PYTS/ASTR 206 – Terrestrial Planet Interiors and Surfaces 1

AnnouncementsHomework #2 due on Thursday

In-class assignmentY th ti lYou can answer the questions as we go along There’ll also be a few minutes at the end to fill this inSave the last question for these final 5 minutesTalk with your neighbors during these last few minutes but not during the lecture

Anyone missing a TI-30XA calculator?

PYTS/ASTR 206 – Terrestrial Planet Interiors and Surfaces 2

Terrestrial Planet Interiors and SurfacesSurfaces

PTYS/ASTR 206 – The Golden Age of Planetary ExplorationShane Byrne – shane@lpl.arizona.edu

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In this lecture…In this lecture…

Internal structure of rocky planetsEarthquakes and what they tell usS f h tSources of heat

How volcanoes work

Wind-related (aeolian/eolian) processesFluvial processesp

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Internal structureInternal structure

Rocky planets have several partsCore – Iron/NickelMantle RockyMantle – RockyCrust – Rocky (different composition)

Strong rocks near surfaceColder rocks = stronger rocksLithosphereRocks are brittle

Weak rocks deeperHotter rocks = weaker rocksAsthenospherepRocks flow

CoreSolid in center inner coreSolid in center – inner coreSurrounded by liquid iron – outer core

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All rocky planets follow the same basic modelDiff i th d t il lik iDiffer in the details – like core sizeSmaller bodies cool off faster – thicker lithospheres

Details for later…Earth has two separate types of crust and plate tectonicsThe Moon may not have a coreMercury’s core is enormous compared to its size

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Compositional vs mechanical termsC t tlCrust, mantle, core are compositionally differentLithosphere, Asthenosphere, Mesosphere, Outer Core and Inner p ,Core are mechanically different

Lithosphere is divided into plates…More on this in the Earth lecture

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How did things get this way?Pl t f d f l di kPlanets formed from solar diskStarted as uniform lumps of rock and metal

P t E th h tProto-Earth was hotMaterial could flowDense material (iron etc..) sinksLighter stuff (rocks) float

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Homogeneous mix of rock and metal

Liquid metal core with rocky mantleTakes only a few million years

Core cools and solidifies from inside outTakes billions of years 12

,800

km

Some planets still have liquid outer coresEarth and Mercury

~

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Hot rock in mantle can convect!Sl l 1 / tl t i l i iSlowly…. ~1 cm/year – mantle material is very viscousMantle is heated from below by convection in the liquid outer core

Mantle also heated throughout by radioactivity

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Cooling is through the surfaceS f i ti l t R2Surface area is proportional to R2

Amount of initial heat and generated heat is proportional to volume, R3

If you double the size of the planetCooling is 4 times faster…but amount of heat is 8 times as much

Bigger planets stay hotter longer

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Why do we still have a liquid core?Why do we still have a liquid core?

Sources of heat for planetary interiorsOriginal heat of formation

Accretion• Impacting objects transfer heat to the proto-Earth

Differentiation• Differentiated object has lower potential energy• Energy difference goes into heat

R di ti itRadioactivityLong lived radioisotopesUraniumThoriumPotassium

Tidal forcesUsually inefficientIo is an exceptionIo is an exceptionIo is stretched when near Jupiter

Io’s shape - 2 orbits

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Melting temperature depends on pressuredepends on pressure

Material temperature depends on

Rate of heat productionRate of heat productionRate of heat release

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Earth is still cooling offT k billi fTakes billions of yearsFreezing iron Solid inner core growing

ConvectionLiquid outer coreMantle (asthenosphere) rocks

ConductionConductionRigid lithosphereEarth’s heat flow ~ 0.08 Wm-2

VolcanoesTransport hot material to theTransport hot material to the surface to cool off

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How do we know all this? - EarthquakesHow do we know all this? - Earthquakes

How do Earthquakes work?Near-surface rocks are brittle and get pushed around by flow of the mantle rocksFaults in rock can break when stresses get too largeTermed “Tectonic activity”

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Difference kinds of tectonic activity – extensional faultsy

Fractures

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Difference kinds of tectonic activity compressive faultsactivity – compressive faults

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Difference kinds of tectonic activity strike slip faultsactivity – strike-slip faults

EEuropa

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Breaking faults releases seismic energyenergy

Travels in waves

Surface waves cause motion in EarthquakesBody waves travel inside the Earth

‘P’ waves and ‘S’ wavesBehave differently

Waves curve back towards the surface

Wave speed depends on rock densityWave speed depends on rock density

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So… how does that help?W ti hWe can time when waves arrive at different placesLater waves sample deeper depthsp pBuild up a map of density variations in the mantle

S waves can’t travelS-waves can t travel through liquids

Liquid outer core leaves a shadow zone with no s-shadow zone with no swaves

• Other evidence for a liquid outer coreq• Earth’s magnetic field – internally generated

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A liquid outer core generates dipole magnetic fieldsmagnetic fields

Earth’s magnetic fieldMercury’s magnetic field

Rotation and convection of liquid iron Moving charges generate magnetic field

Venus has no magnetic fieldHardly any rotation

Mars has no magnetic fieldRotation rate is similar to the EarthRotation rate is similar to the EarthBut has old rocks that are magnetizedCore convection in the past Present core probably entirely solidPresent core probably entirely solid

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On all the terrestrial planets (and then some)VolcanoesVolcanoes

On all the terrestrial planets (and then some)

Mercury – Smooth plains Moon – Maria Venus – Maat Mons

Earth – Mount Augustine Mars – Olympus MonsIo – just about everywhere

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What causes volcanism?Material in mantle risesDecompression causes partial meltingDroplets of molten rock migrate upwards Collect in magma chambersCollect in magma chambers

Rock density decreases as the magma moves towards the surfaceMagma stops when neutrally buoyant

Magma chambers can either:cool off undergroundForms intrusive rock (Pluton) – like graniteSl li bi t lSlow cooling – big crystals

orErupt onto surface to make volcanoesForms extrusive rock – like basaltQuick cooling – tiny crystals

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Different types of volcano

StratovolcanoesSteep-sided Layered ash and lavaLayered ash and lavaFound only on the Earth

Shield VolcanoesLow slopes Layered lavaCoalescing shields called plains volcanismCommon throughout the solar system

Flood VolcanismCommon on Venus & MoonRare on Earth

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Why the different types?All depends on the composition of the lava – silica (Si O2) contentAll depends on the composition of the lava silica (Si O2) content

High silica means high i itviscosity

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Different magmas melt at different temperaturesAlters viscosityAlters viscosityAdding water lowers viscosity

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Silica content determines viscosityTemperature and water-content also play a rolep p y

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StratovolcanoesLots of silica chains – very viscous magmasLots of silica chains very viscous magmasGases trapped in the magma – bubbles can’t riseExplosive eruptionsp pAsh clouds can collapse to form pyroclastic flowsVolcano made from stratified ash and lava llayers

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Mount St. Helens is a famous example

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Splatter cones and cinder conesLots of gas str ggling to escape the magmaLots of gas struggling to escape the magmaLeads to fire fountains and cinders

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Shield volcanoesLess silica chains – non-viscous magmasLess silica chains non viscous magmasRunny magma travels farBuilds low-slope structuresNon-violent eruptionsNon violent eruptions

E.g. Hawaii

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Volcanic calderasVolcanic Collapse features

Collapse pitsWithdrawal of magma from the subsurfaceMagma chamber collapsesMultiple collapses indicate magma chamber filledand emptied several times

Lava tubesSinuous rilles on the Moon and VenusSinuous rilles on the Moon and VenusRun for 100’s of kmCollapsed lava tubesSometimes just portions of the tube collapsecollapse

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The big picture…The big picture…

Earth’s core is still cooling off – drives all the following stepsLiquid core shrinking solid inner core growingLiquid core shrinking – solid inner core growing

This escaping heat drives convection in the mantle

Rising rock can partially melt due to pressure changes

Molten droplets are less dense and rise above rocksMolten droplets are less dense and rise above rocks

Molten material collects at the depth where it’s no longer less dense

These magma chambers can erupt molten rock onto the surface

Type of volcano depends on the viscosity of the rockType of volcano depends on the viscosity of the rockMostly determined by its silica content

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In this lecture…In this lecture…

I t l t t f k l tInternal structure of rocky planetsEarthquakes and what they tell usSources of heatSources of heat

How volcanoes work

Wind-related (aeolian/eolian) processesFluvial processes

Next: CratersNext: CratersReading

Chapter 9.2 to revise this lectureChapter 10 for next lecture