Neptune's Rings

Image of Neptune's Rings taken from Voyager 2 in 1989.

Three rings are easily seen in this image: Adams (outermost), Le Verrier (middle) and Galle (inner ring). To the left of the image, in the Adams Ring, Galatea is visible. Between Adams and Le Verrier, two faint rings can be made out: Arago and Lassell.

Image Credit:

NASA/JPL/Voyager

 

Neptune has five main rings: Galle, Le Verrier, Lassell, Arago, and Adams, named after five important astronomers in Neptune's history. Much like the rings of Uranus, the rings of Neptune are made up of dust particles, and kept in place by shepherd moons, which include Galatea (Adams Ring) and Despina (Le Verrier Ring). It is believed that Neptune has other moons which help keep the rings narrow and stable (so to speak), but they have yet to be discovered.

The Adams Ring is unique in that it contains arcs, which are caused by gravity from Galatea. The arcs were discovered when Neptune occulted a star and where the rings should have been, the star shone through the rings. Close ups by Voyager confirmed that the Adams Ring contained arcs.

Much like the rings around Jupiter, Saturn, and Uranus, the material in the rings of Neptune are not permanent. They are continuously replenished by collisions with the moons of Neptune and are kept in orbit by shepherd satellites. The rings themselves are mostly dust with some ice particles and are covered in organic material (carbon compounds) that make them dark.

Image Credit:

Sky and Telescope

The Rings of Uranus

Image Credit:

NASA

 

 

Image Credit:

NASA

 

Uranus has a faint ring system around the planet. William Herschel thought that he had discovered rings around the planet when he first spotted Uranus, but based on how faint they are, it seems unlikely that what he saw were the rings.

The rings are oriented parallel to the equator of Uranus and much like those of Jupiter and Saturn, are continually replenished by collisions by objects colliding with the moons of Uranus. They are very dark, though they contain icy particles because they are covered with dust from the moons and the lack of sunlight reaching the region around Uranus.

The rings themselves are very narrow, made up of meter-sized particles and smaller. The majority of the rings are no more than 10km wide, with the widest ring only 100km (less than 0.2% the diameter of Uranus). Much like Pandora and Prometheus keeping the F-Ring around Saturn narrow, the rings of Uranus are kept narrow because of shepherd satellites, like Cordelia and Ophelia maintain the ε-Ring (epsilon ring). The ε-Ring is actually very eccentric because of the highly eccentric orbits of Cordelia and Ophelia.

The ring around Uranus were first indirectly discovered in 1977 when Uranus occulted a star, i.e. passed in front of the star. Just before the disk of Uranus passed in front of the star, the star dimmed a little, and just after the Uranus passed in front, the star did not return to its original brightness. Based on the dimming, astronomers were able to conclude, correctly, that Uranus possibly had a ring system. It wasn't unitl Voyager 2 passed Uranus that we were able to image the rings directly, and show that Uranus did have rings.

The Cassini Division

Image Credit: NASA

The Cassini Division is a gap in the ring system of Saturn between the B Ring and the A Ring. It was discovered in 1675 by Giovanni Cassini, hence its name. From Earth, it appears dark and therefore, it was believed that this part of the ring system did not contain any particles. However, when Voyager passed by Saturn, it discovered that there were particles in the division, similar to those in the C Ring. However, they are much more dispersed than in the C Ring, so the division looks empty from Earth.

There are ringlets in the Cassini Division which is caused by resonance with two moons: Mimas which shares a 2:1 resonance with the division (for every one orbit of Mimas, the ringlet makes two) and Enceladus which shares a 3:1 resonance. Mimas and Enceladus tug on the particles in the Cassinin Division, keeping it relatively clear of any particles.

Also, there is a gap in the division called the Huygens Gap. The Huygens Gap contains the ringlet created by the resonance with Mimas.

The Rings of Jupiter

A side view of Jupiter's Rings from Galileo spacecraft

Image Credit:

NASA, JPL, Galileo Project, (NOAO), J. Burns (Cornell) et al.



Saturn is not the only planet with rings. All the Jovian planets have some sort of ring system, though Saturn's rings are the most impressive. Jupiter's rings are so thinly distributed that they were not discovered until 1979 when Voyage I discovered them on its journey through the Solar System.

Jupiter's rings are not as bright as Saturn's rings, which are mostly composed of ice compounds. Jupiter's rings are dark and reddish which tells us that the material making up the rings are of a rocky origin. The ring is located within the Roche limit, the distance from Jupiter where gravity would rip apart a poorly consolidated moon, asteroid, or comet wandering inside that distance. The origin of the ring is probably a moon or asteroid that wandered too close to Jupiter.

 

The rings of Jupiter would have been dissipated long ago if the ring was not continuously replenished by the moons near the rings. The moons get hit by small meteorites, impacting the surface of the moon, and blowing dust out into orbit. This dust then gets incorporated into the rings. We know this by looking at the distribution of dust particles in the rings and see that the portions of the rings near moons are more densely packed with dust than those sections farther away. For example, near the moons Amalthea and Metis, the ring is densest between these moons. Amalthea orbits just outside the ring and Metis, just within the rings diameter.

 

Besides the main ring seen above, there is a much thinner ring outside Amalthea's orbit. This ring is called the gossamer ring because it is so thin. Again, by noting that this ring is densest near Amalthea and Thebe (orbiting farther out from Amalthea), we can conclude that micrometeorite impacts keep the rings intact. Also, Amalthea, Metis, and Thebe are also shepherd satellites of the rings, which will be explain more in detail when we talk about Saturn, its rings, and its moons.