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Lecture 6 The Earth – Evolution of a habitable planet 代寫

PHYS 1160
Introductory Astronomy
and the Search for Life
Elsewhere
Lecture 6
The Earth – Evolution
of a habitable planet
This Lecture
• Age and Formation of the Earth
• Plate Tectonics
• The Earth’s Atmosphere
• Our Satellite – The Moon
2
Formation of the Earth
• As we saw in lecture 5, the Earth and other
planets formed by a process of accretion
from small dust grains within the solar
nebular.
• The Earth was hit by a Mars size object
early in its history to form the Moon.
3
4
Radiometric Dating
• Dates can be assigned to rocks and
minerals through radiometric dating.
• Certain radioactive elements (such
as uranium) decay with well
determined  half-life to form a
daughter element - in this case lead.
• By measuring the uranium and lead
content in certain minerals the time
since the mineral crystallized can be
determined.
• Uranium actually has two isotopes
with different half-lifes providing an
independent check on such dates.
In one half life, half the
uranium will decay to
lead. In two half  lifes
half the remainder will
decay (3/4 in total) etc.
Ages
• The Earth
– Oldest minerals 4.404±0.008 Byr  (Wilde et al. 2001, Nature, 409,
175)
– Age of Earth’s differentiation (U-Pb and Hf-W methods) -
4.49 - 4.53 Byr (McCulloch, 1996, JRSWA, 79, 131, Kleine et al., 2002,
Nature, 418, 952)
• The Moon
– Oldest lunar rocks 4.42 - 4.51 Byr
– Lunar Crust Formation (Hf-W) 4.53 ± 0.01 Byr
• Meteorites
– Oldest Chondrites 4.555Byr
– CAIs in Allende meteorite 4.566 ± 0.002Byr  (Allegre et al. 1995,
GCA, 59, 1445)
• The Sun
– From solar oscillation data - 4.5±0.1Byr  (Guenther & Demarque,
1997, ApJ, 484, 937)
Structure of the Earth
• Lithosphere
– Crust
– Upper Mantle
• Lower Mantle
• Outer Core (liquid)
• Inner Core (solid)
6
7
Plate Tectonics
• Our understanding of the Earth was
revolutionized in the 1950’s and 60’s by the
discovery of plate tectonics.
• Long before this there had been evidence to
suggest that continents moved around on
the Earth’s surface
– The idea had been championed by Alfred
Wegener as early as 1912.
– But at the time it was rejected because no
mechanism could be envisaged.
8
Continental Drift
9
Evidence came from the match of the
coastlines of continents like South
America and Africa.
And from the occurrence of similar
fossils and plant and animal species in
widely separated continents.
For example the  proteaceae ( ( Banskias
and  Grevilleas ) of Australia, are very
similar to the  Proteas  of South Africa
but quite different to any plants in the
northern hemisphere.
Seafloor Spreading
• But the idea only began to be taken
seriously with the discovery of seafloor
spreading.
• It was discovered that in the middle of the
oceans (at sites called mid-ocean ridges),
new crust was being created and spreading
out from the ridge.
10
Seafloor Spreading
11
Subduction
• At other locations (called subduction zones)
crust was disappearing.
• Subduction zones often had a lot of volcanic
activity.
12
Plate Tectonics
13
Plate tectonics is driven by convection in the semi-fluid upper
mantle and is the result of heat coming from the interior of the
Earth
14
The Earth’ ’ s plate boundaries
15
Great Mountain ranges
like the Alps and Himalaya
are thrown up when
tectonic plates collide.
16
17
The Earth’s Atmosphere
• The current atmosphere
is composed primarily
of nitrogen and oxygen.
• Small amounts of H 2 O,
CO 2 , A etc.
18
Atmospheric Temperature Profile
Convective troposphere
extends up to the
tropopause  at about 10km
altitude.
Weather and clouds are
largely confined to this
region.
Temperature rises again
in stratosphere.
Atmospheric Circulation
20
Greenhouse Effect
Earth
Surface
Atmosphere
Solar
Radiation
(visible light)
Radiation From Earth
(Infrared - IR). This
must balance energy
received from Sun.
Gases in atmosphere
Absorb IR
The more IR energy that is
absorbed by greenhouse gases
(and hence doesn’ ’ t escape to
space) - the hotter the Earth
has to be.
Greenhouse (IR
absorbing)
gases are: H
2 2
O, CO
2 2
, ,
CH
4 4
, N
2 2
O, O
3 3
. .
Terrestrial Planet
atmospheres are secondary
atmospheres formed by
volcanoes and impacts of
comets and asteroids.
Composition:
CO
2 2
N N 2 2
CO
H H 2 2 O O
SO
2 2
( ( NOT  O
2 2
) )
The Earth’s Oxygen
23
The Early Faint Sun Problem
Kasting, ,  Toon  & Pollack, 1988
Early Sun was 25-30%
less luminous than now.
With present
atmosphere Earth should
have been frozen before
about 1.5Gyr.
It clearly wasn’ ’ t as
shown by presence of
sedimentary rocks and
fossils, and for most of
the period no evidence
of  glaciation. .
(Except around 2.3Gyr
and 750-600Myr ago).
Early Earth Atmosphere
• Cleary the Earth’s atmosphere was quite
different to that today.
– It must have had substantially more greenhouse
gases.
– Probably more CO 2
– Also possibly CH 4 and other gases.
Geochemical Carbon Cycle
Weathering of silicate rocks:
CO
2 2
+ CaSiO
3 3
=> CaCO
3 3
+ SiO
2 2
CO 2 Thermostat
• The Earth has a CO 2 thermostat which has acted to
stabilize temperature over the long term.
• When temperatures are high the rate of silicate
weathering is high so CO 2 is removed from the
atmosphere.
• When temperatures are low silicate weathering is
slow or stopped. Volcanoes return CO 2 to the
atmosphere.
• Timescale is ~1 million years.
28
The Moon
• The Earth is unique among the terrestrial
planets in having a large satellite – the
Moon.
• The Moon is the only other world in the
Solar System to have been visited by
humans.
• 12 astronauts walked on the Moon during 6
Apollo Moon landings from 1969 to 1972.
29
30
Launch of Apollo
11 – –  16
h th
July 1969
31
Apollo 11 on the Moon —  Buzz  Aldrin  descends the ladder
32
Dark regions,
flat plains with
few craters.
Called Maria
(or Seas) by
early
astronomers.
Light regions
are heavily
cratered
highland
regions ( Terrae
or “ “ lands” ”) )
33
From Apollo, we
know that the
lunar plains
formed about 3.5
Byr  ago.
And are the
younger features
on the Moon.
34
The  cratering  of
the lunar
highlands
occurred about
3.8 – –  4  Byr  ago.
Strom et al., 2005,  Science, 309 , 1847
An Inner Solar System Cataclysm at 3.9Byr?
There is evidence for a cataclysmic bombardment of the Moon, Earth and
Mars about 3.9Byr ago and lasting for ~100Myr.
A recent analysis indicates a different population of  impactors  (main belt
asteroids) were responsible compared with more recent impacts.
Migration of the giant planets into their final orbit could have disrupted
the asteroid belt and triggered this event.
Gomes et al., 2005,
Nature, 435, 466-469.
2:1 resonance between
Jupiter and Saturn
triggers instability that
pushes planets out to
their current locations
and causes asteroid
and comet impacts in
inner solar system.
Jupiter
Saturn
Neptune
Uranus
37
Return to the Moon
• New spacecraft have recently
been sent to the Moon.
• ESA’s SMART-1, Japan’s
Kaguya, China’s Change and
India’s Chadrayaan.
• NASA mission LRO and
LCROSS.
• NASA has plans for renewed
human exploration of the
Moon.
38
NASA’ ’ s Lunar Reconnaissance
Orbiter (LRO)
39
Earthrise on the Moon from  Kaguya
The End
The next lecture will look at the exploration of
the Solar System with spacecraft and Earth-
based telescopes.
 

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