ch 12-2 Saturn rings and moons

saturn’s spectacular ring system

• has an extraordinarily large and complex ring system, which was visible even to the first telescope

• 282,000 km from planet

• 10 m thick

• thought to be secondary objects

overview of the Ring System

• consists of main rings and faint outer rings

• billions of dust like particles from tiny to large

what are the main rings?

• A, B, C

• Cassini division (rings composed of rings between A&B)

• largest and most visible

• ordered by discovery

• B is the brightest

• CBA is actual order in closeness

what are the faint outer rings?

• D, E, F, G

what is the ring particles range in size?

• form fractions of a millimeter of tens of meters

what is the ring composition?

• water ice—like snowballs

• trace: rocky metallic material (colour variation)

• high reflectivity

how do particles travel?

• travel at different speeds and collide and break down into smaller pieces and then fall into atm.

for any given planet and any given moon…

• the critical distance inside of which the moon is destroyed is known as tidal stability limit or Roche limit

What is the Roche Limit?

• The closest distance a moon can orbit a planet without being torn apart by tidal forces.

Why can't large moons exist inside the Roche Limit?

• The planet's tidal forces become stronger than the moon's self-gravity and pull it apart.

Why are most Jovian rings found inside the Roche Limit?

• Material inside the Roche Limit cannot form a large moon and instead remains as ring particles.

How can a ring system form?

• A moon that moves within the Roche Limit can be torn apart, producing ring material.

zone that is too close to the planet for moon formation…

• bc of tidal forces would overcome gravity and tear it apart

what did voyager probes show?

• Saturn’s rings to be much more complex than originally thought

  • composed of thousands of ringlets

    • spokes

    • shepherds moons

ringlets are not actually separated from one another by empty space

• instead rings contain concentric regions of alternating high and low concentration of ring particles

what is the pattern of Saturn’s ring?

• spiral density waves

  • narrow gaps kept clear by action of small moonlets embedded in them

list the rings from closest to farthest

• D, C, B, Cassini Division, A, F, G, E

F ring

• narrow and dynamic

• kinked and clumpy due to gravitational interaction with Shepard Moon

G ring

• further from F

• lots of micro-dust

E ring

• outermost

• far beyond orbit of moon titan

• lots of micro ice and dust

why is F ring considered the strangest ring?

• it appears to have braids and kinks → dense core

• Shepherded by Ephesus and Pandora → channels and snowballs of ice

2 small dark satellites

• Prometheus and Pandora

  • orbit around F ring

  • produce braid

what else did Voyager find?

• radial “spokes” that formed and then dissipated; this probably happens frequently

• freshly formed spokes revolve same rotation of magnetic field and interior of Saturn → tiny dust electrically charged

spokes apperance

• sharp, narrow, suggest short formation times

what is the unofficial theory of spokes?

• think caused by small particles suspended by ring plane and electromagnetic forces responsible for features

What causes divisions (gaps) in Saturn's rings?

• Gravitational resonances with Saturn's moons.

What is orbital resonance?

• A situation where two objects have orbital periods related by a simple ratio, causing repeated gravitational interactions.

How do resonances create gaps in rings?

• Repeated gravitational tugs from moons disturb ring particles and remove them from certain orbits.

What moon is associated with the Cassini Division?

• Mimas.

What causes the Cassini Division in Saturn's rings?

• A 2:1 orbital resonance between ring particles and Saturn's moon Mimas.

  • mimas = 1 orbit

  • ring particle = 2 orbits

what is a “Shepherd” moon?

• defines outer edge of Ring A through gravitational interactions

moon pulls on ring particles to?

• prevent them from dispersing outward by orbiting out

• nudge stray particles back into ring

ring formation: details of formation are unknown

• probably too active to have lasted since birth of solar system

• not all rings may be the same age

• either must be continually replenished or are the result of a catastrophic event

what are Saturn’s many moods appear to be made of?

• water ice

in addition to the small moons, Saturn has

• 6 medium-sixed moons (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus)

• 1 large moon (Titan), almost as large as Jupiter Ganymede

3 groups of Saturn’s moon

1. “small” moons - irregularly shaped ice

2. “medium” moons - spherical bodies - clues of past and present

3. “large” moon - Titan

Titans atmosphere

• thicker and denser than Earth’s; mostly nitrogen 95% and argon

• only moon with substation atm.

• 50% denser than Earth

what does Titans thick atmosphere impact?

• makes surface impossible to see

characteristics of Titan’s atmosphere

• unique, chemically complex due to trace chemicals

• Nitrogen, Methane, Hydrocarbon

• organic rich environment

• more extended than Earth because low gravity

• thick and dense

• unseen surface covered in organic material settled from clouds

• low surface temperatures = thick atm.

• weak gravitational pull

Cassini Huygens observations

• some surface features are visible in infrared image\not possible to see in visible spectrum

• dense rich Nitrogen atm.

what surface features are seen on Cassini?

• internal liquid ocean beneath surface → H2O and Ammonia

• has lakes and liquid methane

  • replenished from rain

Huygens spacecraft landed on Titan

• confirmed presence of liquid ethane in lakes

• methane rain feed lakes

• revealed ice pebbles, icey plains, river like drainage channels

lakes on Titan

• Cassini detected dark reflective smooth regions thought to be lakes

• Northern latitudes (liquid methane and ethane)

• back to radar

  • smooth surface

Titan’s interior: “Water World”

• based on measurements made by Cassini and Huygens, this is the current bets understanding as to what the interior of Titan looks like

  • highly differentiated ocean world

  • thick icey outer crust

  • subsurface liquid ocean

  • rocky core

  • inner may be slushy water instead of open sea

what are Saturn’s Mid-sized moons?

• Mimas

• Enceladus

• Tethys

• Dione

• Iapetus

• Rhea

• orbit between 3 and 9 planetary radii from Saturn

Iapetus

• extreme 2 tone coloration

• 2 km equatorial ridge

• orbits 59 radii away from Saturn

Mimas

• Hershel crater

• “inner icey moons”

• intermediate sized

• orbit between ring system and Titan

• heavily cratered

• water ice

Enceladus

• active geysers

  • heat and potentially habitable

Tethys

• large canyon system

Dione

• tectonic features

• potential subsurface ocean

Rhea

• largest of group

• brightest, heavily cratered

• weak ring system

all mid-sized moons are tidally locked

• heave “leading” and “trailing” surfaces

enceladus

• surface seems oddly youthful

• inset shows icy jets

• large ocean believed to exist under the ice

• white and reflective surface with water ice

• heavily cratered Northern region

• tectonic deformed younger southern

• small

Janus and Epimetheus share a single orbit

• irregularly shape

• unique gravitational dance every 4 years

  • swap positions to avoid collision

• one moves to higher, slower orbit and the other drops lower to faster orbit

telesto and calypso

• two more are at the same Lagrangian points of Saturn’s moon Tethys (trojan moons, 60° behind)

• stable gravitational positions

what did the Cassini spacecraft use to make multiple close passes around Saturn’s moons?

• gravitational slingshots

  • precise orbits are decided on the fly

Saturn and Jupiter rotate

• differentially and is significantly flattened

saturn generates its own heat through?

• helium raindrops

interactions with medium and small moons determine?

• ring structure

the rings are entirely within the?

• Roche limit, where larger bodies would be torn apart by tidal forces

several of he small moons share orbits with?

• either each other or with larger moons