Rocky Worlds
Lecture 10:
The Planets
Turn in Observing Project reports.
© 2005 Pearson Education Inc.,
publishing as Addison-Wesley
Sept. 6
Sept. 6
Sept. 27
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Midterm: Average = 21
A Score of 15 is a low “Pass” and a C-.
>27 is an strong A
>24 is a strong B
7 people got 30/30.
© 2005 Pearson Education Inc.,
publishing as Addison-Wesley
Rocky Worlds
Interstellar Gas and Dust in
our Milky Way Galaxy
Light from
distant stars
is absorbed.
Why?.
Dust and Gas
in
clouds between
the stars!
© 2005 Pearson
Education Inc.,
publishing as AddisonWesley
The Dark Clouds between stars
in the Milky Way
Milky Way
Centaurus A
© 2005 Pearson
Education Inc.,
publishing as AddisonWesley
HST
Formation of Planetary Systems
Observations è Models of Planet Formation
Protoplanetary Disks
Young Stars
of Gas & Dust Observations
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Opaque “disks” around young stars!
Thermal emission (infrared) from
dust in those disks.
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è Mass of disks = 10-100 MJUP
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Disk Lifetime ~ 3 Million years
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© 2005 Pearson
Education Inc., puy
Formation of Planetary Systems
Observations è Models of Planet Formation
Protoplanetary Disks
Young Stars
of Gas & Dust
Theory of
Planet Formation:
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© 2005 Pearson
Education Inc., puy
Dust collides, sticks
and grows è
pebbles/rocks
Gravity helps attract
more rocks
Gravity attracts gas
Theory of Rocky Planet Formation
Inward of 3 AU
Planetesimals (km-sized comets & asteroids)
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Growth of rocks (planetesimals) by collisions and sticking together
Friction circularizes orbits
Big planetesimals gravitationally stir small rocks
Mergers among planetesimals: They grow to Earth-Size
Analytical and N-body:
Building planets inward of 3 AU
•  At 3 AU is “Snow Line” : Inward, it’s hotter than 0C.
•  Only rocks & metals condensed inward into solid objects (not gas or water).
•  Too hot for gases, to stick to rocks (but maybe sticks by gravity).
Hydrogen compounds (H2, H2O, NH3, CH4 ) and Helium are gaseous.
•  è Build only rocky planets inward of 3 AU.
© 2005 Pearson
Education Inc.,
publishing as AddisonWesley
Evidence of Building Planets
By Collisions Among Planetesimals
Evidence of the collisions
by asteroids and comets:
Craters on the Moon
Moon
-Craters: impacts
- Smooth plains:
flows of lava,
after impacts
Small Impact Craters: Bowl Shape
Larger Craters: Complex Shape
Lunar Craters of
Different Sizes
1
Multke Crater: d=7km,
simple bowl shape
3
2
Euler Crater: d=28km, 2.5km deep. Complex
crater with central peak. Ejecta blanket outside!
Bessel Crater: d=16km, 2km deep. Transitional between simple
and complex. Note slumping of rim material into the bowl.
4
King Crater: d=77km, 5 km deep,
high central peak
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Schroedinger Crater: d=320km
(impact basin) , has an inner ring
Copernicus Crater: d=93km, complex peak and flat
How do craters form?
An asteroid (rock) or comet (ice + rock)
collides with a moon or planet.
Computer simulation of impact:
Crater Formation
Experiment: Making Craters
Making Craters:
Different impactors hitting different substances
Dark Lunar Maria:
Lava flood planes
After a giant impact,
lava springs up from below,
spreading out to form
a lava sea” “maria”
Dark Lunar Maria:
Lava flood planes
Different Viscosities of Lava:
Dark Lunar Maria:
Lava flood planes
Different Viscosities of Lava:
When did craters form?
During the first 100 million years of our
Solar System, there were many asteroids
and comets.
Now, most are gone – already hitting a
moon or planet.
You can judge the age of a planet’s
surface by how many craters are there.
How did the Moon Form?
A mars-sized object hit Earth:
Splashed up material to make the Moon
Moon-Forming Impact on Earth
The Four Rocky Planets:
Mercury, Venus, Earth, Mars
- Craters show
collisional history
Collisions and impacts by
planetesimals (asteroids
and comets) formed the
rocky planets!
And caused melting…
Heavy materials sank:
Iron and Nickel
Crust:
Light rocks
Low Density
Internal Structure of the
Terrestrial Planets
Mantle:
FeSiO4
”Silcate”
Core:
Iron-Nickel
--- Dense
Crust: Rocks of lowest density
Lithosphere: Crust and part of mantle (solid)
In the mantle, rock is solid. Near bottom rock deforms, flows.
The core has a liquid outer shell and a solid inner core.
Remarkable diversity in the
terrestrial (rocky) planets?
Mercury: Dense, craters, ridges. No change for last
3 billion years
Venus: Hot and cloudy (but no wind, no rain, no
water, no weather)
Earth: Wet, plate tectonics, life, magnetic field
Mars: Huge volcanoes, once wet but now dry;
no magnetic field
- Craters
- No Atmosphere
- Dense: 5.4 g/cm3
- Ridges !
Mercury has a strong Magnetic Field
Interpretation: Must still have a liquid iron in its core.
Venus
•  Low lands (blue)
must be very young
1.6-0.3 Ga
(recent lava flows)
•  Mountain ranges
(green)
•  Rift valleys
•  Lots of volcanoes
Mars
some craters
volcanoes
ancient river beds
Earth
-volcanoes
-faint craters
-mountains
-liquid water
-plate tectonics
The Earth is
just another rocky planet.
Is it special?
Earth: A Wonderful World
Earth as a Planet
Why is it so different from the other
rocky planets?
1. Earth geologically active
2. Liquid Water
Why is Earth geologically active?
(earthquakes, volcanoes)
The Earth is massive enough to still have a hot
interior. It holds it heat in for billions of years.
“Hot Potato effect”. Surface cools slowly.
So what do we know about the interior of the Earth?
How do we know?
Sources of Internal Heating
-Gravitational potential
energy of accreting
planetesimals
-Differentiation:
Dense material sinks,
heats as it falls.
-Radioactivity (Uranium,
Potassium!)
-Tidal heating
What cools off faster?
A.  A cup of coffee
B.  A teaspoon of the same coffee
What cools off faster?
A.  A cup of coffee
B.  A teaspoon of the coffee
What cools off faster?
A.  A big terrestrial planet.
B.  A tiny terrestrial planet.
What cools off faster?
A.  A big terrestrial planet.
B.  A tiny terrestrial planet.
Objects cool through their surfaces!
Ratio of volume (heat capacity) to surface
area increases with increasing planet size.
Volume = 4/3 π R3
Surface Area = 4 π R2
Heat Flow by Convection
Drives Geological Activity
Convection: Hot rock rises,
cool rock falls.
1 cycle takes ~100 million
years on Earth.
Convection:
Hot material buoyantly rises
carrying heat upward
Heat Drives Geological Activity
Convection: hot rock rises,
cool rock falls.
1 cycle takes ~100 million
years on Earth.
How many times has the
entire Earth mantle turned
over so far?
A)  Not yet once
B)  4 times
C)  45 times
D)  456 times
A large planet…
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Is still warm inside
Has a convecting mantle of solid rock
Has a thinner, weaker lithosphere
Hot solid rock reaches surface, pushes on
crust, moves continents, causes slippage
(earthquakes), melts, and spurts up in
volcanoes!
Geological Activity
Plate Tectonics (Earth) versus Flake Tectonics (Venus)
•  Convection currents in Venus’s interior were more vigorous than
inside the Earth
•  Strong convection prevents the formation of thick crust
•  A thin crust undergoes wrinkling and flaking.
Differentiation:
Dense Material Sinks
Two components
Three components
Why do oil and water separate?
A.  Water is denser than oil, so oil floats on
water.
B.  Oil is more slippery than water, so it slides
to the surface of the water.
C.  Chemical forces drive the separation.
Water is a polar substance, oil is not.
D.  Oil molecules are bigger than the spaces
between water molecules.
Why do oil and water separate?
A.  Water is denser than oil, so oil floats on
water.
B.  Oil is more slippery than water, so it slides
to the surface of the water.
C.  Chemical forces drive the separation.
Water is has electric polar substance, oil is
not.
D.  Oil molecules are bigger than the spaces
between water molecules.
Internal Structure, by density
Differentiation
Planetary-scale Regions of Distinct Composition
Differentiation
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Layers ordered by density
Highest density on the bottom
Is initiated by chemical forces
Gravity sorts materials by density
•  Differentiation converts gravitational
potential energy to heat (recall the
requirement of conservation of energy!)
Internal Structure of the
Terrestrial Planets
Crust: Rocks of lowest density
Lithosphere: Crust and part of mantle (solid)
In the lower mantle, the rock is deformed and flows.
The core has a liquid outer shell and a solid inner core.
III
. Rocks and
Minerals
What are rocky planets made of?
Minerals and Rocks
•  Mineral: naturally occurring solid crystalline
compound with a definite (but not generally fixed)
composition (>3000 minerals)
Quartz (SiO2)
Malachite
CuCO3.Cu(OH)2
Pyrite (FeS2)
Amazonite
What are rocky planets made of?
Minerals and Rocks
•  Quartz: SiO2
Si4+
O2-
What are rocky planets made of?
Minerals and Rocks
•  Calcite: CaCO3
Ca2+
O2C4+
What are rocky planets made of?
Minerals and Rocks
•  Calcite: CaCO3 in a different form
Calcium carbonate will react with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.
CaCO3 + CO2 + H2O → Ca(HCO3)2
What are rocky planets made of?
Minerals and Rocks
•  Olivine: (Mg,Fe)2SiO4
What are rocky planets made of?
Minerals and Rocks
•  Mineral: naturally occurring solid crystalline
compound with a definite (but not generally fixed)
composition (>3000 minerals)
Examples: calcite CaCO3,
quartz SiO2,
olivine (Mg,Fe)2SiO4
•  Rock: solid made of 1 or more minerals
Examples: limestone (calcite),
granite (feldspar, mica, quartz)