Saturn - Lord of the Rings


Low density, mainly composed of hydrogen, spins rapidly, global flipped magnetic field, radiates internal heat, 18 known moons.

Click here for an image of Saturn


Comparison of Jupiter and Saturn



Slow Orbit

29.5 years

11.86 years

Rapid Rotation

10.6 hours

9.9 hours

Large Size

9.5 RE

RE = Earth's Radius

11.2 RE

Low Density

0.71 g/cm3



95 ME

ME = mass of Earth

318 ME

Excess Heat

Twice Solar

(more helium rain)

Twice Solar

(less helium rain)

Liquid Metallic Hydrogen







Saturn's belts and zones blow in the same direction.

Saturn's magnetic field is also flipped (like Jupiter) with terrestrial compass pointing south. Note magnetic axis and rotation axis of Saturn are closely aligned with little tilt between them. Tilt for Jupiter about 10 degrees, for Earth about 12 degrees.

Saturn is smaller mass means it is cooler inside than Jupiter, so more helium rain is needed to generate internal heat of Saturn.

Titan - about size of Mercury - a moon with a substantial, permanent atmosphere.

Composition of atmosphere - lots of nitrogen, some methane.

Surface pressure - comparable to Earth.

Creates hydrogen torus - action of sunlight on methane.

Formed from ices of ammonia, NH3, and methane CH4, on surface of Titan. Jupiter, when hot and young, vaporized these ices from its moons.

Titan - (new) largest moon of Saturn larger than Mercury.

Atmosphere - only satellite possessing a significant dense, thick atmosphere with a surface pressure comparable to that of Earth.

Surface temperature = 94o K - methane can exist as a solid, liquid or gas at this temperature.

Atmosphere - nitrogen, methane and ruddy (orange) organic compound (smog). Methane and ethane can rain down to the surface.

Hubble Space Telescope - infrared radiation penetrates opaque atmosphere (can't "see" through thick atmosphere). Previously unseen surface may contain hydrocarbon lakes or seas. If impacts melt ice, yielding liquid water, amino acids could form by chemical reactions. Prebiological organic chemistry is possible on Titan.

Rings Around Saturn

Discovery - Christiaan Huygens 1655 (26 years old). Thin flat ring nowhere touching the planet and inclined to ecliptic.

Main Properties of Rings

Very wide and incredibly thin only 10 to 100 meters thick (Like piece of paper 40 blocks wide)

Ring particles are made of water ice. (new) Rings lose about 3 tons of water a day due to impact of charged particles and dust.

Rings are inside Saturn's Roche limit at a radius of 2.5Rs where Rs is Saturn's radius. Rings are not solid - consist of innumerable particles (small) in differential orbit about Saturn.



Why Don’t Rings Fall Down?

Explained by James Clerk Maxwell (also 26 years old)

Rings consist of innumerable small particles in differential orbit about Saturn - with speeds that increase closer to planet. (inner particles move faster, outer ones slower)

Why Do Planets Have Rings?

All rings are near planet where gravity is strongest.

Gravity is strongest nearest the planet, so it produces differential "tidal" forces across a nearby satellite.

Saturn's Rings (new)

Saturn's rings are not ancient and enduring; they were not formed with the planet 4.6 billion years ago.

Saturn's rings are a recent, young addition to the solar system, no more than 100 million years old.

Origin of the Rings

Saturn's rings are most likely a moon, or moons, gone to pieces. Collisions with former moons, lying within Roche limit, blasted them to bits. All the material in Saturn's rings when piled together make up an icy satellite no more massive than Saturn's satellite Mimas.

Destruction of the Rings

  1. Saturn's moons exert gravitational tugs on ring particles, stealing energy from them and causing them to slowly spiral toward Saturn.
  2. Constant pelting by cometary dust grains erodes large ring particles away. Also darkens and reddens them so Saturn's bright rings light are young.

Inside Roche limit, at 2.5 planet radius, differential gravitational forces tear a large satellite into pieces.

No large satellite can exist inside Roche limit, but small satellites can because of greater internal cohesion. Unusual small satellites - exchange orbits.

Rings are not primordial leftovers from formation of planets; lifetimes shorter than age of solar system - due to erosion by meteoroid bombardment and gravitational torques exerted by neighboring satellites.

Moonlets embedded in rings are likely source of new rings after collision with meteorites or comets; rings are short-lived products of destruction.

Cussini's gap is not empty.

F ring confined by shepherd satellites called Prometheus and Pandora.

Small satellite creates sharp edge of the A ringlets.

Dark spokes - move at same speed regardless of distance from Saturn-levitated ring particles.