1. Tides & Rings

2. Hopewell Rocks, New Brunswick

3. The Moon: Tides

4. Tides

5. The Moon: Tides Every century the Earth’s day is 0.002 s longer
the Moon is 4 m further away
Four billion years ago
an Earth day was 5-6 hours long
the Moon was 20,000 km away (5% current value)

6. The Moon: Tides One billion years from now
the Earth and Moon will be 1:1 spinlocked
the Earth’s day will equal a lunar month (47 current days)
The Moon will be 526,000 km away (1.37× its current distance)
The Pluto/Charon system already appears to be 1:1 spinlocked.

7. Galilean Satellites

8. Galilean Satellites orbital periods
1.77 days
Europa
3.55 days
Ganymede
7.15 days
Callisto
16.69 days

9. Tidal Heating

10. Io

11. Io

12. Europa

13. Conamara Chaos

14. Enceladus

17. Extreme tides: the Roche Limit
Roche Limit disruption applies
to strengthless satellites.
Typically, the Roche Limit lies
at a distance of 2.4 planetary
radii from the center of a planet.
(Details of satellite density,
viscosity and strength can
affect this).

18. All gas giants have rings
Jupiter: broad, dark, fine particles
Saturn: broad, bright, complex, icy particles
Uranus – narrow, dark, fine particles
Neptune: uneven, fine particles
All consist of independently orbiting small chunks of material within very thin layers. Saturn’s rings span 100,000 miles, are only a few yards thick in places.

19. Why rings?
Tidal forces destroy a large solid moon insides a planet’s Roche limit. Ring systems are always found inside the Roche limit (about 1.44 planet diameters above the surface).
Collisions make rings the final shape for swarms of individual particles in orbit; they sap energy but not momentum.

20. Any object without intrinsic gravity (such as a pile of gravel) will break up inside the Roche limit due to tidal effects; a moon with sufficient mass and under its own gravity need not break up
All rings, and small shepherding moons, lie within the Roche limit; larger moons are outside

21. All major ring systems lie within their respective Roche limits.

22. Stability
Random motions should make some particles leave the rings and limit their lifetime. External effects can help herd stragglers back. Main example: shepherd moons.

27. Internal structure
Rings can be very thin. Radial structures can be produced by gravitational forces (such as tides from nearby moons).
Example: Cassini division in Saturn’s rings. Weaker disturbances and wave patterns can divide a ring into myriads of ringlets.

30. Comet Shoemaker-Levy 9
1994 July 16-22