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Lecture 13

Geology of the Inner Planets

January 7b, 2014

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3MassMean Density kg m

Volume

Clues to the Interior Structure of

Planets

• Mean (Average) Density

– Water ~1000 kg/m3

– Rocks ~2000-4000 kg/m3

– Metals ~7000-10000 kg/m3

– Compare average density to known densities

– The mean density of the Earth is 5500 kg/m3

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Clues to the Interior Structure of

Planets • Many planets act like a

magnet

• Magnetic field caused by a dynamo.

• To produce a magnetic field you need an electrical conductor that is liquid and spinning quickly

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The Earth’s Magnetosphere

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The Van Allen Belts

• Magnetosphere

traps charged

particles emitted

from the Sun in

region called

Van Allen Belts

– Traps high-

energy protons

and electrons

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Aurora

• If solar activity is

strong, some

particles will leak

into the atmosphere

causing aurora

(Northern and

Southern Lights)

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Structure of the Earth

• Average density ~ 5500 kg/m3

• Earth mostly rock and metal

– Silicon, iron, and oxygen very common.

• Interior structure

of Earth can be

determined by

looking at how

earthquake waves

move through the

Earth.

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Interior Structure of the Earth

• Crust: thin, rocky

• Mantle:basalts

• Outer core: Liquid

iron and nickel

– Determined by

seismic studies

– Creates magnetic

field

• Inner core: Solid iron

and nickel

Plate Tectonics on the Earth

• Lithosphere = solid crust and upper mantle.

• Mantle further down is more plastic and can flow.

• Lithosphere is fractured into many plates floating

on denser mantle material

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10

Continental

Drift

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Geologic Activity on the Earth

• Most activity occurs near plate boundaries

– Earthquakes

– Volcanism

Earth’s magnetic field originates in

a so

lid, p

ermanently

ma...

the p

olar re

gions o

f the...

curre

nts o

f molte

n iron...

inte

nse ele

ctric

curre

nts ..

0% 0%0%0%

A. a solid, permanently magnetized core.

B. the polar regions of the magnetosphere.

C. currents of molten iron deep inside the planet.

D. intense electric currents in the Van Allen belts.

0 of 5

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Mercury

• Likely geologically

dead, but at least

some of its core

is liquid

• Many craters

– Highlands ~ 3.8 byrs old

– Intercrater plains ~ older than 3 byrs old

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NASA's Mariner 10 spacecraft made its first flyby of Mercury in March 1974, and was also the only Mariner mission

to visit two planets (the other was Venus).

Images beamed back by the spacecraft from 437 miles above the planet revealed a surface very similar to that of the

moon. However, Mariner 10 only had enough time to map half of the planet -- which is a big reason why NASA

recently sent their MESSENGER spacecraft to Mercury.

Mercury Messenger http://www.nasa.gov/mission_pages/messenger/main/index.html

http://en.wikipedia.org/wiki/MESSENGER

• Launched August 2004

• Arrived 2011

• Found water and water

ice

• Obtained visual

evidence of past

volcanic activity

• Determined the

planetary core is

partially liquid

http://messenger.jhuapl.edu/the_mission/artistimpression/atmercury_br.html

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Messenger Fly-by August 2, 2005 http://en.wikipedia.org/wiki/File:Mdis_depart_anot.ogv

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Interior Structure of Mercury • = 5430 kg/m3 (similar to Earth)

• Earth is only denser because of gravitational compression

– Mass of Earth is greater

– Metals compressed to higher densities

• Mercury has

proportionally much

more metal.

• Weak magnetic field

(1/100 of Earth’s)

little molten metal OR

due to slow spin

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Interior Structure

• Earth uncompressed = 4400 kg/m3

• Mercury uncompressed = 5300 kg/m3

– Mercury formed closer to the Sun, where

higher temperature favored the condensation of

heavier, metallic elements

The central core of Mercury is probably composed of

wat

er and C

O2 ices.

molte

n rock

.

solid

rock

.

solid

or m

olten ir

on.

0% 0%0%0%

0 of 5

A. water and CO2 ices.

B. molten rock.

C. solid rock.

D. solid or molten iron.

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90

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Interior Structure of Venus

• Mass, size and density (5340 kg/m3) similar

to Earth

Composition and internal structure likely similar

• No magnetic field

– Due to slow rotation?

– Pspin= 243 days

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Venus – Magellan Radar Map

Color = elevation. Blue areas are NOT liquid oceans, just low-lying areas

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No Plate Tectonics • No long ridges or faults indicating plate tectonics

• Many local deformations of surface, but not due to

plate tectonics

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Why No Plate Tectonics?

1. Thin lithosphere – could not support

large-scale tectonics so only local

deformations were created.

2. Thick lithosphere

• Not broken into sheets.

• Mantle material not moving enough to move

sheets of material.

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Volcanoes on Venus

• Over 1600 major volcanoes observed

• Many similar to shield volcanoes on the

Earth (e.g. Hawaii)

Image 83 x 73 km

• May currently

be active

• Sulfur dioxide

in atmosphere

.

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Hill Diam ~25 km

Height ~750 m

Alpha Regio (Venus)

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Boann Corona

Diameter: 225 km

Coronae

• Large up-welling

of mantle

• Volcanoes in and

around coronae

• Unique to Venus

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~22x27 km

Lava Channels, Lo Shen Valles

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Craters on Venus

• ~1000 craters have

been observed on

surface.

– Randomly distributed

on surface = surface

all about the same age.

– Surface about 500

million years old.

Venus and Earth have similar

mass

, densit

y, and su

rfa..

mass

and d

ensity.

mass

, densit

y, and m

agn...

magn

etic fi

eld, s

urface

...

0% 0%0%0%

A. mass, density, and surface temperature.

B. mass and density.

C. mass, density, and magnetic field.

D. magnetic field, surface temperature, and

atmosphere.

0 of 5

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90

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• No global magnetic field

• Pspin= 24.6 hours

• = 3900 kg/m3

– no metal in core or

– metal in core is not liquid

interior is mainly rock.

Composition of Mars

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General Surface Features • Northern Hemisphere =

“lowlands”

– Few craters = younger surface

– Lower average elevation

– Evidence for geologic activity

• Southern Hemisphere = “highlands”

– Many craters = much older surface

– Age ~3-4 byrs

Google Mars Map

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Volcanoes on Mars

• Volcanoes mainly found in Northern Hemisphere.

• Largest = Olympus Mons, largest volcano in solar system!

– Diameter = 600 km

– Height = 24 km

– Very few craters on surface

– Possibly younger than 100 million years!

• No evidence for plate tectonics

• Volcanoes likely formed by hot-spot volcanism

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Tharsis Region

Twelve orbits a day provided the

Mars Global Surveyor MOC wide

angle cameras a global snapshot of

weather patterns across the planet

in April, 1999. Here, bluish-white

water ice clouds hang above the

Tharsis volcanoes. This computer

generated image was created by

wrapping a global map onto a

sphere. The center of this sphere is

15 degrees North latitude, 90

degrees West longitude. This

perspective rotates the south pole

(which has no data coverage in the

original map) away from our field

of view. Courtesy NASA

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Olympus Mons

• Size of Missouri

• 3 times height of

Mt. Everest.

Tharsis Volcanoes 34

This is a shaded relief image

derived from Mars Orbiter

Laser Altimeter data, which

flew onboard the Mars Global

Surveyor. Courtesy NASA

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Valles Marineris

• Large canyon

(fracture?) in crust

• 1/5 circumference

of planet

• 7 km deep

• 4 minute tour

http://marsprogram.jpl.nasa.gov/gallery/atlas/

Mars has a rotation period of 24.6 hours and a density of 3900 kg/m3 but it has no global magnetic field. What is its most probable interior composition?

Very

littl

e meta

l, so

lid ro

ck.

Very

littl

e meta

l, m

olten...

Most

ly m

olten m

etal b

u...

Main

ly m

olten m

etal a

n...

0% 0%0%0%

A. Very little metal, solid rock.

B. Very little metal, molten rock.

C. Mostly molten metal but with solid rock.

D. Mainly molten metal and molten rock.

0 of 5

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In-Class Activity:

Cratering on Planetary Bodies