Siarnaq

Siarnaq was discovered on Sept. 23, 2000, by Brett J. Gladman, John J. Kavelaars, Jean-Marc Petit, Hans Scholl, Matthew J. Holman, Brian G. Marsden, Phillip D. Nicholson, and Joseph A. Burns using the 3.6-m Canada-France-Hawaii reflector on Mauna Kea, Hawaii, with adaptive optics. They discovered seven other Saturnian moons at the same time: Tarvos, Ijiraq, Thrymr, Skathi, Mundilfari, Erriapus, and Suttungr.

Siarnaq has a mean radius of about 12.4 miles (20 kilometers), assuming an albedo (a measure of how reflective the surface is) of 0.06. At a mean distance of 11 million miles (18 million kilometers) from Saturn, the moon takes about 896 Earth days to complete one orbit. It rotates once every 10 hours and 9 minutes, which is this is the shortest rotation period of all prograde irregular moons of Saturn.

Siarnaq is one of five known members of the Inuit group of moons, which orbit Saturn at a mean distance of 7 to 11 million miles (11 to 18 million kilometers), at inclinations between 40 and 50 degrees from the plane of Saturn's equator, and with eccentricities of 0.15 to 0.48. (A moon's eccentricity is a number between 0 and 1 which describes the shape of the orbit. The closer to 0, the more circular it is; the closer to 1, the more elongated.)

The Inuit moons all have prograde orbits (they travel around Saturn in the same direction as the planet's rotation), but their deviations from circular orbits and from the plane of Saturn's equator classify them as "irregular" moons. Like Saturn's other irregular moons, they are thought to be objects that were captured by Saturn's gravity, rather than having accreted from the dusty disk that surrounded the newly formed planet, as the regular moons are thought to have done.

The similarities among the Inuit group's orbits suggest a common origin — they may be fragments of a single object that shattered in a collision. The other members of this group are Kiviuq, Ijiraq, Paaliaq, and Tarqeq.

Observations by Tommy Grav and James Bauer using telescopes on Mauna Kea, Hawaii in 2006 (before the discovery of Tarqeq) found that Kiviuq, Siarnaq and Paaliaq all are light red with similar infrared features, further supporting the idea of a common origin.

Originally called S/2000 S3, Siarnaq was named for an Inuit goddess of sea creatures who was also queen of the underworld. Siarnaq is a very important deity because, when angered, she withholds seals, walruses and whales from their Inuit hunters. Siarnaq is better known as Sedna, but that name was already taken by an object, believed to be a dwarf planet, which orbits the sun far beyond the Kuiper Belt (possibly in the inner Oort Cloud).