inner planetary geology i. terrestrial planets the terrestrial planets cooled from molten masses ...
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AST 111 LECTURE 16
Inner Planetary Geology I
Terrestrial Planets
The Terrestrial Planets cooled from molten masses
Acquired structure during cooling
Made primarily of rock and metal
Larger Worlds are Round
Phobos(One of Mars’ two moons)Diameter: ~ 11 miles
EarthDiameter: 8000 miles
NOT ROUND ROUND
Larger Worlds are Round
From the book (p. 260):
Gravity will make any rocky object bigger than about 310 miles into a sphere within about 1 billion years.
What’s inside a planet?
How do we find out what’s inside?We have drilled less than 1% toward Earth’s
center○ Russians drilled 7 miles into the crust!○ 356 oF, drill wasn’t working well
What goes on inside affects what happens on the surface
Seismology Spectroscopists use
spectra to probe atoms and molecules
Seismologists use seismic waves to probe the interior of Earth
On other planets, we’ve only looked at rocks
Seismic Waves
Speed and direction of seismic waves depends on:CompositionDensityPressureTemperatureSolid / Liquid
Send in some waves, see what you get back.
Seismic Waves Waves are
disturbances of a medium
Two types of waves:LongitudinalTransverse
With seismic waves, Earth vibrates as the spring does
Seismic Waves Longitudinal waves:
Go through almost any material in any phase○ Solids, liquids, and
gases
Transverse waves:Inside Earth, only
travel through solids
Seismic Waves
So what does it mean when:
Longitudinal waves from an earthquake reach the other side of the world, while transverse waves do not?
Seismic Waves It means that Earth
has some liquid.
Can figure out the extent via geometry of transmitted / reflected waves
Structure of Terrestrial Planets
Earth’s crust: 30 miles thickEarth’s mantle: 2000 miles thickEarth’s core: 2000 miles thick
Mantle is mostly solid!
Lithosphere is cool and rigid.
Why is it layered? When Earth formed,
it was hot!Mostly liquid
Gravity pulled the heavier elements closer to the center
This is called differentiation.
Strength of Rock
Rock will deform if:You heat it andYou subject it to sustained stress
“Sustained stress” can come from gravity
The rock under the lithosphere is warm and soft (NOT LIQUID). It acts like a really thick sludge!
Geological Activity Change that occurs on the surface
VolcanoesEarthquakesErosion
Results from interior heat reaching the surfaceThings expand and contractThings melt and flow
Geological activity requires:Mantle convectionA thin lithosphere
Interiors: Heating
Planetary interiors have lots of thermal energyThis energy does NOT come from the Sun!Comes from:
○ Heat of accretion (planet formation)○ Heat from differentiation (heavier materials
moving inward)○ Heat from radioactive decay
Interiors: Heating
Which of the following processes are definitely NOT adding thermal energy to Earth today?
○ Heat of accretion?
○ Heat from differentiation?
○ Heat from radioactive decay?
Interiors: Cooling Mantle convection
Hot rock is less dense It gets forced upward
Conduction Inefficient heat exchange
via direct contact between mantle and lithosphere
Surface radiation Atoms and molecules
give up internal energy as EM waves
Interiors: Cooling
Is there evidence that mantle convection is happening today?
Does Earth’s surface radiate energy away? Why or why not? (Think of what causes a continuous spectrum.)
Geological Activity
As the interior cools over millions of years:Lithosphere thickens toward the core
○ Acts as thermal insulationStops convection
Larger planets hold more energy and don’t radiate it as well
Geological Activity
Larger planets stay geologically active longer.
Geological Activity
Is Earth geologically active?
Is the Moon geologically active?
Is Mercury geologically active?
Geological Activity
Earth still has enough warmth in its core to maintain convection. This keeps the lithosphere thin and warm, so heat canreach the surface. This is why Earth is
geologically active.
Geological Activity
The Moon and Mercury couldn’t generate enough
heat to maintain convection. Their lithospheres solidified
and geological activity ceased.
Magnetic Fields Moving charges in the
liquid outer core create a magnetic field
Loss of heat can solidify the core Loss of mag-field
Can look at a planet’s mag. field to learn about liquid core
Planetary Surfaces Impact cratering
Asteroid / comet strikes
Impacts occur around 24,000 – 155,000 mph
Planetary Surfaces
Planetary Surfaces
“Standard” Muddy? Eroded by wind / rain?
(All on Mars.)
Planetary Surfaces
Craters give a very good indication of howgeologically active a surface is.
This is because geological activity erases craters.
Geological Age and Craters
Planetary Surfaces
Why are Mercury and the Moon covered in craters, but Earth is not? (Two reasons.)
Planetary Surfaces Volcanism
Hot, liquid rock (magma) comes up to the surface
Solid rock may squeeze molten rock out
Trapped gases often expand as the magma rises
Planetary Surfaces
Volcanic plains: “runny”lava flow
Shield volcanoes: thickerflow, solidify before they can spread out much
Stratovolcanoes: thickestlava flows. Hardly spreadsbefore solidifying.
Planetary Surfaces
TectonicsStretching, compressing, reshaping
lithosphereCaused mainly by mantle convection
Planetary Surfaces
ErosionBreaking down / transporting rock via wind
or liquid flowMore eroded surfaces are older
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