Gallic group

{{Short description|Category of satellites of Saturn}}

File:Animation of Saturn's Gallic group of satellites.gif

File:Saturn irregular moon orbits a vs. i.png. The X-axis is labeled in terms of Saturn's Hill radius.]]

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 36° and 41°, and their eccentricities between 0.46 and 0.53. 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.{{cite journal |last1=Gladman |first1=B. J. |author1-link=Brett J. Gladman |last2=Nicholson |first2=P. |author2-link=Phil Nicholson |last3=Burns |first3=J. A. |author3-link=Joseph A. Burns |last4=Kavelaars |first4=J. J. |author4-link=John J. Kavelaars |last5=Marsden |first5=B. G. |author5-link=Brian G. Marsden |last6=Holman |first6=M. J. |author6-link=Matthew J. Holman |last7=Grav |first7=T. |author7-link=Tommy Grav |display-authors=etal |date=2001 |title=Discovery of 12 satellites of Saturn exhibiting orbital clustering |journal=Nature |volume=412 |issue=6843 |pages=163–6 |doi=10.1038/35084032 |pmid=11449267 |s2cid=4420031}} 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){{cite journal |last1=Grav |first1=Tommy |last2=Holman |first2=Matthew J. |author-link2=Matthew J. Holman |last3=Gladman |first3=Brett J. |author-link3=Brett J. Gladman |last4=Aksnes |first4=Kaare |author-link4=Kaare Aksnes |year=2003 |title=Photometric survey of the irregular satellites |journal=Icarus |volume=166 |issue=1 |pages=33–45 |arxiv=astro-ph/0301016 |bibcode=2003Icar..166...33G |doi=10.1016/j.icarus.2003.07.005 |s2cid=7793999}} and similar infrared indices.{{cite journal |last1=Grav |first1=Tommy |last2=Holman |first2=Matthew J |author-link2=Matthew J. Holman |year=2004 |title=Near-Infrared Photometry of the Irregular Satellites of Jupiter and Saturn |journal=The Astrophysical Journal |volume=605 |issue=2 |pages=L141–L144 |arxiv=astro-ph/0312571 |bibcode=2004ApJ...605L.141G |doi=10.1086/420881 |s2cid=15665146}}

Remarkably, recent observations revealed that the largest member of the group, Albiorix, actually displays 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.{{Cite journal |last1=Grav |first1=T. |last2=Bauer |first2=J. |author-link2=James Bauer (astronomer) |date=2007-11-01 |title=A deeper look at the colors of the Saturnian irregular satellites |journal=Icarus |volume=191 |issue=1 |pages=267–285 |doi=10.1016/j.icarus.2007.04.020 |arxiv=astro-ph/0611590 |bibcode=2007Icar..191..267G }} Such an impact would require a body with the diameter in excess of 1.25 km and relative velocity of 4.79 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 and of similar inclination, but it orbits much further away from Saturn than the other Gallic moons. This moon will nevertheless also receive a name from Gallic mythology.{{Cite web |last=NASA |date=October 7, 2019 |title=Saturn surpasses Jupiter after the discovery of 20 new moons—and you can help name them |url=https://phys.org/news/2019-10-saturn-surpasses-kupiter-discovery-moonsand.html |website=phys.org |language=en}}

The seventeen members of the group are (in order of increasing distance from Saturn according to JPL mean orbital elements):

class="wikitable sortable" style="text-align:center;"
Name || Diameter (km) || Period (days) || Subgroup{{cite web |title = Orbital and dynamical data for solar system planets and satellites |url = https://www.johnstonsarchive.net/astro/solar_system_orb_dyn_data.html |publisher = Wm. Robert Johnston |access-date = 17 April 2025}}
id=Albiorix bgcolor=#fdf |Albiorix	28.6	783.49	Albiorix
id=S/2020_S_15 bgcolor=#fdf |S/2020 S 15	3	815.52	Albiorix
id=Bebhionn bgcolor=#fdf |Bebhionn	7	834.94	Albiorix
id=S/2007_S_8 bgcolor=#fdf |S/2007 S 8	4	836.90	Albiorix
id=S/2004_S_29 bgcolor=#fdf |S/2004 S 29	5	837.78	Albiorix
id=S/2023_S_18 bgcolor=#fdf |S/2023 S 18	3	844.33	Albiorix
id=S/2023_S_17 bgcolor=#fdf |S/2023 S 17	3	852.50	Albiorix
id=S/2019_S_29 bgcolor=#fdf |S/2019 S 29	3	852.85	Albiorix
id=Erriapus bgcolor=#fdf |Erriapus	12	871.10	Albiorix
id=S/2007_S_11 bgcolor=#fdf |S/2007 S 11	4	871.95	Albiorix
id=S/2019_S_31 bgcolor=#fdf |S/2019 S 31	3	885.96	Albiorix
id=Tarvos bgcolor=#fdf |Tarvos	16	926.37	Albiorix
id=S/2020_S_4 bgcolor=#fdf |S/2020 S 4	3	926.92	Albiorix
id=S/2019_S_34 bgcolor=#fdf |S/2019 S 34	3	925.57	Albiorix
id=S/2005_S_7 bgcolor=#fdf |S/2005 S 7	3	957.36	Albiorix
id=S/2006_S_12 bgcolor=#fdf |S/2006 S 12	4	1035.05	Albiorix
id=S/2004_S_24 bgcolor=#dfd |S/2004 S 24{{refn | group = lower-alpha | Its orbital elements are identical to the other moons in the Gallic group. However, it is more distant}}	3	1341.33	Outlier