An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30
All Jovian planets have rings:
- Jupiter: faint, dusty rings
- Saturn: bright, spectacular rings
- Uranus: dark, thin rings
- Neptune: dark, thin rings & ring arcs
- Bands of orbiting dusty particles and ice balls
- Shepherd moons & orbital resonances
- Roche (tidal) radius
All Jovian Planets have rings
Saturn's rings are bright and broad:
- Discovered in 1659 by Christiaan Huygens.
Uranus & Neptune have thin, dark rings:
- Discovered in 1977 & 1985 by watching stars disappear
behind them (stellar "occultation")
Jupiter has faint, dusty rings:
- Discovered in 1979 by Voyager 1 & 2.
- Can be seen from Earth using infrared images.
Jupiter's Dusty Rings
Jupiter's rings are faint and dusty:
- Made of micron-sized dark dust particles.
- Total mass is <10-12 MEarth.
- Contain virtually no ices.
- Distinct "main ring" surrounded by a
"halo" of fine dust particles
- Faint, thick outer "Gossamer" ring
Dusty ring material may have been knocked off the moons of Jupiter
by micrometeor impacts, or leftover from comets that crashed into
Saturn's Bright, Broad Rings
Saturn has the most elaborate of the Jovian ring systems:
- Broad bands of bright icy material.
- Broad gaps (Cassini Division & Encke Gap)
- Divided into thousands of thin ringlets
- Extend from 73,000 km to 140,000 km from the center of
Saturn (1.2 - 2.3 RSaturn)
The rings are very thin:
- Thickness is <100 meters.
- like a sheet of paper 1 mm thick & 10 km wide!
Saturn's rings are not solid:
- Made of billions of ice balls all in independent orbits
- Range in size from centimeters to 5 meters.
- Total mass ~10-6 MEarth (about the mass of a 100km sized
As the iceballs collide:
The constant collisions keep the iceballs bright and shiny. Old iceballs
would gradually accumulate a dark "patina" of dust that would darken
- Stick together to form larger ice balls, or
- Chip off fragments or break into smaller chunks
Thin Rings of Uranus & Neptune
Uranus & Neptune have narrow, dark rings separated by
- Dark, narrow rings only a few km wide.
- Wide "epsilon ring" is only ~100 km wide.
- Faint, very dark rings only a few km wide.
- Ring "arcs" are clumps of material in the
Thin rings should not last for very long:
- Collisions between ring particles will cause them to
spread out over time, destroying the thin ring.
Thin rings, however, can be gravitationally confined
by a pair of Shepherd Moons:
Subsequent collisions between ring particles then damp out the
eccentricities, resulting in the ring becoming confined between the
- Outer moon's gravity decelerates ring particles,
moving them into lower orbits (eccentric ellipses smaller
than the original near-circular orbit).
- Inner moon's gravity accelerates ring particles,
moving them into higher orbits (eccentric ellipses larger
than the original near-circular orbit)
[Review Lecture 19 on Orbits]
Rings and Resonances
Much of the fine structure within rings is governed by
with the planet's moons.
Example: Saturn's Cassini Division.
- The Cassini division is in a 2:1 resonance with the moon
- Mimas orbits every 22h, while Iceballs in the Cassini Division
orbit every 11h.
- Every 2 orbits, the iceballs in the Cassini Division get
tugged towards Mimas, clearing a gap over time.
The Origin of the Rings
The rings are not very massive:
- Jupiter, Uranus, & Neptune's rings have the mass
of an icy moon ~10 km across.
- Saturn's rings have the mass of a mid-sized icy moon
~200 km across.
Possible origins of the ring material:
- An icy moon that came too close and was disrupted by tidal forces.
- A moon that couldn't form because of tides.
Distance at which the tidal forces from the planet on a moon
equal the gravitational force holding the moon together:
All rings are located inside the Roche Radii of their parent
- If a moon gets closer than the Roche radius of its parent
planet, it will be pulled apart by tides.
- Icy particles orbiting inside the Roche radius of a
planet cannot aggregate into a moonlet.
An excellent source for images of planetary rings and ring
science is the Planetary
Rings Node at the NASA
Ames Research Center.
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Updated: 2007 November 19
Copyright © Richard W. Pogge,
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