Polydeuces (moon)

Discovery

The Cassini Imaging Science Team spotted Polydeuces on October 24, 2004, while checking routine images from the Cassini probe taken three days earlier. They used the blink comparison method, flipping between frames to catch anything moving against the fixed stars. The discovery came from four wide-angle camera shots taken in under six minutes, where Polydeuces shifted three to six pixels per frame. That motion hinted it might orbit Saturn near the large moon Dione, possibly sharing its path in a co-orbital setup.

By November 4, 2004, the team had gathered more images, including two from November 2 and two taken just three hours before the original discovery. These let them calculate a preliminary orbit, confirming Polydeuces as a trojan moon stuck near Dione’s Lagrange point. Using that orbit to predict its position, they found 52 earlier detections in narrow-angle camera images from April 9 to May 9, 2004. The International Astronomical Union announced the find on November 8, 2004.

That same year, Cassini also discovered five other Saturnian objects: Methone, Pallene, S/2004 S 3, S/2004 S 4, and S/2004 S 6. After the announcement, Cassini was redirected in January 2005 for targeted observations to refine Polydeuces’s orbit. In 2006, researchers uncovered even older pre-discovery images from April 2, 2004.

Orbit

Polydeuces is an inner moon of Saturn in a co-orbital configuration with Dione, meaning they share the same orbit. Together with Dione and its other co-orbital companion Helene, Polydeuces orbits Saturn in 2.74 days at an average distance of 377600 km from the planet's center, between the orbits of Tethys and Rhea. Due to gravitational perturbations by other nearby moons of Saturn, Polydeuces's orbital radius can vary by ±7660 km over time. Its orbit is closely aligned with Saturn's equatorial plane with a low orbital inclination of 0.2°.

Polydeuces has a slightly elliptical orbit with an eccentricity of 0.019, which is unusually higher than Dione's eccentricity of 0.002. While Dione's eccentricity is known to result from its 1:2 mean-motion orbital resonance with Enceladus, the effects of this resonance are too weak to explain Polydeuces's relatively high eccentricity. One possible explanation is that Polydeuces has always had an eccentric orbit since its formation because its orbit has not changed much over billions of years.

Polydeuces resides around Dione's Lagrange point trailing 60° behind Dione in its orbit, which makes Polydeuces a trojan moon of Dione. The Lagrange points are locations where the gravitational pulls of Dione and Saturn balance out, allowing for stable co-orbital configurations in Dione's trojans. Dione's other co-orbital moon, Helene, is a trojan residing around the Lagrange point leading 60° ahead of Dione. Trojan moons are not unique to Dione; another large moon of Saturn, Tethys, also has two trojans, named Telesto and Calypso, which reside in its Lagrange points, respectively.

Because of perturbations by other moons of Saturn, Polydeuces does not stay exactly 60° behind Dione; its angular distance from Dione oscillates or librates over time. Of Saturn's four known trojan moons, Polydeuces librates the farthest from its Lagrange point: its angular distance behind Dione oscillates from 33.9° to 91.4° with a period of 790.931 d. In a rotating reference frame with respect to Dione's orbit, Polydeuces appears to travel in a looping path around Dione's point due to its varying relative speed and radial distance from Saturn in its perturbed eccentric orbit.

Polydeuces's apparent looping motion combined with its librating angular distance from Dione forms a tadpole orbit about Dione's point.

Physical characteristics

Polydeuces is a small moon of Saturn, and as of 2020, its most recent size estimate comes from Cassini images taken in 2015. Those measurements put its dimensions at 3.50 by 3.10 by 2.62 kilometers, which works out to a mean diameter of about 3.06 kilometers when you account for its volume. The best Cassini pictures from 2015 show that Polydeuces is elongated, with a smooth outline that doesn't quite match a simple ellipsoid shape. Like Saturn's other trojan moons, it is thought to rotate in sync with its orbit.

Not much else is known about Polydeuces's physical traits because no spacecraft, including Cassini, ever got a close look at it. It is so tiny that its gravitational pull on Cassini or other moons is too weak to measure, so scientists cannot determine its mass or density. Even so, researchers assume its density is similar to that of Saturn's other small inner moons.

Its small size also makes it vulnerable to being shattered by impacts. Based on how often objects hit in the Saturn system, Polydeuces has likely been hit hard enough to break apart at least once in the past billion years. This means it could be a relatively young moon, less than a billion years old, or it could be an ancient moon that keeps reforming from debris after each destructive collision over the system's 4.5-billion-year history.

Polydeuces has a bright, probably smooth surface. This is because fine water ice particles from Saturn's E Ring, which comes from Enceladus's icy plumes, pile up on it. Any craters that might have formed would be completely buried by this material, leaving it looking craterless, much like the moons Methone and Pallene. Its geometric albedo is unknown because it has never been seen at low phase angles.

Cassini images reveal that its brightness is the same on both its leading and trailing sides. Its surface is about as bright as Dione's but darker than Helene's.

A similar brightness difference shows up among Tethys's trojan moons, where Calypso is brighter than Telesto and Tethys. Why these brightness differences exist for the trojan moons of both Dione and Tethys is not understood. Possible reasons include an uneven spread of E Ring particles or recent impacts that made Helene and Calypso brighter.

Did You Know?

• The name Polydeuces was approved by the IAU Working Group on Planetary System Nomenclature on 21 January 2005.
• Polydeuces was previously known by its provisional designation S/2004 S 5.
• Cassini's closest encounter with Polydeuces took place on 17 February 2005 at a distance of 6446.7 km, but no images were taken on that date.