General Astronomy

Rings ‘n’ Things

Rings are swarms of orbiting particles

• Orbits have to be very circular

• Elliptical orbits will result in collisions, destroying the ring

Rings Have to be Very Flat

Inclined orbits will result in collisions and destruction of the ring

Rings have to be Equatorial

• Tilted orbits precess; the plane of the orbit rotates due to the planet’s equatorial bulge, other satellites, and the Sun’s gravity

A tilted ring won’t stay flat very long

• Particle orbits will precess at different rates• Collisions will soon destroy the ring

Rings, Gaps, and Resonances

• When two objects return to the same relative positions regularly, they are said to be in resonance

• Some resonances are stable. Mercury’s 3:2 resonance between its rotation and its orbit is an example.

• Pluto’s period is 3/2 that of Neptune’s. This resonance keeps the two planets from ever colliding even though their orbits cross.

Rings, Gaps, and Resonances

• Some resonances create gaps. There are no asteroids with one-half Jupiter’s period because repeated pulls by Jupiter eventually would change the asteroid’s orbit.

• Gaps in Saturn’s rings are due to resonances with Saturn’s satellites.

Jupiter's Ring System

Jupiter's ring was discovered by Voyager 1 in a single image that was targeted specifically to search for a faint ring system.

The ring is now known to be composed of three major components. The main ring is about 7000 km wide and has an abrupt outer boundary 129,130 km from the center of the planet.

The main ring encompasses the orbits of two small moons, Adrastea and Metis, which may act as the source for the dust that makes up most of the ring.

At its inner edge the main ring merges gradually into the halo. The halo is a broad, faint torus of material about 20,000 km thick and extending halfway from the main ring down to the planet's cloud tops.

Just outside the main ring is a pair of broad and exceedingly faint gossamer rings, one bounded by the moon Amalthea and the other bounded by the moon Thebe.

Jupiter's Ring System

Saturn, Lord of the RingsSaturn's rings make the planet the most spectacular object in the solar

system.

The Saturnian Rings

• The rings are split into a number of different parts, which include the bright A and B rings and a fainter C ring.

• The ring system has various gaps. The most notable gap is the Cassini Division, which separates the A and B rings. The Encke Division splits the A Ring.

• Space probes have shown that the main rings are really made up of a large number of narrow ringlets.

Close up and Personal

Possible variations in chemical composition from one part of Saturn's ring system to another are visible in this Voyager 2 picture. Obtained Aug. 17 from a distance of 8.9 million kilometers (5.5 million miles).

The Voyager project is managed for NASA by the Jet Propulsion Laboratory,

Pasadena, Calif.

Saturn’s Rings• The origin of the rings is obscure.

– One thought is that the rings may have been formed from larger moons that were shattered by impacts of comets and meteoroids.

– Another possibility is a moon/comet which was pulled apart by tidal action

• The ring composition is not known for certain, but the rings do show a significant amount of water. – They may be composed of icebergs and/or snowballs from

a few centimeters to a few meters in size.

Shepherd MoonsMuch of the elaborate structure of

some of the rings is due to the gravitational effects of nearby satellites. This phenomenon is demonstrated by the relationship between the F-ring and two small moons that shepherd the ring material.

Shepherd Moons help maintain the sharp edges of rings

A Shepherd Moon Outside a Ring Tends to Drag Stray Particles Back, Causing Them to Fall Back Into the Ring

A Shepherd Moon Inside a Ring Tends to Speed Stray Particles Up, Causing Them to Rise Back Into the Ring

Outer Shepherd Moon

Inner Shepherd Moon

SpokesRadial, spoke-like features in the broad B-ring were also found by the Voyagers. The

features are believed to be composed of fine, dust-size particles. The spokes were observed to form and dissipate in the time-lapse images taken by the Voyagers. While electrostatic charging may create spokes by levitating dust particles above the ring, the exact cause of the formation of the spokes is not well understood.

Spoke Movie

Particle Size in the RingsPurple > 5 cmGreen < 1 cmWhite: too dense

Can you find Rhea?No, That’s Mimas

Here is Rhea

The Uranian Ring System

• The Uranian ring system was discovered in 1977 during observations of a stellar occultation by the planet. – The star was observed to blink out briefly five times before the

planet and again five times afterward, indicating that the planet was encircled by five narrow rings.

• Subsequent Earth-based observations indicated that there were actually nine major rings.

• Working outward from the planet, they are named 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta, and Epsilon.

• In 1986 the Voyager spacecraft acquired additional occultations and also our best images of the ring system.– The Voyager cameras detected a few additional rings and also

showed that the nine major rings are surrounded by belts of fine dust.

The Uranian Ring System

IR Image of Uranus’ Rings

Another IR image (colorized)

Neptune's Ring System• Evidence for incomplete arcs around Neptune first arose in the mid-

1980's, when stellar occultation experiments were found to occasionally show an extra "blink" just before or after the planet occulted the star.

• Images by Voyager 2 in 1989 settled the issue, when the ring system was found to contain several faint rings, the outermost of which, Adams, contains three prominent arcs now named Liberty, Equality and Fraternity. – The existence of arcs is very difficult to understand because the laws of

motion would predict that arcs spread out into a uniform ring over very short timescales.

– The gravitational effects of Galatea, a moon just inward from the ring, are now believed to confine the arcs.

• Several other rings were detected by the Voyager cameras. – Adams Ring 63,000 km from the center of Neptune– Leverrier Ring is at 53,000 km and the broader– Galle Ring is at 42,000 km. – A faint outward extension to the Leverrier Ring has been named Lassell; it

is bounded at its outer edge by the Arago Ring at 57,000 km.

Neptune's Ring System

Rings Are Probably Short-Lived

• Gravity of the Sun and planet’s satellites constantly disturbs orbits of particles

• Collisions probably happen frequently• Particles should drift out of the ring• Rings around outer planets have

probably come and gone repeatedly during history of Solar System