Transcript geol_311_solar_system[1].

In the beginning…
Geology 311 :
 Formation of the elements
 Formation of the solar system
 Formation of the Earth !
The Big Bang…
 13.7 billion years ago (13,700,000,000 yrs)
 Explosion so powerful that space itself was
propelled outwards almost instantaneously
 200 Million years later, the first stars formed
 Universe is still expanding today
Formation of the Elements
 Cosmological Nucleosynthesis:
• Most H and He formed shortly
(few seconds) after the Big
Bang, when T cooled to 1 billion
degrees
• (H and He make up most of the
universe!)
 Stellar Nucleosynthesis:
• Most heavier elements (metals)
form either in Red Giants or
Supernovae by nuclear fusion
• (but without these there would be
no life!)
Stellar Nucleosynthesis

Red Giants
 Large stars that have exhausted hydrogen fuel in their cores and
go on to ‘burn’ (i.e., fuse) He, C, O, etc.
 Creation of elements up through Fe
 Extra neutrons can be captured to produce heavier elements

Supernovae
 Once core is converted to iron, large stars collapse and then
explode
 Enormous numbers of neutrons produced are captured to
produce heavy elements
Solar System Formation
Formation of the Solar
System:
Slowly rotating nebula
begins to contract,
condense, and spin faster
The Orion Nebula
Gas and dust
particles collide
and coalesce
into
planetesimals
Multiple
collisions and
accretion into
terrestrial
planets
Our Solar System
Terrestrial Planets: Mercury, Venus,
Earth, and Mars
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Rocky (silicate) outer parts
(crust and mantle) and
inner cores of Fe-Ni metal.
Silicates are compounds of
silica (SiO2) and the oxides
of other metals.
Common silicates:
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Olivine: (Mg,Fe)SiO4
Pyroxene: (Mg,Ca,Fe)2Si2O6
feldspar (e.g., (Na,K)AlSi3O8)
mica (e.g., biotite
K(Mg,Fe)3AlSi3O10(OH)).
The Giant Planets: Jupiter and Saturn
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Like the Sun, they consist primarily of H and He. Thus
they are approximately “solar” in composition (though not
exactly).
Small rocky core overlain by metallic H layer, overlain by
molecular H, He layer (gradual transitions from liquid to
gas).
The Icy Planets: Uranus and Neptune

These also consist of largely of H and He, but they are
enriched in C and N compared to Jupiter and Saturn.

Their solid parts consist of ices of methane and nitrogen.
Summary of Planet Formation

The Sun constitutes over 99% of the mass of the Solar System.
Therefore the solar nebula had a “solar composition”.

The giant planets must have formed early, before the gas of the
nebula dissipated, because they are rich in volatiles (H, He).
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Icy planets form more slowly because of lower densities in outer
part of nebula
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The inner planets form after the gas had largely dissipated from
the inner solar system.
 The terrestrial planets are depleted not only in gaseous elements
such as H, He, C, and N, but in “moderately volatile” elements as
well. These include the alkalis (Na, K, Rb, Cs) and elements
such as sulfur, lead, and indium.
 High temperatures in the inner solar system delayed planet
formation.
Observations from Meteorites
• Much of the evidence for how the solar system
formed comes from meteorites.
• Most come from asteroids of the asteroid belt
(e.g., Eros). A few from Mars and the Moon.
• Most meteorites give ages close to 4.56 Ga by
most dating methods. (Ga = billion years)
• Small group of achondrites give much younger
ages (e.g., 1.3 Ga). These meteorites are
thought to be from Mars.
Lunar constraints on planet formation

Moon is the only other planetary body we have directly sampled
and explored

Moon and Earth and closely related, both physically and
chemically

‘Hadean’ record preserved on the Moon
 Oldest rocks from the Moon are 4.45 Ga
 Record of the ‘early Earth’ (Hadean period) is missing, destroyed
by subsequent events
The Moon: some key observations
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No other planet has such a large moon relative to its size
(except Pluto).
Moon has only a very small iron core
Moon has a bulk density about the same as the Earth’s
mantle (suggests compositional similarity).
Highly depleted in highly volatile elements (gaseous
elements); depleted in moderately volatile elements.
Has identical oxygen isotope composition to the Earth
Bottom line: Earth and Moon probably have a shared history
Giant Impact Hypothesis

Idea in a nutshell:
 Body about 1/10 the mass
of the Earth (Mars-sized)
struck the Earth after it is
half or more accreted, and
after the Earth’s core had
at least partially formed.
 Material, mainly from
silicate mantle, is blasted
into orbit around the Earth,
eventually accreting to
form the Moon.
The “late heavy bombardment”

Many impact craters on
Moon date to around 3.9Ga.
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If the Moon was bombarded,
wouldn’t the Earth be as
well since they are very
close to each other?

Why doesn’t the Earth’s
surface look like the Moon?
After all we are still being bombarded…
(Dots show location of known large impact craters)
Chicxulub, Yucatan Peninsula, 65 Ma
Digital shaded relief
image of crater
thought to have
caused extinction of
the dinosaurs
Meteor Crater, AZ 49,000 yrs ago
Earth is a dynamic planet!
Earth is constantly being resurfaced due to plate tectonics and the
rock cycle.
..a really dynamic planet!
Differentiation of the Earth
(a) Early homogenous Earth
(b) Lighter matter “floats” toward surface
(c) Modern structure of the Earth
Earth has a layered structure
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Atmosphere and
Hydrosphere
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Low Density Crust (6-35
km thick)
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Intermediate Density
Mantle (~3000 km thick)
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High density core (~3000
km thick)
 Liquid outer core
 Solid inner core
Earth’s Layers
Thickness
(km)
Volume
1027 cm3
Density
g/cc
Atmosphere
Hydrosphere
Mass
1027 kg
Mass
Percent
0.000005
0.00009
3.80
0.00137
1.03
0.00141
0.024
Crust
17
0.008
2.8
0.024
0.4
Mantle
2883
0.899
4.5
4.016
67.2
Core
3471
0.175
11.0
1.936
32.4
All
6371
1.083
5.52
5.976
100.00
When did the continental crust begin
to form?

Oldest known rocks are
from the Great Slave
Province in Canada and
are approximately 4 Ga
old.
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Oldest known mineral is
a zircon is from
Australian sediments
whose metamorphic age
is 3.5 Ga.
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Inherited zircons are as
old as 4.4 Ga!
Earth’s Atmosphere and Hydrosphere
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When did it form?
How did it form?
Has it evolved with time?
Hypotheses
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(1) The atmosphere (& hydrosphere) formed
immediately by degassing of the Earth’s interior.
 (i.e., about the same time the Earth formed).
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(2) The atmosphere (& hydrosphere) formed
slowly over geological time by degassing of the
Earth’s interior.
Which is right?
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Based on isotopic data, >85% of atmosphere was
produced by “early catastrophic degassing”; the
rest through “continual” degassing.
Volcanic Degassing
contributed large
amounts of H2O, CO2,
and other gases to the
atmosphere
Our unique atmosphere
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Why does our
atmosphere have so
much O2 when Venus and
Mars have hardly any?
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Why is our atmosphere
so poor in CO2 compared
to that of our neighbors?
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Answers to these
questions are related.
Where has all the carbon gone?
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Amount of carbon in sediments exceeds the amount of
carbon in the atmosphere (as CO2) by a factor of 200.
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How did the carbon get there?
 This carbon represents the remains of once living
organisms (almost entirely plants).
 In other words, life, through photosynthesis, is partly
responsible for the low levels of CO2 in the atmosphere.
 Corollary: life is entirely responsible for the presence of
free oxygen in the atmosphere.
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Far more carbon than this is stored in sediments as
carbonate rocks.
When did the oceans form?
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H2O would have been degassed from the Earth’s interior
simultaneously with gases of atmosphere.
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But, when was the Earth’s surface cool enough for oceans
to form?
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The 4.4 Ga zircon has d18OSMOW up to 9‰.
 This suggests the magma from which the zircon
crystallized from reacted with or contained material that
had reacted with liquid water.
 Earth’s surface was apparently cool enough for oceans to
form at 4.4 Ga!
Our unique planet
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Earth’s uniqueness is a
consequence of its
formation
 Formation in the gas-depleted
inner part of solar nebula leads
to moderate size and depletion
in volatile elements
 Its violent early history probably
further contributes to volatile
depletion and atmospheric
composition uniquely suited for
life