AST101 - Terrestrial Planets (Lecture 14)

What are planets inside the frostline composed of?

• 0.2% metal (solid)

• 0.4% Rock (solid)

• 1.4% Hydrogen compounds (gas)

• 98% helium and hydrogen (gas)

• Only metal and rock can form planetesimals

What are planets outside the frostline composed of?

• 0.2% metal (solid)

• 0.4% rock (solid)

• 1.4% hydrogen compounds (Solid ice)

• 98% helium and hydrogen (gas)

• Metal, rock, and hydrogen compounds can form planetesimals

How do craters form?

A planetesimal strikes the surface of an object at a typical speed of 100,00 kmph

Levels of Planetary Interiors From Inside Out

1) Core

2) Mantle

3) Crust

What is the crust?

Low density rock

What is the mantle?

Medium density rock?

What is the core composed of?

Highest density (iron and nickle)

What does the core of the Earth look like?

It has a solid inner core and a liquid outer core

What does the core of the Mercury look like?

It has a partially liquid core

It’s measured through density and spin rate variations

What does the core of Venus look like?

We don’t know much about it

It’s measured through dentity

What does the core of the Moon look like?

It might have a SOLID core

It’s measured with seismometers

What does the core of the Mars look like?

It maybe has a core

It’s measured with seismometers

How do cores develop?

Through differentiatiion

Differentiation

Light materials (lower density rock) float to the surface while heavier materials (iron, nickel) sink to the core

Why are planet interiors hot

Accretion: Dominant during planet formation

Differentiation: More significant earlier

Radioactive Decay: Dominant source of eat today

How Do Planets Cool?

Convection, conduction, radiation

Convection

1. Planet’s mantle is not completely rigid

2. Hot rock weighs less than cooler rock
   Hot rock rises and cooler rock sinks

3. Brings heat up from the core

Conduction

1. Cool crust is rigid so no convection

2. Heat is conducted through the rock (slower than convection)

3. Crust is very thin

Radiation?

1. Infrared light acquires energy away from the surface of the planet

2. If there is more light leaving the planet that coming from the sun, then planet cools

How does the radius impact the volume and area of a planet?

• If you double the radius, volume is 8 times bigger

• The area gets 4 times bigger

Why do smaller planets cool faster?

• Small planets have less mass compared to surface area than a large planet so they cool faster

Mares

• Start with a created surface

• Large impact damaged lithosphere

• Molten lava leaks up, floods the creator

  • Leaves a smooth surface

  • Future impacts create a few new craters

• Happened after heavy bombardment but before moon cooled enough

• Moon and mercury both cooled to the point where this can’t happen anymore

How is a seismometer used to take a look at the core of a planet?

• Using direction and timing, the interior of the planet can be probed

• Earthquakes cause vibrations to travel through the Earth

• Combined data from many earthquakes and seismoters all over the Earth to get a full picture

• Two types of waves are calculated:

  • P waves and S waves

P Waves

• Compression: Compression and stretching in direction of travel

• Can go through water but bends

• Timing for seismometers at different places depends on how far and how fast the wave traveled

S Waves

• Side to side. Vibrate up and down or side to side perpendicular to the direction of travel

• Timing for seismometers at different places depend on how far and how fast the wave traveled

• Can’t go through water

  • Created S-wave shadow zone