1. The Terrestrial Planets
Getting to know our first cousins
The Terrestrial Planets

2. Topics Solar System--the big picture Earth, Moon, Mercury, Venus, Mars
How do we know?
Why do we care?
What is common about the terrestrial planets?
What is peculiar to each of these planets?

3. Models The test of all knowledge is experiment.
We use models to understand how we think the Solar System, including the Sun and planets, formed.
Models can be used to make predictions.
Ultimately the accuracy of the predictions reveal the efficacy of our models.
As we discuss “what happened” remember that these are based on models. Perhaps at some point, experiments will point us to new models.

4. Contents of the Solar System
All masses that orbit the Sun plus the Sun!
One star - called the Sun
nine planets
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto
more than 60 moons (often called natural satellites)
tens of thousands of asteroids
countless comets
dust and gas
Our Sun constitutes nearly 99.44% of the mass of the Solar System

5. Terrestrial planets (Earth-like): Mercury, Venus, Earth, Mars

6. What makes them similar?
small--1/100 radius of the Sun
Size
orbit at 0.4 to 1.5 AU
Location
Moons
few
none
Rings
composition
dense rock and metal

7. Density density = mass/volume Density of water = 1.0 g/cm3
Density of wood =
0.5 g/cm3
Density of silicate rock =
3.0 g/cm3
Density of iron =
7.8 g/cm3

8. Composition? Density Mercury 5.4 g/cm3 Venus 5.2 g/cm3 Earth 5.5 g/cm3
Mars
3.9 g/cm3
So what are these planets mostly made of?

9. Earth
Mass and radius give mass/volume = bulk density, about 5.5 times water
Key to composition, internal structure, verified by seismic waves
Metals: bulk density about 8 g/cm3; rocks: about 3 g/cm3; earth: about metals/rocks

10. How do we measure density?
Mass & spherical shape (Newton’s law of gravitation)
Radius (from angular size and distance)
Bulk density (mass/volume) => infer general composition

11. Evolution of a planet - internal effects
Energy flow from core to surface to space
Source: Stored energy of formation, radioactive decay
Results in volcanism, tectonics

12. Evolution of a planet - external effects
Impact cratering: Solid objects from space
Bomb-like explosion; many megatons (H-bomb!)
Creates circular impact craters on solid surfaces

13. Earth Composition Volcanism Plate tectonics Atmosphere Craters
Magnetic field

14. Aurora
caused by charged particles emitted from the Sun interacting with the Earth’s atmosphere
charged particles are most highly concentrated near the poles due to their motion in the earth’s magnetic field.

15. Craters Barringer meteor crater
Largest, most well-preserved impact crater
Fist crater recognized as an impact crater (~1920s)
49,000 years old

16. Earth’s layers Core (metals) Mantle (dense rocks)
Crust (less dense rocks)
Partially or fully melted material separates by density (differentiation)
Age of earth ~ 4.6 Gy ~age of meteorite material and lunar material
Astronomy: The Evolving Universe, Michael Zeilik

17. Earth’s age
Radioactive dating: Decay of isotopes with long half-lives; for example, uranium-lead, rubidium-strontium, potassium-argon.
Gives elapsed time since rock last melted and solidified (remelting resets clock)
Oldest rocks about 4 Gy Gy for earth’s formation => about 4.5 Gy for earth’s age

18. Earth’s Tides
due to the variation of the gravitational force of the moon on the earth
two tides per day

19. Tides The Sun also has an effect on the tides.
Eventually the earth and moon will slow down and the moon will recede.

20. Moon Origin maria craters
fission?
capture?
condensation?
ejection of a gaseous ring?
maria
craters
similar in density to Earth’s mantle but proportion of elements is not exactly like the Earth’s

21. Mercury rotational period is 2/3 of its orbital period -- hot and cold
hard to view from Earth
highly elongated orbit
iron core
small magnetic field
thin atmosphere, mostly sodium
it looks like the Moon

22. Venus ...where the skies are cloudy all daayyyy.
dense atmosphere, mostly CO2
high surface pressure and temperature
rotation (117 E-days), revolution (225 E-days)
rotates about its axis in the “wrong direction”
similar density and size as Earth
two continents, one continental plate
no moons

23. Mars small in size two moons thin atmosphere, mostly CO2
4 seasons (why?)
smaller density (what would this mean?)
polar caps (mostly CO2, some water)
canyons (evidence of flowing water?)

24. What’s important? similarities of terrestrial planets
peculiarities of terrestrial planets
how we know things like the period of rotation, composition, and age of a planet, to name a few

25. For Practice
Looking through this chapter, make a list of similar features and different features of the terrestrial planets.
Identify each instant where the book described something we know about a planet and how we know it.