Download Birth of Solar System and Terrestrial planets bb

Birth of the Solar System and
Terrestrial Planets
According to the nebular theory, our solar system formed
from a giant cloud of interstellar gas.
(nebula = cloud)
Galactic Recycling
• Elements that
formed
planets were
made in stars
and then
recycled
through
interstellar
space.
Conservation of Angular Momentum
• The rotation speed of the cloud from which
our solar system formed must have
increased as the cloud contracted.
• Rotation of a contracting cloud speeds up
for the same reason a skater speeds up as
she pulls in her arms.
Flattening
• Collisions between particles in the cloud
caused it to flatten into a disk.
• Collisions between gas particles in a cloud
gradually reduce random motions.
• Collisions between gas particles also reduce
up and down motions.
• The spinning cloud flattens as it shrinks.
Collapse of the Solar Nebula
Evidence from Other Gas Clouds
We can see stars
forming in other
interstellar gas
clouds, lending
support to the
nebular theory.
Disks Around Other Stars
•
Observations of disks around other stars
support the nebular hypothesis.
Motion of Large Bodies
• All large bodies
in the solar
system orbit in
the same
direction and in
nearly the same
plane.
• Most also rotate
in that direction.
Two Major Planet Types
• ____________
planets are
rocky, relatively
small, and close
to the Sun.
• _______ planets
are gaseous,
larger, and
farther from the
Sun.
Why are there two major types of
planets?
Conservation of Energy
• As gravity causes the cloud to contract, it
heats up.
• Inner parts of the disk are hotter than outer
parts.
• Rock can be solid at much higher
temperatures than ice.
Temperature Distribution of the Disk and the Frost Line
Fig 9.5
Inside the ________: Too hot for hydrogen compounds to form ices
Outside the __________: Cold enough for ices to form
Formation of Terrestrial Planets
• Small particles of rock and metal were
present inside the frost line.
• Tiny solid particles stick to form
________________.
• Gravity draws planetesimals together to
form planets.
• This process of assembly is called
_________.
Accretion of Planetesimals
•
Many smaller objects collected into just a
few large ones.
Summary of the Condensates in the Protoplanetary Disk
Heavy Bombardment
• Leftover
planetesimals
bombarded
other objects
in the late
stages of solar
system
formation.
Why have the planets turned out
so differently, even though they
formed at the same time from the
same materials?
Terrestrial Planet Interiors
• Applying what we have learned about Earth’s
interior to other planets tells us what their interiors
are probably like.
__________________
• Gravity pulls
high-density
material to center
• Lower-density
material rises to
surface
• Material ends up
separated by
density
_____________
• A planet’s outer
layer of cool, rigid
rock is called the
lithosphere.
• It “floats” on the
warmer, softer
rock that lies
beneath.
Strength of Rock
• Rock stretches when
pulled slowly but
breaks when pulled
rapidly.
• The gravity of a large
world pulls slowly on
its rocky content,
shaping the world into a
sphere.
Heat Drives Geological Activity
Convection: hot rock
rises, cool rock falls.
One convection cycle
takes 100 million
years on Earth.
Sources of Internal Heat
1. Gravitational
potential energy of
accreting
planetesimals
2. Differentiation
3. Radioactivity
Heating of Interior over Time
• Accretion and
differentiation
when planets
were young
• ____________
_______is most
important heat
source today
Cooling of Interior
• Convection
transports heat as
hot material rises
and cool material
falls
• Conduction
transfers heat
from hot material
to cool material
• Radiation sends
energy into space
Planetary Magnetic Fields
Moving charged particles create magnetic fields.
A planet’s interior can create magnetic fields if its
core is electrically conducting, convecting, and
rotating.
Earth’s Magnetosphere
Earth’s magnetic fields protects us from
charged particles from the Sun.
The charged particles can create aurorae
(“Northern lights”).
Tectonics
• Convection of the mantle creates stresses in the crust called
tectonic forces.
• Compression forces make mountain ranges.
• A valley can form where the crust is pulled apart.
Role of Size
• Smaller worlds cool off faster and harden earlier.
• Moon and Mercury are now geologically “dead.”
Surface Area to Volume Ratio
• Heat content depends on volume.
• Loss of heat through radiation depends on surface
area.
• Time to cool depends on surface area divided by
volume:
surface area to volume ratio
4r 2 3
=

4 3 r
r
3
• Larger objects have a smaller ratio and cool more
slowly.

Impact Cratering
• Most cratering happened
soon after the solar system
formed.
• Craters are about ___
times wider than objects
that made them.
• Small craters greatly
outnumber large ones.
The Production of a Crater
Impact Craters
Meteor Crater (Arizona)
Tycho (Moon)
Volcanism
• Volcanism happens
when molten rock
(magma) finds a path
through lithosphere to
the surface.
• Molten rock is called
lava after it reaches the
surface.
Lava and Volcanoes
Runny lava makes flat
lava plains.
Slightly thicker lava
makes broad shield
volcanoes.
Thickest lava makes
steep stratovolcanoes.
Outgassing
• Volcanism also releases gases from Earth’s interior
into the atmosphere.
Plate Tectonics on Earth
• Earth’s continents
slide around on
separate plates of
crust.
Plate Tectonics on Earth
Effects of Atmosphere on Earth
1.
2.
3.
4.
Erosion
Radiation protection
Greenhouse effect
Makes the sky blue!
Erosion
• Erosion is a blanket term for weather-driven
processes that break down or transport rock.
• Processes that cause erosion include
— Glaciers
— Rivers
— Wind
Radiation Protection
• All X-ray light is
absorbed very high in
the atmosphere.
• Ultraviolet light is
absorbed by ozone
(O3).
Earth’s atmosphere absorbs light at most wavelengths.
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.
Sun
• Over 99.9% of solar system’s mass
• Made mostly of H/He gas (plasma)
• Converts 4 million tons of mass into energy each second
Mercury
• Made of metal and rock; large iron core
• Desolate, cratered; long, tall, steep cliffs
• Very hot and very cold: 425°C/ 797°F (day),
–170°C/ -274°F (night)
Cratering of Mercury
• Mercury has a mixture of heavily cratered and
smooth regions like the Moon.
• The smooth regions are likely ancient lava flows.
Cratering of Mercury
The Caloris basin is
the largest impact
crater on Mercury
Region opposite the
Caloris Basin is
jumbled from
seismic energy of
impact
Tectonics on Mercury
• Long cliffs indicate that Mercury shrank early in its
history.
Venus
• Nearly identical in size to Earth; surface hidden by clouds
• Hellish conditions due to an extreme greenhouse effect
• Even hotter than Mercury: 470°C/ 878°F, day and night
Cratering on Venus
• Impact craters, but fewer
than Moon, Mercury,
Mars
Volcanoes on Venus
• Many volcanoes,
including both shield
volcanoes and
stratovolcanoes
Tectonics on Venus
• Fractured and
contorted surface
indicates tectonic
stresses
Erosion on Venus
• Photos of rocks
taken by lander
show little
erosion
Does Venus have plate tectonics?
•
•
Most of Earth’s major geological features
can be attributed to plate tectonics, which
gradually remakes Earth’s surface.
Venus does not appear to have plate
tectonics, but its entire surface seems to
have been “repaved” 750 million years
ago.
Why is Venus so hot?
The greenhouse effect on Venus keeps its
surface temperature at 470°C.
But why is the greenhouse effect on Venus so
much stronger than on Earth?
Atmosphere of Venus
• Venus has a very thick carbon dioxide
atmosphere with a surface pressure 90 times
that of Earth.
• Reflective clouds contain droplets of
sulfuric acid.
• The upper atmosphere has fast winds that
remain unexplained.
Greenhouse Effect on Venus
• Thick carbon
dioxide atmosphere
produces an
extremely strong
greenhouse effect.
• Earth escapes this
fate because most of
its carbon and water
are in rocks and
oceans.
Runaway Greenhouse Effect
More evaporation,
stronger greenhouse effect
Greater heat,
more evaporation
• The runaway greenhouse effect would account for
why Venus has so little water.
Earth
Earth and
Moon to scale
• An oasis of life
• The only surface liquid water in the solar system
• A surprisingly large moon
What unique features of Earth are
important for life?
1.
2.
3.
4.
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
Continental Motion
• Motion of continents can be measured with
GPS
Carbon Dioxide Cycle
1. Atmospheric CO2
dissolves in
rainwater.
2. Rain erodes minerals
that flow into the
ocean.
3. Minerals combine
with carbon to make
rocks on ocean floor.
Carbon Dioxide Cycle
4. Subduction carries
carbonate rocks
down into the
mantle.
5. Rock melts in
mantle and outgases
CO2 back into
atmosphere through
volcanoes.
Long-Term Climate Change
• Changes in Earth’s axis tilt might lead to ice ages.
• Widespread ice tends to lower global temperatures
by increasing Earth’s reflectivity.
• CO2 from outgassing will build up if oceans are
frozen, ultimately raising global temperatures again.
These unique features are intertwined:
•
•
•
•
Plate tectonics create climate stability
Climate stability allows liquid water
Liquid water is necessary for life
Life is necessary for atmospheric oxygen
Moon
craters
smooth plains
Earth’s Moon
• Diameter of 3475 kilometers (2150 miles) is
unusually large compared to its parent planet
• Density
• 3.3 times that of water
• Comparable to Earth's crustal rocks
• Perhaps the Moon has a small iron core
Earth’s Moon
•
•
•
•
Gravitational attraction is ____of Earth's
No atmosphere
Tectonics no longer active
Surface is bombarded by micrometeorites from
space which gradually makes the landscape
smooth
Moon’s Formation
• Before Apollo missions, three hypotheses of the
Moon’s origin:
– Moon originally a small planet orbiting the Sun and
was subsequently captured by Earth’s gravity during a
close approach (capture theory)
– Earth and Moon were twins, forming side by side from
a common cloud of gas and dust (twin formation
theory)
– The Moon spun out of a very fast rotating Earth in the
early day of the Solar System (fission theory)
Moon’s Formation
• Each of these hypotheses gave different
predictions about Moon’s composition:
– In capture theory, the Moon and Earth would be
very different in composition, while twin theory
would require they have the same composition
– In fission theory, the Moon’s composition
should be close to the Earth’s crust
Moon’s Formation
• Moon rock samples proved surprising
– For some elements, the composition was the
same, but for others, it was very different
– None of the three hypotheses could explain
these observations
Impact Theory of Moon’s
Formation
– Moon formed from debris blasted out of the Earth by
the impact of a Mars-sized body
– Age of lunar rocks and lack of impact site on Earth
suggests collision occurred at least 4.5 billion years ago
as the Earth was forming
– The impact would vaporize low-melting-point materials
(e.g., water) and disperse them explaining their lack in
the Moon
– Only surface rock blasted out of Earth leaving Earth’s
core intact and little iron in the Moon
– Easily explains composition difference with Earth
Giant Impact
Giant impact stripped matter from Earth’s crust
Stripped matter began to orbit
Then accreted into Moon
Moon
• Some volcanic activity 3 billion years ago must have
flooded lunar craters, creating lunar maria.
• The Moon is now geologically dead.
Mars
• Looks almost Earth-like, but don’t go without a spacesuit!
• Giant volcanoes, a huge canyon, polar caps, and more
• Water flowed in the distant past; could there have been
life?
Mars versus Earth
•
•
•
•
•
50% Earth’s radius, 10% Earth’s mass
1.5 AU from the Sun
Axis tilt about the same as Earth
Similar rotation period
Thin CO2 atmosphere: little greenhouse
• Main difference: Mars is SMALLER
Seasons on Mars
• Seasons on Mars are more extreme in the southern
hemisphere because of its elliptical orbit.
Storms on Mars
• Seasonal winds on Mars can drive huge dust storms.
The surface of Mars appears to have ancient riverbeds.
Eroded
crater
The condition of craters indicates surface history.
http://fti.neep.wisc.edu/neep602/FALL97/LEC15/Fusion-19.JPG
Low-lying regions may once have had oceans.
Opportunity
Spirit
• 2004 Opportunity Rover provided strong evidence for abundant
liquid water on Mars in the distant past.
• How could Mars have been warmer and wetter in the past?
Today, most water
lies frozen
underground (blue
regions)
Some scientists
believe accumulated
snowpack melts
carve gullies even
today.
Climate Change on Mars
• Mars has not had
widespread surface
water for 3 billion
years.
• The greenhouse
effect probably kept
the surface warmer
before that.
• Somehow Mars lost
most of its
atmosphere.
Climate Change on Mars
• Magnetic field may have preserved early Martian
atmosphere.
• Solar wind may have stripped atmosphere after field
decreased because of interior cooling.
Martian Surface
– Martian Polar Ice Caps
• Change in size with seasons (Mars tilt
similar to Earth’s)
• Southern cap
– frozen CO2 (dry ice)
– diameter
» 5900 km (winter)
» 350 km (summer)
Martian Surface
• Northern
– surface layer of CO2
– primarily water ice
– separate layers indicative of climate
cycles (including “ice ages”)
– Shrinks to 1000 km in summer
• Far less water than Earth’s caps
So, Are there Martians?
Censored
Fossils of ancient Martian life? The tiny rod-shaped structures look similar to primitive
fossils found in ancient rocks on Earth. However, some scientists think these
structures formed chemically. (Courtesy NASA.)
Are there Martians?
• Maybe, extremophiles live on Earth and
may exist at northern pole on Mars.
• Extremophiles exist at the “fringes” of
livable conditions.
– Tube worm communities at hydrothermal vents
in the deep ocean (Chemosynthetic)
– Bacteria communities under ice in Antarctica
http://www.jasonproject.org/expeditions/jason9/guaymas/images/tue21/marked_mat.jpg
http://www.jasonproject.org/expeditions/jason9/guaymas/images/tue21/worm_bunch.jpg
What makes a planet habitable?
• Located at an optimal distance from the Sun
for liquid water to exist
What makes a planet habitable?
• Large enough for geological activity to
release and retain water and atmosphere
Planetary Destiny
Earth is
habitable
because it is
large enough to
remain
geologically
active, and it is
at the right
distance from the
Sun so oceans
could form.