Starship Asterisk*

Foozinator wrote:Janus looks like it might be on the same plane as the rings (and may or may not be orbiting on the ring plane or just crossing at the time), but Rhea appears to be orbiting several degrees off the plane of the rings. How many of Saturn's moons have orbits at angles to the plane of the rings?

Chris Peterson wrote:... Most of the moons are inclined by a few tenths of a degree; there are also a bunch of tiny ones with extreme inclinations. Janus has an inclination of 0.163°, and Rhea has an inclination of 0.345°. ...

primordial_punt wrote:Sorry folks. But the info accompanying this photo is wrong. The photo is actually being taken from BELOW the ring plane. Prometheus is in front of the rings while Rhea is behind. I wish APOD would be more careful.

primordial_punt wrote:Sorry folks. But the info accompanying this photo is wrong. The photo is actually being taken from BELOW the ring plane. Prometheus is in front of the rings while Rhea is behind. I wish APOD would be more careful.

Click to view full size image

Saturn's rings occupy the foreground of this image. The small moon Janus appears to hover above, while the far larger moon Rhea is partially obscured by the rings.

Janus appears to be located directly over the rings, but the moon is actually further away, at a range of about 1.1 million kilometers (684,000 miles) from the Cassini spacecraft. Rhea is 1.6 million kilometers (994,000 miles) from the spacecraft. This view looks toward the trailing hemisphere of Janus (179 kilometers, or 111 miles across) and the Saturn-facing side of Rhea (1,528 kilometers, or 949 miles across).

This view looks toward the northern, sunlit side of the rings from just above the ringplane.

Credit: NASA/JPL/SSI

owlice wrote:The non-obvious moon is Janus.

moons.jpg

DavidLeodis wrote:I think I've worked it out! I thought the main object was Saturn but I now think it is Rhea and that Saturn is off screen to the right. Apologies for my mental confusion regarding the image. but still :)ing.

http://en.wikipedia.org/wiki/Rings_of_Rhea wrote:
<<Voyager 1 observed a broad depletion of energetic electrons trapped in Saturn's magnetic field downstream from Rhea in 1980. These measurements, which were never explained, were made at a greater distance than the Cassini data. In August 2007, Cassini passed through Rhea's plasma shadow again, but further downstream. Its readings were similar to those of Voyager 1.

On November 26, 2005, Cassini made the one targeted Rhea flyby of its primary mission. It passed within 500 km of Rhea's surface, downstream of Saturn's magnetic field, and observed the resulting plasma wake as it had with other moons, such as Dione and Tethys. In those cases, there was an abrupt cutoff of energetic electrons as Cassini crossed into the moons' plasma shadows (the regions where the moons themselves blocked the magnetospheric plasma from reaching Cassini). However, in the case of Rhea, the electron plasma started to drop off slightly at eight times that distance, and decreased gradually until the expected sharp drop off as Cassini entered Rhea's plasma shadow. The extended distance corresponds to Rhea's Hill sphere, the distance of 7.7 times Rhea's radius inside of which orbits are dominated by Rhea's rather than Saturn's gravity. When Cassini emerged from Rhea's plasma shadow, the reverse pattern occurred: A sharp surge in energetic electrons, then a gradual increase out to Rhea's Hill-sphere radius. These readings are similar to those of Enceladus, where water venting from its south pole absorbs the electron plasma. However, in the case of Rhea, the absorption pattern is symmetrical. The simplest explanation for the symmetrical punctuations in plasma flow are "extended arcs or rings of material" orbiting Rhea in its equatorial plane. These symmetric dips bear some similarity to the method by which the Rings of Uranus were discovered in 1977. The slight deviations from absolute symmetry may be due to "a modest tilt to the local magnetic field" or "common plasma flow deviations" rather than to asymmetry of the rings themselves, which may be circular.

In addition, the Magnetospheric Imaging Instrument (MIMI) observed that this gentle gradient was punctuated by three sharp drops in plasma flow on each side of the moon, a pattern that was also nearly symmetrical.>>

Possible Rhean rings Ring 	orbital radius (km)
disk 	< 5900
1 	≈ 1615
2 	≈ 1800
3 	≈ 2020

http://en.wikipedia.org/wiki/Rings_of_Rhea wrote:
<<The Saturnian moon Rhea may have a tenuous ring system consisting of three narrow, relatively dense bands within a particulate disk. This would be the first discovery of rings around a moon. The discovery was announced in the journal Science on March 6, 2008.

In October 2009, it was announced that a set of small ultraviolet-bright spots distributed in a line that extends three quarters of the way around Rhea's circumference, within 2 degrees of the equator, may represent further evidence for a ring. The spots presumably represent the impact points of deorbiting ring material. There are no images or direct observations of the material thought to be absorbing the plasma, but the likely candidates would be difficult to detect directly. Further observations are planned for Cassini's first mission extension, with a targeted flyby scheduled for March 2, 2010.

Cassini's flyby trajectory makes interpretation of the magnetic readings difficult. The obvious candidates for magnetospheric plasma-absorbing matter are neutral gas and dust, but the quantities required to explain the observed depletion are far greater than Cassini's measurements allow. Therefore the discoverers, led by Geraint Jones of the Cassini MIMI team, argue that the depletions must be caused by solid particles orbiting Rhea: An analysis of the electron data indicates that this obstacle is most likely in the form of a low optical depth disk of material near Rhea’s equatorial plane and that the disk contains solid bodies up to ~1 m in size.

Simulations suggest that solid bodies can stably orbit Rhea near its equatorial plane over astronomical timescales. They may not be stable around Dione and Tethys because those moons are so much closer to Saturn, and therefore have much smaller Hill spheres, or around Titan because of drag from its dense atmosphere. Several suggestions have been made for the possible origin of rings. An impact could have ejected material into orbit; this could have happened as recently as 70 million years ago. A small body could have been disrupted when caught in orbit about Rhea. In either case, the debris would eventually have settled into circular equatorial orbits. Given the possibility of long-term orbital stability, however, it is possible that they survive from the formation of Rhea itself. For discrete rings to persist, something must confine them. Suggestions include moonlets or clumps of material within the disk, similar to those observed within Saturn's A ring.>>