Top Qs
Timeline
Chat
Perspective
Norse group
Category of satellites of Saturn From Wikipedia, the free encyclopedia
Remove ads
The Norse group is a large group of retrograde irregular satellites of Saturn. Their semi-major axes range between 12 and 27 Gm, their inclinations between 136° and 178° and their eccentricities between 0.06 and 0.63. Unlike the Inuit and Gallic groups, the orbital parameters are widely dispersed and the group is likely to be composed from a number of dynamically related subgroups with more homogeneous orbital and physical parameters. The International Astronomical Union (IAU) reserves names taken from Norse mythology (mostly giants) for the satellites, with the exception of Phoebe (Greek mythology), the largest, which was discovered long before the others.
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 (3.1 miles) in diameter. 17 of these are thought to fit into the Norse group. One of these is the most distantly orbiting satellite of Saturn. A public naming contest for the satellites was announced, restricted to names from Norse mythology.[1] Ten of the satellites received official names in August 2022.[2]
Remove ads
Subgroups
Summarize
Perspective
Several attempts have been made over the years to divide the retrograde satellites, 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 besides Phoebe, Gladman et al. separated by inclination four of the five newly discovered retrograde satellites (later known as 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.[3]
In 2003, Grav et al. performed an analysis and determined that Ymir's colour and semi-major axis made it incompatible with having originated from Phoebe in a collision. They concluded that Gladman et al.'s Phoebe group was most likely composed of two dynamical groups instead of one, centred on the moons Phoebe and Ymir, both around 174° in inclination. They did not attempt to classify the other three moons in the Phoebe group with either Phoebe or Ymir.[4]
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 retograde satellites after figures in Norse mythology, but did not treat them as a group of their own. These 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 the nine newly discovered satellies at the time all fell into their inclination groupings but did not specify which moons went into which groups.[5]
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, 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 instead proposed a total of six retrograde "families" (not necessarily covering all 27 of the known retrograde satellites at the time), each containing two or three moons, by assuming that moons in a group have a shared collisional origin with realistic dispersion velocities (≤200 m/s). The six groups were:[6]
- 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 could be groups composed of multiple generations of collisional fragments. They commented on two clusters that they call A and B: Cluster A, composed of Mundilfari's group, Jarnsaxa, and Aegir's group, and Cluster B, similar to Cluster A but also including Bergelmir's group.[6]
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:[7]
- 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.[7]
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 moons were not in these two subfamilies: Phoebe, which was by itself, and Fenrir and Loge, which they linked together as a pair.[8]
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,[6] 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.[9]
Ashton et al. (2025) opted to distribute the Norse group into subgroups based on subtle features in the cumulative inclination disribution 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 group.[10] The subgroups (each named after their largest member) proposed were:
- The "Phoebe subgroup", made up of 21 moons with inclinations greater than 172.5°. This criteria was in effect very similar to that of the Phoebe subgroup in Ashton et al. (2021), as all of the moons in the 2021 Phoebe subgroup ended up in the 2025 Phoebe subgroup. They deemed it unlikely that this group originated from a single collision event, noting 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 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 highighted Ymir's relative isolation within the subgroup.
- The "Mundilfari subgroup", made up of moons with inclinations between 157° and 172.5°. 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, Greip and Saturn LVIII.
- The "Kari subgroup", made up of moons with inclinations between 151° 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, with inclinations less than 151°. Of these, they considered Narvi and S/2019 S 11 probably related, and Skathi and Hyrrokkin possibly related, with Bestla remaining alone.[10]
Remove ads
List
Summarize
Perspective
The following is a table of the members of the Norse group, along with their estimated diameters, their orbital periods, and which subgroups they belong to as described in Ashton et al. (2025).[10] For the 128 newly discovered satellites not included in the paper, they are sorted into their categories based on their stated inclination criteria. By default they are sorted in order of increasing distance from Saturn, according to JPL's mean orbital elements.[11][needs update]
Remove ads
Notes
- May be a fragment of Phoebe, since it orbits at close proximity to Phoebe and could potentially be a part of the Phoebe subgroup.
See also
References
External links
Wikiwand - on
Seamless Wikipedia browsing. On steroids.
Remove ads