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