The Terrestrial Planets

1. The Terrestrial Planets Chapter 6 Getting to know our first cousins

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 few Moons 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 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 50-50 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 + 0.5 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 • 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.