Pallene (moon)

Lore & Background

Pallene was discovered by the Cassini Imaging Team in 2004, during the Cassini–Huygens mission, and given the temporary designation S/2004 S 2. In 2005, the name Pallene was provisionally approved by the IAU Division III Working Group for Planetary System Nomenclature, and was ratified at the IAU General Assembly in 2006. After the discovery, it was realized that Pallene had been first photographed on August 23, 1981, by the space probe Voyager 2, appearing in a single photograph and provisionally named S/1981 S 14, but its orbit could not be computed at the time.

Pallene is visibly affected by perturbations from the much larger Enceladus, though not as large as Mimas' perturbations on Methone. These perturbations cause its osculating orbital elements to vary with an amplitude of about 4 km in semi-major axis, and 0.02° in longitude (about 75 km). Eccentricity changes between 0.002 and 0.006, and inclination between about 0.178° and 0.184°. It was previously thought to be trapped in a 19:16 orbital resonance with Enceladus, but is now known to be very close to but not actually in resonance.

In 2006, images taken in forward-scattered light by the Cassini spacecraft revealed a faint dust ring around Saturn that shares Pallene's orbit, now called the Pallene Ring. The ring has a radial extent of about 2,500 km, sourced from particles blasted off Pallene's surface by meteoroid impacts. Unlike other ring-embedded moons such as Methone, Anthe, and Aegaeon, Pallene has no associated arc structure, because those moons are in orbital resonances while Pallene is not. Pallene, along with Aegaeon and Methone, are smooth bodies with an ellipsoidal shape, and have darker surfaces than expected, possibly due to high-energy radiation. Pallene is darker on its leading hemisphere than its trailing one by roughly 10–20%.

Reader's Guide

Pallene's significance lies in its role as a small moon that generates its own faint dust ring, the Pallene Ring, discovered in 2006 by the Cassini spacecraft. This ring, with a radial extent of about 2,500 km, is formed by meteoroid impacts ejecting particles from Pallene's surface. Unlike other ring-embedded moons such as Methone, Anthe, and Aegaeon, Pallene lacks an arc structure because it is not in an orbital resonance, allowing ejecta to spread into a complete ring rather than being confined. The moon's orbit is visibly perturbed by Enceladus, though not as strongly as Mimas affects Methone, and its orbital elements vary in semi-major axis, eccentricity, and inclination. Pallene was first photographed by Voyager 2 in 1981 but not identified until after its 2004 discovery by the Cassini Imaging Team. The Cassini spacecraft performed two flybys of Pallene, on 16 October 2010 and 14 September 2011, at distances of 36,000 km and 44,000 km respectively. Its physical characteristics include a smooth, ellipsoidal shape and a darker surface than expected, with a leading hemisphere 10–20% darker than its trailing hemisphere, possibly due to high-energy radiation.

Did You Know?

• Pallene was first photographed by Voyager 2 on August 23, 1981, but was not identified as a moon until after its 2004 discovery.
• It is associated with a faint dust ring called the Pallene Ring, which has a radial extent of about 2,500 km.