Siarnaq was discovered on 23 September 2000 by astronomers Brett Gladman and John Kavelaars at the Mauna Kea Observatory in Hawaii. Gladman and Kavelaars were using the observatory's 3.6-meter Canada-France-Hawaii Telescope to search for distant irregular satellites around Saturn, as well as to perform follow-up observations of Gladman's previous discoveries Ymir and Paaliaq. During their search, Gladman and Kavelaars found two new satellite candidates, Siarnaq and Tarvos. Preliminary orbit calculations ruled out the possibility that the satellite candidates could be foreground asteroids, and the discoveries were shortly reported to other astronomers for further confirmation. The discovery of Siarnaq and Tarvos was formally announced in a notice published by the International Astronomical Union on 25 October 2000, on the same day as the announcement of Ymir and Paaliaq. The discovery of the two satellites raised Saturn's known moons to 22, surpassing Uranus's moon count of 21 at the time. Follow-up observations of Siarnaq and Tarvos were later conducted at the Kitt Peak National Observatory in December 2000. The discovery of Siarnaq formed part of an observational campaign coordinated by Gladman to search for distant irregular satellites around Saturn. This campaign consisted of an international team of eight astronomers using various ground-based telescopes to survey Saturn's Hill sphere, the region within which satellites can have stable orbits around the planet. From August 2000 to February 2001, the team surveyed 90 percent of Saturn's Hill sphere down to a limiting magnitude of 23 and discovered 12 irregular satellites including Siarnaq, raising Saturn's known moons to 30 and surpassing Jupiter's moon count of 28 at the time.
Naming
The moon is after the giant Siarnaq of Inuit mythology. The name was announced by the International Astronomical Union on 8 August 2003.
Physical characteristics
From infrared observations by the Wide-field Infrared Survey Explorer spacecraft, Siarnaq is estimated to be in diameter. It is light red in color, and the Siarnaupian spectrum in the infrared is very similar to the Inuit-group satellites Paaliaq and Kiviuq, supporting the thesis of a possible common origin in the break-up of a larger body.
Shape and rotation
The rotation period of Siarnaq was measured by the Cassini spacecraft to be 10.19 hours; this is the shortest rotation period of all prograde irregular moons of Saturn. Siarnaq displays a light curve with three maxima and minima over a full rotation, implying a roughly triangular shape. From Cassini observations of Siarnaq at different phase angles, the orientation of its north rotational pole has been determined to be pointing toward 98° ecliptic latitude and −23° ecliptic longitude. This corresponds to a sideways axial tilt, indicating that Siarnaq experiences long, extreme seasons similar to the planet Uranus.
Orbit
Siarnaq orbits Saturn at an average distance of in 897 days. The moon has been found to be in a secular resonance with Saturn, involving the precession of its periapsis and that of the planet. The studies of these resonances are key to understand the capture mechanism for the irregular satellites and, assuming a common origin of a given dynamical group in the break-up of a single body, to explain today's dispersion of the orbital elements.
