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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. 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. 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. The four members of the group are (in order of increasing distance from Saturn): *Albiorix *Bebhionn *Erriapus *Tarvos 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)〔Grav, Tommy; Holman, Matthew J.; Gladman, Brett J.; Aksnes, Kaare ''Photometric survey of the irregular satellites'', Icarus, 166,(2003), pp. 33-45. (Preprint )〕 and similar infrared indices 〔 Grav, Tommy; and Holman, Matthew J.; ''Near-Infrared Photometry of the Irregular Satellites of Jupiter and Saturn'', The Astrophysical Journal, 605, (2004), pp. L141–L144 (Preprint )〕 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.〔 Grav, Tommy; and Bauer, James; ''A deeper look at the colors of Saturnian irregular satellites'', (Preprint )〕 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. ==References== *Ephemeris (from MPC ) *Mean orbital parameters (from JPL ) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Saturn's Gallic group of satellites」の詳細全文を読む スポンサード リンク
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