Animation of Saturn's Gallic group of satellites Saturn Albiorix · Bebhionn · Erriapus · Tarvos Diagram illustrating the orbits of the irregular satellites of Saturn, with major groups and moons labeled. The inclination and semi-major axis are represented on the Y and X-axis, respectively. The satellites with inclinations below 90° are prograde, those above 90° are retrograde. The X-axis is labeled in terms of Saturn's Hill radius.
The Gallic group is a dynamical grouping of progradeirregular satellites of Saturn. They are thought have a common origin in a captured asteroid that was later involved in a collision, making them a likely collisional family. Most of the moons follow similar orbits, although one moon, S/2004 S 24, has a much higher semi-major axis and a much lower eccentricity than the others, so it is not known whether it originated from the same parent body as the rest of the group.[1] The International Astronomical Union (IAU) reserves names taken from Gallic mythology for these moons, though only four of them have names at present.
Similar mean orbital elements led the discoverers to postulate a common origin for the group in a breakup of a larger body.[2] 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)[3] and similar infrared indices.[4]
Later 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. Rather than directly originating from the common progenitor, it was postulated that Tarvos and Erriapus could instead be fragments of Albiorix, leaving a large, less red crater.[5] Such an impact would require a body with the diameter in excess of 1.25km and relative velocity of 4.79km/s, resulting in a large crater with a radius of 12km. Numerous, very large craters observed on Phoebe (an irregular moon in the Norse group) prove the existence of such collisions in the Saturnian system's past.
There is also a theory that the progenitor moon suffered a head-on collision, which could explain some trends in the orbital elements of the Gallic group. The orbital parameters are fairly compact; the semi-major axes mostly range between 16 and 19 million km, their inclinations between 36° and 41°, and their eccentricities between 0.46 and 0.53. Within the Gallic group, relative to Albiorix, all other moons orbit at a larger distance, and almost all of them have greater inclinations. The eccentricity somewhat increases with semi-major axis.[1]
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.[6]
List
The seventeen members of the group are (in order by date announcement):[7]
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