Download Planetary geology The Terrestrial Planets

Planetary geology
The Terrestrial Planets
Dr. Ken Rice
Discovering Astronomy S
The Terrestrial Planets
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Mercury, Venus, Earth (Moon), Mars
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Located between 0.4 and 1.52 AU
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Masses (Earth masses)
– Mercury (0.055), Venus (0.82), Earth (1.0), Mars (0.11)
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Radii (km)
– Mercury (2440), Venus (6051), Earth (6378), Mars (3397)
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Densities of ~ 5 g/cm3 (Mars – 3.93 g/cm3)
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Interiors
Differentiation – Layering by density
• Core
– densest material – metals such as nickel and iron
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Mantle
– Rocky material of modest density – minerals containing for example silicon and
oxygen
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Crust
– Lowest density rocks – eg., granite, basalt
– Lithosphere
• Crust and upper part of mantle
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How do we know?
• Earthquakes produce two types of waves
– P waves – longitudonal
– S waves – transverse
• Molten outer core stops S waves, but
allows P waves through.
– S waves can only travel through solids.
• Analysis of seismic waves allows us to
develop a model of the Earth’s interior!
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Longitudinal and transverse waves
Longitudinal wave – P wave
Transverse wave – S wave
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Terrestrial planet surfaces
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The surfaces of the terrestrial planets are all very different
– Mercury (and the Moon) are heavily cratered
– Earth, Venus and Mars have relatively few craters
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Must have all looked the same when they were young
– All subjected to heavy bombardment
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Geological activity!!!
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What causes geological activity?
The primary driver of geological activity is interior heat.
• Heat of accretion
– Accreting planetesimals release gravitational potential energy.
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Heat from differentiation
– As dense material sinks it also releases gravitational potential energy.
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Heat from radioactivity
– Rocks and metals contain radioactive isotopes that release energy when they
decay.
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How do planets cool?
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Convection
– Hot material expands and rises while cool
material contracts and falls (very slow –
1cm/year).
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Conduction
– Hot material transfers heat to cooler material.
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Radiation
– Energy can be lost by radiating light into
space
Small planets cool faster than big
planets.
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Surface area to volume
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Heat escapes into space from the planet’s surface
– Surface area
A = 4πr 2
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The amount of internal energy will depend on the volume of the planet
– Volume
4
V = πr 3
3
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The surface area to volume ratio is therefore
A 4πr 2 3
Surface area to volume ratio = =
=
4
V
πr 3 r
3
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If we have two planets with the same initial temperature, but different radii, the
one with the smaller radius will lose its internal heat first.
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Impact cratering
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Form when an asteroid of comet hits the
surface of a planet
– Typical velocity – 40000 – 250000 km/hr
– Crater generally 10 times as wide as the object
and has a depth similar to the object size.
Tycho crater - Moon
Meteor crater - Arizona
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Volcanism
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Occurs when underground molten rock – magma - finds its way through the
lithosphere to the surface
– Molten rock is less dense than solid rock so rises
– The Earth’s interior is NOT molten, so a magma chamber may be squeezed by the
surrounding rock
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•
•
Runniest lava flattens out and forms plains – e.g., Moon
Lava that is somewhat thicker form shield volcanoes - not very steep – Mars.
Thickest lava forms stratovolcanoes – tall, steep - Earth.
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Tectonics and Erosion
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Tectonics refers to any surface reshaping process
– Stretching or compression of the lithosphere
– Creates mountains or valleys (e.g., Great Rift Valley in Africa)
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Erosion is the breakdown of rock through the action of ice, liquid or gas
– Grand Canyon
– Need volcanoes to produce outgassing and create an atmosphere
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Moon
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Heavily cratered
– Geological dead – interior must have cooled
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Lunar highlands
– Many craters
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Lanar maria
– Smooth (very few craters) – formed by a flood of lava
The lava that filled the lunar
maria came from heat released
by radioactive decay a few
hundreds million years after the
moon formed
• Early heavy bombardment
• Very few impacts afterwards
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Mercury
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Similar to the Moon
– Heavily cratered
– Some lava plains
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Caloris Basin
– Huge crater – almost half of Mercury’s radius
– Massive impact after heavy bombardment
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Some very high cliffs
– Contraction of the core and mantle,
deforming the crust.
Looks very like the moon
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Venus
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•
Similar in size and mass to the Earth
Surface mapped by radar
– Crater count suggest surface age of 750 Million years
• No small craters – small impactors burnt up in dense atmosphere
– Abundant lava plains and volcanoes
– Crust is quite contorted - tectonics
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Plate tectonics on Venus
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Earth’s lithosphere broken up into a number of different plates.
– Probably resulted from the forces of the underlying mantle convection.
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Venus shows no evidence for plates
– Thicker and stronger lithosphere
– Could be because water evaporated faster on Venus than on Earth, strengthening
the lithosphere.
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Mars
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Southern hemisphere more heavily cratered than the north
– Southern surface older
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Geological activity has erased craters in the North
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Geological activity on Mars
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Volcanism
– Olympus Mons – 26 km high
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Tectonics
– Valles Marineris
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Erosion
– Winds and water?
Olympus Mons –
26 km high but not
very steep –
medium thickness
lava.
Valles Marineris – rift valley 4 times deeper than the Grand
Canyon
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Water on Mars
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Mainly in the form of ice
– Poles
– Top metre or so of surface
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Ancient water flows
– Sedimentation
– “blueberries” (seas or oceans)
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Recent water flows
– Gullies and craters
– Liquid water is unstable
Map showing hydrogen content of Martian soil
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Summary
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