Saturn's Trojan moons
Saturn's Trojan moons are four small moons that share the orbit of a larger moon, sitting near the stable Lagrange points 60° ahead of or behind it. Telesto and Calypso accompany Tethys; Helene and Polydeuces accompany Dione. Saturn is the only planet known to have Trojan companions of its moons, a configuration found elsewhere for planets themselves, such as Jupiter's Trojan asteroids.
Contents
What a Trojan moon is
In a system of two bodies, such as Saturn and one of its moons, there are five points where a third, much smaller object can keep a fixed position relative to the other two. These are the Lagrange points, named for the 18th-century mathematician Joseph-Louis Lagrange. Two of them, L4 and L5, lie on the moon's orbit, 60° ahead and 60° behind, each forming an equilateral triangle with Saturn and the moon. Objects placed there are stable: if nudged, they drift around the point rather than escaping.
The name "Trojan" comes from the asteroids found at Jupiter's L4 and L5 points, which were named after heroes of the Trojan War. By extension, any small body sharing an orbit at those points is called a Trojan. Saturn's Trojan moons do not sit exactly on the points; they oscillate, or "librate", around them over periods of hundreds of days to years, tracing tadpole-shaped paths when viewed in a frame rotating with the host moon.
This is different from the arrangement of Janus and Epimetheus, which have similar masses and swap orbits instead of keeping a fixed spacing.
Telesto and Calypso, the companions of Tethys
Tethys, a 1,060 km moon made largely of water ice, has two Trojans. Telesto leads it by about 60° and Calypso trails it by about 60°. Both were discovered from Earth in 1980, during the ring-plane crossing that dimmed the rings' glare: Calypso in March 1980 by Dan Pascu, Kenneth Seidelmann, William Baum and Douglas Currie at the US Naval Observatory, and Telesto in April 1980 by Bradford Smith, Harold Reitsema, Stephen Larson and John Fountain. Voyager confirmed both later that year.
Cassini images show Telesto as a remarkably smooth body, about 33 × 24 × 20 km, with relatively few visible craters, as if blanketed in fine dust. Calypso, about 30 × 23 × 14 km, is irregular and among the most reflective objects in the solar system, a property attributed to coating by icy particles of the E ring, which is fed by the geysers of Enceladus.
In mythology, Telesto and Calypso were daughters of the Titans Tethys and Oceanus; Calypso is better known as the nymph who held Odysseus on her island for years in Homer's Odyssey.
Helene and Polydeuces, the companions of Dione
Dione, about 1,120 km across, also has two Trojans. Helene, leading Dione at L4, was discovered on 1 March 1980 by the French astronomers Pierre Laques and Jean Lecacheux, observing from the Pic du Midi observatory. It was the first Trojan moon ever found. Measuring roughly 36 × 32 × 30 km, it was imaged at close range by Cassini in 2010 and 2011. The images show a surface partly covered by smooth, fine-grained material, with gully-like features that suggest the material has flowed downslope under Helene's very weak gravity.
Polydeuces, trailing Dione at L5, is tiny, only about 3 km across. Cassini's imaging team discovered it in images from October 2004. Its libration is unusually large: rather than staying near 60° behind Dione, it wanders over tens of degrees along the orbit. The name comes from Polydeuces (Pollux), the brother of Helen of Troy, a fitting pairing with Helene.
Why only Saturn?
No other planet is known to host Trojan moons. Saturn's mid-sized moons, which are massive enough to create stable Lagrange points but have nearly circular, uninclined orbits, provide favorable conditions. How the Trojans got there is still discussed: they may be fragments left over from the formation of their host moons, or debris from impacts that was captured in the stable zones. Their smooth surfaces may be related to the steady rain of E ring particles in this region of the system.
For a full overview of Saturn's moons and their groups, see how many moons Saturn has and the list of Saturn's moons.
FAQ
What are Saturn's Trojan moons?
They are Telesto and Calypso, which share Tethys's orbit, and Helene and Polydeuces, which share Dione's orbit. Each sits near a point 60° ahead of or behind its host moon.
Does any other planet have Trojan moons?
No other planet is known to have them. Trojan objects are common for planets, such as Jupiter's Trojan asteroids, but Saturn is the only planet whose moons have their own Trojans.
Which was the first Trojan moon discovered?
Helene, the leading Trojan of Dione, discovered on 1 March 1980 by Pierre Laques and Jean Lecacheux from the Pic du Midi observatory in France.
Why is Calypso so bright?
Calypso orbits within Saturn's E ring, and its surface is thought to be coated by fresh icy particles from that ring, which is fed by the plumes of Enceladus.
Sources
- NASA Science: Saturn's moons
- NASA: Cassini mission
- Porco, C. C. et al., "Cassini imaging science: initial results on Saturn's rings and small satellites", Science 307 (2005)
- Thomas, P. C., "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission", Icarus 208 (2010)