Nov. 25th - Tides & Rings

Question 1

The Moon’s Tidal Force

Why does tidal force affect different parts of Earth slightly differently:

Tidal Force

Answer 1

• Because the strength of gravity declines with distance, the gravitational attraction of each part of Earth to the Moon becomes weaker as we go from the side of Earth facing the Moon to the side facing away from the Moon.
• This difference in attraction creates a “stretching force,” or tidal force, that stretches the entire Earth to create two tidal bulges, one facing the Moon and one opposite the Moon

Question 2

What parts of the earth do tides affect?

Answer 2

• Land
• Ocean (generally noticed more, since water flows easier)

Question 3

Why are there two tidal bulges? What does this result in?

Answer 3

• Earth’s rotation carries any location through each of the TWO tidal bulges each day
• This is why high tide occurs

Question 4

When and why do low tides occur?

Answer 4

Low tides occur when the location is at the points halfway between the two tidal bulges.

Question 5

How long are the cycles between high and low tides?

Answer 5

• as the moon’s highest position returns every 24 hours and 50 minutes, as opposed to solely 24, each high tide occurs approximately 12.25 hours apart

Question 6

What factors can affect the timing and height of tides?

Answer 6

• Latitude
• Orientation of coastline (north facing, south facing)
• Depth and shape of the channels the rising tide must flow

Question 7

What can also have a tidal force on earth?

Answer 7

THE SUN

Question 8

How strong is the tidal effect of the sun?

Answer 8

• Despite having a larger mass, the sun has a weaker tidal effect on earth than the moon - also despite having a stronger gravitational attraction (making the earth orbit around the sun)
• However, the weaker tidal effect is due to distance; there is less of a noticeable pull

Question 9

When do the tidal forces of the moon work together? What happens?

Answer 9

• Occurs every full/new moon
• Create spring tides, which “spring up” from earth

Question 10

When do the tidal forces of the moon counteract each other? What happens?

Answer 10

• Every first and third quarter moon
• Creates neap tides, relatively small

Question 11

What is Tidal Friction?

Answer 11

• As the earth is being stretched by tidal forces, this creates friction
• HOW?
  1. The moon attempts to keep earth’s tidal bulges on the earth-moon line (which would be possible if the earth didn’t rotate)
  2. However, earth’s rotation wants to pull the tidal bulges along with it
  3. THE NET RESULT: the friction between the two causes the bulges to always be slightly ahead of the earth-moon line

Question 12

What does the misalignment of the tidal bulges with the earth-moon line cause?

2 effects:

Answer 12

1. The moon’s need to realign the bulges with the earth-moon line actually slows earth’s rotation down
2. The gravity of the bulges pulls the moon ahead of its orbit, adding orbital energy that causes the moon to move farther from earth

Question 13

The moon’s synchronous rotation

a natural consequence of tidal friction

Answer 13

• Because earth’s tidal effects on the moon are stronger, the moon also has two tidal bulges
• RE: the moon slows earth’s rotation
• Thus, the moon must have rotated much faster, but the earth slowed its rotation down a significant amount - having such a strong force
• Once it reached the point at which the moon and its bulges rotated at the same rate, there was no further source for tidal friction

Question 14

Tidal Effects on Other Worlds

Answer 14

• Synchronous rotation is common: Jupiter’s four moons (Io, Europa, Ganymede, and Callisto), Pluto’s Charon

Question 15

What are Saturn’s rings like?

Answer 15

• From earth, look like big sheets
• From imaging, actually made of individual rings - each separated by narrow gaps
• Made of icy particles, sometimes clumped together by their mutual gravity

Question 16

Ring particle characteristics

Answer 16

• Made mostly of relatively reflective water ice
• The rings look bright where they contain enough particles to intercept sunlight and scatter it back toward us.
• Each individual ring particle orbits Saturn independently in accord with Kepler’s laws
• The individual ring particles are so close together that they collide frequently.
• However, the collisions are fairly gentle: Despite the high orbital speeds of the ring particles, nearby ring particles orbit at nearly the same speed and in the same direction, and therefore touch only gently when they collide
• The rings are so thin that they disappear from view when we see Saturn edge-on, as we do around the equinoxes of its 29.5-year orbit of the Sun.

Question 17

Rings and gaps - how are they caused?

Answer 17

• By particles bunching up at some orbital distances and being forced out at others.
• This bunching happens when gravity nudges the orbits of ring particles in some particular way.
• One source of nudging comes from small moons located within the gaps in the rings themselves, sometimes called gap moons.
• The gravity of a gap moon can effectively keep the gap clear of smaller ring particles while creating ripples in the ring edges
• The ripples appear to move in opposite directions on the two sides of the gap, because ring particles on the inner side orbit Saturn slightly faster than the gap moon, while those on the outer side orbit slightly slower than the gap moon.
• Ring particles also may be nudged by the gravity from larger, more distant moons.
• The periodic nudges reinforce one another and clear a gap in the rings—in this case, the large gap visible from Earth (the Cassini division). This type of reinforcement due to repeated gravitational tugs is another example of an orbital resonance, much like the orbital resonances that make Io’s orbit elliptical

Question 18

How do other jovian ring systems compare to Saturn’s?

Answer 18

The rings of Jupiter, Uranus, and Neptune are so much fainter than Saturn’s that it took almost four centuries longer to discover them. Ring particles in these three systems are far less numerous, generally smaller, and much darker.

Question 19

Similarities of Jovian & Saturn rings

Answer 19

• Similarities:
  1. All rings lie in their planet’s equatorial plane
  2. Particle orbits are nearly circular, with small orbital tilts relative to the equator
  3. Individual rings and gaps are probably shaped by gap moons, shepherd moons, and orbital resonances

Question 20

How were Uranus’ rings discovered?

Answer 20

• Uranus’s rings were discovered in 1977 during observations of a stellar occultation—a star’s passage behind Uranus as seen from Earth.
• During the occultation, the star “blinked” on and off nine times before it disappeared behind
• Uranus and nine more times as it emerged.
  Scientists concluded that these nine “blinks” were caused by nine thin rings encircling Uranus.

Question 21

How were Neptune’s rings discovered?

Answer 21

• Similar observations of stars passing behind Neptune yielded more confounding results: Rings appeared to be present at some times but not at others.
• Could Neptune’s rings be incomplete or transient?

Question 22

One key clue to the origin of ring systems comes from the fact that…

Answer 22

• Rings always lie within two to three planetary radii of their planet, which is a region where the tidal forces tugging an object apart become comparable to the gravitational forces holding it together (This region is often called the Roche tidal zone.)
• Only small objects held together by nongravitational forces—such as the electromagnetic forces that hold solid rock, spacecraft, and human beings together—can avoid being ripped apart in this region.

Question 23

What are the 2 rejected theories of ring formation?

Answer 23

1. A moon was brought into the roche tidal zone and torn apart - however moons don’t just wander out of their orbits
2. The particles were leftover from the disks surrounding young planets - however wouldn’t have lasted this long, as it would’ve been puvlerized by micrometeorites