Download Geology of the Inner Planets

Lecture 9
The Geology of the Inner
Planets
October 5, 2015
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2
Clues to the Interior Structure of
Planets
• Mean (Average) Density
Mass
Mean Density 
Volume
–
–
–
–
–
 kg 
 3
m 
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
3
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 moving
electrical charges, such as
liquid metals spinning
quickly in the core of a
planet.
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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
– protons and
electrons
– high energies
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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
– magnetic field
– seismic studies
• Inner core: Solid iron
and nickel
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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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Geologic Activity on the Earth
• Most activity occurs near plate boundaries
– Earthquakes
– Volcanism
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Earth’s magnetic field originates in
A.
B.
C.
D.
a solid, permanently magnetized core.
79%
the polar regions of the magnetosphere.
currents of molten iron deep inside the planet.
intense electric currents in the Van Allen belts.
6%
A.
11%
4%
B.
C.
D.
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Mercury
• Likely geologically
dead
• Many craters
– Highlands ~ 3.8 byrs old
– Intercrater plains ~ older than 3 byrs old
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.
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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
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Mercury Messenger
• Launched August
2004
• Arrives 2011
http://messenger.jhuapl.edu/the_mission/artistimpression/atmercury_br.html
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Messenger Fly-by August 2, 2005
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The central core of Mercury is probably composed of
A.
B.
C.
D.
water and CO2 ices.
molten rock.
solid rock.
solid or molten iron.
85%
3%
A.
7%
B.
6%
C.
D.
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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
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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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Volcanoes on Venus
• Over 1600 major volcanoes observed
• Many similar to shield volcanoes on the
Earth (e.g. Hawaii)
• May currently
be active
• Sulfur dioxide
in atmosphere
Image 83 x 73 km
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Coronae
• Large up-welling
of mantle
• Volcanoes in and
around coronae
• Unique to Venus
Boann Corona
Diameter: 225 km
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Lava Channels, Lo Shen Valles
~22x27 km
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Craters on Venus
• ~1000 craters
observed on surface.
– Randomly distributed
on surface = surface
all about the same age.
– Surface about 500
million years old.
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Venus and Earth have similar
A.
B.
C.
D.
mass, density, and surface temperature.
mass and density.
93%
mass, density, and magnetic field.
magnetic field, surface temperature, and
atmosphere.
3%
0%
A.
B.
C.
4%
D.
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Composition of Mars
• No global magnetic field
• Pspin= 24.6 hours
•  = 3900 kg/m3
– Some metal in the core but
not liquid.
– interior is mainly rock
compared to Venus and
Earth.
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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
–
–
–
–
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.
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Tharsis Volcanoes
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/
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Comparison of Inner Planet Geologies
37
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?
A.
B.
C.
D.
Very little metal, solid rock.
80%
Very little metal, molten rock.
Mostly molten metal but with solid rock.
Mainly molten metal and molten rock.
10%
8%
1%
A.
B.
C.
D.