Epimetheus (moon)
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Epimetheus is a small icy moon of Saturn with a volumetric mean diameter of approximately 116 km. It travels between the F and G rings on almost the same orbit as Janus. Approximately once every four years, their mutual gravity causes the moons to exchange their inner and outer orbits.[1][3][6]
In Cassini images, Epimetheus is an irregular and densely cratered body. The south-polar region is noticeably flattened, probably by the remains of a large impact basin. Dark smooth terrain lies beside brighter, fractured icy surfaces.[7]
Size and physical characteristics
Epimetheus has a volumetric mean radius of 58.2 km and a mass of approximately 5.27 × 1017 kg. Its diameter is about one and a half times smaller than Janus's, while its mass is 3.6 times lower. Compared with Mimas, Epimetheus is almost 3.4 times smaller and approximately 71 times less massive.[1]
Its mean density is 0.64 g/cm³, substantially below that of solid water ice. This is consistent with an ice-rich body containing a large volume of voids. Like Janus, Epimetheus may be a loosely bound collection of fragments.[1][2]
The moon is far from spherical: its elongated outline is interrupted by craters and broad depressions. Its approximate dimensions are 135 × 108 × 105 km.[2]
A spherical estimate gives a surface gravity of approximately 0.0104 m/s², almost 950 times weaker than Earth's. The calculated escape velocity is about 35 m/s, or 125 km/h. Local gravity varies with the body's irregular shape.[1]
Orbit and rotation
Epimetheus orbits approximately 151,000 km from Saturn's centre. One revolution takes about 16.7 hours. The orbit is only slightly inclined to the planet's equatorial region and lies outside the narrow F ring.[3]
Rotation is synchronous on average, but the moon's orientation oscillates slightly relative to uniform rotation. Physical librations and their changes during the orbital exchanges have been studied through surface images and dynamical models.[4]
Its orbital elements do not remain fixed over decades: each co-orbital exchange changes the semi-major axis and orbital period. Epimetheus alternates between lying closer to Saturn than Janus and farther from the planet.[6][4]
Orbital exchange with Janus
The pair is in a 1:1 orbital resonance. Their characteristic orbital radii differ by approximately 50 km, less than the mean radius of either moon. The bodies themselves do not pass within 50 km of each other: the difference between the orbits is not their separation during an encounter.[6]
The inner moon moves slightly faster and gradually catches the outer one. During the approach, mutual gravity changes their energy and angular momentum. The inner body moves to a higher, slower orbit, while the outer body drops to a lower, faster orbit; the moons then separate again.[6]
Because Epimetheus is less massive, its orbit changes more than that of Janus. The exchange recurs approximately every four years, and the original relative configuration returns after about eight years. In a frame rotating with the pair's mean motion, the relative trajectory has a horseshoe shape.[4]
Cassini observed exchanges in 2006, 2010 and 2014. These events allowed calculated motions to be compared with direct observations of both moons and with changes in Saturn's ring structures.[8]
Surface and impact structures
Craters cover almost the entire visible surface. Large impacts occupy a substantial fraction of the moon's diameter, while smaller craters overlap older terrain. Smoothed areas occur beside bright scarps and fractured surfaces.[7][9]
Cassini obtained particularly detailed views of the south-polar region during a flyby in December 2007. A broad depression occupying much of the hemisphere may be the remnant of an ancient crater and can explain the pronounced flattening of the south.[7]
Colour images distinguish dark, relatively smooth terrain from brighter, slightly yellowish fractured surfaces. Dark material probably moves down slopes and contains less ice than the exposed bright material, although both terrains are considered rich in water ice.[7]
Two craters have official names: Hilairea and Pollux. Both belong to the mythological cycle of Castor and Pollux, also used for surface features on Janus.[10][2]
Dust ring
A faint dust ring surrounds the shared orbital region of Janus and Epimetheus. Its particles are most visible at very large phase angles, when the viewing geometry favours strong forward scattering.[11]
Numerical models give individual particles lifetimes of no more than a few decades, so the dust must be replenished continually. Micrometeoroid impacts on the two moons can eject material and sustain the ring.[11]
Effect on the main rings
Janus and Epimetheus produce resonant perturbations in Saturn's rings. As they exchange orbits, the resonance locations shift and the response of the ring material changes. The outer edge of the A ring, near the 7:6 resonance with the co-orbital pair, has been studied in greatest detail.[12]
Observations from 2006 to 2014 showed that the prominent seven-lobed edge pattern present while Janus occupied the inner orbit disappeared after the 2010 exchange. The rearrangements have also been linked with solitary waves travelling through the rings.[12][8]
Discovery and name
Richard Walker observed an object near the orbit of Janus on 18 December 1966. Observations of two bodies were initially assigned to a single moon, producing inconsistencies in the calculated orbit.[2]
In 1977, John Fountain and Stephen Larson recognised a separate moon and determined a suitable orbital period. Dale Cruikshank confirmed Epimetheus in 1980, and Voyager 1 observations clarified the structure of the pair.[5][2]
The moon is named after Epimetheus, the brother of Prometheus in Greek mythology. It is also designated Saturn XI and S/1980 S3.[5]
Cassini observations
On 3 December 2007, Cassini photographed the south-polar region. These frames were combined into a colour image of the moon.[7]
On 6 December 2015, Cassini again imaged the Saturn-facing hemisphere in detail from approximately 35,000 km.[13]
During the encounter on 21 February 2017, the spacecraft photographed the anti-Saturn hemisphere from approximately 15,000 km. The images resolve small craters and details of the irregular surface that cannot be observed from Earth.[9]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Planetary Satellite Physical Parameters // NASA JPL. Epimetheus: gravitational parameter from the SAT415 solution. Mass calculated from GM using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 Epimetheus // NASA Science.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Planetary Satellite Mean Elements // NASA JPL. Epimetheus: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 4.2 4.3 Robutel P., Rambaux N., Castillo-Rogez J. Analytical Description of Physical Librations of Saturnian Coorbital Satellites Janus and Epimetheus // Icarus. 2011. Vol. 211, issue 1. P. 758–769.
- ↑ 5.0 5.1 5.2 5.3 Planet and Satellite Names and Discoverers: Saturn XI (Epimetheus) // USGS/IAU.
- ↑ 6.0 6.1 6.2 6.3 O'Neill S., Hay K., deMattos J. Theoretical and Computational Models for Saturn's Co-Orbiting Moons, Janus and Epimetheus // arXiv:2312.13442. 2023.
- ↑ 7.0 7.1 7.2 7.3 7.4 Epimetheus Revealed // NASA/JPL/Space Science Institute. 11 January 2008.
- ↑ 8.0 8.1 Rehnberg M. E. et al. A Traveling Feature in Saturn's Rings // Icarus. 2017. Vol. 279. P. 104–110.
- ↑ 9.0 9.1 Zoom-in on Epimetheus // NASA/JPL-Caltech/Space Science Institute. 3 July 2017.
- ↑ Epimetheus: Nomenclature // Gazetteer of Planetary Nomenclature. USGS/IAU.
- ↑ 11.0 11.1 Winter O. C. et al. Particles Co-Orbital to Janus and to Epimetheus: A Firefly Planetary Ring // The Astrophysical Journal. 2018. Vol. 852. Article 14.
- ↑ 12.0 12.1 El Moutamid M. et al. How Janus' Orbital Swap Affects the Edge of Saturn's A Ring? // Icarus. 2016. Vol. 279. P. 125–140.
- ↑ Examining Epimetheus // NASA/JPL-Caltech/Space Science Institute. 8 December 2015.