The Terrestrial Planets Chapter 6 Getting to know our first cousins

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?

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.

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

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

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

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

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?

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

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

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

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

Earth Composition Volcanism Plate tectonics Atmosphere Craters Magnetic field

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.

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

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

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

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

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

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

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

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

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?)

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

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.