Lecture 13 - Terrestrial Interiors

Inside the frost line, what materials can form planetesimals?

Metal and rock

What can form planetesimals outside the frost line?

Metal, rock, and hydrogen compounds (ice).

Frost Line

The distance from the Sun where it’s cold enough for hydrogen compounds to freeze.

What are planetesimals?

Small solid seeds of planets

What was the “Heavy Bombardment”?

A period ~4 billion years ago when leftover planetesimals heavily impacted planets.

Why do Mercury and the Moon have more craters than Earth?

They lack erosion, tectonics, and atmosphere

Mares (Maria)

Smooth, dark regions on the Moon created by lava flooding impact craters.

What caused some lunar craters to become Mares?

Molten lava flooded the craters after large impacts.

What does the number of craters indicate about a planet’s surface?

Its age

Why do Mercury and the Moon no longer have active volcanism?

They cooled and their lithospheres became rigid.

Differentiation

The process where denser materials sink to the core while lighter materials rise.

Which layer of a planet stops S-waves?

Liquid Core

What do P and S waves tell us about Earth’s interior?

They reveal solid and liquid layers through how they travel.

Which wave type travels through liquids?

P waves

Seismology

The study of vibrations (seismic waves) to understand a planet’s interior.

List three heat sources for planets.

Accretion, differentiation, radioactive decay.

Which heat source dominates today?

Radioactive Decay

Convection

Heat transfer through rising hot material and sinking cool material in the mantle.

Conduction

Slow transfer of heat through the rigid crust.

Radiation

Heat loss from a planet’s surface via infrared light.

Which planets cool faster: small or large?

Small

Why do smaller planets cool faster?

They have a larger surface area relative to their volume.

After 1 billion years, which would be cooler: a planet or its small moon?

The planet

Terrestrial Planets

Small, rocky planets with thin atmospheres, few moons, and heavy elements.

List the terrestrial planets.

Mercury, Venus, Earth, Mars (and Earth’s Moon).

Rhea, a moon of Saturn, is mostly:

Ice with some rock and metal

Why is Rhea mostly ice?

It formed beyond the frost line where ice could condense.

Which technique do scientists use to study planetary interiors?

Ultrasound (seismic waves)

Why can’t we drill to Earth’s core?

The deepest hole is only ~12 km; Earth’s radius is 6371 km.

Seismometer

An instrument that measures vibrations from quakes to infer interior structure.