Gallic group

Lore & Background

The Gallic group is named after figures from Gallic mythology, as reserved by the International Astronomical Union, though only four of its moons have names at present. 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; it will nevertheless receive a name from Gallic mythology.

Similar mean orbital elements led the discoverers to postulate a common origin for the group in a breakup of a larger body. The group was later found to be physically homogeneous, all satellites displaying light-red colour and similar infrared indices. Later observations revealed that the largest member, 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. Such an impact would require a body with a diameter in excess of 1.25 km and relative velocity of 4.79 km/s, resulting in a large crater with a radius of 12 km.

The Gallic group is not centred on Albiorix; within the group, relative to Albiorix, all other moons orbit at a larger distance, and almost all have greater inclinations. The eccentricity somewhat increases with semi-major axis. The group has a moderately large orbital spread and is unusual in that it has several large or medium size moons and fewer smaller members, casting doubt on whether it is a true dynamical family. One study found the group is unlikely to have a shared collisional origin unless fragments were ejected asymmetrically or some other mechanism later modified the moons' semi-major axes. Another study simulated a head-on collision scenario and found it plausible, but dispersion velocities only fit realistic ranges when S/2004 S 24 was excluded, leading to alternative explanations for that moon. A further study suggested the group may have been affected by the near-orbital resonance between Jupiter and Saturn known as the Great Inequality, which may have depleted the population rather than spreading it out.

Reader's Guide

The Gallic group is significant as one of the dynamical groupings of Saturn's irregular moons, notable for its prograde orbits and likely collisional origin. The article highlights that the group's largest member, Albiorix, shows two different colours, suggesting that Tarvos and Erriapus may be fragments from an impact on Albiorix, leaving a large crater. This scenario is supported by evidence of similar large collisions on Phoebe, an irregular moon in the Norse group. However, the group's status as a true collisional family is debated: its unusual size distribution—several large or medium moons and fewer small ones—and the outlier S/2004 S 24 raise doubts. Studies have proposed that a head-on collision could explain the group's orbital spread, but only if S/2004 S 24 is excluded, leading to suggestions that it may have originated from a secondary collision or be unrelated. Additionally, the Great Inequality resonance between Jupiter and Saturn may have eliminated most original fragments, leaving the present members as survivors. The group's legacy lies in illustrating the complexity of satellite formation and evolution in the Saturnian system, with ongoing uncertainty about its precise origin.

Did You Know?

• The Gallic group is also called the first inclination group or the 34° inclination cluster.
• The largest member of the group, Albiorix, displays two different colours, one compatible with Erriapus and Tarvos.
• S/2004 S 24 has a much higher semi-major axis and lower eccentricity than other Gallic moons, and its origin is uncertain.