Pandora (moon)
Pandora is a small moon of Saturn that orbits immediately outside the narrow F ring. Its elongated icy body is covered by impact craters and fine loose material. One orbit takes slightly more than 15 hours, and the moon's volumetric mean diameter is approximately 81 km.[2][1][3]
Pandora was long described as the outer shepherd of the F ring, acting together with Prometheus. Cassini observations changed this picture: Prometheus is now considered the more important body in confining the ring, while Pandora contributes to its gravitational disturbance.[4]
Size and physical characteristics
Pandora measures approximately 104 × 82 × 63 km. Its long axis is almost 1.7 times the length of its short axis. Large depressions and projections shape the entire body, giving it an outline distinctly different from a sphere.[5]
Its volumetric mean radius is 40.6 km, and its mass is approximately 1.39 × 1017 kg. Pandora is slightly smaller and about 14% less massive than Prometheus. Its mean density is about 0.50 g/cm³, approximately half the density of solid water ice. This low value is consistent with a porous interior.[1]
A spherical estimate gives a surface gravity of approximately 0.0056 m/s², about 1,700 times weaker than Earth's. The calculated escape velocity is approximately 21 m/s, or 77 km/h. Because of the body's irregular shape, local gravity varies across the surface.[1]
Orbit
Pandora's semi-major axis is approximately 141,700 km from Saturn's centre. One orbit takes 0.631369 Earth days, or about 15 hours 9 minutes. Its orbit is nearly circular, with an eccentricity of about 0.004, and lies close to the plane of the rings.[3]
Prometheus orbits closer to Saturn at a mean distance of approximately 139,400 km. The F ring lies between the paths of the two moons. Their semi-major axes differ by about 2,300 km, and the inner moon Prometheus moves more rapidly than Pandora.[3]
Pandora and Prometheus are linked by the 121:118 mean-motion resonance. Four overlapping components of this resonance produce chaotic variations in their motion. Their observed positions gradually diverged from earlier predictions; dynamical models reproduce both the long-term drift and abrupt changes recorded around 2000.[6]
Surface and topography
Pandora's surface contains craters, grooves and ridges. Fine icy material covers even its impact depressions and softens their outlines. Compared with the sharply defined craters of Hyperion, parts of Pandora's terrain appear smoother.[2]
Images obtained during the 2016 close approach show narrow parallel grooves away from a broad equatorial region. Smooth deposits conceal the grooves in some places. The underlying fractures may have formed through tidal stresses exerted by Saturn.[5]
Some surface particles may originate in the neighbouring F ring. Pandora can collect fine ring material, but the depth of the loose surface layer is probably uneven. This may explain why smooth areas occur beside terrain where grooves remain clearly visible.[5]
Interaction with the F ring
The original model treated Prometheus and Pandora as two shepherd moons that jointly confined the F ring between their orbits. Cassini data showed that their combined gravity also disturbs the ring material, producing complex structures that change with time.[4]
Models discussed by the mission team indicate that Prometheus plays the dominant role in confining the main part of the ring because of the geometry of its orbit. Pandora contributes less to confinement than once assumed, so the traditional description of it as a shepherd moon does not fully represent its interaction with the ring.[4]
Discovery and exploration
Pandora was discovered by the Voyager 1 science team in October 1980 and initially designated S/1980 S26. The name Pandora was approved in 1985 after the figure from Greek mythology. The moon is also designated Saturn XVII.[2][7]
On 18 December 2016, Cassini made its closest approach to Pandora during the entire mission, passing approximately 20,000 km above its surface. The spacecraft's optical instruments observed the moon for about two hours.[8]
One of the most detailed images from this sequence was taken at a distance of approximately 40,500 km. It reveals large depressions, Pandora's irregular outline and fine details of its icy terrain.[9]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Planetary Satellite Physical Parameters // NASA JPL. Pandora: GM = 0.00926 ± 0.00002 km³/s² from the SAT415 solution. Mass calculated using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Pandora // NASA Science.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Planetary Satellite Mean Elements // NASA JPL. Pandora: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 4.2 Pandora, the Would-Be Perturber // NASA/JPL-Caltech/Space Science Institute. 6 November 2017.
- ↑ 5.0 5.1 5.2 Far-Out: Pandora Has Smooth and Groovy Terrains // NASA Science Mission Directorate. 2017.
- ↑ Farmer A. J., Goldreich P. Understanding the Behavior of Prometheus and Pandora // Icarus. 2006. Vol. 180. P. 403–411.
- ↑ Saturn Moons // NASA Science. Table of moons and discovery circumstances.
- ↑ Cassini Significant Events 12/14/16–12/27/16 // NASA Science. 28 December 2016.
- ↑ Pandora Up Close // NASA/JPL-Caltech/Space Science Institute. 21 December 2016. PIA21055.