Transcript Chapter 7 - Earth and the Terrestrial Worlds

Homework #4
 Due Wednesday, February 24, 11:59PM
 Covers Chapters 6 and 7
 Estimated time to complete: 1 hour
 Read chapters, review notes before starting
Telescope Viewing Tonight!
Come see Jupiter and its moons through the Gallalee
Hall 16” Telescope tonight from 7-10 PM.
From this classroom, go right down the hall, turn right
and find stairwell. Go up two flights of stairs and to
the left to get out to the roof (4th floor).
Be sure to sign the sign-up sheet to receive an extra
days worth of clicker credit.
Can Rock Flow?
 Rock stretches when pulled
slowly (especially when
very warm) but breaks
when pulled rapidly.
 The gravity of a large world
pulls slowly on its rocky
content, shaping the world
into a sphere.
 Bodies over 500 km in
diameter will become
spherical in ~1 billion years
by slow, slow deformation
of rock by gravity.
Lithosphere
 A planet’s outer layer
of cool, rigid rock is
called the lithosphere
(crust + upper part of
mantle).
 Rock in lithosphere is
cold and does not
deform like silly putty,
while warmer mantle
rock below it does
deform.
Lithosphere can be broken into
plates.
 Rigid lithosphere
“floats” on the warmer,
softer rock that lies
beneath.
Heat Drives Geological Activity
Rock heated by
radioactivity in core.
Hard, rigid lithosphere
rock does not circulate
Heat moved through
mantle by convection.
Convection: Hot rock
rises, cool rock falls.
One convection cycle
takes 100 million
years on Earth.
Rock actually “flows”
from inner mantle to
outer mantle and back!
Why Does Lack of Heat Matter?
Once the internal heat is gone, the once-soft hot
rock cools off  lithosphere grows thicker and
thicker.
Cold rock will not flow and rise to the surface
through cracks in the lithosphere
 no more geological activity such as volcanoes
This is what has already happened on Mercury and the
Moon long ago, and is happening to Mars now.
Role of Size
Since internal heat drives geological activity, a total loss
of heat leads to a total loss of geological activity.
Smaller worlds cool off faster and solidify earlier.
The Moon and Mercury are now geologically
“dead” because they have already lost all their
internal heat.
Important!
Surface Area–to–Volume Ratio
Heat content depends on volume.
Loss of heat through radiation depends on
surface area.
Cooling rate depends on surface area divided
by volume:
2
p
r
4
=3
Surface area–to–volume ratio =
4 pr 3 r
3
Bigger objects
cool more
slowly
• Easy to remember: a hot pea cools off much faster
than a hot potato at the same initial temperature
What key quality is responsible for the Moon
and Mercury being geologically dead?
A) their distance from the Sun
B) their chemical composition
C) their size
D) their lack of atmosphere
What key quality is responsible for the Moon
and Mercury being geologically dead?
A) their distance from the Sun
B) their chemical composition
C) their size
D) their lack of atmosphere
Because of their small size, they have higher
surface area-to-volume ratios  cooled off faster
(think hot pea vs. hot baked potato)
What processes shape
planetary surfaces?
Processes That Shape Surfaces
 Impact cratering
Impacts by asteroids or comets
 Volcanism
Eruption of molten rock onto surface
 Tectonics
Disruption of a planet’s surface by internal stresses
 Erosion
Surface changes made by wind, water, or ice
Last 3 processes stop once the internal heat is gone.
Impact Cratering
Most cratering happened soon
after the solar system formed
(“Period of Heavy
Bombardment”).
Craters are about 10 times wider
than object that made them,
and 10-20% as deep as they are
wide (impact at 40,000-250,000
km/hr).
Small craters greatly outnumber
large ones.
Impact Craters
Meteor Crater (Arizona)
Tycho Crater (Moon)
(Note central rebound peak)
Volcanism
Volcanism happens
when molten rock
(magma) finds a
path through crack
in the lithosphere to
the surface.
Molten rock is called
lava after it reaches
the surface.
Outgassing
Mount St. Helen’s
Volcano National
Park
 Volcanism also releases gases from Earth’s interior into
the atmosphere (outgassing). This is where the
terrestrial planets got their atmospheres.  key
 Atmospheres diffuse away if not continuously replenished
 explains lack of atmospheres on dead planets
Tectonics
Convection of the mantle creates stresses in
the crust called tectonic forces.
Compression of crust creates mountain ranges.
Valley can form where crust is pulled apart.
Plate Tectonics on Earth
Earth’s continents slide around on
separate plates of crust.
Lithosphere is fractured into more
than a dozen plates  continents sit
on some of these plates
Apparently unique to Earth in the
Solar System today.
Continental Motion
The idea of
continental drift
was inspired by
the puzzle-like fit
of the continents.
Mantle material
erupts where the
seafloor spreads,
causing plates to
slowly drift across
the globe.
Continental Motion
Plates are
pieces of
Earth’s
fractured
lithospher
e
Motion of the continents (on the pates) can be
measured with GPS – few centimeters/year.
Surface Features
The Himalayas
formed from a
collision
between
plates.
Surface Features
The Red Sea is
formed where
plates are
pulling apart.
Rifts, Faults, Earthquakes
The San
Andreas fault in
California is a
plate boundary.
In 20 million years, SF and LA will
be near each other.
Sideways
motion of
plates can
cause
earthquakes.
Plate Motions
Measurements of plate motions tell us past
and future layout of the continents.
Pangaea – 250 million years ago
Rodinia – 750 million year ago
http://pubs.usgs.gov/publications/text/dynamic.html
Cryolophosaurus ellioti found in
Antarctica
Erosion
Erosion is a blanket term for weather-driven
processes that break down or transport rock.
Processes that cause erosion include:
glaciers
rivers
Wind
All dependent on the presence of an
atmosphere.
Erosion by Ice
Glaciers
carved the
Yosemite
Valley.
Erosion by Water
The Colorado
River continues
to carve Grand
Canyon.
Erosion by Wind
Wind wears
away rock and
builds up sand
dunes.
Erosional Debris
 Erosion can
create new
features such as
deltas by
depositing debris.
Erosion also builds, not always
destroys
 Sediment is
transformed onto
sedimentary rock
– most common
type of rock on
Earth
Which of the following is the only process
capable of shaping the surface of a dead
world?
A) volcanic activity
B) plate tectonics
C) erosion
D) impact cratering
Which of the following is the only process
capable of shaping the surface of a dead
world?
A) volcanic activity
B) plate tectonics
C) erosion
D) impact cratering
First three depend (ultimately) on internal heat.
What is an atmosphere?
An atmosphere is a layer of gas that surrounds a
world.
Source: outgassing from volcanic activity
Earth’s Atmosphere
About 10 kilometers
thick
Analogy: as thick as
a dollar bill wrapped
around a standard
globe.
Consists of:
Nitrogen: 77%
Oxygen: 21%
Argon: 1%
H2O, CO2: trace
Effects of Atmosphere on Earth
1) Erosion
- rain, wind, ice shapes surface
2) Radiation protection
- harmful X-rays, gamma-rays blocked
3) Greenhouse effect
- keeps Earth warm (too warm?)
4) Makes the sky blue!
Radiation Protection
All X-ray light is
absorbed very high
in the atmosphere.
Ultraviolet light is
absorbed by ozone
(O3) – explains
concern about holes
in the ozone.
The Greenhouse Effect
Visible (yellow) light
passes through the
atmosphere easily and
warms a planet’s surface.
Planet absorbs optical
light and emits thermal
blackbody infrared (red)
radiation.
The atmosphere absorbs
infrared radiation from the
surface, trapping heat like a
blanket.
Greenhouse
effect:
Certain
molecules let
sunlight
through but
trap escaping
infrared
photons.
The Greenhouse Effect
(H2O, CO2,
CH4) 
greenhouse
gases
Greenhouse Effect: Good or Bad?
Because of the greenhouse effect, Earth
is considerably warmer than it would be
without an atmosphere (above the
freezing point of water)…but so is Venus.
Without the greenhouse effect, Earth
would be too cold for liquid water to exist
on its surface  very bad for life!
Why the sky is blue
Atmosphere scatters
blue light from the
Sun, making it
appear to come from
different directions.
Sunsets are red
because less of the
red light from the
Sun is scattered.