Saturn's Gallic group of satellites

The Gallic group is a dynamical grouping of the prograde irregular satellites of Saturn following similar orbits. Their semi-major axes range between 16 and 19 Gm, their inclinations between 35° and 40°, and their eccentricities around 0.53.

Irregular satellites of Saturn. The diagram illustrates the Gallic group in relation to other irregular satellites of Saturn. The eccentricity of the orbits is represented by the yellow segments (extending from the pericentre to the apocentre) with the inclination represented on Y axis.
Animation of Saturn's Gallic group of satellites   Saturn    Albiorix  ·    Bebhionn  ·    Erriapus  ·    Tarvos

The International Astronomical Union (IAU) reserves names taken from Gallic mythology for these moons.

Similar mean orbital elements led the discoverers to postulate a common origin for the group in a breakup of a larger body.[1] The group was later found to be physically homogeneous, all satellites displaying light-red colour (colour indices B − V = 0.91 and V − R = 0.48)[2] and similar infrared indices [3]

Remarkably, recent observations revealed that the largest member of the group, Albiorix, displays actually two different colours: one compatible with Erriapus and Tarvos, and another less red. Instead of the common progenitor, it was postulated that Tarvos and Erriapus could be fragments of Albiorix, leaving a large, less red crater.[4] Such an impact would require a body with the diameter in excess of 1 km and relative velocity close to 5 km/s, resulting in a large crater with the radius of 12 km. Numerous, very large craters observed on Phoebe, prove the existence of such collisions in the Saturnian system's past.

The discovery of 20 new moons of Saturn was announced in October 2019 by a team led by Scott S. Sheppard using the Subaru Telescope at Mauna Kea. One of them, S/2004 S 24, is also prograde, but it orbits much further away from Saturn than the four known Gallic moons. This moon will nevertheless also receive a name from Gallic mythology.[5]

The four members of the group discovered before 2019 are (in order of increasing distance from Saturn):

References
  1. Gladman, B. J.; Nicholson, P.; Burns, J. A.; Kavelaars, J. J.; Marsden, B. G.; Holman, M. J.; Grav, T.; et al. (2001). "Discovery of 12 satellites of Saturn exhibiting orbital clustering". Nature. 412 (412): 163–6. doi:10.1038/35084032. PMID 11449267.
  2. Grav, Tommy; Holman, Matthew J.; Gladman, Brett J.; Aksnes, Kaare (2003). "Photometric survey of the irregular satellites". Icarus. 166: 33–45. arXiv:astro-ph/0301016. Bibcode:2003Icar..166...33G. doi:10.1016/j.icarus.2003.07.005.
  3. Grav, Tommy; Holman, Matthew J (2004). "Near-Infrared Photometry of the Irregular Satellites of Jupiter and Saturn". The Astrophysical Journal. 605: L141–L144. arXiv:astro-ph/0312571. Bibcode:2004ApJ...605L.141G. doi:10.1086/420881.
  4. Grav, Tommy; and Bauer, James; A deeper look at the colors of Saturnian irregular satellites, Preprint
  5. Saturn surpasses Jupiter after the discovery of 20 new moons—and you can help name them, NASA, phys.org, October 7, 2019
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