Transcript The Earth and its Moon - Mid

The Earth and its Moon
10
The Earth
Basic Properties
• Terrestrial planet (the original!)
• Composition
– rocky (heavy elements: oxygen, silicon, iron)
– very different from sun and Jovian planets
• Orbit
– nearly circular
• Climate
– Not too hot, not too cold: JUST RIGHT!
• only planet with liquid water
• water necessary for life
General Features
Mass: MEarth = 6 x 1024 kg
Radius: REarth = 6,378 km
Density:  = 5,500 kg/m3
Age: 4.6 billion years
Liquid Water!
Atmosphere!
Question
• Why does the Earth have an
atmosphere, while the moon does not?
Escape Speed
• Earth: 11 km/s
• Moon: 2.4 km/s
• Explains why moon
has no atmosphere!
Convection
Convection also
occurs when you boil
water, or soup. Think
of Earth's surface as a
boiling pot!
Question
• Neglecting other effects, the Earth
should reradiate all of the energy
received from the Sun, giving the Earth a
surface temperature of 250 K = -23o C.
• Why is the average temperature of Earth
so much (about 40o C) higher?
The Greenhouse Effect
Main greenhouse
gases are H2O
and CO2 .
If no greenhouse
effect, surface
would be 40 oC
cooler!
Questions
• What are the major layers in the interior
of the Earth?
• How does density change with depth?
• What about temperature?
Formation of Earth
• Link to
formation of
Earth
• Link to video
Questions
• What does the differentiation exhibited
by the Earth tell us about its past?
– (Hint: The Earth was much hotter in the
past.)
• What were the two main sources of
heating experienced by the Earth early in
its history?
Earth's Interior Structure
Average density
5,500 kg/m3
Crust
Mantle
Core
3,000 kg/m3
5,000 kg/m3
11,000 kg/m3
Density increases with depth => "differentiation"
Earth must have been molten once, allowing denser
material to sink, as it started to cool and solidify.
Heating due to (i) bombardment and (ii) radioactivity
Bombardment by what? What important result do we
see today?
Density Stratification
•Density = Mass/Volume
•As Earth condensed its internal heat became so
intense that it became molten.
•Denser elements sank towards the core, lighter
floated to the surface causing density stratificationmaking layers of the Earth’s internal structure
according the density.
•These layers can be characterized by either their
chemical or physical composition.
Chemical Composition
• Chemically the Earth can be divided into 3
layers.
1.Crust – a thin 20 mi. layer composed of
low-density rocks made of silica minerals
-two types: oceanic and
continental crust
2.Mantle -1800 mi. thick made of high
density iron and magnesium silicate rock
3.Core- 1800 mi. made of higher density
metal ( Ni and Fe)
Crust
Mantle
Most volume
Structure of the earth video
Core
Earth's Internal Structure
Mantle is semi-solid rock.
Cracks allow material to
rise => volcanoes.
Core temperature is 6000
K. Outer - Molten.
Inner - solid.
Metallic
How do we obtain
information about the
structure of the Earth's
interior?
Like all waves, seismic waves bend or refract.
S-waves are unable to travel in liquid.
Measurement of seismic wave gives info on density of Earth's
interior and which layers are solid/molten.
Interior of Earth Activity
• http://www.classzone.com/books/earth_sci
ence/terc/content/investigations/es0402/es
0402page03.cfm
Plate Tectonic
Question
• What is the driving force behind the
motion of the plates?
• What types of geological features or
processes would you expect near plate
boundaries?
Plate Tectonics
•Volcanic and earthquake activity occurs on plate boundaries
What causes the drift?
Convection! Plates ride on top of convective cells.
One cycle takes millions of years => heat loss is extremely slow.
Mantle Convection
•
•
•
•
Heat escapes core; heats mantle
Hot mantle rises
Convection
Cold mantle sinks
Convection flow carries crust sideways
Rift Zone
Subduction
Subduction
Three Types of Boundaries
1. Transform -A transform boundary occurs
where two plates slide against each
other.
2. Divergent –Plates Move Apart
3. Convergent- Plates move together
Transform Boundary
• A transform boundary occurs where two plates slide
against each other. But rather than sliding smoothly,
the plates build up tension, then release the tension
with a spurt of movement. This movement is felt as an
earthquake.
Transform plate boundary video
Video
Divergent Boundary
Also known as spreading boundary, a divergent
boundary occurs where two plates move apart, allowing
magma, or molten rock, to rise from the Earth's interior
to fill in the gap. The two plates move away from each
other like two conveyor belts moving in opposite
directions.
For more on divergent boundary, go to The Sea Floor
Spread.
Rift Valley in Iceland
• National Geographic Video
• East African Rift
Valley
Convergent Boundary
Also known as subduction boundary, a convergent
boundary occurs where one plate slides under another
as the two are pushed together. If there is land at the
edge of one of these plates, the ocean plate will
subduct, or slide under that plate.
For more information, go to The Continental Slide.
3 Subtypes of Convergent
Boundaries
• Ocean-Ocean
• Ocean-Continental
• Continent-Continent
• Animation
Animation
Collisional Boundary
• A collisional boundary occurs where two
land masses on plates are pushed
together. Trying to occupy the same
space, the land masses buckle and fold,
creating mountain ranges.
For more information, go to The
Continental Crush
Uplift: Building
Mountain Ranges
1. Plate Tectonic activity 1
Present
Repaving the Earth
• Ocean floor renewed in few 100 million yrs
• Continents last longer
– Surface features erased by erosion
(water and wind)
• Evidence of craters erased in time
– Oldest rocks on earth about 3.8 billion yrs old
• Radioactive dating
Volcanoes are usually found in places
where:
A. the low pressure of the atmosphere
pulls the lava/magma to the surface.
B. earthquakes occur from oceanic
plates colliding with continental
plates.
C. deep-rooted mountains have cracked
Earth’s crust.
D. Earth’s rotation has caused weak
spots in its crust.
The change in position of the continents
over time is primarily caused by
A. continental plates floating on the
ocean.
B. mantle material circulating inside
Earth.
C. Earth’s slow shrinking as it cools.
D. global wind patterns and sustained
ocean currents.
Earth’s Atmosphere
• Composition
– Mostly nitrogen, N2 (78%)
– Some oxygen, O2 (21%)
– A little water, H2O
– Teeny, tiny bit of carbon dioxide CO2
• Atmospheric Pressure
– 14.7 pounds per square inch (1 bar)
– Produced by weight of atmosphere above us
Atmospheric Protection
• Ozone, O3
– Ozone absorbs ultraviolet light
– Protects us from sunburn
– Ozone destroyed by chlorine
• NOTE: Ozone destruction NOT same
problem as global warming!
Ozone and
CFCs
2000 Ozone hole
Over Antarctica
Change in CFC-11
Last 20+ years
First Atmosphere
Composition - Probably H2, He
• These gases are relatively rare on Earth compared to
other places in the universe and were probably lost to
space early in Earth's history because
– Earth's gravity is not strong enough to hold lighter
gases
– Earth still did not have a differentiated core (solid
inner/liquid outer core) which creates Earth's
magnetic field (magnetosphere = Van Allen Belt)
which deflects solar winds.
• Once the core differentiated the heavier gases could be
retained
Second Atmosphere
• Produced by volcanic out gassing. Gases produced
were probably similar to those created by modern
volcanoes (H2O, CO2, SO2, CO, S2, Cl2, N2, H2) and NH3
(ammonia) and CH4 (methane)
• No free O2 at this time (not found in volcanic gases).
• Ocean Formation - As the Earth cooled, H2O produced
by out gassing could exist as liquid in the Early Archean,
allowing oceans to form.
– Evidence - pillow basalts, deep marine sediments in
greenstone belts.
– Link to Goldilocks principle activity
Greenhouse Effect
• Sunlight absorbed by ground
• Heat emitted using infrared
light
• Infrared light trapped by
gasses in Earth’s atmosphere
– Greenhouse gasses:
• Carbon Dioxide (CO2)
• Water (H2O)
• Heat cannot escape
• Earth’s surface gets hotter
– about 23 degrees C (40 degrees
Global
Warming
Atmospheric Protection
• Meteors (“shooting stars”)
– Small rocks melt/burn up before hitting
surface
– Larger objects can get through
1833 Leonid Storm
1998 Leonids
1997 Leonids
Seen by MSX
Meteor
Showers
Causes of Meteor Showers
Peekskill
Meteorite
1992
Perils from Space
Effects of Impacts
• Where are the craters on Earth?
– erased by geologic activity
• volcanoes, earthquakes, plate tectonics
– erased by erosion
• weather, rain, wind
• Evidence for impacts?
– some craters still exist (Meteor Crater, AZ)
– geologic evidence of old craters
– occasional impacts today
• Effects of impacts?
– global climate changes
• mass extinctions? (dinosaurs)
– water in oceans?
Questions
• What causes the tides?
• When are the highest (and lowest) tides
seen?
The Moon
Mass = 0.012 MEarth
Radius = 0.27 REarth
Density = 3,300 kg/m3
(Earth 5,500 kg/m3)
Gravity = 1/6 that of Earth
Tides
Tides are due to Moon's gravitational pull being stronger on
side of Earth closest to it (Sun also influences tides).
Two high and two low tides per day. Why?
Tides (cont.)
Lunar Structure
Moon composed of material very similar to Earth's mantle!
The Lunar Surface
Dark areas: "maria"
●More recent lava flows.
●
Lighter areas at higher
elevation: "highlands".
●
Many craters (due to
meteorite impacts). Only
important source of
erosion!
●
Highlands have 10x the
crater density of maria =>
Highlands are older!
●
maria
highlands
The Moon
• Mass 1/80 of Earth’s
mass
• Gravity 1/6 of Earth’s
• Atmosphere
– no real atmosphere
– few volatiles (elements
that evaporate at relatively
low temperatures; e.g.
water)
Apollo 17
Dec 1972
Lunar Surface
• Dominant Features
– Craters
– Maria = “seas”
• Dark features
• Radioactive dating (moon rocks)
– age 3.3 - 4.4 billion yrs
– older than rocks on Earth
• Earth-Moon system about
4.5 billion years old
Geological Features
• Surface dominated by impacts
• Highlands
– heavily cratered, light colored
• oldest parts of surface
– silicate rocks
• Maria
– 17% of surface, mostly on near
side
– dark material, fewer craters
• youngest parts of surface
– volcanic plains of basalt
• lava-filled impact basins
Composition & Structure
• Average density
– 3.3 g/cm3
• lower than Earth’s mantle
(5.5 g/ cm3)
• Similar to Earth’s crust
• Composition
– mostly lighter silicates
– depleted in iron
– similar to Earth’s crust
• Mantle
– solid
– little seismic activity
Not geologically active (now)
• Core
– Small, possibly iron rich
– solid and cold
Differentiation
Moon’s interior molten in
Impact Craters
• Not erased by erosion (no
atmosphere)
– preserved record of impacts
– indicator of solar system history
• Crater Origin
– not volcanic
– meteor impacts
• surface “explosions”
• creates circular craters
– typical characteristics
• bowl-shaped
• turned up rims
• central peaks
Crater Counts
• Number of craters indicates age
– many craters = old
– few craters = young
• Ongoing impacts
– during last 3.8 billion yrs
• Compare: highlands vs. maria
– maria formed 3.8 billion years ago
• Based on number of craters
– highlands have many more craters
• highlands older
• Conclude:
– Period of heavy bombardment
• prior to 3.8 billion years ago
A moon covered with numerous and very old
craters created by meteorite impacts
likely
a)
b)
c)
d)
has no ocean to cover the craters.
orbits a large Jupiter sized planet.
has a cold, solid interior.
has no protective magnetic field.
How did the Moon form?
We're not quite sure! Three older theories:
1) "Fission": The material that would be the Moon was thrown
off the Earth. Problem: Earth not spinning fast enough.
2) "Coformation": The Moon and Earth formed out of the
same material at the beginning of the Solar System. Problem:
Moon has different density and composition.
3) "Capture": The Moon was a stray body captured into orbit
around Earth. Problem: an extremely unlikely event.
So now, Impact theory preferred:
Early in Solar System, a Mars-sized object hit the forming Earth,
ejecting material from the mantle which coalesced to form Moon.
Computer simulations suggest this is plausible.
Formation of Moon
Giant Impact Theory