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

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QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

What makes them similar?

small--1/100 radius of the Sun

orbit at 0.4 to 1.5 AU

few

none

Size

Location

Moons

Ringsdense rock and metalcomposition

Density

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

density = mass/volume

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

TidesThe Sun also has an effect on the tides.

Eventually the earth and moon will slow down and the moon will recede.

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

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.