Voyager 2 is the only spacecraft to visit all four outer planets.

Spacecraft to the Outer Solar System

Flybys: Pioneer 10, 11 Voyager 1, 2

Orbiters/ : Galileo, CassiniLanders (Jupiter) (Saturn)

What Are The Rings Like?

All four jovian planets have ring systems, Saturn merely the most obvious.Others have smaller, darker ring particles than Saturn

Saturn’s Rings: Type Example

Saturn has 7 ring regions, named A through G.

Rings named in order of discovery, not distance from Saturn, Ring order is: D, C, B, A, F, G, E.

(image color created using ultraviolet, infrared and clear images then adjusting to resemble natural color.)

Two new rings are detectable plus Enceladus is feeding ice particles to the outermost E ring.

Enceladus

E AG

F

new ring

BCD

E

AGF

new ring

new ring

1st Ring: in the orbits of the moons Epithemius and Janus. Inside the G and E rings and beyond the A ring.

new ringPallene

2nd Ring: in the orbit of the moon Pallene, between the G & E rings.

The B ring is sandy colored, while the outer A ring is white from ice.

Rings made of thousands of small rings of ice particles like fluffy snowballs

Rings in UVC ring B ring

Dirty ice cleaner ice

Older and more stained?

Younger and cleaner?

Ring Location in Gas Giants

• Rings occur within 2-3 radii away (inside the Roche Limit)

– Region where gravity of a planet is so strong that materials can’t hold together to form a moon.

• Planet’s gravity pulls in material, moons’ gravities push and pull in opposite direction.

• Tug-o-war holds rings in place (for a while)

Some rings are held in place by the gravity of tiny moons. -They are called 'Shepherd Moons'

F ring has three shepherds.

Keeping Rings in Place 1: Shepherd Moons

close-up of the F ring and two shepherds. It is held between the gravity of these two moons.

Prometheus (moves inside the ring)

Pandora(moves outside the ring)

Keeping Rings in Place 2: Resonances

Orbital resonance with the gravity of a larger moon can push the particles, producing a ring or a gap

Ring Formation in Gas Giants• Rings aren’t leftover from planet formation

because the particles are too small to survive this long in a strong gravity field.

• There must be a continuous replacement of tiny particles.

• The most likely source is impacts with the gas giants’ moons.

Ring Formation in Gas Giants

• Impacts continually occur on the many small moons, chipping off fine material

• Fine material spreads out in orbit around the gas giant

• The gravity effects from the moons then hold the bits in place as rings.

What have we learned?• What are the rings like?

– The rings are tiny pieces of fluffy snow balls held in orbit 2-3 radii away from a gas giant world.

– They occur because gas giants have lots of moons:

• Impacts chip off material that becomes rings.• Material is nudged into resonance orbits by the gravity

of the moons, forming rings• Some rings are held by the gravity of tiny shepherd

moons.

© 2014 Pearson Education, Inc.

Why don't ring particles form a moon?

A. They collide too violently to accrete into a moon.

B. Tidal forces from moons prevent them from accreting.

C. Tidal forces from the planet prevent them from accreting.

D. Their masses are too small for them to accrete.