Transcript Today`s Powerpoint - Physics and Astronomy

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Chapter 5: The Earth
Clicker Question:
Sunlight absorbed by the Earth’s surface is
reemitted in the form of?
A: radio waves
B: infrared radiation
C: visible radiation
D: ultraviolet radiation
E: X-ray radiation
Earth's Internal Structure
How do we know? Earthquakes. See later
Crust: thin. Much Si and Al
(lots of granite). Two-thirds
covered by oceans.
Mantle is mostly solid, mostly
basalt (Fe, Mg, Si). Cracks in
mantle allow molten material
to rise => volcanoes.
Core temperature is 6000 K.
Metallic - mostly nickel and
iron. Outer core molten, inner
core solid.
Atmosphere very thin
Earthquakes
They are vibrations in the solid Earth, or seismic waves.
Two kinds go through Earth, P-waves ("primary") and S-waves ("secondary"):
How do they measure where Earthquakes are centered?
seismic stations
*
*
*
Like all waves, seismic waves bend when they encounter changes in
density. If density change is gradual, wave path is curved.
S-waves are unable to travel in liquid.
Thus, measurement of seismic wave gives info on density of Earth's
interior and which layers are solid/molten.
Zone with no S waves:
must be a liquid core
that stops them
But faint P waves
seen in shadow zone,
refracting off dense
inner core
No P waves too:
they must bend sharply
at core boundary
Curved paths of
P and S waves:
density must slowly
increase with depth
Earth's Interior Structure
Average density
5.5 g/cm3
Crust
Mantle
Core
3 g/cm3
5 g/cm3
11 g/cm3
Density increases with depth => "differentiation"
Earth must have been molten once, allowing denser
material to sink, as it started to cool and solidify.
Earthquakes and volcanoes are related, and also don't occur at random
places. They outline plates.
Plates moving at a few cm/year. "Continental drift" or "plate tectonics"
When plates meet...
1) Head-on collision
(Himalayas)
side view
2) "Subduction zone"
(one slides under the other)
(Andes)
3) "Rift zone"
(two plates moving apart)
(Mid-Atlantic Ridge)
4) They may just slide past each other
(San Andreas Fault)
top view
=> mountain ranges, trenches, earthquakes, volcanoes
The Mid-Atlantic Ridge is a
rift zone.
What causes the drift?
Convection! Mantle slightly fluid and can support convection.
Plates ride on top of convective cells. Lava flows through cell
boundaries. Earth loses internal heat this way.
Cycles take ~108 years.
Plates form lithosphere (crust and solid upper mantle).
Partially melted, circulating part of mantle is asthenosphere.
Pangaea Theory: 200 million years ago, all the continents
were together!
Convection
Earth's surface heated by Sun. What would happen if it couldn't get rid
of the energy as fast as it gets in?
Convection also
occurs when you boil
water, or soup. Think
of Earth's surface as a
boiling pot!
Convection causes both small-scale turbulence and large scale circulation
patterns. It also occurs within Earth, on other planets, and in stars.
The Greenhouse Effect
Main greenhouse
gases are H2O and
CO2 .
If no greenhouse
effect, surface
would be 40 oC
cooler!
Clicker Question:
The Greenhouse effect would not occur if:
A: The Earth had no atmosphere.
B: The amount of carbon dioxide doubled.
C: We got rid of all the forests.
D: The Earth didn’t have an ocean.
The Moon
Mass = 7.4 x 1025 g
= 0.012 MEarth
Radius = 1738 km
= 0.27 REarth
Density = 3.3 g/cm3
(Earth 5.5 g/cm3)
Gravity = 1/6 that of Earth
We always see the same face of the Moon.
This means: period of orbit = period of spin
Why?
Tidal Locking
The tidal bulge in the solid
Moon elongates it slightly (2-3
km) along an axis pointing to
Earth.
Top view of Moon
orbiting Earth
Earth
If orbit period faster than spin
period, tidal bulge would have to
move around surface of Moon,
creating friction, which slows
the Moon’s spin down until tidal
bulge no longer migrates around.
Dark side of the moon
Tides
A feature of oceans (but solid material has small tides too).
Two high and two low tides per day.
Tides are due to Moon's gravitational pull being stronger on
side of Earth closest to it (Sun causes smaller tides).
Earth-Moon gravity keeps them
orbiting each other. But side of Earth
closest to Moon has slightly stronger
pull to Moon => bulges towards it.
Other side has weaker pull => bulges
away compared to rest of Earth.
The Earth spins once a day while the
bulge always points towards and away
from the Moon => high and low tides.
Tides
Tides
The Lunar Surface
- Large, dark featureless areas:
"maria" or "seas".
- Lighter areas at higher
elevation: "highlands".
- Loads of craters (due mostly
to meteorite impacts). No
winds to erode them away.
- Highlands have 10x the crater
density of maria.
maria
highlands
Lunar Volcanism (long ago)
Remember: volcanism is a way of losing internal heat
Evidence:
- Maria: result of old,
widespread lava flows
(filled in largest, early
impact craters)
- "Rilles": ditches
indicating old lava flows
- Linear chains of craters
(not formed by impacts),
probably marks
ancient fault, collapsed
lava domes
Cratering
- Impact speeds
several km/sec
- "Ejecta blanket" of
pulverized rock
surrounds crater
- Impacts =>
"regolith": ~20 m thick
layer of pulverized
rock covering Moon.
Cratering Rates
Small meteroids common, large ones rare. So same true for craters:
Crater size
Occurrence
10 km
every 10 million years
1m
every month
If no other processes (erosion, lava flows) change the surface, the number
of craters in an area tells you the age of the surface.
Moon's History
Age: 4.5 billion years
3.9 billion years ago:
heaviest meteoritic bombardment
ended
3.9 - 3.2 billion years ago:
volcanism created maria. Maria
are just the largest craters, filled in.
3.2 billion years -> present
no volcanism, cratering continued
at lower rate, geologically dead!
Lunar Structure
(from Apollo seismic data and theoretical arguments)
Core and
asthenosphere
take up small
fraction of
volume
compared to
Earth case –
the Moon is
more rigid
And no atmosphere, so no wind or erosion.
Surface reflects geologic history well.
Clicker Question:
When do the largest high tides occur?
A: When the Moon is at first quarter
B: When the Moon is full.
C: When the Earth is at aphelion in its orbit.
D: When the Moon is at 3rd quarter.
Clicker Question:
The surface gravity of the moon is 1/6 that
of Earth. If Matt weighs 120 lbs on Earth,
how much does he weigh standing on the
moon?
A: 120 lbs
B: 60 lbs
C: 30 lbs
D: 20 lbs
E: 10 lbs
Clicker Question:
Suppose the Moon was half as dense, but the
same size. How much would Matt (120 lbs
on Earth) weigh?
A: 120 lbs
B: 60 lbs
C: 30 lbs
D: 20 lbs
E: 10 lbs