Section 12: Rings, Moons and Pluto

Moons compositon

around 200+

• Composition:

  • Rocky, icy and terrestrial → due to being formed further from the sun 

  • Organic compounds

Moon orbits

• ⅓ have prograde orbits

• ⅔ have retrograde orbits 

Prograde

• Orbits in the same direction as the main bodies rotatioin along its axis

Retrograde

Orbits in the opposite direction as the main bodies rotatioin along its axis

Orbital Resonance

 where two or more orbiting bodies exert regular, periodic gravitational influence on each other → they all line up

• Keeps all orbits elliptical 

Jupiters Moon Io 

• Volcanoes

  • 100 recent volcanic activity

• Lava planes

  • Magma with hot silicate

  • Sulfur and sulfur dioxide → get heated and rise but then fall due to cooling

  • Makes it colourful

• No impact craters due to it volcanically active

• Low gravity

• Internal Heat Source: Tidal Heating 

• molten core

Volcanic Plume

•  a column of ash, gas, rock, and other volcanic materials ejected into the atmosphere during an explosive eruption 

• Tidal Heating 

The continous contraction and expansion of a moon caused by the gravity of the planet as it reaches aphelion or perihelion

Icy Galilean Moons of Jupiter

• Europa Ganymede and Callisto

  • Composition:

    • Ice water 

    • Might have subsurface Oceans 

Jupiters moon: Europa

• Icy surface but more rocky

  • Due to jupiter being hotter during its formation 

  • Cracks in the ice hypothesised to it floating on an ocean of water 

• Fewer impact craters: indicate geological resurfacing 

  • Younger surface 

• Small Magnetic field → similar to those caused by a liquid water ocean 

• Internal heating: from tidal heating like io just not as strong 

• Water plume:

• From cryo volcano that ejects water

Jupiters moon: Ganymede

• Largest moon in the solar system

• Water ice surface 

• Cratering 

  • On half and not the other half

• Partially molten interior

• Magnetic field

• Has tectonic and volcanic forces → Caused by tidal heating

Jupiters moon: Callisto

• Heavily cratered ice ball 

  • Brighter patches are impact craters on the moon

• Geologically dead → no activity 

• Mainly ice instead of rock and metal 

• Not fully differentiated despite the various melting points 

• Has a magnetic field → suggesting subsurface ocean

  • HEating of the ocean is unknown since it's too far to be affected by tidal heating 

Saturn's Moon titan

• Saturn's Largest moon 

• Only moon that has a dense atmosphere 

  • Composed of nitrogen and hydrocarbons creating a haze 

  • Has a methane cycle → Low temp equivalent 

• Seas and rain with methane and ethane 

• Huygens: A probe from cassini that landed on titan

  • The most distant yet still controlled landing ever 

Neptunes Moon Titron

• Composition: 75% rock, 25% water ice

• Thin atmosphere: mainly nitrogen

• Low temperature of 35-40K

• Has a retrograde motion and is inclined 

  • Neptune’s tidal bulge is behind triton causing it to spiral inward → slowly reaching its roche limit

  • Like basically the bulge on neptune is behind so its slowing down the moon and pulling it in → since its losing energy by accelerating neptune by pushing it forward 

• Recent geological activity

  • Resurfacing of some of the impact craters 

  • Heating from absorbing solar energy rather than tidal heating

Pluto

• Discovered in 1930 by Clyde Tombaugh - using blink-comparator method, examining pairs of photographic plates

  • Attempt to see objects that moved relative to background stars

• Very eccentric orbit which was tilted on the plane of all other planets 

Pluto’s Moon Charon

•  helped calculate pluto’s mass which was too small to be considered a planet 

  • Charon is very large (half of plutos size) 

  • Formation: From a giant impactor creating a large collision that ejected materials that coalesced into pluto 

  • Double tidal Lock system: Pluto and charon are both tidally locked to one another

    • Each always shows the same face to the other

pluto properties

• Surface temp being extremely cold 

• Density similar to outer planet moons implying rock and water ice composition

• Composition: 

  • Geologically active 

  • Colour variation showing different chemical compositions on the surface 

    • Dark areas: Hydrocarbons and organic compounds 

    • Light areas: Frozen nitrogen  

• Surface temp being extremely cold 

• Density similar to outer planet moons implying rock and water ice composition

• Composition: 

  • Geologically active 

  • Colour variation showing different chemical compositions on the surface 

    • Dark areas: Hydrocarbons and organic compounds 

    • Light areas: Frozen nitrogen  

• Trans Neptunian OBjects

• Terrestrial-like: despite it being outside the solar system (should’ve been icy)

• They are small icy bodies beyond neptunes orbit 

• Prompted the reevaluation of planetary classification

Dwarf Planet

•  a celestial body resembling a small planet but lacking certain technical criteria that are required for it to be classed as such.

Ring Systems:

• In all four jovian planets

• Ring composition: billions of small icy fragments ranging in size

Origin of Planetary Rings

1. Break-up Hypothesis

2. Leftover Material Hypothesis

Break-up Hypothesis

• Suggests that rings are remnants of a shattered moon.

• When a moon moves inwards towards the roche limit and the planets tidal forces become stronger than the moons gravity causing it to break apart - can’t hold itself together  (normally 2.5 planetary radii) 

  • Debris then spreads out to form rings 

  • All ring systems lie within their planet’s Roche limit

• Why can only small moons exist within the inside limit of the roche limit → less likely to be affected by the planets tidal forces 

Roche Limit/ Tidal Stability Limit:

• When a moon moves inwards and the planets tidal forces become stronger than the moons gravity causing it to break apart - can’t hold itself together  (normally 2.5 planetary radii) 

1. Leftover Material Hypothesis

• Rings could also be leftover material from the planet’s formation that never coalesced into a moon due to tidal forces or collisions

Motion of Ring Particles

• Each individual particle follows Kepler’s laws of planetary motion 

  • Inner particles - closer to the planet and moves faster 

  • Outer particles - move slower 

• Mutual Gravitational Interactions/ Collisions:

  • Change orbital speeds slightly: Nearby particles can pull on each other, exchanging small amounts of momentum and energy.

  • Spread the ring out: Over time, these gravitational “nudges” cause the ring to gradually spread

  • Create wave patterns: Between particle collisions creating wave- like ripples across the rings 

  • Collisions: randomize motion and keep particles moving in nearly circular, flat orbits and dissipate energy - reducing eccentricities 

Mutual Gravitational Interactions/ Collisions

• The attractive force that exists between any two objects with mass

Saturns Rings: 

• Ring Names: Outer → E, g, f, a, b, c, d ← Inner

  • B = brightest ring

  • Cassini Gap: Gap between A and B ring

    • Caused by Mimas the moon (whcih is outside the roche limit it just has a strong enough gravity to effect it) 

  • E is very weak and tenuous 

  • F is very thin

• Shepard Moons on Ring F: Pandora and Prometheus 

• Orbital Resonance: Moons that produce gaps in the rings

  • At those special locations, the moon’s gravity tugs on the same ring particles repeatedly — always at the same point in their orbits creating a gap by moving it asidehe resonance disturbs the particles’ orbits, changing their eccentricities (making them more elliptical

ORbital resonance

An orbital resonance occurs when two orbiting objects have periods that form a simple ratio

eg One object goes around twice for every one orbit of another.

Uranus’s Rings: 

• Discovery

  • Dimmed multiple time behind a background star showing that the rings cover up light and are dark and narrow 

    • Made of hydrocarbon compounds that absorb light

Neptunes Rings:

• Weaker and more tenuous than uranus rings 

• Shepherd Moons

Shepherd Moon

• Gravitationally interact with the rings to maintain its structures 

  • tiny moons whose gravity helps shape and confine the ring material