Tides & Rings

Hopewell Rocks, New Brunswick

The Moon: Tides

Tides

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)

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.

Galilean Satellites

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

Tidal Heating

Io

Io

Europa

Conamara Chaos

Enceladus

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).

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.

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.

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

All major ring systems lie within their respective Roche limits.

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.

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.

Comet Shoemaker-Levy 9 1994 July 16-22