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1
Creation of the Universe, Solar System, Planets,
and Satellites
Creation Mythologies
1.1.1 Pre-Civilization, Primitive Peoples
- Cultural and religious traditions
- Ancient mythical explanations of the origins of humanity, nature and the
cosmos.
1.1.2 Civilized Man
- Religions and Cultural Mythologies

Epic of Gilgamesh – Sumerian mythology (oldest known recorded
story of man)
 Written on 12 clay tablets

Ancient Greece – spontaneous creation from Chaos

Judaism, Christianity, Islam – model of creation based on a God who
decides to create something

The Haida peoples – creation by the Raven trickster

Shinto (Japan) – Izanami and Izanagi – Japanese equivalent of Adam
and Eve
- Philosophical Mythologies

Neocreationism – Intelligent Design Hypothesis
 The universe is so tuned for existence that the conscious designer
must have had some purpose
- Scientific Mythologies

Quantum fluctuation of the nothing – a spontaneous creation is possible

Big Bang – The universe explodes out of nothing

Inflationary Big Bang

Concordance model – the world is being pushed forward by Dark
Energy
2
A Scientific Narrative
Early Attempts at Quantifying the Moment of Creation
-
Kant – How long has the Sun been burning (chemistry)
Hutton – geology
Von Helmholtz – The gravitational energy of the Sun could account for 20
million years (physics)
Kelvin – less than 100 million years (measured how quickly the earth was
cooling)
Joly – 90-100 million years
Walcott – at least 1.6 billion years (paleontology – fossils and geological
clock)
Modern Physics – The Radioactive Clock
- Becquerel discovered radioactivity – provided the “clock” with which to
measure the “age” of rocks
- Modern physics uses the radioactive clock to determine the earth’s age.
- Radioactive materials decay at a rate that is proportional to the amount of
material (atoms are lost at a rate that is proportional to the amount of
material).

Half life of carbon-14 = 5730 years

This is the isotope used in carbon dating.
-
Half Life (t1/2) – The time in
which it takes half of atoms
had decayed into other
elements
-
Potassium-40
decays
spontaneously to
argon-40
Argon + Potassium
= constant (if system
is kept closed)
-
The Setting of the Clock and the “Oldest Materials”
- “Age” in the context of rock and minerals = time
- Method by which rocks are dated = Uranium-238 to Lead-206
-
-
Oldest mineral = zircon – 4.4 x 109 years old (Australia)
Oldest rocks

The Acasta Gneiss – 3.96 x 109 to 4.03 x 109 years old (metamorphic
rock found in Northern Canada and Greenland)

Igneous rock – 3.86 x 109 years (Porpoise Cove, Quebec)
But, even older…
- Meteorites – 4.54 x 109 years old

Tagish Lake meteorite – fell on the lake in BC in 2000 is believed to
represent the most primitive materials from the condensing solar
nebula.

Meteorites contain all the organics that seem to be necessary for
constructing life.
-
4.54 x 109 years seems to be a good age to assign the formation of the
Terrestrial Planets
Nucleosynthesis and the Sun’s Fire
THE BIG BANG
- Big Bang – Energy explodes out of nowhere, condenses into matter, and
then the matter recombines into some form of energy

13.5 – 13.9 x 109 years ago
-
The Big Bang forms largely hydrogen and helium
It describes the abundance of chemistry we have.’
What Happened…
1) 1027 K
An incredibly hot, extremely compressed ball of energy exploded
into our universe
2)
Adiabatic Expansion – The heat (that was compressed) is now
being spread over a larger volume – cools down
3) 1012 K
1 second – leptons and quarks condense out of the energy

Leptons – light particles, weak nuclear force (ie. electrons)

Quarks – small charged particles that can combine into
baryonic matter (the materials and particles (protons and
neutrons) that form everything else)
Leptons and Quarks are the two basic constituents of matter.
3 or 4 seconds – All the baryonic matter of the universe has largely
formed into protons and neutrons

4)
1010 K
5)
Neutrons are not stable when alone (half life = 12 minutes) – start
decaying into electrons and protons
6)
Some neutrons attach to protons to form 2H (deuterium, the atomic
mass 2 isotope of hydrogen)
-
We now have a neutrally charged plasma (mix of particles) of protons,
neutrons, 2H (deuterium) nuclei, electrons, and other leptons and “exotic
particles”
-
Nucleosynthesis – The formation of the nuclei of all known chemistry – this
happens in the first few seconds following the Big Bang
In the next 500 or so seconds…
The Net Result:
-
The Be and Li are produced, then used up. However, traces of this material
remain in the plasma.
-
This reaction = exothermic

It is the same reaction that is happening at the core of the Sun
(production of He)

The energy released in this reaction is carried away in gamma rays
such as heat and light – these gamma rays account for the mass lost in
the net reaction.
The hydrogen fusion reaction requires atoms to be close to one another.
-
After 500 seconds…
- The universe is too cool and too expanded for the reaction to continue.
-
We are still left with hot plasma (95% H, 5% He, and traces of elements such
as Be and Li)

This plasma remains opaque to the transmission of light and
electromagnetic radiation for 380 000 years.

When the plasma stabilizes and forms atoms, it becomes transparent.
2.2.1 Where Do All The Other Elements Comes From?
- 500 seconds after the Big Bang, nucleosynthesis stops (the universe is too
cool and too expanded)
Nucleosynthesis in the Stars and the Sun
- The ordinary matter that was created by the Big Bang has mass – this mass
is attracted to other mass by gravitational force

Material begins to congregate under gravitational forces

Gravity compresses material together again

The material is heated (adiabatic compression)

When it gets hot and dense enough, nucleosynthesis starts again
Massive protostars form
Protostars = early phase of star formation

It’s believed that stars began to form 100-200 million years after the Big
Bang.
-
Elements other than H, He, and other light metals are built up in the synthesis
(creation) of stars
Supernovae can create all the elements.
The very chemistry of what we are made came out of the explosion of stars.
The Sun
- Nucleosynthesis started about 5 x 109 years ago (and will continue for another
6 to 7 billion years)
- The Sun is currently in its hydrogen burning stage
- Mass = 1.99 x 1030 kg
- Composition = (92% H, 7% He, around 2% everything else)
- Power (how quickly it releases heat) – 3.86 x 1026 J/s
-
-
Within the Sun, the largely hydrogen mantle is compressed by overlying
materials.
It gets compressed to such a high density and temperature that we go back to
the conditions similar to the first 500 seconds of the Big Bang – hydrogen is
continuously being converted into helium.
This process is maintained because the compression is maintained.
The energy that is produced from hydrogen-helium burning is emitted off the
surface of the sun.
Further Nucleosynthetic Reactions Within Stars
- Stars smaller than the Sun

Produce no elements beyond the small amounts of Li and Be due to the
hydrogen fusion process

Burn slowest and live longest
- Stars comparable to or larger than the Sun

CNO (carbon-nitrogen-oxygen) process
- If a star is larger enough, helium (which is denser than hydrogen) sinks into
the core.

The helium burning stage occurs if the star is large enough that the
overlying mass compresses this core to temperatures exceeding 10 10 K.

This He core grows and the layer of burning moves out to lower
pressures and lower temperatures

The He core eventually synthesizes into carbon.
-
Our sun can’t go past the carbon core (core can’t hold itself up anymore by
radiating energy)
-
Larger stars…

H  He  C  O  Si  Fe
-
Fusion comes to an end when it is no
longer an exothermic process
-
Elements beyond Fe are produced
through
the
slow
endothermic
processes in smaller stars (around
size of Sun) – This results in the
creation of lighter elements
-
Heavier elements come from novae
and supernovae.
Novae and Supernovae
- Only happen with larger stars
- When fusion can’t continue, the star collapses upon itself
-
-
If the star is sufficiently massive (10 solar masses), it violently explodes. This
is a massive star supernova or a type II supernova

Tremendous energy available for neutron nucleus fusion via r-process
(“rapid” process)
Stars that are smaller than about 1.5 solar masses collapse into white dwarf
stars after passing through a red giant carbon-burning phase at the end of
their lives
-
-
White Dwarfs are very dense and can pull material away from nearby objects
and become more massive.

When their mass is sufficient to compress the core to temperatures and
pressures that ignite the fusion fire, the star can explode.
This kind of explosion is known as a carbon-oxygen bomb, a white dwarf
supernova, or a type Ia supernova.
This kind of supernova contains no heavy elements

Brightness can be used to measure distance.
-
If a star isn’t large enough to explode, it more gently collapses into a dwarf, a
neutron star or black hole

It doesn’t nucleosynthesize beyond Fe.
-
Theory of Nucleosynthesis = Burbidge, Burbidge, Fowler and Hoyle
2.3
-
The Big-Bang Scenario
Best evidence for Big Bang – the universe is presently expanding as if it has
always been expanding from some original point
The universe seems to be expanding in a linear function
The universe was at “zero size” 13.7 billion years ago.

We cannot see or calculate the universe at “zero size”
Two Grand Theories of Physics
- Gravitation Theory – Gravitation not due to a force, but mass-energy and
momentum
- Quantum Mechanics – Waves can be measured in particle-like packets
called quanta.
-
Our physics describes the interactions between “things” according to four
fundamental forces
1) Strong Interaction
Affects only quarks and gluons
Holds protons and neutrons together
2) Weak Interaction
Holds atomic nuclei together
3) Electromagnetic Force
Holds atoms together, involved in the spread of light
4) Gravity
Attracts bodies together according to their mass
-
Based on our current understanding of physics, we can back up time to
describe the universe’s earliest conditions

But, we can only see back to about 380 000 years after the beginning

Earlier than that, the universe is opaque to our vision
- 1 Planck time = smallest calculable increment of time (1.6 x 10-43 seconds)
-
-
-
-
We think we can tell
what is going on in this
380 000 year circle
Our physics fails when
the universe is so hot
and so dense (1010
Planck times) that our
understanding of the
physical world no
longer applies
We do know that for the
universe to look as it
did at 380 000 years,
the universe must have
expanded at an
extremely rapid rate
(inflation)
Inflation – the universe
expanded faster than
the speed of light
No way to prove or disprove inflation theory – our physics isn’t applicable.
3
-
The Size and Age of Our Universe
We know the solar system condensed as long as 4.54 x 109 years ago.
We also know that the solar system is comprised of materials that must have
been formed in earlier phases of stellar evolution.
Measuring Astronomical Distances and Time
-
The Ancients Greeks and Astronomy
Western astronomical science is based in the early Greek civilization

Thales – promoted the understanding of nature

Anaximander – universe as consequent of the basic element, water

Pythagoras – planet was a sphere, heavenly bodies moved in circles

Anaxagoras – moon’s light was reflected from the Sun

Eudoxus – mathematical-geometrical cosmology

Aristotle – proof that Earth was a sphere

Aristarchus – Sun as the centre of the universe
-
Ptolemaic theory of an Earth-centred universe = the Almagest
-
Two thousand years ago, we knew the Earth was round. We knew its size.
We have measured the relative distances to the Moon and the Sun. We had
discovered the heliocentric reference frame
-
-
Scales in the Solar System
Copernicus – heliocentric view
Brahe – made important measurements
Kepler – used Brahe’s measurements to discover the laws of planetary
motion.
Galileo – provided supporting evidence for Kepler’s work.
Parallax Triangulation
For nearby stars, we can use parallax measurement to determine their
distance
For the most distant objects that we can observe, the most distant quasars,
we need to use other methods

Quasars = sources of electromagnetic radiation – high energy output
-
-
The Earth orbits the Sun in
an orbital diameter of 2 AU
(300 x 106 km)
-
By observing the same star
6 months apart, we see it
from a different perspective
(across a distance of 2 AU)
Parallax second (parsec) – The distance corresponding to a parallax angle
of 1 arcsec (1”)
1 pc = 3.08 x 1013 km
1 pc = 3.26 light years
Distance (parsec) = 1/
Where  = the parallax angle (arcsec)
-
Using the Hipparcos telescope, we can now determine the distance of stars to
about 300pc.
Distance Measured by Apparent Brightness of Stars
Awavefront = 4r2
-
Light emitted from a source spreads over an area which is proportional to the
distance from the source
r = distance to
1  1/r2
-
As the area increases (the distance from the source increases), the intensity
of light decreases
The Apparent Magnitude Scale
- The Scale of Apparent Magnitude – Hipparchus

Brighter star = 0

Dimmest star = 6
- Sun’s apparent magnitude = -26.8
- The magnitude of the brightness of light can depend on colour.
-
-
-
Each step in increase of magnitude represents a dimming of the apparent
brightness of the object by a factor of 2.5

A star with a magnitude of 3 is 2.53 times dimmer than a star of
magnitude 0.
The Brightest Stars, Distance, and the Absolute Magnitude Scale
The inherent brightness of a source is just as important as the apparent
brightness that we observe.
Rigel and Deneb – two of the brightest stars that we see in the sky (60 000
times brighter than the Sun)

They’re extremely far away – don’t appear as bright
Proxima Centauri – closest of all known stars, but its apparent magnitude is
11.5 – we can’t see it with the naked eye
 Centauri A – closest star we can see (1.29 pc, apparent magnitude 0)
 Centauri B – 1.29 pc, apparent magnitude 1.5
-
Absolute Magnitude Scale – measures the brightness of stars as if they
were all placed at a distance of 10pc.

Sun’s absolute magnitude = 4.83
We probably wouldn’t be able to see it at a distance of 10pc.

Rigel and Deneb’s absolute magnitude = -7.1
-
The brightest stars usually shine with the whitest or bluest light
Cooler stars (Betelgeuse, late red giant phase) are also quite bright
Generally, blue = bright, red/brown = dim
-
During the middle age of a star, it’s said to be a main sequence star
It lies on a trend in the Hertzsprung-Russell diagram of stellar luminosity as a
function of temperature or colour.

All stars in main sequence are in the hydrogen-fusion stage

Blue stars = hot, high luminosity, low absolute magnitude, massive

Red stars = opposite of blue.
3.1.6 Stellar Evolution
- A star spends most of its life on the main sequence – it produces energy
through hydrogen to helium fusion

It slowly becomes hotter and brighter
- It moves off the main sequence as it begins helium fusion (typically becomes
larger, cooler, and brighter)
-
Large stars move through stages quickly

Stars larger than 8 solar masses finish their lives as supernovae
Stars the size of the sun move into a red giant phase before gently
collapsing into white dwarves

If they’re a bit larger they can collapse into neutron stars or black holes.
-
Small stars (smaller than 5 solar masses) collapse under their own gravity

They become smaller and temporarily hotter as gravitational energy
converts into heat – white dwarves

They can also pull nearby material in with their gravitational force,
producing a Type Ia supernova
-
Very large stars (between 5-20 solar masses) go through a yellow-supergiant,
Cepheid-variable phase late in their lives

Variation in brightness is related to absolute magnitude.
3.1.7 A Distance Scale Based on the Intrinsic Brightness of Stars
- From H-R Diagram, we can know how intrinsicly bright a star is.
- By comparing the apparent and absolute magnitudes, we can determine the
distance to the star.
3.1.8 A Distance Scale Based on the Intrinsic Brightness of Galaxies
- For the closest galaxies, we can determine their size if we know their
distance.
- Larger galaxies = brighter galaxies
-
-
We measure the distance of galaxies by using the apparent brightness of the
brightest resolvable stars in the galaxy

We can now measure distances to galaxies out to 10 Mpc
One of the brightest kinds of stars is the Cepheid variable type

Extremely large, yellow stars thought to be in the stage of life following
hydrogen fusion.

Polaris (our North Star) is a Cepheid.
-
If we can recognize the kind of star, and we know how far away they are, we
can figure out how intrinsically bright they are
-
At great distances, we run into another kind of galaxy, the Quasar.

They produce enormous amounts of light and energy

They are probably enormous young galaxies in formation that have
nearby galaxies or black holes consuming enormous amounts of mass
from them.
Quasars are very far away and very old.
The extreme brightness of quasars allow us to extend our distance scale out
to the boundaries of our universe (3000 Mpc).
-
3.1.9 Hubble: “Red-shift” and the “Age” of the Universe
- In the 1920s, astronomers noticed a spectral shift in galaxies that were
moving away.

The spectrum tended to shift towards the red end as they appeared
smaller in the field of vision.
-
Edwin Hubble used the luminosity-period relationship of Cepheid variable
stars as a means of measuring distance to many galaxies.

The farther away the galaxy, the more red light it emitted

The red shift of their spectral lines corresponded to their distance

This Doppler Shift of light towards a reddening is caused by the galaxy
moving away from us

The amount of red shift is proportional to the velocity at which the
galaxy is receding.
-
Hubble computed the Hubble age, the age of the universe, from this straightline relationship between distance and speed of recession

He was about 12 billion years off
The Velocity of Recession vs. Distance from Earth
Hubble Constant (Ho) = v /d
Where v = velocity (km/s)
And d = distance (Mpc)
-
Recent determinations of the Hubble Constant give us a measurement of
about 71 km / s  Mpc

This constant actually has units of 1/time (the units of distance cancel
out with one another)
-
One way of explaining the separation of galaxies is to regard them as starting
from the same place – the farthest away moved the fastest
-
Today, we generally accept that…
Ho-1 lies somewhere between 11 and 15 billion years
-
Ho-1 counts to the time when all the galaxies were in the same place – the
centre of the Big Bang
Hubble Age = Age of the universe
-
Einstein’s Theory of Relativity – masses (or massless particles like
photons) traveling through space cannot be faster than the speed of light

Therefore, c x Ho-1 is the farthest distance that light could have traveled
since the beginning of the universe

By this theory, the “radius” of the universe is 13.7 x 10 9 light years, or
4200 Mpc.
-
From another perspective, we can regard space to be expanding with the
Hubble constant

Our galaxies aren’t moving through space, they are moving with
expanding space.
-
While we can perhaps agree on the “age” of the universe, there’s much
argument as to the “radius” of the universe.
-
Using the distance scaled based on the apparent brightness of Type Ia
Supernovae, it seems that the slope of the Hubble plot is getting steeper

If true, this means that the universe is expanding at an increasing rate
Dark Energy – introduced to describe that which is forcing this apparent
acceleration.

70% of everything in the Universe is comprised of this undetectable
dark energy
Dark Matter – Unseen mass either in the form of exotic matter (WIMP) or
cold ordinary matter (MACHO)

Does not emit or reflect enough electromagnetic radiation to be
detected directly – we can infer their presence from the gravitational
effects that they have on visible matter.

25% of everything in the universe is thought to be Dark Matter
The remaining 5% of the universe is what we see and know about
-
-
-
The 70-25-5 ratios are described by the Concordance Cosmological Model
(variant of the Inflationary Big Bang Model)
4
-
The Condensation of our Sun and the Accretion of
the Planets
The Big Bang created H, He, Li, Be, and little else
All the more massive nuclei were created in the nuclear fire of stars
From the dust of this nucleosynthesis, the elements of our Earth were formed
and from these our Earth and Sun and planets condensed.
4.1 The Condensation of the Solar System
-
About 5 billion years ago (8 billion years after Big Bang), at some gravitational
centre, material from the clouds of dust and gas left behind by supernoval
explosions began to assemble the mass of our Solar System
-
All elements known to exist in our solar system, except Promethium (Pm) preexisted in this condensed cloud

Half-life of Pm = 30 years.

All of the Pm would have been quickly lost to the cloud, and the now
cool cloud wasn’t hot enough to produce its replacement.
1) Material from the clouds of dust and gas began to assemble
2) A great mass of gas (largely H and He) at the centre of this condensing
cloud began to form a proto-sun
3) About the proto-sun, local centres of condensation formed orbits and
concentrated the planets

These pro-planets were brought into orbit by angular momentum

As gravitational condensation increased and more material was
attracted in, the angular momentum accelerated (just as a figure skater
pulls in arms to spin faster)
4) Conserving angular momentum, the infalling dust and gas starts revolving
faster

Angular Momentum ( ) is a conserved quantity of physics
-
-
We can’t account for the exact distribution of the planets about the Sun, but
any condensation would have caused the planets to revolve around the
gravitational central Sun in the same direction
Exceptions: Venus, Uranus, and Pluto, which all rotate about their own axis
(one that is contrary to that of the Sun)
Bode’s Law and the Titius-Bode Relationship
- There seems to be some “order” to the distribution of planets
Bode’s Law
rk = a + b2k
Where k = planetary number counting from the sun
a and b = parameters which best fit the planetary
distribution from the Sun
Titius-Bode Relationship
rk = ro  pk-1
Where ro = 0.4AU = 6 x 107 km (the orbital distance of
Mercury from the Sun)
p = 1.73 (best average fit through the whole
planetary system when we count the belt
of asteroids b/w Mars and Jupiter)
4.1.1 Gravitational Energy Retained as Heat in a Condensing Planet or the
Sun
-
Von Helmholtz recognized that the gravitational energy contained within the
Sun could account for its shining for between 20-40 million years
How do we estimate this energy?
1) Start from an extended, “absolutely” cold (0K) cloud
2) Somewhere a small mass Mc of radius r assembles (perhaps under
electrostatic or magnetic forces)
3) Volume of centre (sphere) = (4/3)(r3)
4) Mass = pV, where p= density
5) Suppose that at some great distance Rstart, a small element of mass,
dm, is waiting to fall in upon this gravitating centre
6) Now, if our small element of mass were to fall in towards our
condensation centre, it would accelerate, gaining velocity and kinetic
energy of motion equal to its continuing loss of potential energy
7) Eventually it would – perhaps moving very fast – hit our central mass
centre and release all of its kinetic energy in heat.
8) Mc increases slightly in volume
9) The energy contributed by the infalling dm is dE= G(16/3)(p22r4),
where G is the universal Cavendish gravitational constant
-
Layer by layer, the Earth forms.
It becomes hotter and hotter as more gravitational energy is converted to
heat.
There is a lot of energy largely contained as heat.
4.1.2 Internal Temperature of a Condensing Planet
- The temperature of a material that is containing energy in the form of heat
depends on its heat capacity
- Water is one of the most efficient materials for holding heat

1000 calories heats 1L of water 1C

Heat capacity = 1000cal/L/K

1 calorie = 4.180 Joules of energy
-
-
The heat capacity of rocks and metals is quite a lot less than that of water

Rocks ~ 1000 J/kg/K
If the Earth had retained all the gravitational energy at condensation, its
temperature would have started at 37000C
We now think that the interior of the proto-Earth was quite cool – at 37000C,
all Earth materials would have vapourized, and all atoms and molecules
dissociated into plasma.
We know that the Earth must have condensed quite cool.
It seems that more than 95% of the gravitational energy of condensation was
re-radiated into space.

We have good evidence that Earth condensed cool and subsequently
heated up.
4.2
-
-
-
-
The Accretion and Differentiation of Earth
The terrestrial planets condensed into orbits around the proto-sun about 4.54
billion years ago.
Most of the H and He from that primordial cloud condensed into the central
Sun whose nuclear fires started to burn in its core.
Many lighter elements (H,He) and the elements which do not easily
chemically combine (He, Ne) were not easily contained by the gravitational
field of the condensing Terrestrial Planets, but were easily held by the
enormous Sun.
The Terrestrial Planets (Mercury to Mars, then the asteroids) held the heavier
elements that formed them as rocky-metallic bodies.
Beyond the Terrestrial Planets and the asteroids, the environment was much
cooler in the early solar system

Lighter elements such as H and He, as well as H20 were available to
condense into the formation of gas giants (Jupiter, Saturn, Uranus,
Neptune)
-
Uranus and Neptune – water giants
Pluto – largely water-ice
-
Beyond Pluto – Kuiper Belt, then Oort
Cloud

Oort Cloud – a spherical cloud of
comet-like bodies – snowballs of
water, ice, and original cosmic
dust.
-
Hydrogen is the most abundant element in the solar system

Then He, O, Ne, N, C, Si, Mg…Fl
Implications of Elemental Abundance
- Almost all of the elements listed are produced in the nucleosynthetic fusion
processes
- Nickel and Cobalt (elements with nuclei more massive than Iron) are also
quite abundant

Along with Iron, these are some of the most stable of all nuclei
- Ni and Co are among the first elements to be produced in the r-process of
neutron capture in supernoval explosions
- The elements with massive nuclei – only produced by supernovae.
Earth contains by mass…
- 35% Fe
- 30% O
- 15% Si
- 13% Mg
- Combined in this ratio, these elements form the mineral olivine ([Mg,Fe]SiO4
4.2.1 The Accretion (Growth) of a Terrestrial Planet – Earth
- Birth of Earth – 4.54 billion years ago

Material attracted to a gravitational centre in orbit about the proto-Sun
- Late in the formation (condensation) of Earth, one last large object (size of
Mars?) crashed into proto-Earth

This happened before 4.44 billion years ago years ago

This catapulted up an enormous volume of material which began to orbit
Earth – MOON!
- Oldest zircons = 4.4 billion years old
- Oldest rocks on moon – 4.44 billion years old
The Earth at the Time…
- Differentiation – Denser elements/materials sink to the centre and lighter
ones rise to the surface
- The moon contains much less Fe than Earth

But, the surface rocks on the moon contain more Fe than the surface
rocks on Earth
- Oldest rocks on the Moon are older than the oldest minerals and rocks on
Earth (40 million years older than minerals, 400 million years for rocks)

The Moon solidified quickly after collision

The surface of the Earth remained largely molten for another 200+
million years.
- The greater amount of iron in lunar rocks tells us something about the degree
of differentiation of Earth that had already happened by the time of the
collision
Cold (Slow) Accretion Model
- Cold Accretion Model (Hanks and Andersen)

The original temperature of proto-Earth never exceeded 2000C
anywhere

Temperature of Earth’s core now = 6000C
-
The Cold Accretion Model suggests that the Earth was originally homogenous
Over time, the Earth began to differentiate and heat up
How has the Earth heated up since its initial formation?
1) Heat from Gravitational Compression
- Growing weight of matter to surface caused compression of the interior, which
raises the temperature
2) The Big Whack
- The collision that splashed the moon into orbit
- The earth heated up as a result of this collsion
3) Radioactive Decay
- 26Al decays into 26Mg by emitting a + particle
- Aluminum is very abundant on Earth
- Enough of 26Al condensed into Earth to produce sufficient heat to start the
physical differentiation
- Other isotopes (isotopes with short half-lives) of the lighter elements must
have also condensed with the cloud and contributed to a rather rapid heating
of the Earth

If the cloud had been formed in a supernoval explosion, it is very likely
that there were many short-lived isotopes
-
Radionuclei now contribute significantly to the internal heating of Earth
-
The early heat has not yet been entirely radiated away from the Earth’s
surface – the high internal temperature of the planet still partially derives from
early radioactive heating.
Now, internal heating derives from the continuing geochemical differentiation
and from the release of the latent heat of fusion of iron as the Earth’s core
slowly freezes
- 40K alloys with iron at very high temperatures and pressures – it could also be
contributing to the heating process
Other Arguments…
- The Formation of the Moon

This collision accounts for the Earth’s high angular rotation rate

Such a collision would have caused global and deep melting of the
mantle of the Earth

Earth would have been covered in a deep magma ocean – the melted
iron would sink deeper
-
Why is There Less Iron on the Moon?
There are several possible reasons…
1) Radioactive heating of Earth had already occurred and started the migration
of iron towards the Earth’s core – there would have been less iron on the
outer regions of the Earth
2) The colliding object was poorer in iron than the Earth and diluted the iron
abundance of the splashed-up mix of material
3) Both bodies were already differentiated with an iron core – most mantle layer
materials from both objects contributed to the splash
4) The colliding body was enriched in iron, but left that iron behind to sink deep
into the Earth
Top
Homogenous Accretion
Bottom
Planetesmal Differentiation
-
There is still debate over
whether iron was homogenously
distributed or whether the
planetesmals that formed the
proto-Earth
were
already
differentiated
Debate over the environment in which the solar system was formed…
- Cold, slow accretion model – requires a stable environment
- A more violent and chaotic environment (such as that within the Crab Nebula)
has been argued as more probable

The violent environment could account for the small distance to the
edge of our Kuiper belt and for the fact that Uranus and Neptume have
much less H and He than Jupiter and Saturn

It could also account for the possible partial differentiation of iron before
accretion
-
Crab Nebula – Remnant of a supernoval explosion observed by Chinese
astronomers in 1054

A rapidly rotating neutron star or pulsar exists within the nebular
debris cloud

White dwarf supernoval explosions don’t leave behind cores – this must
have been a massive star supernova.
4.3
-
Geochemical Differentiation of an Earth-like Planet
Many proposals for the earliest condition of Earth

Substantially heated in a post-accretion collision

If accretion had happened sufficiently quickly the Earth would have
retained much of its gravitational energy of accretion

Geochemical differentiation of the Earth occurred from a cold,
homogenous accretion, and that the collision of the Mars-sized object
occurred after differentiation.

We presume the Earth accreted with an internal temperature not
exceeding 2000K anywhere

Surface temperature around 270K

Temperature at depth around 2000K (adiabatic)
As short-lived radionuclei decayed, the Earth heats up, until…
- Iron melts at a shallow zone
- The iron (dense and now liquid) works it way deeper into the Earth, releasing
gravitational potential energy
- Heating occurs as the Earth differentiates, the radionuclei decay, and the
Earth’s inner core slowly freezes (latent heat of fusion of iron)
4.3.1 The Warming Earth and Iron Melting
- Rapidly decaying isotopes in homogenous Earth heat up the Earth – it is
warmed relatively evenly to a temperature that melts iron at a depth of a few
hundred kilometers.
-
The dense iron works its way deeper into Earth, displacing the lighter
minerals and elements, which then rise to shallower depths
Silicates = metal cations with SiO44- anion
Oxides – metal cations with O2- anion
From outside in…
- Atmosphere
- Oceans
- Crust (granite, basalt)
- Silicate Mantle (majority of
Earth’s volume)
- Liquid Iron Outer Core (with
Ni, Co, S, O, C)
- Solid Iron Inner Core (almost
pure Fe)
-
This differentiation of Earth
arose from heat
The Major Source of Interior Heat at Present
1) Compression brings about the freezing of iron

Compression brings most materials to solidify (except water)

At higher pressures, the temperature at which materials solidify or
freeze increases

Iron can be frozen by applying pressure
The pressure freezing of iron in the core accounts for 5-10% of the
heat flowing from the Earth’s interior
This pressure freezing of iron releases its latent heat of fusion
2) Density differentiation in the liquid outer core

Liquid Iron Outer Core = Fe, Ni, Co, and lighter elements

As iron or iron-nickel-cobalt freezes onto the inner-core, the outer core
is depleted of denser materials, thus lowering its overall density

This density differentiation releases gravitational potential energy in the
form of heat
This probably accounts for as much or more release of heat than
the latent heat of fusion iron freezing onto the inner core.
3) Radioactive Decay
- Radioactive elements (U, Th, and K) probably still account for a major source
of internal heating
- J. Marvin Herndon: There is a U-Th fission reactor deep within the frozen
inner core.
4.3.2 Geophysical-Geochemical Differentiation and the Formation of the
Earth’s Core
Iron in Differentiation
-
-
Early Earth lost most of its volatiles (H, He, Ne) – its gravitational force wasn’t
enough to hold these elements
Iron (35% Earth’s mass), Si, Al, and Mg retained because of its density and
lack of volatility
Oxygen (30% of Earth’s mass) retained because it was largely bound with
silicon in silicates (SiO44-)
Iron was probably quite evenly distributed through the body of the proto-Earth
(the cold homogenous accretion model)
As the iron reaches the deep interior of the Earth, pressures begin to
compress it into a solid.
A solid inner core forms surrounded by a still liquid outer core, both largely
composed of iron.
The heat from the pressure-freezing of iron (latent heat of fusion) raises the
temperature of the liquid iron outer core and helps to maintain its liquid state.
The Geodynamo in the Earth’s Core
- Geodynamo – the mechanism that produces the Earth’s magnetic field
-
Heat flows from hot to cold – the core to the surface

Organized convective motion of the core’s fluids
The outer core fluid (largely iron), conducts electricity

Faraday’s Law – If a conductor is moved through a magnetic field, an
electrical current is generated in the conductor.

Ampere’s Law – Electrical current flows in closed loops, and a loop of
current generates a magnetic field.
Magnetic Field  Current  Magnetic Field = feedback loop
-
The Earth’s spin maintains the magnetic field of the Earth
-
Convective motion in the outer forces the feedback loop.
The convection is powered by the heat escaping from inner core (iron
freezing or radioactive decay)
-
This geodynamo (the source of the Earth’s magnetic field) must have started
by 3.5 billion years ago – mineral crystals found in South Africa show remnant
magnetic fields
This also means that Earth must have been differentiated by that time.
-
-
The Moon
- The collision with the Earth splashed up material from the outer shells of the
Earth – this formed the material of the moon
-
The Moon is less rich in iron than Earth

The argument is that the moon is less dense, and that Fe is the only
abundant dense element.
-
At lunar formation, the Earth had already partially differentiated

Much of its Fe was already in the core – didn’t splash up from the
Earth’s outer layers.
4.3.3 Mantle, Crust, the Continents and Oceans
- The silicate mantle extends from a depth of about 3000km almost to the
surface
- The lightest materials which froze out of the magma ocean (which were also
silicates), formed the Earth’s overlying crust.

Granite = Continents

Basalt = Ocean basins (denser)
4.3.3 The Waters of the Oceans and the Atmosphere
Oceans
- There is debate over where ocean water came from

Possibly (and at least partially) through hydrogen and oxygen being
released from the Earth’s interior during differentiation

Cometary bombardment probably brought vast amounts of water to the
Earth’s surface
-
Presently, the Earth’s oceans are saline, with a content of dissolved salts of
about 3.5% by mass.

This salt came from the continents, and was also released by the
Earth’s interior by volcanism (about 4 billions years ago)
Why is there no ocean on the Moon, Mercury, Venus, or Mars?
- Moon and Mars

Gravity wasn’t great enough to hold the water onto its surface
- Mercury and Venus

Too hot for water to remain on the surface and it largely escaped into
space.
Just because there’s no ocean doesn’t mean there’s no water…
- Mars

Did have surface waters and perhaps large oceans, but its gravitational
force wasn’t enough to hold it
- Moon

May be water in the regolith (the mineral soils)
- Mercury

Water ice detected in permanently shaded craters
- Venus

Water exists in the very high atmosphere
Atmosphere
- The lighter elements that don’t condense into liquid at the Earth’s temperature
and pressure form a thin atmosphere enveloping the planet
The atmosphere is composed of…
By volume…
- 78% N2
- 21% O2
- less than 1% Ar
-
A vary amount of water vapour evaporating from surface liquid water and
plant life
An increasing amount of CO2
Traces of NO2 and CH4 – strong infrared absorbers (greenhouse gases)
Infrared radiation absorbers (H2O, CO2, CH4, N2O) maintain the surface
temperature of the Earth about 35K warmer than it would be if none of these
gases were present

The Earth would be hard frozen everywhere without the greenhouse
effect trapping heat near the surface.

2.2 billion years ago – evidence of a completely frozen Earth (only
recovered because CO2 from volcanoes resulted in greenhouse
warming)
5
The Present Solar System
Our Solar System at present…
- 9 planets (all but two have moons)
- A belt of 6000-10000 asteroids mostly between Mars and Jupiter
- Kuiper Belt – 200 million comets spreading from Pluto to 20x Sun’s distance
- Oort Cloud – Several billion comets
5.1
Collisions with Comets and Asteroids?
5.1.1 Comets
- Most comets come from the Kuiper Belt, and sometimes from the Oort Cloud
- Comets enter the inner solar system in elliptical (almost parabolic) orbits
- Speeds as high as 80 000km/hr
-
Halley’s Comet – trapped within the Solar System

Aphelion (farthest distance from Sun) – as far as Pluto

Perihelion (closest distance to Sun) – as close as Venus
5.1.2 Asteroids
- Most are gathered in the orbital ring beyond Mars
- Biggest asteroid = Ceres, then Pallas, Juno, Vesta, Eugenia, Siwa, Kleopatra
The Chicxulub Impact Event
- 64.98 million years ago
- A large asteroid (10km diameter?) struck Earth at Chicxulub, Mexico

Caused an 180km diameter crater
- The atmosphere was filled with dust

Solar insolation was blocked – Earth became very cold

Dinosaurs starved to extinction
- When the dust clouds settled, CO2 from vapourized rocks remained and the
greenhouse shielding heated up the Earth to very high temperatures
- Asteroidal Winter became a Greenhouse Summer
- Only Darwinian winners survived
- Another such event occurred about 35 million years ago
Comet in Russia
- Trees were laid down radially, and the impact was heard 1000km away, but
there is no evidence that any materials reached the ground at impact centre.
-
-
The high velocity of a comet (higher than asteroids because comets have
fallen in from greater distances) causes its explosion in the upper
atmosphere.
The effects on the surface were caused by an atmospheric shock wave from
the explosion of the comet
5.2
What are Comets, Meteoroids, and Asteroids?
Comets
- Sometimes display bright, long tails
- Records of returns of periodic comets has provided us with some of the best
evidence for the known dynamics of the Earth’s and Moon’s orbits and
rotations.
- Comets are the primordial material from which our Solar System condensed.

“Dirty snowballs” – water, methane ice, dusts, gases
- When their orbits bring them close to the sun, the ice vapourizes

This releases dust and gases, which are carried away from the comet in
a direction out from the Sun by a high-speed solar wind
Asteroids and Meteoroids
- Metallic or rocky debris which clutter the Solar System
Asteroid
- Most asteroids are organized into a near-circular orbit about the Sun between
the orbits of Mars and Jupiter (about 420 x 106km)
- Largest known asteroid = Ceres (940km diameter)
- Asteroids are thought to be debris left behind when planetesmal pieces are
unable to combine due to periodic disturbances from Jupiter’s gravitational
field.
Meteoroids
- Smaller pieces of rocky and metallic debris not organized into semi-stable
orbits – smaller asteroids (diameter less than 100m)
- May arise from asteroids colliding with one another

Could be so small that they could fall out of the asteroidal orbital belt.
- When they strike the Earth’s atmosphere, small ones burn up leaving trails of
light – meteors
- Larger meteoroids produce brighter trails – fireballs
Meteorites
- If a meteoroid descends to the surface of Earth, a meteorite is the remnant
that it leaves on Earth
- Their ages date back as much as 4.545 billion years ago (we can’t find any
older material on Earth)
Meteor Showers…
- Perseid Shower (August 12) – Earth crosses debris that is orbiting about the
Sun
- Leonid Shower (mid-November) – Meteoroids derived from a recently
evaporated comet whose debris hasn’t yet evenly distributed itself along the
orbital path. Big ones occur on a period of 33-35 years.
5.2.1 Types of Meteorites
Iron Meteorites
- Mostly iron, some nickel, and a bit of iridium (Ir)

Ir is a siderophile element – likes to be associated with Fe
- Internal structure is made of very large crystals

This suggests that the iron cooled and froze extremely slowly
- Must have formed under a thick layer of insulation (to have been protected
from losing heat more quickly)
- Cooled within the cores of bodies
Stony-Iron Meteorites
- When iron is mixed into Stony meteorites (duh.)
Stony Meteorites
- Small crystal structure – suggests much more rapid cooling
- Probably froze at shallow depths
Chondrites
- Characterized by glassy, bead-like chondrules (mineral grains)
- Glass forms when solids freeze too quickly to organize a crystal structure

Chondrules melted and solidified before they could crystallize
- It is believed that chrondules come from the surface of bodies that are
constantly colliding with other bodies – the surface of the body melts and
solidifies extremely rapidly.
- Carbonaceous chondrites contain amino acids (build protein)
- Enstatite chondrites – thought to represent the most primitive materials that
joined in the primordial cloud that formed the sun

These materials further assembled into our terrestrial planets
Achondrites
- Contain no chondrules
- Look more like igneous rocks
Orbital Dynamics & Kepler’s Law
6
-
-
Planets and asteroids mostly follow elliptical orbits in the ecliptic plane (the
plane of Earth’s orbit about the Sun)
Copernicus and Galileo promoted the heliocentric viewpoint
The Sun is so enormous compared to other masses that it is not affected by
gravitational forces – it’s at a fixed point
Mercury orbits the Sun most quickly
Pluto (then Neptune) has the highest inclination angle
6.1
-
Kepler’s Laws
Kepler (student of Brahe) used Brahe’s observations to establish three laws:
1) The planets move about the Sun in ellipses
2) The orbits sweep out equal areas in equal amounts of time

The planet travels faster when closer to the Sun

Perihelion = closest

Aphelion = farthest
This variation in the speed of orbit has an effect on when the Sun
appears to rise or set (In the Northern Hemisphere, winter solstice
(shortest day, Dec. 21), and summer solstice)
3) The period (p) of the orbit is proportional to the semi-major axis (a)

P 2  a3
6.2
-
Newtonian Mechanics Allow Us to “Weigh” the Sun
Newton recognized that gravity was responsible for the ordered, Keplerian
motion of the planets

The force of gravity contained planets into precisely Keplerian orbits
F = (Gmm)/(r2)
Where G= Newtonian constant of universal gravitation
-
Centripetal Force – the force that opposes gravitational force
F = (mv2)/r
-
For planets to stay in orbit, gravitational force = centripetal force
6.2.1 Is there a Supermassive Black Hole at the Centre of our Milky Way?
- Ghez – Observed the orbits and periods of stars in the direction of where the
galactic centre is known to be
- Ghez and her colleagues determined that there is an invisible supermassive
gravitational centre (mass = 3 x 106 solar masses) at the centre of our Milky
Way

Stars orbit this supermassive black hole
6.2.2 The Orbit of Earth’s Moon: Weighing Earth
Full Moon Phase
- Happens once every lunar month
- The line from Sun to Earth to Moon is nearly in a straight line.
- The face of the Moon (as seen from Earth) is fully lighted by the Sun.
Lunar Eclipse
Moon – Earth – Sun
-
(Full Moon Phase)
Lunar eclipses occur, on average, about twice a year

Can be seen anywhere on the Earth where the Moon can be seen
during the period of shadowing
New Moon Phase
- When the line from the Sun to Moon to Earth is most nearly a straight line.
- If the alignment is just right, we may observe a solar eclipse at New Moon.
Solar Eclipse
Earth – Moon – Sun
(New Moon Phase)
6.2.3 Moon’s Orbit
- Lunar Month

The period between phases of the Moon (ie. period between two Full
Moons)

Also called synodic month

Not the period of the Moon’s orbit about Earth (it’s actually a bit more)
- Siderial Month

The interval between two successive times at which the Moon appears
in the same location relative to the distant stars in the sky

Earth “month” = mean siderial month
- Draconitic Month

The Moon’s orbit is inclined to Earth’s equator by 18.3

The Draconitic Month takes this inclination angle into account.
- Tropic Month

The revolution period between points fixed in declination
6.2.4 Tides
- Gravitational forces caused by the Sun and Moon raise tides
- Not only are there tides raised on the oceans, but also on the solid body of
the Earth
-
-
Lunar Tides – The Moon contributes to about 2/3 of the tidal disortion of the
Earth (on a 24 hour 51 minutes period)
Solar Tides – The Sun contributes to about 1/3 of the tidal distortion (on a 24
hour period – there are 12 hours between successive highs and lows)
Beating of the Tidal Periods

At New Moon and Full Moon phases (when the Sun and Moon are
aligned), tidal forces are at the highest – solar and lunar tides add up

7
When the Sun and Moon are most misaligned, tidal amplitudes are
lowest.
Rotational Dynamics of the Planets and their
Satellites
-
Planets rotate on their own axes (as do the moons of the planets)
-
The Earth wobbles as it rotates

Partly due to the seasonal movement of mass over the Earth (seasonal
wobble)
The Moon librates (swings back and forth) as it orbits the Earth
-
Key Points:
- Jupiter rotates fastest
- Venus rotates slowest
-
-
-
7.1
Venus, Uranus, and Pluto are
inclined more than 90 to their
orbital plane.
Pluto and Uranus rotate with
their axes lying almost in their
orbital plane
Venus has highest inclination
angle (it rotates backwards in
relation to its revolution about
the Sun
Moments of Inertia of the Planets
-
A rotating body tends to stay in rotation unless acted upon by an external
torque.
-
Angular Momentum ( ) = The quality and quantity which determines the
continuing tendency to rotates
Rate of Rotation ( ) = In radians, determines the number of turns per unit
time (rad/s)
-
-
If we look at the example of a spinning bicycle wheel, we find that
(angular
momentum – the tendency of the wheel to keep spinning) is proportional to
the radius of the wheel (a), the mass of the wheel (mw) and the rate of rotation
( )
 mw
 a2

-
The properties inherent to the wheel are assembled as the moment of
inertia (its inertia with respect to rotational motion) of the wheel:
I = mw a2 (mass x radius2)
-
The angular momentum of the spinning wheel is:
L = I
-
If we can determine I for a planet, we can learn something useful about how
mass or density is distributed within

Perfectly spherical planet of uniform density
I = (2/5)(ma2)

Perfectly spherical planet with mass concentrated in thin surface layer
I = (2/3)(ma2)

Perfectly spherical planet with all mass concentrated at its centre
I=0
-
The lower the moment of inertia, the more concentrated the mass it at the
centre.
I < 0.4ma2  mass concentrated towards centre
I = 0.4ma2  uniform density
I > 0.4ma2  mass concentrated towards surface
-
The lower the moment of inertia, the more mass is concentrated at the depth.
7.2
Determining I by Astronomic Observations
-
We can neither brake nor accelerate the rotation of a planet in order to
measure its moment of inertia – this can be determined by observing a
planet’s response to astronomical forces and torques.
-
Inertial space = background reference (frame of reference) provided by the
phenomenon of inertia)
When a body rotates on an axle, it maintains its angular momentum vector
fixed in inertial space

The axis of rotation (the centre around which the Earth spins) orbits the
Sun in an ellipse.

The axis of rotation is fixed in inertial
-
-
Torque = The angular force that causes a change in rotation.
Torque is the force that is, at a distance, applied to the axis of rotation
When a torque is applied to the axle, a reactive torque exactly 90 to applied
torque results in maintaining torque balances.
The Moon applies a twisting torque to the axle upon which the Earth rotates –
the reaction torque causes the Earth’s axis to move in a direction 90 to the
applied torque.
The Earth is a slightly flattened ellipsoid (it’s equatorial radius is greater than
its polar radius)

-
-
-
The Moon’s present location is not aligned with the equator of the Earth.
On the side that is closer to the moon, there is a greater gravitational pull.
-
-
- The rate of precession,
-
p
The Moon produces a torque
on the axis of rotation.
In reaction to this torque, the
Earth precesses (tilts its axis)
to about 23.5
Earth will keep tilting. In 12 000
years it will be tilted closer to
40
is proportional to the applied torque twisting the
axle of rotation.
p is inversely proportional to the angular momentum of the Earth’s spin,
which is proportional to I.

Example: As a top spins and begins to topple…
The rate of precession increases
Applied torque increases
Angular momentum decreases
7.2.1 The Moment of Inertia for Earth (Iz)
- By analyzing the gravitational pull-induced torques on the Earth due to the
Moon, as well as the rate of precession of Earth, we were able to determine
the Earth’s moment of inertia about the rotation axis.
- Iz = 0.3308 ma2 (*note: Iz < 0.4ma2)
7.2.2 The Moment of Inertia for the Other Terrestrial Planets
- Only Earth has a massive enough Moon to apply a significant torque on its
rotation.
Mercury and Venus
- No moons
- Rotate very slowly (thus, torques from Sun and nearby planets are very small
and produce enormously long precession periods)
- Venus is essentially invisible from Earth because of Venus’ thick atmospheric
covering clouds
- We don’t have good measure of I for these two planets
Mars
- Rotates quickly (every 25 hours)
- Small Moons (Deimos and Phobos) produce immeasurably small torques
- Jupiter is close enough to occasionally apply significant torques which do
present a precession.
- Measurements recently made by looking at the position of Viking Landers
(1970s) and comparing it to the position of the Mars Pathfinder Lander
Another Way to Determine I…
- Measure the gravity field about a planet (observed by analyzing the
disturbance that the field has on satellites orbiting the planet)
- Accomplished recently for Mars and Venus
- Mars Iz = 0.366ma2
- Venus Iz = 0.33ma2
The Moon
- Rotates slowly but is under enormous gravitational torque applied by the
Earth
- Libration = apparent back and forth movement of the Moon due to this
gravitational pull
- Moon’s Iz = 0.394ma2
Mercury
- Can only obtain estimates – we haven’t yet mapped its gravitational field
- Iz = approx. 0.33ma2 – 0.39ma2
- This is a reasonable guess based on geochemical knowledge based on
surface geological materials.
Iz for the Terrestrial Planets
Venus = approx. 0.33ma2
Earth = 0.3308ma2
Mercury = approx. 0.33ma2 – 0.39ma2
Mars = 0.366ma2
Moon = 0.394ma2
***Remember: the lower the moment of inertia the more mass is concentrated at
the depth (less than 0.4ma2 – the mass is more concentrated at the centre)
-
-
-
For the Earth, Moon, and Mars, we have excellent evidence that density is
concentrated at the centre.

This supports the belief that Earth is mostly iron at the core

The Moon’s moment of inertia suggests that it contains less iron, and
that there is less compression to high density at depth

Mars supports an iron core as well.
It is believed that Venus’ interior is like that of Earth – it’s believed that the two
planets were formed of similar materials
Mercury is almost as dense (on average) as Earth

Mercury is much smaller than Earth – this suggests that Mercury
contains a large iron core that perhaps accounts for 50%
Earth, among all the planets, has the greatest average density
All the terrestrial planets are geophysically and geochemically differentiated

On Earth, the process of differentiation continues to the present.
8 Geophysical Processes in Planetary Differentiation
-
Geophysics = The physics of the Earth
Geochemistry = The chemistry of the Earth
Cosmochemistry = The science of chemistry (applied to the Universe as a
whole)
Selenophysics = The physics of the Moon
-
The differentiation of the terrestrial planets, asteroids, and gas giants
depends upon both geophysical and geochemical processes.
-
The geophysical processes involved in differentiation depend largely upon the
different densities of materials (can be inherent, or dependent on
temperature)

Buoyant materials rise, dense materials sink

The buoyancy of mantle materials and the fluid core (caused by
temperature) is largely due to the latent heat of fusion (freezing of the
inner iron core)
7.1
-
The Internal Structure of Earth, Moon, and the Terrestrial
Planets
Earth, Moon and Mars are all denser at the centre

We infer the same for Venus and Mercury (no clear evidence)
Typical Structure of a Typical Terrestrial Planet / The Moon (Earth as Model)
- Thin outer crust of lighter silicates (granitic) and denser silicates (basaltic)
-
-
Deep mantle of silicates (denser with depth)
Core (largely composed of Fe, Ni, and some lighter alloys)

Core may be frozen solid (Earth and Mercury), or have an overlying
melted shell
If a planet’s gravity is strong enough to hold volatiles against evaporation into
space, it may have an atmosphere (Earth has atmosphere and ocean)
The crust and mantle is enriched in Ca, Na, K, Al, Si, and O (they have little
Fe and Ni, which have mostly sunk into the core)
-
-
-
-
By mass, Fe is most
abundant on Earth,
then O, then Si
The crust consists
mostly of O, then Si.
Oxygen (like He and
H) was not easily
contained to Earth
by its gravitational
pull during the early
evolution
Sulfur and Potassium seem to have significantly lower abundances on Earth
– this could be because they alloy with iron in the fluid outer core.
The core has some lighter alloying (S, O, C)
Structurally, the Earth is now well differentiated…
- Crust (0 – 33km)
- Mantle (33 – 2900km)

Comprises about 85% of the Earth’s volume

Composed mostly of silicates with metal oxides (MgO, FeO, etc.)
- Core (2900km – centre)

90% Fe, 5% Ni, and maybe C, Si, O, S, and H.

35% of the Earth’s total mass is at the core

The inner core is almost pure Fe.
The Moon
- Has less iron that Earth.
- Relatively smaller iron core.
Venus
- Similar size to Earth
- Iron core (probably 35-40% of totally mass)
- Probably (at depth) similarly differentiated to Earth, with an iron core (either
frozen or liquid)
Mars
- Iron core (30+% mass) overlain by silicate material
Mercury
- Large mass for its size
- Probably about 50% iron
Earth
- Highest density of all the planets

Compression of overlying materials squeezing iron core to high density.
The Gas Giants (Jupiter, Saturn, Uranus, Neptune)
- These planets are far enough from the Sun and massive enough to have high
gravitational fields.
- These planets contain the lightest elements of condensation
Jupiter
- Small core of rocky silicate material
- Deep iron core that is perhaps the size of the Earth
- Deep iron core is surrounded by a ocean of liquid metallic hydrogen, which in
turn is surrounded by liquid molecular H2-hydrogen.
- Thick atmosphere of methane (with possible some water and CO2)
- More than 70% of mass is H
- 25% of mass is He
Saturn
- Composed similarly to Jupiter over an Earth-like central zone
Uranus and Neptune
- Silicate cores overlain by an atmosphere of volatiles
- Atmosphere has much more H2O, CH4 and N2 than Jupiter and Saturn
Pluto
- Probably most water-ice
The Earth’s Crust
- The crust is differentiated over the surface into continents and ocean basins
- Continents = low density granitic materials (average elevation = 0.9km)
- Ocean basins = higher density basaltic materials (average depth = 4.5km)
-
How do we account for these large-scale differences in elevation? Isostasy
8.2
Isostasy
-
Explains the differences of elevation on the Earth’s surface
Explains the reason for the depth of the ocean floors and the generally highstanding continents
-
J.H. Pratt (1855) – light materials “float” higher than dense materials, and are
thus at a higher elevation.
G.B Airy – The high-standing regions have deeper roots, but density is
similar to lower standing regions
-

-
Accounts for the height of icebergs – the deeper the root, the higher the
iceberg
If the rocks of the crust of the Earth are floating, then they must be floating on
a fluid mantle.
8.2.1 Post-Glacial Rebound and Isostatic Adjustment
- It has been observed that the coasts of Sweden and Finland in the Gulf of
Bothnia are slowly rising relative to sea level.

In the last 5000 years, this coast of Sweden and Finland has risen by
more than 100m.
Post Glacial Rebound
- Up until about 7000 years ago, this region was overlain by an the
Fennoscandia icesheet to a depth of over 3km.
- For 30 000 years, it depressed the Earth’s lithosphere.
-
The region had
come into isostatic
adjustment with the
load
-
About 10 000 years ago, this ice sheet began to melt quickly.
The heavy load of ice had depressed the underlying continent.
-
Relieved of the heavy ice load, the whole continental region began to rebound
– it bounced back up
-
The same happened with the Laurentian icesheet over Hudson’s Bay.
8.2.2 Viscosity of the Fluid Mantle
- The fluid mantle flows extremely slowly

It has a very high viscosity (self-stickiness, )
- Water -  = 10-3 Pas
- Maple syrup -  = 2.5 Pas
- Glass -  = 1012 Pas
-
The viscosity of the mantle fluid = 1020 – 1023 Pas

Flows extremely slowly
On short time scales, the mantle is a very hard solid (we can observe its
fluidity over a time scale of hundreds or thousands of years)
-
The fluid nature of the mantle allows for the convection process and the
continuing geophysical differentiation of the planet.
8.3
Mantle Convection
-
The continents move across the surface of the Earth
Observations by several scientists seem to come to the conclusion that the
continents had once been joined.
-
Expanding Earth Hypothesis – The theory that Earth’s volume had
increased, and that the earlier surface was no longer large enough to cover
the surface of the greater volume.
8.3.1 Earth’s Mantle is Fluid
- The Earth’s fluid mantle can be brought into circulation if the Earth’s interior
temperature is sufficiently hot
- Post-glacial rebound suggests that the mantle is fluid on long time scales
- The mantle (as a fluid) can efficient transport heath from depth by a process
called convection
- Convection drives the drift.
8.3.2 The Adiabatic Gradient
- If we compress an object very quickly, it doesn’t have time to come to thermal
equilibrium

If we insulate it so that heat energy can’t flow in or it, it can’t come to
equilibrium – we are just compressing the heat into a smaller and
smaller volume
- Adiabatic Compression = the compression of an object that conserves the
entropy of the material.
-
-
Pressure acts evenly in all directions
-
The dimensions of the cube shorten
– it responds to the pressure stress
with a deforming strain.
-
If no energy is allowed to escape
from the volume, the energy in the
volume is also compressed (and it
becomes hotter)
The change in temperature is proportional to the change in pressure
T  P
T  T
T  1/p
-
The constant of proportionality

If we were to heat the gas under constant pressure, it would want to
expand as
T  p
-
where p = density of the material
where p is the volumetric expansion
ratio per unit of temperature (K)
Heat capacity at constant pressure (CHp) = its capacity to hold heat.

Determines what temperature increase would be caused by the entering
of heat – the material will more or less expand.

For a material of very high CHp, it warms less and expands less.
- When we compress the gas under some increase of pressure (P), its
temperature rise (T) is greater according to its volumetric expansion ratio
(p), and lesser according to 1/CHp
T  (p)/(CHp )
-
Pressure within the Earth’s mantle increases with depth.
If we know the temperature at the top of the mantle and we know the
thermodynamic constants appropriate to mantle material, we can determine
the adiabatic temperature profile to the base of the mantle.
-
Because the mantle is fluid-like, the mantle is in constant convection – this
brings the mantle temperature toward the adiabatic temperature profile.
Heat can be carried through fluids by convection but can only be carried
through solids by conduct.
The outer elastic shell of the Earth, the lithosphere, acts as a heatconductive layer with very poor heat conductivity.

It acts as insulation between the cold exterior of the Earth and its hot
interior.

The temperature gradient is very steep (300K at surface to 1500K at
base)
-
8.3.3 The Adiabatic Gradient and Convection
-
Adiabatic temperature profile – tells us that pressure and temperature
within the mantle are related


-
If we move a volume of material upwards to lower pressure, its
temperature decreases.
The material expands to remain in exact temperature equilibrium with
the lower pressure.
Adiabatic Temperature Profile = The theoretical temperature of the
mantle/crust at certain pressures
Temperature Gradient = The actual temperature within the mantle/crust
The temperature gradient is much steeper than the adiabatic
Because Earth’s mantle is in such vigorous convection, the actual
temperature may not correspond with the theoretical temperature (as outlined
in the adiabatic temperature profile
-
-
As a volume of material
moves upwards, it
follows the slope of the
adiabat (the dotted black
lines)
In the crust and the part of the mantle that makes up the lithosphere, the
temperature gradient is much steeper adiabatic
In this zone, conduction transports heat (not convection)
The actual temperature profile (green line) is a little steeper than the adiabatic
temperature profile (red line)
“Steeper” = temperature increases more rapidly towards depth than would be
predicted by the adiabatic gradient.
-
-
Adiabat – the adiabatic gradient that a volume of material follows as it is
moved upwards and cools.

But as the material moves upwards, it finds itself warmer than the other
material at its new (shallower) depth and lower pressure

It has expanded in comparison with local material, but it is warmer,
therefore, its density is lower (it is relatively buoyant)
If we move a volume of material upwards through a temperature gradient
which is steeper than the adiabatic, it wants to move even further upwards
due to its buoyancy

The reverse is true for a volume of material moving towards greater
depth. It wants to continue sinking.
***Remember: As a volume of material moves upwards, it cools following the
adiabatic gradient (the adiabat)
Convection
- The upwards and downwards movement of material as its temperature and
density change
- The whole material of the mantle is set into continuing motion as heat is
carried from depth towards the surface by the convecting fluid
- The Earth’s mantle has been convecting for the past 4.5 billion years.
-
-
The motion would eventually stop if the actual temperature profile cooled and
settled down to the adiabatic gradient.

When no more excess heat maintains the steep temperature gradient,
convection will cease.
Much of the heat emanating from the core to the base of the mantle is being
released by the latent fusion of iron.

The rest is the residual heat from earlier decay of radionuclei and from
the gravitational potential energy released as heat during the
differentiation of the Earth.
-
When the whole core is frozen, convection of the mantle will stop
-
It is probable that the mantles of Mercury, Mars and the Moon are no longer
convecting
8.3.4 The Rayleigh Number
-
The vigour of the convection is determined by a ratio of forces, the Rayleigh
Number:
R = (buoyant forces)/(viscous forces)
-
The buoyant forces push the convection while the viscous (sticky) forces slow
them down.
-
Leon Knopoff (1964)

When R > 2380, heat transfer is primarily in the form of conduction
Approximates the geometry of Earth’s mantle
The relatively vigorous convection of the Earth’s mantle would require that
R > 105
The evidence for this convection is the rapid motion of tectonic plates across
the surface of the Earth
Because the motion is rapid, R must be very high and the buoyant forces
must dominate the viscous forces that are resisting convection.
-
-
-
-
The buoyancy forces are high when the deep interior of the Earth is at
temperatures much higher than that accounted for by adiabatic compression
(when the temperature gradient is high)
While the temperature of the core-mantle boundary needs to be only 2100K
(under an adiabatic gradient), we believe the actual temperature to be around
2800-3200K.
-
8.4
The Earth will convect for a long time
into the future.
Mantle Convection and Plate Tectonics
-
Starting in the 1960s, geophysicists started measuring the rates of motion of
various plates floating over the Earth’s surface
8.4.1 Seismic and Tectonic Boundaries
Seismology = The geophysical science that is concerned with earthquakes and
the radiating wave fields that earthquakes produce.
Tectonics = The geological science that is concerned with the continuing slow
motions of the mantle and lithospheric plates
The Seismic Boundaries
- Crust = The outer skin, typically 30km thick

Relatively low velocity for seismic sound waves (P-Waves)
P-Waves are one of the two types of elastic body waves that are
produced by earthquakes and recorded by seismometers.
P-Waves have the highest velocity of all seismic waves

Its base is characterized by a sharp increase in the P-wave velocity to
about 8km/s
- Mantle = Comprises most of the volume of the Earth (silicate, rocky)

Layered structure that affects seismic wave velocities

The structure is caused by phase changes of the mantle minerals as
they are compressed at depth into higher density forms.

Most distinct boundaries…
About 440km where olivine becomes compressed into the spinel
structure
About 670km where spinel becomes compressed into perovskite,
then periclase
- Outer Core = A liquid mix of Fe, Ni, and some S, O, C
- Inner Core = At the centre is frozen, almost pure iron, inner core

Radius is 1222km
-
The outer and inner core contain about 35% of the Earth’s total mass in less
than 1/8 of its volume.
The Tectonic Boundaries
- Lithosphere = The outer elastic shell of Earth (includes crust and upper
mantle)

Elasticity = A property of a material where it deforms under stress, but
then returns to its original shape when the stress is removed.

Appears to be quite solid

Lithospheric plates (between 0km – 100km) are on the surface due to
convective forces

The layer is elastic on long time scales. The underlying asthenosphere
is plastic on short time scales
- Asthenosphere = Right below the lithosphere (deeper part of the upper
mantle). Plastic on short time scales

Plasticity = A property of a material where once a force is applied to it,
the change is non-reversible

This layer is the softest or most easily deformable and flowing region of
the upper mantle.

Appears to be extremely hard over short periods of time.
- Mesosphere – Below about 700km – the mantle becomes much less viscous
by a factor of about 100
- Core- Mantle Boundary Zone – A mixed zone of varying thickness (10200km thick) that lies at the base of the mantle

May be lithospheric materials that have been subducted to depth by
convection
-
The rigid lithosphere is broken up into 11 major and several minor plates

They are floating on the athenosphere, driven into circulation by
convection.
8.4.2 Earthquakes, Volcanoes, and Tectonics
- Earthquakes and volcanoes seem to be assembled along lines around the
world, especially rimming the Pacific Ocean (The “Ring of Fire”)
- “Ring of Fire” – characterizes boundaries between colliding and laterally
sliding tectonic plates.
Plate Margins
Convergent Margins
- The “Ring of Fire” follows zones where an oceanic plate was converging
towards a continental plate and subducting (moving downwards under the
other plate)
- Most of the largest earthquakes occur along the converging margins

West Coast (BC, Washington, Oregon) forms a subduction zone of
convergence where the Juan de Fuca plate is being drawn down under
the North America plate
Cascadia Megathrust (1700) – largest earthquake ever known to
have occurred on Earth
Divergent Margins, Spreading Ridges
- Occurs when two tectonic plates are moving away from one another
- Mid-Atlantic Ridge – the centre of spreading of the Atlantic Ocean
- When the plates are pulled apart, material from the mantle rises due to
convection, and a ridge (a sort of underwater mountain range) is formed.
- Most divergent zones are found in mid-ocean, though soon (ie. East Africa
Rise) are spreading continents apart
- Earthquakes that occur along spreading ridges tend to be smaller than those
on convergent margins
- Volcanism of the ridges is characterized by dense basaltic magmas – this is
different from those volcanoes rimming the Pacific.
Transform Margins
- When the edges of plates slip against each other
- The San Andreas fault (California) and the Anatollian fault (Northern
Turkey) are both dextral transform faults (where, when looking across at the
other fault, one sees a motion to the “right” – sinistral fault = opposite
direction)
- Volcanoes can also be associated with transform faults
- Many transform boundaries are locked in tension before suddenly releasing,
and causing earthquakes
Hot Spots
- Hot spots are locations on the Earth’s surface that have experienced active
volcanism for a long period of time
- There are isolated volcanoes which are neither associated with ridges or rims
- Hawaii

Hawaii appears to be caused by mantle material rising and penetrating
the surface of the Earth

As a tectonic plate moved across a fixed hot sot, the Hawaiian Islands
were slowly formed
- Most hot spot volcanoes are basaltic because they erupt through oceanic
lithosphere
-
All of these motions suggest that the underlying mantle is convecting
There is a variation of density within the mantle due to variations in the
geochemistry and mineralogy of the mantle at different depths
-
The power driving the convection heat engine of Earth comes largely from the
latent heat of fusion given up at the inner core
In carrying heat to the surface, the mantle is forced into convective circulation,
which pushes the floating lithospheric plates across the Earth’s surface
7.4.3 Earthquakes, Volcanoes, and Tectonics on Other Planets and the
Moon
-
-
“Moonquakes” have been observed

Shallow fractures that are thought to be left-over stress being relieved
through faulting

At depth, due to the monthly cycle of tidal stressing as the Moon’s orbit
about the Earth varies in distance.
No “Venusquakes” have been observed
One small “Marsquake” has been recorded

Thought to have been caused by a fracture due to long-standing
stresses in the crust of Mars, or due to a landslide
-
Lunar seismic activity doesn’t necessarily suggest tectonics
There is no evidence of seismic activity on Venus or Mars
-
Active volcanism is another measure of tectonic activity
Volcanic activity has only been observed on Earth, Io and Europa (two of
Jupiter’s Moons), and Triton (Saturn’s Moon)
Io is the most volcanically active body discovered so far in our solar system
Europa’s surface of water-ice shows thousands of moving plates
-
8.5
Measuring Mantle Circulation
8.5.1 Heat Flow
- The elastic lithosphere rides over the fluid asthenosphere
- There is essentially no change in mineralogy from the base of the lithosphere
into the upper asthenosphere
-
-
-
What distinguishes the lithosphere from the asthenosphere is a temperature
at which the viscosity (stickiness) of the mantle changes quite abruptly
The lithosphere appears elastic on long time scales, while the asthenosphere
appears liquid
Under continents, the lithosphere is thick (> 100km)

Heat flow through the continents is lessened by the thick layer of
lithospheric material (less than 50mW per m2)
Through the thinner lithosphere (around 50km), heat flows more easily to the
surface (about 100mW per m2)
-
The rate of heat flow to the surface can be used to determine the thickness of
the lithosphere
8.5.2 Loads on the Lithosphere
- When a load of mass lies on the lithosphere, the load depresses the
lithosphere directly under the load, but allows it to bulge upwards at some
distance lateral from the load

ie. Hawaiian Island Chain
8.5.3 Paleomagnetic Evidence for Plate Motions
- The Earth’s magnetic field looks like one produced by a strong magnet buried
at great depth (dipolar magnetic moment – the magnetic force has two
poles)
-
On the surface of the Earth, the N on a compass tends to point geographically
northwards.

Actually it is pointing towards the North Geomagnetic Pole (now in the
Arctic Islands of Canada)
-
The Earth’s geomagnetic field is generated by the geodynamo
While the magnetic poles are almost always nearly aligned with the rotation
axis, their polarity changes irregularly.
Magnetization of Rocks
- When a hot rock cools, whatever natural magnetic field exists at that time and
place is “frozen into the rock”

The rock takes on the magnetic field direction which holds at their time
of freezing
- When sedimentary rocks assemble at water bottom, small grains of the
mineral magnetite assemble into the rock

The grains of magnetite tend to align with the magnetic field

Sedimentary rocks can also show the direction of magnetization at the
time of their sedimentation
-
The polar orientation of the magnetic field is frozen into rocks as they cool or
sediment
-
Polar Wander: By mapping the direction towards geomagnetic North in
different rocks, we can determine how the magnetic pole has wander over
time.
Magnetic “striping” of the ocean basins: When rocks come from the
interior of the Earth and solidify along ocean ridges, they quickly cool and
-
take on the momentary direction and intensity of the Earth’s geomagnetic
field.

At a divergent ridge, magma is brought to the surface and freezes,
capturing the magnetic field’s orientation
8.5.4 Motion of Plates over “Plumes”, “Hotspot Volcanism”
- At several places on the Earth, the lithosphere seems to be penetrated at hot
spots, where magma issues to the surface from somewhere in the mantle
- Hot spots are seemingly fixed in position.
-
Where a plume penetrates the lithosphere, its magma forms great shield
volcanoes (ie. Mauna Loa in Hawaii)
-
It is believed that the Hawaiian islands were formed as the Pacific Plate was
being carried northwestward

Thus the islands get older as we move NorthWest.
8.5.5 Age and Depth of the Ocean Floor
- Ocean basins are oldest when farthest from the ridge axis at which they were
generated.
- Two areas symmetrically apart from the ridge at which they were generated
are the same age.
The Deepest Part of the Ocean Basins are the Oldest…
- As the oceanic lithosphere ages and cools, it freezes.
- The frozen lithosphere is denser than the magma that is rising up, so it
“floats” deeper than the rising magma.
-
Nowhere is there ocean basin older than 220 million years
The ocean basins subduct into the mantle at convergence
8.5.6 Other Measures of Plate Motions
- Geometric surveying over time can establish spreading and convergence
rates
Geodetic methods that have been applied to the problem…
- Radio-Interferometry

By looking at quasars (the most distant objects) with radio telescopes
which are sitting on different tectonic plates, the change in baseline
distance between the telescopes could be measured
- GPS (Geographical Positioning Systems)

In the Cold War, the US and USSR (as well as some other countries)
launched a series of satellites carrying extremely precise atomic clocks
which allowed for precise geographical positioning
8.5.7 Continents and Ocean Basins
-
When a plate is diverging in one area, it must be converging at another point
– the Earth is not growing in size
-
While the oceanic plates are relatively young, the continents are generally
very old.

The continents, never having been recycled into the interior of the Earth,
have probably floated on the surface for the past 4 billion years

The oceanic plates are being continuously recycled through the Earth’s
interior (mostly through subduction)
8.5.8 Earthquakes and Seismotectonics
- Very deep earthquakes occur at many places on Earth
- They commonly follow a trend to depth that makes an angle of about 45
degrees with the surface.
Earthquakes on Subduction Zones
- Earthquakes in Benioff zones (zones of deep-trending earthquakes) occur
within a subducting lithospheric plate
- Focus = central point from which an earthquake starts faulting
-
-
Tonga Trench (ocean plate subducts beneath another oceanic plate)

One area where deep-focus earthquakes lie

Along the Tonga trench, the Pacific Plate, converges upon the
Australian-Indian Plate (upon which Australia floats)

The Pacific Plate is sinking at an angle of about 45 degrees

A deep trench forms at the line of contact along the ocean between the
plates.

Along this 45 degree contact surface, very large earthquakes occur as
the plates stick, then slip at irregular intervals of time.
South America (ocean plate subducts beneath a continental plate)

On the west coast of South America

The Nazca Plate is subducting under the South American Plate
Largest earthquake every instrumentally recorded on Earth
occurred along this convergent margin (magnitude = 9.5)
Tidal waves (tsunami) spread away from that earthquake, traveling
across the Pacific Ocean to Japan, reflected off Japan’s coastline,
then traveled back across the Pacific to South and North America.
The Cascadia Subduction Zone
- A convergent margin which extends along the west coast from central
Vancouver Island to about mid-Oregon

Believed to have been the location of the largest event known to have
occurred on Earth (great mega-thrust earthquake of 1700)

Believed to have a magnitude approaching 10
Volcanism on Subduction Zones
- Subduction zones are characterized by frequent and very large earthquakes,
as well as by volcanism
- When a plate is subducting, light continental granitic materials which have
slowly sedimented on the ocean floor are carried down to depth with the
plate.
- These granitic materials eventually melt and break their way back to the
surface, causing volcanoes.
-
Tonga Island is formed of such volcanism
-
These volcanoes are generally not characterized by dense basaltic rock, but
of low density continental materials

Mt. Baker, Garibaldi, Mt. St. Helens, etc. are all such volcanoes

They are formed of the convergence of the Explorer Plate with the North
American Plate
Japan, Taiwan, Borneo, and the Indonesian islands are all volcanically
formed near or along convergent margins.
-
-
Wherever you find volcanoes issuing andesitic or rhyolitic rock, and
wherever you find earthquakes aligning along Benioff zones, we have
convergent margins
Earthquakes at Continent-Continent Collisions
- India vs. Asia

The low density continental material of the Indian subcontinent couldn’t
be brought to sink into the mantle because it is so light and buoyant.

The Indian subcontinent plowed into the Asian and pushed up the
Himalayan mountains and plateau (highest elevation region on Earth)
The Himalayas are still being pushed up, and then eroded down by
the work of water and ice (this will continue for another 100 million
or so years)
-
Large earthquakes occur as a consequence of continent-continent collison
Earthquakes on Transform Margins
- Transform faults often arise along long convergence margins

San Andreas Fault (though California)
- Anatolian Fault

Across northern Turkey and right through Istanbul

Long fault lines of great shallow earthquake activity
-
Earthquakes on transforms faults are caused by the rapid release of tension
as the faults are locked together
9
-
-
-
Seismology and the Internal Structure of Earth and
the Planets
Except for the deepest deep-focus events, earthquakes are caused by a
fracturing or faulting of the elastic rock of the lithosphere
Deep earthquakes could be caused by extremely rapid mineralogical changes
of phase (ie. pressure driven change of olivine into spinel)
Focus = The source of the earthquake

The point at which an earthquake starts to fracture or at which
mineralogical phase begins to change
Epicentre = The place on the surface directly above the focus
Fault Plane = The surface over which the fracture occurs
Seismic Waves = The waves that issue from an earthquake

Can help seismologists to determine the plane of faulting, the amount of
direction of slip, and the amount of energy released in the event
9.1
-
-
Seismic Waves
Much of our knowledge of Earth’s interior has been obtained by studying
seismic waves generated by earthquakes as they travel out through the
interior and across the surface
Seismic waves are detected by seismographs
The records of earthquake waves are called seismograms
P-waves (Primary Waves)
- The seismic waves that travel most rapidly from a source to a distant
seismograph receiver
- P-waves move through the Earth in a slinky-like motion
- P-waves are equivalent to sound waves which travel through the body of the
Earth (called body waves)
S-Waves (Secondary Waves)
- A slower traveling body wave type that is also generated by earthquakes
- S-waves follow the P-wave along a path through the interior of the Earth
- The S-wave carries a motion which is oscillatory (perpendicular to the
direction of the wave)

Unlike the P-wave, which moves in a slinky-like motion, the S-wave
moves up and down (like a rope)
- S-waves do not travel through liquids

This led to the discovery that the outer core is liquid and that the inner
core is solid

Though these waves don’t exist in the outer core, they do in the inner
core
-
-
-
Using these body waves and measuring their travel times from distance
earthquakes to seismographs, one can obtain the average velocity of the
waves
The body waves that travel from an earthquake to a nearby seismograph
don’t travel very deeply into the Earth – ones that travel greater distances do.

Those that travel further sample the velocities of materials at greater
depth within the Earth
From body waves, seismologists have obtained a good idea of the elastic
properties of the interior of the Earth
Surface Waves
- Earthquakes also produce disturbances on the surface which cause waves to
travel out over the surface of the Earth
- Rayleigh Waves – Travel out over the surface like ripples travel across the
surface of water (though it is a bit different – ripples on water are driven by
gravity forces, Rayleigh waves are dominated by elastic forces)

With Rayleigh waves traveling across solid surfaces, elastic forces
dominate

The wave represents an elastic distortion of the surface which is later
restored

Rayleigh wave velocities are typically the same as the average S-wave
Motion on the Surface…
- Gravity Waves on Water

The waves are traveling in a forward direction at the crest and in the
reverse direction in troughs
- Rayleigh Waves

The crests are moving backwards as the wave moves under it

The surface of the troughs move forward.
-
Love Waves – produce often-strong horizontal motions of the surface

Are mostly normal (to right and left) in the direction of travel of the wave


-
-
Love waves are really S-waves that have become trapped by the
layered structure of the Earth near the surface
Love wave velocities are typically faster than Rayleigh waves
Surface waves are scaled in amplitudes according to the strength of an
earthquake

A larger earthquake typically causes larger amplitude Rayleigh waves.
But, earthquakes can occur at great depths, and disturb the surface less
The surface waves not only depend on the size of the earthquake, but also on
their depth
9.2
Magnitude Scales for Earthquakes
The Richter Scale
- For every increase in magnitude by 1, the amplitude increases by 10x
- For every increase in magnitude by 1, the surface wave energy increases by
100x
- For every increase in magnitude by 1, the source energy increase by 63x
-
As earthquakes become larger, they spread their energy over an ever
broader range of frequencies
9.3
-
-
Elastic Properties of Earth’s Interior
P-wave velocity generally increases with depth in the Earth
-
P-waves travel faster in more
rigid materials
-
The velocity of P-waves
increase towards the mantle as
the compressed materials
within the mantle become
increasingly more rigid
-
The velocity drops drops at the
core-mantle boundary due to
the outer core’s fluidity
Because the solid inner core contributes some rigidity, the velocity jumps
back up.
-
-
-
S-wave velocity depends only upon rigidity and density (the more rigid the
medium and the lower the density, the faster the s-wave)
-
In the outer core, the rigidity
falls to 0. Therefore, the
velocity of the S-wave falls to
0 as well
-
S-waves do travel in the inner
core – this means that this
region has rigidity.
Density
Both P-waves and S-waves have velocities which are inversely proportional
to the square-root of local material density

The higher the density, the slower the wave
The density of the Earth increases with depth
Rigidity (shear modulus)
- Rocks are solid and resistant to shearing (breaking off) stresses
- Rigidity is a measure of hardness
- Liquids have no resistance to shearing stresses
- The viscosity of the mantle brings the mantle to respond with the nature of a
solid to seismic shear waves (but ONLY on time scales of a few seconds)
-
-
From the base of the lithosphere to the asthenosphere, the rigidity seems to
decrease (the most “fluid” region of the mantle has a rigidity almost as great
as a diamond)
Rigidity increases through the asthenosphere to the base of the mantle
Deep in the mantle is very rigid (though on long time scales, it appears to be
fluid)
In the outer iron core, the rigidity drops to zero (low viscosity)
The inner core is solid
Incompressibility (bulk modulus)
- The measure of a material’s resistance to changing its volume under
pressure.
- Gases are very compressible (therefore small incompressibility)
- Hard rocks have high incompressibility
Pressure
- Pressure increases with depth
8.4
Other Bodies in the Solar System and Beyond
-
The Apollo 11 (1969)

Mission placed magnetometers and seismographs on the moon.

First seismographs installed on a body beyond Earth
-
Viking I and II (1976)

Viking I’s seismographs failed

Viking II only recorded one possible marsquake

Not enough information about the solid or fluid mechanical properties of
the planet
-
Venera Landers

Both landers failed due to Venus’ incredible heat
-
Helioseismology = Used to determine interior properties of the Sun
Explosions on the Sun set off trains of “seismic” waves through the Sun
10
Mineralogy and Geological History of the Terrestrial
Planets
10.1 Mineralogy and Minerals
-
-
-
-
Molecules – stoichiometrically precise relative composition of elements (ie.
water has 2 atoms of H for every atom of O)
Minerals – do not follow a precise stoichiometry.

More like structures in which certain atoms can be easily secured
The minerals compose rocks in the Earth’s crust are typically silicates,
oxides, and sulfides with various metallic cations.
Silicates and oxides form lithophiles (“like to be” rock)
Chalcophiles are metallic elements that combine easily with sulphur.
Olivine = One of the most important minerals in the solar system (ranges
between Mg2SiO4 and Fe2SiO4, allowing for any possible mixed composition
of magnesium and iron.
The physical properties of olivine change as the Mg/Fe ratio changes

Forsterite (Mg-rich olivine) freezes at a much higher temperature than
Fayalite (Fe-rich olivine)
-
Quartz (SiO2) = A mineral which is a common constituent of rocks

In pure form it is a colourless crystal – though it can take on colours

The colour is a consequence of traces of metallic elements in the quartz
Quartz is a characteristic mineral in granite (igneous, continental crustal rock)
Basalts have little or no quartz mineralization
-
Shield volcanoes are basaltic rock
West-coast volcanoes are andesitic (of continental, granitic material)
-
10.1.1 Condensation of Minerals in the Primordial Solar Nebula
- As the solar system was condensing with a proto-sun as the gravitational
centre, heat from the igniting proto-sun warmed up the disk which would
spawn the planets
- Near the proto-sun, the temperature of the cloud increased
- High temperature refractory minerals such as perovskite and spinel
condensed into minerals at temperature probably more than 1400K.

The inner regions of the nebula became relatively richer in high
temperature refractory minerals – other minerals couldn’t form and were
swept farther out into the solar nebula by thermal radiation and the
proto-solar win.
- In cooler temperatures (800-1400K), iron and nickel condensed into metallic
crystals, and olivine crystallized, as did feldspars, pyroxenes, and silica.
- Lighter, volatile minerals and molecules (carbon dioxide, methane) are most
common in the distant regions of the now condensed solar system – they only
formed when temperatures fell below 300K.
-
The terrestrial planets are largely composed of the refractory minerals,
metals, and high temperature silicates.
10.2 Composition of the Terrestrial Planets
The Moon
- Moon rocks were found to be similar to Earth rocks, but not identical
- Moon basalts were less rich in SiO2 than basalts on Earth.
- Crustal rocks on the moon had higher iron content
- Its compositional difference tells us something about its geological past
-
-
Higher iron content in crustal rocks on the moon is consistent with a model of
a Moon that splashed up in a giant collision of the proto-Earth with a Marssized body.
4.44 billion years ago, Earth had not yet differentiated completely
The Moon (being small), quickly froze with little internal heat generation
except from radioactive decay – it was not able to start mantle convection in
order to bring about further differentiation.
Mercury
- Mariner 10 Probe – revealed that the surface of Mercury is less rich in iron
than the crustal rocks on Earth
- This suggests that Mercury went through a more complete and early
differentiation
-
Mercury shows no evidence of active tectonics, but it does possess a
magnetic field that indicates that its iron core is in circulation
-
-
-
This further suggests that an inner core might still be freezing out within
Mercury, releasing a latent heat of fusion which drives the fluid circulations
generating the magnetic field
It is possible that a small scale convection allowed for further differentiation of
the mantle.
There is still no evidence though, that the crust of Mercury has been
resurfaced or recycled into the interior in the past 3.9 billion years.
The density of craters on Mercury give us a good estimate of the age of its
crust.
10.2 The Crater-Density Clock
The Moon
- During the Apollo missions to the Moon, the astronauts returned rock samples
to Earth

Maria Basins (basaltic rocks) – rocks no younger than 3.3 billion years
(about 3.7-3.9 billion years)

Highlands (granitic rocks) – older than 4.44 billion years (probably age
of solidification of the Moon’s crust)
- The Highlands are heavily cratered, while the maria basins aren’t as much.
- The maria basins were flooded with basalts subsequent to the impacts which
produced them, filling them with younger material
-
-
There is an “order” to the cratering process

A small crater which is seen to be within another larger crater must have
been created after the larger one.
By ordering craters, and then by calibrating their “age” with the radio-isotope
determination, we can calibrate a “crater-density clock.”
-
Early in the Solar System’s history, the rate of impacts was much higher than
it is now

There were also relatively larger impacts than occur now.
-
The Highlands are very old
The youngest crater on the moon (Mare Imbrium) is about 1.8 billion years old
The moon has been geologically inactive for the past 1.8 billion years
Mercury
- Maria basins have been flooded with basalts subsequent to the impacts which
produced them.
Venus and Earth
- Magellan orbiter used a SAR (synthetic aperture radar) which could penetrate
the planet’s thick cloud cover
- The surface is very young and there is a lack of small craters on the surface
-
Venus’ dense atmosphere (90x that of Earth) protects its surface from being
hit by small impactors

The same holds for Earth, especially over the ocean covered basins
-
Venus’ surface has been completely resurfaced by flooding basalts in
relatively recent geological time
On earth, the ocean basins have all been completely renewed in the past 210
million years (average age of ocean basins is about 100 million years)
-
-
-
Because Earth is so continually geologically active, the ocean basins are
younger than the basaltic basins of the other planets
The continents on Earth are under continuing attack by erosional processes –
the record of cratering (except for the largest and deepest) is lost
On Earth, there are very few craters smaller than 8km in diameter.


Geological activity erases the cratering record
Partially due to the “shielding effect” of the atmosphere for the smallest
impactors, and well as the shielding effect that ocean over the ocean
basin.
Mars
- There is evidence that there is still erosion caused by water and wind on Mars
- The highland regions of Mars, however, have not been much affected by
these erosional processes

Here, the cratering record is good evidence for highland age.
- However, the Southern Highlands show one apparent age for large craters
and a young one for small.

There appear to be fewer small craters than expected
- This may suggest that there used to be some atmospheric protection on Mars
(but it is now gone)
-
Linae Planium – Large region of Mars’ northern hemisphere which may have
been an ocean basin until 2 billion years ago, when the possible oceans
finally evaporated from the surface of Mars.
Mars’ gravity is so much less than Earth’s that warm volatile water can’t be
gravitationally bound to the planet
It may be that until about 4 billion years ago, Mars had a substantial
atmosphere
Evidence of lakes and oceans have recently been found on Mars
-
Generally, Mercury and the Moon have the oldest surfaces
Mars is somewhere between
Earth and Venus have the youngest surfaces
-
-
-
The highlands of all the terrestrial planets might be regarded as the long-term,
high-standing, granitic-like crustal masses
The lowlands can be regarded as denser basaltic masses
-
Earth (the most geologically active) shows the youngest age of its basins
-