Download Quiz #2

Quiz #2
! 
Wednesday 9/9
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Figure 3.22 in
Bennett & Shostak
(available on
Learn)
Early Earth
Processes shaping Earth, and enhancing habitability
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Reshaping of surface, plate tectonics
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Magnetic fields,
The Earth at its earliest stages
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Molten at the surface, very hot
! 
Sources of heat
◦  Impacts transfer energy as heat,
melts rock (surface active)
◦  Radioactive decay
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Hadean Eon:
◦  Heavy bombardment
◦  Inclusion of oxygen – seen in, e.g., Zircon mineral grains
◦  Released into atmosphere in compound gases
"  overall much less abundant than what is found in the atmosphere today!
origin of the
solar system
oldest rocks
on Earth - end of
heavy bombardment
Hadean
billions of
years ago:
4.56
rise in
oxygen first multiplate
tectonics?
cellular fossils
Archean
3.8
Proterozoic
2.5
Cambrian
Explosion
Phanerozoic
0.55
present
Life in the Hadean era
Interplanetary Dust
10 µm diameter
! 
Atmosphere mostly CO2, not much free O2
◦  We could not breathe this early atmosphere
◦  Microbes could survive
! 
Effects of impacts?
◦  Size matters!
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Impacts are common
◦  Small impacts ~10,000 tons of interplanetary
dust every year
◦  Giant impacts are rare, over 100 craters
identified
Manicouagan Crater, Quebec
70 km diameter
206-214 Ma
Size distribution of craters
microscopic Moon craters
modern Earth
Size matters
Meteor Crater impactor 50-60 m,
Chicxulub Crater impactor 10-15 km.
350-400 km diameter:
!  Could vaporize entire oceans
!  Raise surface temperatures by 2000˚C
!  Sterilize Earth’s surface
!  Could have been a couple
Source of temperature increase:
!  Heat of impact
!  Vaporization of rock (radiates heat into the atmosphere)
!  Vaporized water absorbs sunlight and heats the atmosphere
150-190 km diameter:
!  Vaporized top few hundred meters of the oceans
!  Killed many things except those in deep oceans or deep subsurface
!  Probably several/many
Large impactors through time
Interior structure of Earth
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! 
! 
! 
Crust
Mantle
Outer core
Inner core
Earth is differentiated
Denser material
closer to core
Rocky
Metallic
! 
Convection currents in astenosphere
bring material up to surface from
below
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Structure determined via seismology
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Different types of seismic waves behave differently depending on the
material
◦  Compressional P waves travel through solid and fluid materials
◦  Shear S waves can only travel through solid materials
"  fluids can’t support the side-to-side particle motion for S-waves
P-wave: primary wave
S-wave: Secondary wave
Plate tectonics
! 
The litosphere has broken up into ~dozen plates due to convection in
the mantle
◦  Plates moving a few cm/year
◦  Evidence: GPS and VLBI measurements, ‘puzzle’ fits, seafloor spreading,
different types of crusts (sea – basalt, continents – lower density rock)
Two continental plates colliding into each other, creating
the Himalayas.
The Andes were formed by pieces of oceanic and
continental crust colliding.
Rio Grande Valley – A rift valley
! 
Crust and litosphere pulled
apart
! 
Due to earth movements when
the North American and Pacific
plates scraped against each
other ~35 Myrs ago.
Volcanic activity
! 
May arise where hot mantle material rises up through hot spot
◦  Hawaiian Islands formed from volcanic material from a hot spot beneath
the Pacific Ocean. Plate tectonics carries the Pacific plate over the hot
spot, forming a chain of islands.
◦  Continental hot spots – geysers and hot springs.
Timescales
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Slow motion, a few cm/yr
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About 200 km in 100 Myrs
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225 Myrs ago a single
continent called Pangea
Plate tectonics in the Solar System
! 
Earth the only planet with ongoing plate tectonics!
◦  Moon, Mercury, and Mars have cooled so rapidly that they do not have
enough internal convection to break up their thick crusts.
◦  Venus is almost the same size as the Earth, but due to a much higher
temperatures at its surface, its crust has dried up and thickened so it
cannot break up into plates.
Volcanically active planets and satellites
More on planetary atmospheres
! 
Volcanism helps to create atmosphere of Earth, but gas being lost.
! 
If gas molecules move sufficiently fast and exceed the escape velocity they
escape into space
! 
Escape velocity is the minimum speed needed before a body has enough
kinetic energy to escape from the surface of a planet (overcome the
gravitational field)
2
mvesc
GMm
Ek = E p ,
=
2
r
2GM
vesc =
r
M = mass of planet
m = mass of object/molecule
r = radius of planet
G = gravitational constant
! 
So, if an atmosphere will be retained or not depends on the speed of the
molecules in the atmosphere
! 
This speed is determined by the temperature T and the molecular
weight.
! 
The gas escapes if
2
2
mvthermal
mvesc
k BT =
≥
2
2
k BT
⇒m< 2
vesc
kB = Boltzmann’s constant
T = temperature
m = molecular mass of gas
! 
Different gases have different
molecular masses
◦  Their average speeds are
different at a given
temperature
! 
For a body to retain a
particular component of the
atmosphere for a long period
of time (the age of the Solar
System for example…), the
average speed of the
molecules in the gas must be
much less than the escape
velocity
! 
! 
! 
A planetary atmosphere can be lost
◦  Via thermal escape
◦  Impacts
◦  Solar wind stripping
Smaller bodies more prone to atmospheric loss via impacts, since they
have weaker gravities and smaller escape velocities
The sweeping of atmospheric gas particles into space by the Sun is called
solar wind stripping
The magnetic field of the Earth
! 
! 
Solar wind stripping can be prevented by a magnetic field
To have a global magnetic field a planet must have
◦  An electrically conducting fluid in its interior
◦  The fluid must undergo convection
◦  The fluid must undergo reasonably rapid rotation
! 
The Earth’s magnetosphere is cavity carved out in the solar wind by the
magnetic field
◦  It deflects most solar wind particles, and channeling a few towards the
poles – creating auroras.
◦  Charged particles trapped within are forming Van Allen belts.
Magnetic fields in the Solar System
! 
Only Earth and Mercury amongst terrestrial planets have magnetic fields.
◦  Mercury – small and has slow rotation (1 rot in 59 Earth days), but
have a very large metal core
◦  Mars and the Moon have no magnetic fields due to core not liquid
enough. Mars lost most atmosphere when its interior cooled
◦  Venus rotates too slow (1 rot in 243 Earth days)