Terrestrial Worlds

Orin Harris and Greg Anderson Department of Physics & Astronomy Northeastern Illinois University

Spring 2021

c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future – slide 1 / 84 Outline

Terrestrial Interiors Magnetic Fields Geologic Processes Terrestrial Worlds Terrestrial Atmospheres Review

c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future – slide 2 / 84 Terrestrial Planets

c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future – slide 3 / 84

Outline Terrestrial Planets Terrestrial Planets Terrestrial Interiors Sources of Evidence Onion article Planetary Densities S and P Waves Terrestrial Interiors S and P Waves Differentiation Interiors of Terrestrial Plants Q: Differentiation Q: Planetary Cores Earth’s Interior Sources of Internal Heat Earth Heat Evolution Heat Transfer Q: Cooling Volume to Surface Ratio Q: Cooling Planets c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future – slide 5 / 84 Q: Rank Sources of Evidence

How do we know what is inside? • Average density • Earthquakes • Magnetic fields • Lava flows • Local gravity variations

c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future – slide 6 / 84

Planetary Densities (ρ = M/V )

5.52 5.43 5.24 5.0

4.0 3.92 ) 3

3.0

2.0 Density (g/cm

1.0 Mercury Venus Earth Mars Saturn Uranus Neptune tNOs 0.0 Asteroids Jupiter

c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future – slide 8 / 84

Differentiation

• Gravity pulls high-density material to the center. • Lower-density material “floats” to the surface. • Material ends up sorted by density.

least dense

denser

densest

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 11 / 84 Interiors of Terrestrial Plants

Earth Venus Mars Mercury Moon

Core Mantle Crust highest density medium density lowest density

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 12 / 84 Q: Differentiation

What is necessary for differentiation to occur in a planet? A) It must have metal and rock in it. B) It must be a mix of materials of different density. C) Material inside must be able to flow. D) All of the above E) B and C

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 13 / 84 Q: Differentiation

What is necessary for differentiation to occur in a planet? A) It must have metal and rock in it. B) It must be a mix of materials of different density. C) Material inside must be able to flow. D) All of the above E) B and C

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 13 / 84 Q: Planetary Cores

The terrestrial planet cores contain mostly metal because: A) the entire planets are made mostly of metal. B) metals condensed first in the solar nebula and the rocks then accreted around them. C) metals sank to the center during a time when the interiors were molten throughout. D) radioactivity created metals in the core from the decay of uranium.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 14 / 84 Q: Planetary Cores

The terrestrial planet cores contain mostly metal because: A) the entire planets are made mostly of metal. B) metals condensed first in the solar nebula and the rocks then accreted around them. C) metals sank to the center during a time when the interiors were molten throughout. D) radioactivity created metals in the core from the decay of uranium.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 14 / 84 Earth’s Interior

3 Earth ρave =5.5 g/cm

Core: Fe, Ni,...

ρ = 10 − 13 g/cm3

Mantle: O, Si, Mg, ...

ρ =3.3 − 5.7 g/cm3

Crust: O, Si, Al, ...

ρ =2.7 − 3.3 g/cm3

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 15 / 84 Sources of Internal Heat

• Primordial: from planetary formation. Accretion: Kinetic and potential energy of impactors converted into heat. Differentiation: As accreted material separates by density, gravitational potential energy is converted to heat.

• Radiogenic: Radioactive decay of uranium, potassium, ...kinetic energy of decay products converted into heat. • Tidal Heating: Not important for terrestrial planets.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 16 / 84

Q: Cooling

What cools off faster?

A) A large cup of coffee

B) A small cup of coffee

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 19 / 84 Q: Cooling

What cools off faster?

A) A large cup of coffee

B) A small cup of coffee

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 19 / 84 Volume to Surface Ratio

Heat Energy ∝ Volume (V) Heat Loss ∝ Surface Area (S)

Cooling Time tcool

V ∝ Stcool

Surface Area Volume to Surface Ratio: S =4πr2 V 4πr3 r t ∝ = 3 = Volume cool S 4πr2 3 4 V = πr3 3 c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 20 / 84 Q: Cooling Planets

What cools off faster? A) A big terrestrial planet B) A tiny terrestrial planet C) They cool at the same rate

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 21 / 84 Q: Cooling Planets

What cools off faster? A) A big terrestrial planet B) A tiny terrestrial planet C) They cool at the same rate

Smaller worlds cool off faster and harden earlier. The Moon and Mercury are now geologically dead.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 21 / 84 Q: Rank terrestrial planets

Rank the five terrestrial worlds in order of size from smallest to largest: A) Mercury, Venus, Earth, Moon, Mars. B) Mercury, Moon, Venus, Earth, Mars. C) Moon, Mercury, Venus, Earth, Mars. D) Moon, Mercury, Mars, Venus, Earth. E) Mercury, Moon, Mars, Earth, Venus.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 22 / 84 Q: Rank terrestrial planets

Rank the five terrestrial worlds in order of size from smallest to largest: A) Mercury, Venus, Earth, Moon, Mars. B) Mercury, Moon, Venus, Earth, Mars. C) Moon, Mercury, Venus, Earth, Mars. D) Moon, Mercury, Mars, Venus, Earth. E) Mercury, Moon, Mars, Earth, Venus.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 22 / 84 Bergmann’s Rule

“The body mass of a particular species increases with latitude” –German biologist Carl Bergmann 1847.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 23 / 84 Outline Terrestrial Planets Terrestrial Planets Terrestrial Interiors

Magnetic Fields The Earth’s Magnetic Field The Earth’s Magnetic Field Magnetic Fields The Van Allen Belts Q: Strongest Mag Field Venusian B-field Geologic Processes Terrestrial Worlds Terrestrial Atmospheres

Review

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 24 / 84 The Earth’s Magnetic Field

On the surface

N S B N

m N W E

S S

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 25 / 84 The Earth’s Magnetic Field

Aurora Borealis (Northern Lights)

Click for movie

B

Aurora Australis (Southern Lights)

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 26 / 84 The Van Allen Belts

Image from: Addison Wesley Longman

James Van Allen 1914-2006

Van Allen Belt: Doughnut shaped region of energetic charged parti- cles trapped by the Earth’s mag- netic field. Discovered by Van Allen in 1958. Image from: http://www.wikipedia.org c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 27 / 84 Q: Strongest Mag Field

Which of the terrestrial worlds has the strongest magnetic field? A) Mars B) Earth C) the Moon D) Venus E) Mercury

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 28 / 84 Q: Strongest Mag Field

Which of the terrestrial worlds has the strongest magnetic field? A) Mars B) Earth C) the Moon D) Venus E) Mercury

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 28 / 84 Q: Venus’s Magnetic Field

Which of the following most likely explains why Venus does not have a strong magnetic field? A) It does not have a metallic core. B) Its rotation is too slow. C) It is too close to the Sun. D) It is too large. E) It has too thick an atmosphere.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 29 / 84 Q: Venus’s Magnetic Field

Which of the following most likely explains why Venus does not have a strong magnetic field? A) It does not have a metallic core. B) Its rotation is too slow. C) It is too close to the Sun. D) It is too large. E) It has too thick an atmosphere.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 29 / 84 Outline Terrestrial Planets Terrestrial Planets Terrestrial Interiors

Magnetic Fields Geologic Processes Earth’s Interior Geologic Processes Geological Processes Plate Tectonics Plate Tectonics Plates Ocean Floor Q: Heavily Cratered Q: Tectonic Activity Terrestrial Worlds Terrestrial Atmospheres

Review

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 30 / 84 Earth’s Interior

rigid crust Depth(km) Layer

“plastic” 0-60 Lithosphere 0-35 Crust mantle 35-60 UpperMantle 35-2,890 Mantle liquid core 2,890-6,360 Core Fe-Ni 2,890-5,150 Liquid Outer Core solid 5,150-6,360 Solid Inner Core

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 31 / 84 Geological Processes

Processes which shape planetary surfaces: Impact cratering: From Comets or asteroids striking a planet surface. > 150 on Earth. Origin of water? Craters ∼ 10 times as wide as object that create them. Volcanism: Molten rock erupts on the surface. Outgassing. Tectonics: Stretching, compression or movement of lithosphere. Erosion: Breakdown and transport of surface rock from the action of ice, liquid or gas i.e. wind, rain, glaciers.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 32 / 84 Plate Tectonics

convection • Earth’s surface: covered by plates • Mantle convection: currents move crustal plates in different directions. • Source of heat: radioactivity. • Motion: a few cm per year • Ocean floors: continually moving, spreading from the center, sinking at the edges. • Seafloor: recycled through subduction every 200 million years.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 33 / 84

Q: Heavily Cratered

When we see a region of a planet that is not as heavily cratered as other regions, we conclude that:

A) there is little volcanic activity to create craters.

B) the planet is rotating very slowly and only one side was hit by impactors.

C) the planet formed after the age of bombardment and missed out on getting hit by leftover planetesimals.

D) the surface in the region is older than the surface in more heavily cratered regions.

E) the surface in the region is younger than the surface in more heavily cratered regions.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 37 / 84 Q: Heavily Cratered

When we see a region of a planet that is not as heavily cratered as other regions, we conclude that:

A) there is little volcanic activity to create craters.

B) the planet is rotating very slowly and only one side was hit by impactors.

C) the planet formed after the age of bombardment and missed out on getting hit by leftover planetesimals.

D) the surface in the region is older than the surface in more heavily cratered regions.

E) the surface in the region is younger than the surface in more heavily cratered regions.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 37 / 84 Q: Tectonic Activity

A planet is most likely to have tectonic activity if it has A) low surface gravity. B) high surface gravity. C) low internal temperature. D) high internal temperature. E) a dense atmosphere.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 38 / 84 Outline Terrestrial Planets Terrestrial Planets Terrestrial Interiors

Magnetic Fields Geologic Processes Terrestrial Terrestrial Worlds Worlds Full Moon Geology of the Moon Late Heavy Bombardment Late Heavy Bombardment Late Heavy Bombardment Geology of Mercury Mercury Topograpy Mercury Photos Mercury Scarp Geology of Venus Venus (Magellan Radar Image) Venus: Sapas c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future slide 39 / 84 Mons – Venus: Maat Full Moon Geology of the Moon

• Giant Impact Hypothesis: The Moon formed from debris ejected when the proto-Earth collided with a Mars sized planet (Theia) 4.4 Ga. • Late Heavy Bombardment (Lunar Cataclysm): The Moon was completely resurfaced by impacts around 4 – 3.85 billion years (Ga) ago. • Lunar Maria: ancient floods of basaltic lava which filled impact basins 3.5–3 Ga. • Small molten core • Very weak magnetic field: no dynamo.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 41 / 84 Late Heavy Bombardment

Hypothesis: • Isotopic dating of Moon rocks brought back by Apollo indicate that the Moon was completely resurfaced by impacts around 4.1 – 3.8 billion years (Ga) ago: • All of the terrestrial planets would have been heavily impacted. • This period of impacts may have been produced by the migrations of gas giants.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 42 / 84

Geology of Mercury

• Almost no atmosphere • Made of metal and rock with an unusually large iron core. Giant Impact? Vaporization of surface rock by protosun? Mercury did not accrete lighter material? • Geologically inactive for billions of years: Surface dominated by impact craters and plains from magma eruptions (similar to lunar maria) • Compression: long tall, steep cliffs from cooling and contraction. • Surprise: Magnetic field 1.1% of Earth. Implies molten core with dynamo.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 45 / 84 Mercury’s Topography MESSENGER Mosaic MESSENGER Mosaic MESSENGER Mosaic Mercury Scarp: Long cliffs from cooling & contraction Geology of Venus

• Dense atmosphere: 96.5% CO2, pressure 90 times Earth’s, extreme greenhouse effect: T ≈ 470◦ C, opaque clouds of sulfuric acid. • Surface imaged with radar (Magellan) • No tectonic plates. • Geologically active: more volcanoes (1,600) than any planet in our solar system. • Few impact craters compared to Mercury & Moon. – Dense atmosphere burns up small meteoroids. – Periodically resurfaced by lava flows.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 49 / 84 Venus (Magellan Radar Image)

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 50 / 84 Venus Magellan NASA/JPL-Caltech Sapas Mons shield volcano, Venus. Maat Mons shield volcano, Venus.

Venera 9 Venera 13

Earth

Oasis of Life • Surface liquid water • Large Moon • atmosphere contains oxygen, ozone. • moderate greenhouse ef- fect • Composition: rocks, metals • density 5.52g/cm3

Earthrise from Apollo 8

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 56 / 84 Unique features of Earth

• Surface liquid water • Atmospheric oxygen • Plate tectonics • Long term climate stability

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 57 / 84 Unique features of Earth

• Surface liquid water • Atmospheric oxygen • Plate tectonics • Long term climate stability

71% of Earth’s surfaces is cov- ered by water. Liquid wa- ter (H2O) is possible because of Earth’s distance from the Sun and a moderate greenhouse effect.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 57 / 84 Unique features of Earth

• Surface liquid water • Atmospheric oxygen • Plate tectonics • Long term climate stability

By volume, our atmosphere con- tains 21% oxygen. Photosynthesis is required to make high concentrations of oxy- gen.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 57 / 84 Unique features of Earth

• Surface liquid water • Atmospheric oxygen • Plate tectonics • Long term climate stability

Plate tectonics can change and stabilize global climate over long time scales (∼ 106 − 108 years).

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 57 / 84 Unique features of Earth

• Surface liquid water • Atmospheric oxygen • Plate tectonics • Long term climate stability

Over very long time scales, (tens of millions of years) the geologic carbon cycle may act like a ther- mostat.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 57 / 84 Geology of Mars

• Dichotomy: Northern (newer, lower, flat) and southern (older, higher, rugged, magnetized) hemisphere surfaces are very different. Giant Impact? • Past geologic activity: largest volcanoes in solar system, equatorial canyon system, Present? • Very weak “fossil” magnetic field (3,000) weaker than Earth. No current dynamo, but Mars had one in the past. • Large impact basins. 60% of the surface of Mars shows a record of impacts from LHB.

• Polar ice caps (CO2) • Evidence of past (and some present) flowing water.

• Atmosphere more than 100 times thinner than Earth, 95% CO2

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 58 / 84 Mars panorama

• Panorama by curiosity rover link 1 • Panorama by curiosity rover link 2 • Panorama by curiosity rover link 3

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 59 / 84 Mars ISS

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 60 / 84 Olympus Mons, Mars

Olympus Mons: Martian shield volcano three times as tall as Mt. Everest.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 61 / 84 Valles Marineris, Mars: Largest canyon in the Solar System. Hubble Hubble Viking composite Spirit Rover Tracks NASA’s Spirit & Opportunity Rovers: mineral evidence of past liquid water on Mars. NASA/JPL: Water on Mars Warm Season Flows

NASA/JPL: Warm Season Flows, Salty Liquid Brine?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 67 / 84 Warm Season Flows

NASA/JPL: Warm Season Flows, Salty Liquid Brine?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 67 / 84 Warm Season Flows

NASA/JPL: Warm Season Flows, Salty Liquid Brine?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 67 / 84 Warm Season Flows

NASA/JPL: Warm Season Flows, Salty Liquid Brine?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 67 / 84 Warm Season Flows

NASA/JPL: Warm Season Flows, Salty Liquid Brine?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 67 / 84 Warm Season Flows

NASA/JPL: Warm Season Flows, Salty Liquid Brine?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 67 / 84

Outline Terrestrial Planets Terrestrial Planets Terrestrial Interiors

Magnetic Fields Geologic Processes Terrestrial Terrestrial Atmospheres Worlds Terrestrial Atmospheres Escape Velocity Terrestrial Atmospheres Atmosphere Q: Substantial Atmosphere Q: Least Atmosphere Atmospheric Effects Atmospheric Penetration Greenhouse Gases Q: Greenhouse Gasses? Mars c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 69 / 84 Mars Escape Velocity

Escape velocity:

2GM v = esc r R Thermal velocity:

3T v ∼ rms r m Loss of atmospheric component: 1 v > v rms 6 esc

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 70 / 84

Composition of Earth’s Atmosphere

By Volume

N2 (78%) Ar (1%)

CO2 (0.035%)

O2 (21%)

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 72 / 84 Q: Substantial Atmosphere

Which of the following worlds has the most substantial atmosphere? A) Mercury B) Venus C) the Moon D) Mars E) Earth

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 73 / 84 Q: Substantial Atmosphere

Which of the following worlds has the most substantial atmosphere? A) Mercury B) Venus C) the Moon D) Mars E) Earth

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 73 / 84 Q: Least Atmosphere

Which of the following planets has the least substantial atmosphere? A) Venus B) Earth C) Mars D) Neptune E) Mercury

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 74 / 84 Q: Least Atmosphere

Which of the following planets has the least substantial atmosphere? A) Venus B) Earth C) Mars D) Neptune E) Mercury

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 74 / 84 Atmospheric Effects

• Radiation protection • Greenhouse effect • Erosion – Glaciers – Rivers – Wind

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 75 / 84 Atmospheric Penetration

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 76 / 84 Greenhouse Gases

Greenhouse gases absorb infrared light. Gas Formula Contribution

water vapor H2O 36-72%

carbon dioxide CO2 9-26%

methane CH4 4-9% ozone O3 3-7 % This table reflects both the radiative effects and the natural abundance of gases on Earth.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 77 / 84 Q: Greenhouse Gasses?

What are greenhouse gases? A) gases that absorb visible light B) gases that absorb ultraviolet light C) gases that absorb infrared light D) gases that transmit visible light E) gases that transmit infrared light

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 78 / 84 Q: Greenhouse Gasses?

What are greenhouse gases? A) gases that absorb visible light B) gases that absorb ultraviolet light C) gases that absorb infrared light D) gases that transmit visible light E) gases that transmit infrared light

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 78 / 84 Mars

• a =1.52 AU

• R =0.53REarth

• M =0.11MEarth • giant volcanoes, huge canyons • polar ice caps • water flowed in the past. • life? • Two tiny moons: Phobos Deimos (captured asteroids)

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 79 / 84

Q: Red Planet

Why is Mars red? A) It is made primarily of red clay. B) Its surface rocks were rusted by oxygen. C) Its atmosphere scatters blue light more effectively than red light. D) Its surface is made of ices that absorb blue light. E) Its surface is made of ices that absorb red light.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 81 / 84 Further Study

• Plate Tectonics • Time Lapse Aurora Video • USGS Mars Geologic Map • Why is Mars half magnetized?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 82 / 84 Outline Terrestrial Planets Terrestrial Planets Terrestrial Interiors

Magnetic Fields Geologic Processes Terrestrial Review Worlds Terrestrial Atmospheres

Review Review

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 83 / 84 Review

• List Earth’s moon and the four terrestrial planets from the smallest to the largest. • What are the core, mantle, and crust? • What is differentiation? • Why do terrestrial planet cores contain metal? • List the three principle sources of internal heat on planets. • Which terrestrial world has the largest magnetic field? • Which terrestrial world has most substantial atmosphere? • Why are there few craters in the lunar maria? • Why is Mars red? • How does size affect cooling?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 84 / 84