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The Terrestrial Planets
Chapter 6
Getting to know our first cousins
Topics
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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
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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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What makes them similar?
small--1/100 radius
of the Sun
orbit at 0.4 to 1.5
AU
few
none
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 (Hbomb!)
• Creates circular
impact craters on solid
surfaces
Earth
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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 wellpreserved impact
crater
• Fist crater recognized
as an impact crater
(~1920s)
• 49,000 years old
Earth’s layers
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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
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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.