Janus (moon)
Janus is a small icy moon of Saturn orbiting just beyond the F ring. Its volumetric mean diameter is approximately 178 km. Together with Epimetheus, Janus forms a co-orbital pair: the moons travel at almost the same distance from Saturn and exchange their inner and outer orbits approximately once every four years.[1][2][6]
Their mutual gravity alters their motion during each approach and prevents a collision. The exchange also affects Saturn's rings because resonances associated with Janus shift and change the perturbations in the ring material.[7]
Size and physical characteristics
Janus has a volumetric mean radius of 89.2 km and a mass of approximately 1.90 × 1018 kg. Its diameter is about 2.25 times smaller than that of Mimas, while its mass is nearly 20 times lower. Janus itself is approximately 3.6 times more massive than Epimetheus.[1]
The moon is elongated and irregular. The diameter of 178 km describes a sphere of equal volume; its actual dimensions differ by direction. Large depressions and protrusions visibly alter the outline.[1][8]
The mean density is only 0.64 g/cm³, less than that of solid water ice. Together with the highly reflective surface, this points to an ice-rich and substantially porous body, possibly a loosely bound collection of fragments.[1][2]
A spherical estimate gives a surface gravity of approximately 0.016 m/s², about 620 times weaker than Earth's. The calculated escape velocity is approximately 53 m/s, or 192 km/h. Local gravity varies because of the irregular shape.[1]
Orbit and rotation
Janus orbits approximately 151,500 km from Saturn's centre, in the region between the F and G rings. One revolution takes about 16.7 hours. Its orbit is nearly circular and only slightly inclined to the equatorial region of the Saturn system.[3][2]
Rotation is synchronous on average, so the same side faces Saturn. Small oscillations in orientation, known as physical librations, are superimposed on this motion. The co-orbital exchanges with Epimetheus introduce further changes into both moons' orbital and rotational motion.[4]
Janus and Epimetheus are in a 1:1 resonance and have nearly identical orbital periods. Their characteristic orbital radii differ by only about 50 km, less than the mean radius of either moon. This is a difference between the orbits, not the distance between the bodies during an encounter.[6]
The moon in the inner orbit moves slightly faster and gradually catches the outer one. As they approach, their mutual gravity transfers energy and angular momentum: the inner moon moves to a higher, slower orbit, while the outer moon drops to a lower, faster orbit. They then move apart again.[6][4]
Because Janus is approximately 3.6 times more massive, its orbit changes less than that of Epimetheus. Encounters recur approximately every four years, and the pair returns to the same relative configuration after about eight years.[4][7]
In a frame rotating with the pair's mean orbital motion, the relative trajectory has a horseshoe shape. This differs from Trojan moons, which oscillate near particular points ahead of or behind a larger body.[9]
Surface and craters
The surface is densely cratered. Several impact structures exceed 30 km in diameter, a substantial size for a body less than 200 km across. Many crater rims appear softened, and some areas may be covered with loose, fine-grained icy material.[2][8]
Cassini images show light and dark terrain, irregular ridges and broad transitions between craters. Long shadows under low illumination reveal relief that is less apparent when the Sun is high.[10]
Four craters have official names: Castor, Idas, Lynceus and Phoibe. They belong to the mythological cycle of Castor and Pollux. The crater Phoibe is named after a daughter of Leucippus and is unrelated to the distant Saturnian moon Phoebe.[11]
Janus–Epimetheus ring
A faint dust ring occupies the region of the shared orbit. Fine particles scatter light strongly in the forward direction, so the ring becomes visible only under particular arrangements of the Sun, spacecraft and ring plane.[12]
Individual particles may remain in the region for less than a few decades, so the ring must be continually replenished. Micrometeoroid impacts ejecting material from Janus and Epimetheus are a principal source of the dust.[12]
Effect on the A ring
Janus influences the outer edge of Saturn's A ring through a 7:6 orbital resonance. Ring particles in the resonant region complete approximately seven revolutions for every six by Janus. The repeated gravitational perturbations create a regular departure of the ring edge from a circle.[7]
From 2006 to 2010, when Janus occupied the inner orbit, the ring edge displayed a pronounced seven-lobed pattern with an amplitude of approximately 12.8 km. After the 2010 orbital exchange, the resonance shifted and the stable pattern disappeared. The change was traced through more than 5,000 images and 170 occultations obtained from 2006 to 2014.[7]
The rings therefore preserve an observable response to the moons' orbital rearrangement. Changes at the edge provide independent evidence of resonant interactions between small moons and Saturn's ring particles.[7]
Discovery and exploration
Audouin Dollfus observed Janus on 15 December 1966. Observations of another nearby object followed within days, but the two were initially forced into a single orbit. Recognition of two bodies travelling along almost the same path resolved the inconsistencies.[2]
In 1980, observations by Dan Pascu confirmed Janus, and images from Voyager 1 established the structure of the co-orbital pair. The moon is named after Janus, the two-faced Roman god of beginnings and transitions.[5][2]
Cassini photographed Janus repeatedly and observed mutual events with Epimetheus. On 14 February 2010, for example, the spacecraft recorded a sequence in which Epimetheus passed in front of Janus. Such observations helped refine the orbits.[13]
During a flyby on 27 March 2012, Cassini photographed the craters of Janus in detail from approximately 45,000 km. The images show the moon's shape, impact terrain and variations in surface material.[10]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Planetary Satellite Physical Parameters // NASA JPL. Janus: gravitational parameter from the SAT415 solution. Mass calculated from GM using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Janus // NASA Science.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Planetary Satellite Mean Elements // NASA JPL. Janus: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 4.2 4.3 Analytical Description of Physical Librations of Saturnian Coorbital Satellites Janus and Epimetheus // arXiv:1003.0557. 2010.
- ↑ 5.0 5.1 5.2 Planet and Satellite Names and Discoverers: Saturn X (Janus) // USGS/IAU.
- ↑ 6.0 6.1 6.2 Epimetheus // NASA Science.
- ↑ 7.0 7.1 7.2 7.3 7.4 El Moutamid M. et al. How Janus' Orbital Swap Affects the Edge of Saturn's A Ring? // Icarus. 2016. Vol. 279. P. 125–140.
- ↑ 8.0 8.1 Janus against Saturn, PIA08296 // NASA/JPL/Space Science Institute. Cassini image from 25 September 2006.
- ↑ Theoretical and Computational Models for Saturn's Co-Orbiting Moons, Janus and Epimetheus // arXiv:2312.13442. 2023.
- ↑ 10.0 10.1 Janus' Craters // NASA/JPL-Caltech/Space Science Institute. 7 May 2012.
- ↑ Nomenclature Search Results: Janus // Gazetteer of Planetary Nomenclature. USGS/IAU.
- ↑ 12.0 12.1 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.
- ↑ Cruising Past Janus // NASA/JPL/Space Science Institute. 1 April 2010.
