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Chapter 9 - Planetary Geologyof Earth and the other Terrestrial Worlds

When you visit a planet, it ceases to be an astronomical object…When you visit a planet, it ceases to be an astronomical object…

9.1 Planetary Interiors and Surfaces

• Learning Goals:– Common processes on “solid” worlds

•• How do these things work?How do these things work?

What are terrestrial planets like on the– What are terrestrial planets like on the inside?

– What is geological activity and what causes it?

– Why do some planetary interiors create magnetic fields?

Common Processes• Terrestrial planets all have solid surfaces

–– Dominated by Dominated by rockyrocky materialsmaterialsRocks are naturally occurring aggregates of mineralsRocks are naturally occurring aggregates of minerals

Minerals are naturally occurring homogeneous solids with a Minerals are naturally occurring homogeneous solids with a definite chemical composition and ordered atomic arrangementdefinite chemical composition and ordered atomic arrangement

All t l d i i lAll t l d i i lAll metals and ices are mineralsAll metals and ices are minerals

• Solid surfaces can break, stretch, compress, or melt.

• Stable for long periods of time (millions to billions of years) → Preserves ancient geologic history!

• High melting points → Silicate volcanism

Sharing a Family History of:

• Impact Cratering• Volcanism• Faulting• Resurfacing

Different surface expressionsDifferent surface expressions

Don’t get distracted by the surface differences!Don’t get distracted by the surface differences!Most of that is due to atmospheres (or not)Most of that is due to atmospheres (or not)

They’re They’re almostalmost identical on the inside!identical on the inside!

And a common genetic history…And a common genetic history…

• Applying what we have learned about Earth’s interior to other planets tells us what their interiors are probably like

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A word about…A word about…Planetary InteriorsPlanetary Interiors

GravitationalGravitationalInteractions tell Interactions tell us a great dealus a great dealabout how a bodyabout how a bodyabout how a bodyabout how a bodyis put together.is put together.

Surface CrustSurface Crust

Plastic MantlePlastic Mantle

Solid/Liquid CoreSolid/Liquid Core

Interiors of the Big 4Interiors of the Big 4• Mercury is densest and

largest core relative to planet size

• Venus has relatively ythick crust

• Earth has relatively thin crust

• Mars has extremely thick crust and very large core.

Q: How do we know?Q: How do we know?A: Seismic WavesA: Seismic Waves

• Vibrations that travel through Earth’s interior tell us what Earth is like on the inside

• Ditto for moonquakes and marsquakes

Special Topic:How do we know what’s inside a planet?

• P waves push matter back and forth

• S waves shake matter side to side

Special Topic:How do we know what’s inside a planet?

• P waves go through Earth’s core but S waves do notwaves do not

• We conclude that Earth’s core must have a liquid outer layer

Scientific Visualization

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Earth’s InteriorEarth’s Interior• Core: Highest

density; nickel and iron

• Mantle: Moderate density; silicon, oxygen, etc.

• Crust: Lowest density; granite, basalt, etc.

• Gravity attracts high mass (high-density) material to center

• Lower-density material is displaced

DifferentiationDifferentiation

pto surface– Atmospheric gases– Silicate rocks– Nickel/Iron core

• Material ends up segregated by density

LithosphereLithosphere• A planet’s outer

layer of cool, rigid rock is called the lithosphere

• It “floats” on the warmer, softer rock that lies beneath

Strength of RockStrength of Rock• Rock stretches when

pulled slowly but breaks when pulled rapidly

• The gravity of a large world pulls slowly on its rocky content, shaping the world into a sphere

High Density InteriorsHigh Density Interiors• Hot accretion

• Differentiation when planets were youngwere young

• Radioactive decay is most important heat source today

Cooling of InteriorCooling of Interior• Convection

transports heat as hot material rises and cool material fallsfalls

• Conductiontransfers heat from hot material to cool material

• Radiation sends energy into space

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Role of SizeRole of Size

• Smaller worlds cool off faster and harden earlier• Moon and Mercury are now geologically “dead”• Fully cooled planets dubbed “rocks”

Surface Area to Volume RatioSurface Area to Volume Ratio

• Heat content depends on volume• Loss of heat through radiation depends on surface

area• Time to cool depends on surface area divided by

volumevolume

surface area to volume ratio = 4r2

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3r3

3

r

• Larger objects have smaller ratio and cool more slowly

Magnetic FieldsMagnetic Fields Sources of Magnetic FieldsSources of Magnetic Fields

• Motions of charged particles

icreates magnetic fields

Sources of Magnetic FieldsSources of Magnetic Fields• A world can have

a magnetic field if charged particles are moving inside

• 3 requirements:– Molten interior– Convection– Moderately rapid

rotation

So What?So What?

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9.2 Shaping Planetary Surfaces9.2 Shaping Planetary Surfaces

• Learning Goals:– What processes shape planetary surfaces?

– Why do the terrestrial planets have different geological histories?

– How does a planet’s surface reveal its geological age?

Why the Different Surfaces?

Surface Expressions• History of interior processes

– Still molten → Still active volcanism– Lava resurfacing

• AtmospheresAtmospheres– Outgassing– Retention of volatiles– Erosion

• Cratering History• Tectonics

Impact CrateringImpact Cratering• Most cratering

happened soon after solar system formed– Early heavy a y eavy

bombardment

• Craters are about 10 times wider than object that made them

• Small craters greatly outnumber large ones

Impact CratersImpact Craters VolcanismVolcanism• Volcanism happens

when molten material (magma) finds a path thro gh lithosphere tothrough lithosphere to the surface

• Molten material is called lava after it reaches the surface

• Can be any composition

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Volcanic EdificesVolcanic Edifices

Runny lava makes flat lava plains

Slightly thicker lava makes broad shield volcanoes

Thickest lava makes steep stratovolcanoes

OutgassingOutgassing

• Volcanism releases gases from planetary interior into t hatmosphere

TectonicsTectonics

• Convection of the mantle creates stresses in the crust called tectonic forces

• Compression forces make mountain ranges• Extension forces (pulling apart) create valleys

Plate Tectonics on EarthPlate Tectonics on Earth

ErosionErosion• Generic term for processes that break down or

transport rock• Erosion Processes include

– Rain & Rivers– Wind– Wave action– Micrometeorite gardening– Viscous Relaxation (including glaciers)

Fluvial ErosionFluvial Erosion

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Glacial ErosionGlacial Erosion Aeolian ErosionAeolian Erosion

Micrometeorite ErosionMicrometeorite ErosionGardeningGardening

Only effective with thin atmospheres

Clues to ErosionClues to Erosion

• Erosion followed by deposition shows up in bedded rock deposits

Different Surfaces indicate Different Surfaces indicate different historiesdifferent histories

Different Surfaces indicate Different Surfaces indicate different historiesdifferent histories

Size MattersSize Matters

• Smaller worlds cool off faster and crystallize earlier• Larger worlds remain warm inside, promoting

volcanism and tectonics• Larger worlds also have more erosion because their

gravity retains an atmosphere

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Distance from SunDistance from Sun

• Planets close to Sun are too hot for rain, snow, ice and so have less erosion– More difficult for hot planet to retain atmosphere

• Planets with liquid water have most erosion

• Planets far from Sun are too cold for liquids, limiting erosion

Role of RotationRole of Rotation

• Planets with slower rotation have less weather and less erosion

• Planets with faster rotation have more weather and more erosion

How old are the surfaces?How old are the surfaces?

We have a Rosetta Stone…We have a Rosetta Stone…

Cratering of MoonCratering of Moon

• Some areas of Moon are more heavily cratered than othersthan others

• Younger regions were flooded by lava after Early Heavy Bombardment

History of CrateringHistory of Cratering

• Most cratering happened in first billion years

• More Craters = Older Surface

• Heavilty-cratered surfaces have not changed much in last 3-4 billion years

• Distinction: Surface age versus rock age?

Calibrating the Cratering CurveCalibrating the Cratering Curve

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Cratering CurvesCratering Curves• Calibrated curves from the Moon are the

basis of all inner solar system surface ages– Why not for the outer solar system?

Different flux ratesViscous relaxation much more common

• Age of surface is not the same thing as age of the rock– What happens when a big impact wipes out the

existing surface?

9.3 Geology of the Moon and Mercury9.3 Geology of the Moon and Mercury

• Learning Goals– What geological processes shaped our

Moon?Moon?

– What geological processes shaped Mercury?

Both are Geologically Both are Geologically DeadDead

• Geologically “dead” because major geological processes have virtually stopped:– No plate tectonics– No atmospheric erosion– No flowing surface

water– Interiors frozen solid?

Lunar GeologyLunar Geology

•• Oldest units are lunar highlands (similar to Oldest units are lunar highlands (similar to Earth mantle)Earth mantle)

•• Intermediate age are lunar maria made by Intermediate age are lunar maria made by floods of runny lava from late major impactsfloods of runny lava from late major impacts

•• Youngest are smaller bright impacts (e.g. Youngest are smaller bright impacts (e.g. Tycho)Tycho)

•• Oldest units are lunar highlands (similar to Oldest units are lunar highlands (similar to Earth mantle)Earth mantle)

•• Intermediate age are lunar maria made by Intermediate age are lunar maria made by floods of runny lava from late major impactsfloods of runny lava from late major impacts

•• Youngest are smaller bright impacts (e.g. Youngest are smaller bright impacts (e.g. Tycho)Tycho)

Lunar InteriorLunar Interiorfar near farfar near far

• Giant impact theory says Moon is mostly formed from Earth’s mantle material

• Initially molten• Strong gravity from Earth pulled fluid mass to near side• Dominance of maria on near side

Lunar MariaLunar Maria• Smooth, dark

lunar maria are less heavily

t d thcratered than lunar highlands– younger

• Maria were made by flood of runny lava

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Formation of Lunar MariaFormation of Lunar Maria

Large impact crater weakens crust

Heat build-up allows lava to well up to surface

Early surface covered with craters

Cooled lava is smoother and darker than surroundings

Tectonic FeaturesTectonic Features

• Formed after lava floods

• Wrinkles arise• Wrinkles arise from cooling and contraction of lava flood

Geology of MercuryGeology of Mercury

Cratering on MercuryCratering on Mercury

• A mixture of heavily cratered and smooth regions like the Moon

• Smooth regions are likely ancient lava flows

Caloris BasinCaloris Basin

Caloris basin is largest impact crater on Mercury

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Almost a planet killerAlmost a planet killer

The Caloris impact blew off the antipodes: Chaotic Terrain

Tectonics on MercuryTectonics on Mercury

• Long cliffs indicate that Mercury shrank early in its history

9.4 Geology of Mars9.4 Geology of Mars

• Learning Goals:– The Myth of Mars

j l i l– Major geological features of Mars?

– Geological evidence of water on Mars?

The Myth of MarsThe Myth of Mars

• Percival Lowell misinterpreted surface features seen in telescopic images of Mars

The Myth of MarsThe Myth of Mars

• Original observation of “caneli” in Italian (channels)

• Known similarity of seasons and length of day

The Myth of MarsThe Myth of Mars

original

deliberate fakery

g

recent high resolution image

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Mars EnvironmentMars Environment

• Extremely thin (6 mbar) atmosphere• Very cold – Highs of 40°F at Noon on Equator

Major Geological FeaturesMajor Geological Features

Cratering & the Crustal DichotomyCratering & the Crustal Dichotomy

• Amount of cratering differs greatly across surface• Many early craters have been erased

Volcanism on MarsVolcanism on Mars• Many large shield

volcanoes

• Olympus Mons isOlympus Mons is largest volcano in solar system

Tectonics on MarsTectonics on Mars

• System of valleys known as Valles Marineris thought to originate from tectonics

Evidence of WaterEvidence of Water

• Orbital images show many examples ofexamples of what appear to be dried-up riverbeds

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Shorelines?Shorelines?

Where did all the water go?Where did all the water go?

Evidence of WaterEvidence of Water

• Much evidence of a planetwide permafrost layer

• Enough to coverEnough to cover all of Mars to depth of 20m

Erosion of CratersErosion of Craters

• Details of some craters suggest they were oncethey were once filled with water

• Stratified sedimentary deposits

The Record in the RocksThe Record in the Rocks

• Mars rovers have found rocks that appear to have formed in water

Martian RocksMartian Rocks

• Exploration of impact craters has revealed that Mars’ deeper layers were affected by water

Hydrogen ContentHydrogen Content

• Map of hydrogen content (blue) shows that low-lying areas contain more water ice

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Present Day Water FlowPresent Day Water Flow Term Exam #3

• 35 objective questions at 2 points each

• Covering Chapters 7, 8, 9, and 10

• 3 essay/short answer questions @ 10 points each• 3 essay/short answer questions @ 10 points each.– Select 3 from 5

• Open note and open book, but…– Never memorize anything you can look

up…except where to look it up.