by neufer » Thu Jul 15, 2010 4:00 pm
Chris Peterson wrote:Bill in Tennessee wrote:
Why do both satellites of Saturn (Rhea and Janus) have orbits that are off the plane of the rings? I would think that because the rings are made up of debris that have found homeostasis in their own orbit as determined by the gravity of Saturn, that the moons would have the same orbit. Is it that the moons have a slightly different orbit and actually cross through the rings on a periodic basis? As the photo is shown, it appears that the rings have one orbit, and that the two satellites shown have orbits, one above the plane of the rings, and the other below the plane of the rings. Is this an optical illusion? I am just curious as to why the different bodies would seem to have different orbital planes.
There is no mechanism to force the moons into orbits with zero inclination. The rings, however, behave like a fluid. Individual particles collide with one another, transferring angular momentum. This is quite unlike the moons, where only gravity is significant in determining orbital characteristics.
Tidal forces from Saturn, the rings, and other moons keep most
lunar inclinations within one degree for the first 2 million kilometers.
(Such small inclinations are simply exaggerated in this APOD.)
http://en.wikipedia.org/wiki/Moons_of_Saturn wrote:
Code: Select all
Moon semi-major axis Period (d) inclination
-----------------------------------------------------------
Pan 133,584 0.57505 0.001°
Daphnis 136,505 0.59408 ≈ 0°
Atlas 137,670 0.60169 0.003°
Prometheus 139,380 0.61299 0.008°
Pandora 141,720 0.62850 0.050°
Epimetheus 151,422 0.69433 0.335°
Janus 151,472 0.69466 0.165°
Aegaeon 167,500 0.80812 0.001°
Mimas 185,404 0.942422 1.566°
Methone 194,440 1.00957 0.007°
Anthe 197,700 1.03650 0.1°
Pallene 212,280 1.15375 0.181°
Enceladus 237,950 1.370218 0.010°
Tethys 294,619 1.887802 0.168°
Telesto 294,619 1.887802 1.158°
Calypso 294,619 1.887802 1.473°
Dione 377,396 2.736915 0.002°
Helene 377,396 2.736915 0.212°
Polydeuces 377,396 2.736915 0.177°
Rhea 527,108 4.518212 0.327°
Titan 1,221,930 15.94542 0.3485°
Hyperion 1,481,010 21.27661 0.568°
Iapetus 3,560,820 79.3215 7.570°
[quote="Chris Peterson"][quote="Bill in Tennessee"]
Why do both satellites of Saturn (Rhea and Janus) have orbits that are off the plane of the rings? I would think that because the rings are made up of debris that have found homeostasis in their own orbit as determined by the gravity of Saturn, that the moons would have the same orbit. Is it that the moons have a slightly different orbit and actually cross [b]through [/b]the rings on a periodic basis? As the photo is shown, it appears that the rings have one orbit, and that the two satellites shown have orbits, one above the plane of the rings, and the other below the plane of the rings. Is this an optical illusion? I am just curious as to why the different bodies would seem to have different orbital planes.[/quote]
There is no mechanism to force the moons into orbits with zero inclination. The rings, however, behave like a fluid. Individual particles collide with one another, transferring angular momentum. This is quite unlike the moons, where only gravity is significant in determining orbital characteristics.[/quote]
Tidal forces from Saturn, the rings, and other moons keep most
lunar inclinations within one degree for the first 2 million kilometers.
(Such small inclinations are simply exaggerated in this APOD.)
[quote=" http://en.wikipedia.org/wiki/Moons_of_Saturn"]
[code]Moon semi-major axis Period (d) inclination
-----------------------------------------------------------
Pan 133,584 0.57505 0.001°
Daphnis 136,505 0.59408 ≈ 0°
Atlas 137,670 0.60169 0.003°
Prometheus 139,380 0.61299 0.008°
Pandora 141,720 0.62850 0.050°
Epimetheus 151,422 0.69433 0.335°
Janus 151,472 0.69466 0.165°
Aegaeon 167,500 0.80812 0.001°
Mimas 185,404 0.942422 1.566°
Methone 194,440 1.00957 0.007°
Anthe 197,700 1.03650 0.1°
Pallene 212,280 1.15375 0.181°
Enceladus 237,950 1.370218 0.010°
Tethys 294,619 1.887802 0.168°
Telesto 294,619 1.887802 1.158°
Calypso 294,619 1.887802 1.473°
Dione 377,396 2.736915 0.002°
Helene 377,396 2.736915 0.212°
Polydeuces 377,396 2.736915 0.177°
Rhea 527,108 4.518212 0.327°
Titan 1,221,930 15.94542 0.3485°
Hyperion 1,481,010 21.27661 0.568°
Iapetus 3,560,820 79.3215 7.570°[/code][/quote]