir and S/2006 S 1
• Mundilfari, S/2004 S 13, and S/2004 S 17
• Kari and S/2006 S 3
• Aegir, S/2004 S 12, and Hati
• Fornjot, Loge, and Fenrir
• Narvi and Bestla

They also made larger groups of moons that merged multiple families and previously left-out moons that they called "clusters", that they said could be groups composed of multiple generations of collisional fragments. One of the clusters passed their acceptance criterion, "cluster A", which was composed of Mundilfari's group, Jarnsaxa, and Aegir's group.^[5]

Denk et al. (2018) decided to again split the retrograde moons into six families based on their orbital elements, similar to Turrini et al. They did acknowledge the retrogrades were sometimes treated as a group all together called the "Norse group". They left out fewer moons but still did not put all 29 retrograde moons known at the time in a group. Their groups were:^[9]

• Narvi and Bestla
• Greip, S/2007 S 3, Suttungr, and Thrymr
• S/2004 S 13, Mundilfari, Jarnsaxa, S/2004 S 17, Hati, S/2004 S 12, Aegir, and S/2004 S 7
• S/2006 S 1, Bergelmir, and Farbauti
• Kari and S/2006 S 3
• Fenrir, Surtur, Loge, Ymir, and Fornjot

Besides their main groups, they also speculated on links between Hyrrokkin and Greip, and between Thrymr and S/2004 S 7. They noted that other than Phoebe being distinctly separate from all other moons, clustering in the Norse group was not obvious.^[9]

Also in 2018, Holt et al. used a technique called cladistics to split the retrograde satellites into two main parts. They acknowledged the Norse group, or what they called the "Phoebe Family", as a distinct class of moons, on equal ranks with the "Siarnaq Family" and the "Albiorix Family" (Inuit group and Gallic group respectively). They separated the Norse group into two "subfamilies", which they named them the "Aegir subfamily" and the "Ymir subfamily", named after their largest members. The Aegir subfamily contained the following ten moons: Mundilfari, Jarnsaxa, Bergelmir, Suttungr, Farbauti, Aegir, Fornjot, S/2006 S 1, S/2007 S 2, and S/2007 S 3. The Ymir subfamily contained the following 16 moons: Skathi, S/2004 S 13, S/2004 S 17, Greip, Hyrrokkin, Hati, Ymir, Narvi, Surtur, S/2004 S 12, Skoll, Thrymr, Kari, S/2006 S 3, Bestla, and S/2004 S 7. Only three Norse moons were not in these two subfamilies: Phoebe, which was by itself, and Fenrir and Loge, which they linked together as a pair.^[10]

Ashton et al. (2021) chose to split off the Norse satellites with inclinations within 3° of Phoebe's into a "Phoebe subgroup", and suggested that the remainder of the Norse group could likely be subdivided further. They based this on the fact that Phoebe by itself should be involved in half of all collisions between Saturn's irregular moons,^[5] and concluded from this that a collisional family resulting from these events could feasibly exist. The subgroup contained 11 out of 46 Norse moons, and they were Phoebe, Ymir, Suttungr, Thrymr, Greip, Angrboda, Skrymir, Gerd, Alvaldi, S/2007 S 2, and S/2007 S 3.^[4]

Ashton et al. (2025) opted to distribute the Norse group into three subgroups, each named after their largest member, based on subtle features in the cumulative inclination distribution of the moons. Though they noted that defining the subgroups on inclination alone was likely to produce some interlopers that were put in the wrong subgroup, they did not claim that each subgroup was representative of a collisional family.^[1] The subgroups proposed were:

• The "Phoebe subgroup", made up of 21 moons with inclinations greater than 172°. This criteria was in effect very similar to that of the identically named Phoebe subgroup in Ashton et al. (2021), as all of the moons in the 2021 subgroup ended up in the 2025 subgroup. It was deemed unlikely that this group originated from a single collision event. They noted a cluster of moons with semi-major axes around 20 million km that could be dynamically related, the moons with the lowest semi-major axes (S/2006 S 9, S/2006 S 20, S/2019 S 2, and S/2007 S 2) that could be related with Phoebe, and a cluster of four moons (Angrboda, Alvaldi, S/2019 S 13, and S/2004 S 46) that could be related to each other. They also highlighted Ymir's relative isolation within the subgroup.
• The "Mundilfari subgroup", made up of moons with inclinations between 157° and 172°. This inclination subgroup was distinguished because it had a significantly larger fraction of small moons than the others, possibly evidence of a recent collision. They also identified two moons that could be part of this subgroup instead of the Phoebe subgroup, S/2007 S 2 and Saturn LVIII.
• The "Kari subgroup", made up of 12 moons with inclinations between 151.7° and 157°. Most of the moons in this subgroup share a similar semi-major axis, besides a few with appreciably lower semi-major axes, possibly evidence of a collisional family.

The remaining five moons were denoted the low-inclination moons, but not a group per se, with inclinations less than 151.7°. They considered Narvi and S/2019 S 11 probably related, and Skathi and Hyrrokkin possibly related, with Bestla remaining alone.^[1]

The following is a table of the members of the Norse group, along with their estimated diameters, their semi-major axes, eccentricities, and inclinations, and which subgroups they belong to according to various sources. Their subgroup is denoted by their primary moon or largest member and a unique colour. By default they are sorted in order of their announcement date.

• List of natural satellites
• Moons of Saturn
• Collisional family

• Scott S. Sheppard - SaturnMoons
• Outer Moons of Saturn - tilmanndenk
• Robert Johnston: Orbital and dynamical data for solar system planets and satellites