1B11 Foundations of Astronomy The Earth as a planet

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1B11 Foundations of AstronomyThe Earth as a
planet

• Liz Puchnarewicz
• emp_at_mssl.ucl.ac.uk
• www.ucl.ac.uk/webct
• www.mssl.ucl.ac.uk/

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1B11 The Earth as a planet
This is an image of London and the Home Counties
taken from the Space Shuttle. The Earth is the
third planet from the Sun. radius 6380km mass
6 x 1024 kg mean density 5.5g/cm3
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1B11 Cross-section through the Earth
asthenosphere (partially molten)
lithosphere (includes crust)
The outer crust is about 30km thick.
mantle silicates slow convection
outer core liquid (Fe, Ni)
inner core solid (?)
7000K
5000K
3800K
1300K
250km
6380km
5000km
2900km
100km
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1B11 Basic parameters
The cross-section through the Earth shows that it
is internally differentiated. Metal core density
10 - 13 g cm-3 Silicate mantle density 3.3 -
5.5 g cm-3 Silicate crust density 2.7 3.0 g
cm-3 Atmosphere P105 N m-2 (1 bar) 78 N2
21 O2 One natural satellite Hydrosphere Biosphe
re
unique in the Solar System?
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1B11 The fluid Earth
Continental masses are mostly granite and float
on the basalt.
Surrounding the mantle is the crust, mostly rocks
which have solidified from molten lava. These are
basalt and comprise the ocean basins and the
subcontinent sections of the crust. It floats on
the mantle.
The mantle is rock made of iron and magnesium
combined with silicon and oxygen. The density is
about 4 g cm-3 and at these temperatures and
pressures, it flows like a liquid.
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1B11 Seismology
Seismology is the study of the passage of waves
through the Earth. Earthquake seismology reveals
much about the structure of the Earth.
Body waves Travel through the body of the
Earth.
Surface waves Both are transverse
Rayleigh waves describe the vertical motion.
Slowest waves.
Love waves describe the horizontal motion.
P-waves (pressure or primary) are compressional
waves. Move fastest, 6km/s
S-waves (shear or secondary) transverse waves,
travel at 2km/s
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1B11 Seismic wave transport
ground motion
time
seismometer
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1B11 Properties of seismic waves
P-waves move much faster ( 2x) than
S-waves S-waves cannot propagate through a
fluid Rayleigh wave velocity is 0.9x
S-waves Love waves travel faster than Rayleigh
waves
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1B11 Rock types

• Igneous rocks formed from molten lava (magma)
  eg, basalts (oceanic crust) and granites
  (continental crust).
• Sedimentary rocks
  produced by the erosion and
  re-deposition of igneous rocks (generally
  underwater), eg sandstone
• Metamorphic rocks
  igneous or sedimentary rocks altered by
  high temperatures and/or pressures

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1B11 Dating rocks
Most rocks contain trace quantities of
radioactive elements. Radioactive isotopes have a
half-life which is the time taken for 50 of
the material to decay into daughter isotopes.
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1B11 Cross-section through the Earth
asthenosphere (partially molten)
lithosphere (includes crust)
The outer crust is about 30km thick.
mantle silicates slow convection
outer core liquid (Fe, Ni)
inner core solid (?)
7000K
5000K
3800K
1300K
250km
6380km
5000km
2900km
100km
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1B11 The outer layers upper mantle
continental shelf
0km
sea level
continental crust
oceanic crust, r2.9g cm-3
5km
30km thick, r2.7g cm-3
Lithosphere (rigid)
continental root
40km
Asthenosphere (plastic)
r3.3 g cm-3
250km
base of upper mantle 400km below
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1B11 Lithosphere as a condensate
melting temp
Temp (K)
temp
0 1000 2000 3000 4000
asthenosphere
lithosphere
lower mantle
0 500 1000 1500 2000
depth (km)
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1B11 Plate tectonics
The lithosphere is divided into roughly 10 large
plates which move in response to convection in
the mantle. This is the cause of continental
drift and seismic and volcanic activity.
volcanoes
mid-oceanic ridge (new crust)
oceanic lithosphere
continental crust
melting due to release of pressure
Mantle convection
earthquakes
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1B11 Plate boundaries

• Spreading ridges
• The rise of molten material from the mantle
  creates new oceanic crust in the lithosphere.
• 2. Convergent boundaries
• Plates are subducted back into the mantle in
  subduction zones. At a continental boundary, this
  causes folding and the creation of mountains, and
  volcanism.
• 3. Translational boundaries
• Plates slide past each other along transform
  faults.

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1B11 Evidence for plate tectonics

1. Continental shapes
2. Biological and fossil evidence
3. Earthquake and volcano distribution
4. Topography of the ocean floor
5. Direct measurement

satellite
radio telescope
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1B11 Radiogenic heating
The main source of the Earths internal heat
comes from the decay of radioactive isotopes. The
most important isotopes are 238U, 235U, 232Th
and 40K and together these provide approx. 28 x
1012 W. Other possible heat sources original
heat from planet formation growth of the inner
care (latent heat, gravitational potential
energy) gravitational contraction
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1B11 Geothermal heat flow
The average geothermal heat flow is 0.06 W
m-2. Over the whole Earth, this is 30 x 1012 W
which is in good
agreement with estimated radiogenic
values. BUT There are sources of heat loss, eg
hydrothermal vents at ocean ridges, so taking
these into account, the output may be as high as
40 x 1012 W. This would then imply a significant
non-radiogenic heat source, which is most likely
to be residual heat from the Earths rotation.
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1B11 The Earth is cooling
Note that radiogenic heat must be decreasing with
time Today 28 x 1012 W 4.5 billion years ago
120 x 1012 W So there must have been much more
vigorous geological activity (ie plate tectonics)
in the past.
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1B11 The age of the Earth
Radioactive dating indicates an age for the Earth
of 4.6 billion years. The oldest rocks on the
Earths surface are younger about 4.0 billion
years. These are igneous rocks ie they have
formed out of molten material. It is estimated
that it would have taken 0.5 billion years for
these first rocks to form. Meteorites are
generally 4.55 billion years old and the Moon is
4.6 billion years old (from radioactive
dating). This is similar to the age of the Sun
thus it seems that the solar system formed at the
same time about 4.6 billion years ago.
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1B11 Useful isotopes for dating rocks
87Rb -gt 87Sr 48 x 109 yrs
238U -gt 206Pb 4.5 x 109 yrs
40K -gt 40Ar 1.3 x 109 yrs
235U -gt 207Pb 0.71 x 109 yrs
By measuring the relative proportions of these
isotopes in rocks it is possible to fate
them. Note however that melting resets the clock
so the ages relate to the time that a rock was
last molten.
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1B11 How old is the Earth?
17th Century Archbishop Ussher 4004 BC 1788
James Hutton The abyss of time no vestige of
a beginning, no prospect of an end 1859 Darwin
more than 300 million years old 1900 Best
estimates were about a billion years 1956
Patterson 4.6 billion years from radiogenic
lead isotopes. This agrees with astronomical
estimates for the age of the Sun (estimate
independently from the H-R diagram) and with
meteorites. Note that most surface rocks are much
younger, with ages less than 600 million years.
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1B11 Surface processes
Continental drift, fold mountains, volcanism and
earthquakes
Plate tectonics Weathering Biology Meteorite
impacts
Wind, rain and ice form new sedimentary rocks
Some erosion and sedimentation processes.
Atmospheric evolution.
More important in the past evidence removed by
weathering
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1B11 Structure of the atmosphere
150
ionosphere - dissociation and heating by solar
UV and X-rays
thermosphere
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Height (km)
mesosphere
50
ozone layer (heating)
stratosphere
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troposphere
Ground heated by sunlight
200 240 280 320
Temp (K)
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1B11 Atmosphere cont.
The ground is heated by sunlight to a temperature
of approx 300OK. In the troposphere, the
temperature gradient falls off by about 6O per
km. Pressure
where P(h0) 1.01 x 105 Pa
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1B11 The magnetic field
The Earths magnetic field is a dipole (bar
magnet) inclined at 12O to the rotation axis.
Field strength B 4 x 10-5 T (small toy
magnets are 0.02T).
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1B11 Origin of the magnetic field
The metallic core of the Earth is a conducting
fluid. The (nonuniform) rotation and convection
currents in the core are believed to generate
organized currents, and thus a magnetic
field. The Earths magnetic field reaches far
beyond the planet itself, and traps the charged
particles which are emitted in the solar wind.
The particles become trapped in the magnetic
field, in the Van Allen belts. The influence of
the magnetic field reaches out even further, for
many hundreds of Earth radii. This region is
called the magnetosphere, which engulfs the Earth
and channels most solar wind particles away from
the Earth.
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1B11 The Magnetosphere
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1B11 History of life on Earth
Origin of multi-celled animals (600M)
1
Eukaryotic cells appear (large, complex cells
containing a nucleus (with DNA) and organelles
which perform respiration and photosynthesis).
Reproduce sexually.
Time (x 109 years ago)
2
Atmospheric O2 increases
3
Oldest micro-fossils / prokaryotic cells - the
simplest form of carbon based life. Reproduce by
cloning.
Origin of life
4
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1B11 Photosynthesis
CO2 H2O
CH2O O2
chlorophyll
O2 is a waste product of photosynthesis and is
toxic to most photosynthesising
organisms. However it is probably required for
large, multi-cellular animals.
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1B11 The Moon essential facts
Radius 1738km (1/4 of the radius of the
Earth) Mass 1/80 of the mass of the Earth Mean
density 3.3 g cm-3 Distance from the Earth
400,000 km No atmosphere!! Midday temp 120OC
Midnight temp -180OC Sidereal rotation period
27.3 days orbital period Surface dominated by
impact craters No magnetic field!!
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1B11 Exploration summary
1959 First impact Luna2 1959 First far-side
images Luna 3 1966 First orbiter Luna
10 1964-65 5 Surveyor landers 1966-67 5
Lunar Orbiters 1969-72 Apollo 1994
Clementine 1998 Lunar Prospector
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1B11 Lunar surface features

• Highlands
• Maria
• Impact Basins
• Regolith
• Rilles
• The highlands are bright and heavily cratered and
  cover 84 of the surface of the Moon. They are
  very old (at least 4 billion years) and are the
  original lunar crust.
• The maria are seen only on the near side. They
  are dark regions with fewer craters and cover 16
  of the surface. They are relatively young (3-3.8
  billion years old) and are basaltic flood lavas
  which have filled impact basins.

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1B11 Impact basins
Impact basins are very large impact craters,
measuring at least 300 km in diameter. They are
surrounded by concentric mountain ranges and are
found all over the Moon. They are only flooded
on the nearside (by lava to form the maria). This
implies that the near side of the Moon has a
thinner crust. The basins formed 3.9-4 billion
years ago, but the final flooding occurred up to
800 million years later.
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1B11 Impact craters
central peak
slumping
ejecta blanket
rim
flat floor
secondary crater
Impact energies Meteorite
D5km, r3g cm-3 and v20
km/s KE 4x1022J 107 MT TNT gt Crater
50-100 km across
Extent of transition cavity
10s of km
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1B11 More surface features
A regolith is where the surface is covered by a
layer of dust (soil) produced by
micro-meteorite impacts. Approx 0.5mm every
million years Rilles are sinuous valleys cut by
flowing lava.
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1B11 Impact cratering rate
The flux of impacting meteorites decreased
rapidly in the Moons early history.
origin of basins
number of craters
flooding of basins
Curve calibrated using dated Apollo rock samples
origin of the Moon
Heavy bombardment epoch
4 3 2 1 0
Age of surface (x109 years)
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1B11 Basic Lunar geophysics

• Seismicity
  is very low,
  approx 2 x 1010J/yr (compared to the Earth,
  approx 2 x 1018 J/yr)
• b. Heatflow
  measured at Apollo
  15 and 17 sites to be approx 0.02 W m-2,
  consistent with radiogenic heating
• c. No evidence for current volcanic or tectonic
  activity
• d. No magnetic field, so if a metal core exists,
  its probably solid (more seismic data are needed)

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1B11 Is there ice on the Moon?
In 1998, data from the Lunar Prospector indicated
that water ice is present at both the north and
south lunar poles, in agreement with Clementine
results for the south pole reported in November
1996. The ice could represent relatively pristine
cometary or asteroid material which has existed
on the Moon for millions or billions of years.
Deposits of ice on the Moon would have many
practical aspects for future manned lunar
exploration. Humans need water (!) and could
provide hydrogen and oxygen for rocket
fuel. However in 2003, radar signals beamed from
the Arecibo Observatory in Puerto Rico penetrated
to depths of 20ft - but found no sign of thick
layers of ice.
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1B11 Lunar cross-section
crust
iron-poor mantle (density 2.9 g cm-3)
To Earth
zone of moonquakes (homogeneous material)
iron-rich core (density 3.5 g cm-3)
mare