Janus, also known as Saturn X, is a natural satellite of the outer planet, Saturn. Janus was discovered on December 15, 1966 by the late French astronomer Audouin Dollfus.
Discovery and naming[]
Janus was identified by Audouin Dollfus on 15 December 1966 and given the temporary designation S/1966 S 2, Previously photographed on 29 October 1966 without realizing it. On 18 December, Richard Walker observed an object in the same orbit as Janus, but whose position could not be reunited with the previous observations. Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain realized that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits, Walker is now credited with the discovery of Epimetheus. Voyager 1 confirmed this orbital configuration in 1980.
Janus is named after the two-faced Roman god Janus. Although the name was proposed soon after the initial 1966 discovery, it was not officially adopted until 1983, when Epimetheus was also named.[1]
Orbit[]
Janus's orbit is co-orbital with that of Epimetheus. Janus' mean orbital radius from Saturn was, as of 2006, only 50 km less than that of Epimetheus, a distance smaller than either moon's mean radius. Each day, the inner moon is an additional 0.25° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction increases the inner moon's momentum and decreases that of the outer moon. This added momentum means that the inner moon's distance from Saturn and orbital period are increased, and in exchange the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that the moons effectively swap orbits, never approaching closer than about 10,000 km. At each encounter Janus's orbital radius changes by ~20 km and Epimetheus's by ~80 km: Janus's orbit is less affected because it is four times as massive as Epimetheus.[2]
Phyical Characteristics[]
Janus is extensively cratered with several craters larger than 30 km and has few linear features. Janus's surface appears to be older than Prometheus' but younger than Pandora's. Janus has a very low density and relatively high albedo, meaning that it is likely icy in composition and structurally a rubble pile object.
Features[]
Craters on Janus, like those on Epimetheus, are named after characters in the legend of Castor and Pollux.
Interactions with rings[]
A faint dust ring is present around the region occupied by the orbits of Janus and Epimetheus, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006. The ring has a radial extent of about 5000 km. Its source is particles blasted off their surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.
Along with Epimetheus, Janus acts as a shepherd moon, maintaining the sharp outer edge of the A Ring in a 7:6 orbital resonance. The effect is more obvious when the more massive Janus is on the resonant (inner) orbit.
Gallery[]
| Geology of Janus and Epimetheus | ||
|---|---|---|
| Janus | Craters | Castor • Idas • Lynceus • Phoibe |
| Epimetheus | Craters | Hilaeira • Pollux |
| Moons of Saturn | ||
|---|---|---|
| Inner Moons | Ring Moonlets | S/2009 S 1 • Aegaeon |
| Ring Shepherds | Pan • Daphnis • Atlas • Prometheus • Pandora • Epimetheus • Janus | |
| Alkyonides | Methone • Pallene • Anthe | |
| Major Moons and Trojans | Large Moons | Mimas • Enceladus • Tethys • Dione • Rhea • Titan • Hyperion • Iapetus |
| Tethys/Dione Trojans | Tethys Trojans (Telesto • Calypso) Dione Trojans (Helene • Polydeuces) | |
| Inuit | Kiviuq Subgroup | S/2023 S 6 • S/2023 S 1 • S/2019 S 1 • S/2004 S 54 • S/2019 S 22 • S/2019 S 23 • S/2020 S 11 • S/2019 S 25 • Kiviuq • S/2023 S 2 • S/2004 S 55 • S/2005 S 4 • S/2020 S 12 • S/2020 S 1 • Ijiraq • S/2019 S 24 • S/2007 S 10 • S/2019 S 26 • S/2020 S 13 • S/2023 S 7 |
| Paaliaq | Paaliaq | |
| Siarnaq Subgroup | S/2023 S 19 • S/2004 S 31 • S/2023 S 3 • S/2019 S 32 • Tarqeq • S/2019 S 14 • S/2020 S 19 • Siarnaq • S/2005 S 6 • S/2020 S 3 • S/2004 S 58 • S/2006 S 23 • S/2019 S 6 • S/2020 S 5 • S/2023 S 22 | |
| Gallic | Albiorix • S/2020 S 15 • Bebhionn • S/2007 S 8 • S/2004 S 29 • S/2023 S 18 • S/2023 S 17 • S/2019 S 29 • Erriapus • S/2007 S 11 • S/2019 S 31 • Tarvos • S/2020 S 4 • S/2019 S 34 • S/2005 S 7 • S/2006 S 12 • S/2004 S 24 | |
| Norse | Low-inclination | Skathi • S/2020 S 17 • S/2023 S 20 • Hyrrokkin • Narvi • S/2023 S 27 • S/2019 S 37 • Bestla • S/2019 S 11 • S/2023 S 30 • S/2020 S 27 • S/2023 S 46 |
| Kari Subgroup | S/2019 S 28 • S/2006 S 25 • S/2006 S 1 • S/2004 S 45 • S/2023 S 33 • S/2020 S 30 • S/2006 S 3 • Kari • Geirrod • S/2019 S 17 • S/2019 S 19 • S/2019 S 18 • S/2004 S 21 • S/2004 S 36 • S/2023 S 45 • S/2019 S 20 | |
| Mundilfari Subgroup | S/2004 S 56 • S/2023 S 8 • S/2023 S 11 • S/2006 S 21 • S/2023 S 9 • S/2006 S 22 • S/2023 S 10 • S/2023 S 12 • S/2007 S 5 • S/2007 S 7 • S/2004 S 37 • S/2004 S 47 • S/2004 S 40 • S/2020 S 14 • S/2019 S 27 • S/2023 S 13 • S/2019 S 3 • S/2020 S 16 • S/2023 S 14 • S/2023 S 16 • S/2020 S 7 • S/2023 S 15 • S/2023 S 50 • Skoll • S/2020 S 18 • S/2019 S 30 • S/2020 S 2 • S/2023 S 4 • S/2019 S 4 • S/2004 S 41 • S/2004 S 57 • S/2020 S 20 • S/2006 S 24 • S/2004 S 42 • S/2019 S 35 • S/2004 S 13 • S/2007 S 6 • Mundilfari • S/2023 S 21 • S/2020 S 21 • S/2019 S 33 • S/2004 S 43 • S/2006 S 10 • S/2019 S 5 • S/2004 S 59 • S/2023 S 23 • S/2020 S 22 • Gridr • Bergelmir • Jarnsaxa • S/2023 S 24 • S/2006 S 27 • S/2004 S 44 • S/2020 S 23 • Hati • S/2004 S 17 • S/2023 S 25 • S/2004 S 12 • Eggther• S/2006 S 13 • S/2023 S 28 • S/2019 S 36 • S/2007 S 9 • Farbauti • S/2023 S 26 • S/2019 S 9 • S/2023 S 48 • S/2020 S 24 • Aegir • S/2019 S 10 • Beli • S/2020 S 25 • S/2023 S 29 • S/2004 S 61 • S/2019 S 12 • S/2023 S 32 • S/2006 S 14 • Gunnlod • S/2019 S 15 • S/2023 S 31 • S/2020 S 6 • S/2004 S 7 • S/2005 S 5 • S/2023 S 34 • S/2020 S 32 • S/2020 S 26 • S/2006 S 16 • S/2006 S 15 • S/2023 S 38 • S/2023 S 36 • S/2004 S 28 • S/2020 S 29 • S/2020 S 8 • S/2023 S 35 • S/2023 S 49 • S/2023 S 39 • S/2004 S 48 • S/2023 S 40 • Fenrir •S/2004 S 50 • S/2019 S 38 • S/2006 S 17 • S/2004 S 49 • S/2023 S 42 • S/2020 S 28 • Surtur • S/2006 S 18 • S/2020 S 34 • S/2020 S 31 • Loge • S/2004 S 39 • S/2019 S 16 • S/2020 S 36 • S/2004 S 53 • S/2020 S 40 • S/2020 S 33 • S/2023 S 43 • Thiazzi • S/2020 S 38 • S/2019 S 42 • S/2019 S 39 • S/2004 S 34 • S/2019 S 40 • S/2020 S 39 • S/2020 S 42 • S/2019 S 41 • Fornjot • S/2023 S 5 • S/2004 S 51 • S/2020 S 10 • S/2020 S 41 • S/2020 S 9 • S/2006 S 29 • S/2019 S 44 • S/2020 S 43 • S/2023 S 44 • S/2019 S 21 • S/2004 S 52 • S/2023 S 47 • S/2019 S 43 • S/2020 S 44
| |
| Phoebe Subgroup | Phoebe • S/2006 S 20 • S/2006 S 9 • S/2007 S 2 • S/2019 S 2 • Greip • S/2006 S 26 • Suttungr • S/2004 S 60 • S/2007 S 3 • S/2006 S 11 • S/2019 S 7 • S/2019 S 8 • Thrymr • S/2004 S 46 • Angrboda • Gerd • S/2019 S 13 • Skrymir • S/2023 S 37 • Alvaldi • S/2023 S 41 • S/2006 S 28 • Ymir • S/2020 S 37 • S/2020 S 35 • S/2006 S 19 • S/2004 S 26 | |
| Other Moons | Hypothetical | Chiron • Themis |
| Unconfirmed Moonlets | S/2004 S 6 • S/2004 S 4 • S/2004 S 3 | |
| Propeller Moonlets | Peggy • Bleroit • Earhart • Santos-Demont | |