Transcript 9_Origin_earth

Memorial for Columbia Astronauts
• “This cause of exploration and
discovery is not an option we
choose. It is a desire written
in the human heart. We are
that part of creation which
seeks to understand all
creation. We find the best
among us, send them forth
into unmapped darkness, and
pray they will return.“
--- President Bush
Iain Clark, son of Laurel Clark comforted by his father
Origin of Earth
Lecture Nine, Feb. 5, 2003
Course Project
• Topic of your choosing related to class.
• Proposal (1-2 pages) Feb. 14, 2003.
• Project background (3-5 pages) Mar. 28,
2003.
• Project (10 pages) April 25, 2003.
Project
• Almost any topic of interest to you and related
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to the class is possible.
Try and discuss as much of the science, covered
in class, as possible.
Example of a project: “The Movie 2001 and The
Evolution of Man”, by C Horowitz. See paper
copy or class web site. You should read this
paper! Note, how it discusses the movie “2001
A Space Odyssey” in terms of many of the topics
covered in class.
Ideas for Projects
• Big Bang/ creation of Universe.
• Supernovae, neutron stars, and or black holes.
• Impacts of comets and asteroids.
– Death of dinosaurs (See for example, “T.Rex and the
crater of doom” by L. Alvarez).
– Impact of Shoemaker Levy 9 with Jupiter (See for
example, “Impact Jupiter” by D. Levy)
– Present day impact threat.
Project Ideas Continued
• Mars
– Robotic exploration
– Manned mission (See for example, “The Case For
Mars”, R. Zubrin)
– Colonization or terrafarming (See text)
• Discuss Science in a Science Fiction Story.
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2001, 2010… by A. C. Clark
“Andromeda Strain” by M. Crichton
“Red, (Green, Blue) Mars” by K. S. Robinson
Lots and Lots to chose from…
• Space exploration
Ideas Continued
• Origin of Life
• Extrasolar planets (see for example,
“Worlds Unnumbered” by D. Goldsmith)
• Interstellar Flight and Colonization.
• SETI
•…
Alchemy of The Heavens
• Origin of the chemical elements.
• Chemical element has atomic number Z (number of
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electrons orbiting nucleus).
Big Bang Nucleosynthesis: after the first 3 min. Universe
was about 25% He and 75% H and little heavier
elements.
First generation stars had no heavy elements for rocky
planets or life.
First generation stars make elements in core.
New elements released into interstellar medium when star
dies either as planetary nebula or supernova.
Life of a Star
• Birth: collapse of gas cloud forms protostar.
• Main sequence: center of star becomes hot
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enough to burn Hydrogen into Helium. Our Sun
will be on main sequence for 10 billion years.
Red Giant: Outer part of star expands and
cools. Core contracts and starts to burn He into
Carbon and other heavier elements.
Star dies: either as a planetary nebula (low mass
star) or as a supernova (high mass star).
Supernovae
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Gigantic stellar explosions.
Eject new chemical elements into space.
Make neutron stars and black holes.
Accelerate cosmic rays (energetic particles
constantly hitting Earth’s atmosphere from
space).
Shock waves can help gas clouds to collapse and
form new stars.
Deadly to life (within say ~1-300 light years???)
Origin of Earth
• Why are there planets? To conserve angular momentum
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(amount of spin).
(Linear) momentum is mass times velocity.
Angular momentum is momentum times distance from
rotation axis.
Think of an ice skater in a spin. With her arms apart she
spins slowly (her hands have a small momentum times a
large distance from her rotation axis). As she brings her
hands in she spins much faster. Angular momentum is
conserved. Because her hands are nearer the rotation
axis they must have a higher momentum and so she
spins faster.
Collapse of Primitive Solar Nebula
• Very extended gas cloud has, in general, a
very small but nonzero rotation.
• As the cloud collapses the conservation of
angular momentum causes it to rotate
faster.
• Gas cloud can more easily collapse along
rotation axis so a round cloud flattens into
a disk.
False color image of disk around
Beta Pict.
Formation of Solar System
• The center of the disk collapses to form a
star.
• While the rest of the disk forms a system
of planets.
• Planets and star formed at same time.
• All planets are expected to orbit around
the star in the same direction.
Formation
of Star
and
Planets
Planets versus Double Stars
• Two ways to conserve angular
momentum:
– Form a single star with planets.
– Form two or more stars orbiting each other
(indeed many stars are observed to be in
binary or multiple star systems, for example
the nearest star Alpha Cent. is a triple star
system)
• Expect planets to be common.
Earth Formed Hot
• Large impacts continued.
• Heat of formation melted rocks.
• Dense Iron and other materials sank to form
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core of Earth leaving mantle depleted in dense
elements.
Radioactive decay of Uranium, Thorium and
Potassium helped to heat interior of earth.
Core cools by convection. Hot material rises and
cold material sinks. This leads to rotating
motions which carry heat away from core.
The Earth Cools by Convection
• Hot material near the core
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expands and becomes less
dense. It then rises. As it
cools it becomes denser and
sinks back to the core.
This rotary motion transports
heat from the core.
These motions can power
plate tectonics where large
plates move carrying many
surface features with them.
Convective currents also
generate earth’s magnetic
field.
Core
Plate Tectonics
Formation of
New Crust
• As plates move
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apart, the
Atlantic ocean is
getting wider
and new crust is
forming in mid
ocean.
Age of crust:
Evolving Earth Crucial for Life
• Magnetic field shields cosmic rays (radiation)
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from space.
Early atmosphere rich in CO2.
Greenhouse effect of CO2 very important for
Earth temperature. Gases trap Sun’s heat and
keep it from being reradiated back into space.
CO2 is transparent to visible light but absorbs
infrared radiation.
Without greenhouse effect, Earth would be
some 35 C colder (average temp. below -20 C
instead of +12 C) and a frozen ice ball.
Earth’s Thermostat
• Amount of CO2 in atmosphere controlled by
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absorption to make carbonate rocks or outgassing from volcanoes.
This changes greenhouse effect and Earth’s
temperature.
Earth’s temperature has been remarkably stable
over its history despite an increase in the Sun’s
output.
There must have been more CO2 at early times
and this somehow decreased in a way that
compensated the increase in the Sun’s output.
We don’t understand this
Thermostat
• We don’t understand many of the mechanisms
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which regulate CO2.
How much more CO2 will lead to a runaway
greenhouse effect such as what happened to
Venus?
What will the ultimate effect of human produced
CO2 be (global warming)?
How close did Earth come to losing CO2 and
going into a permanent global ice age
(“snowball earth”)?
Venus is Hot as Hell
• Atmosphere is 90
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times thicker then
Earth and mostly CO2.
Greenhouse effect
raises temp. by 400 C
to about 470 C (hot
enough to melt lead).
Not a likely place for
life.
Mars was once Warmer and Wetter
• In the past, Mars
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atmosphere had much
more CO2.
This allowed greenhouse
effect to raise temp.
above 0 C.
Mars has since lost most
of its atmosphere for
unknown reasons,
perhaps absorption in
rocks.
Formation of Earth
• Planets and Stars from together from
collapse of rotating gas cloud.
• Sun and planets of same age.
• Planets all orbit Sun in same direction.
• Earth formed hot. Rocks melted.
• Denser material such as Iron sunk to
center and formed Earth’s Core.
• Radioactive decay continues to heat core.
Earth is Geologically Alive.
• We are lucky that Earth’s core is still hot and this heat
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flow drives convection, plate tectonics and generates
magnetic field.
Plate tectonics may help regulate CO2 in atmosphere.
Note, no other planet in solar system appears to have
plate tectonics.
Earth’s temp. remained remarkably stable despite
warming Sun.
Timeline: Big Bang 13 Bya, creation of chemical
elements 12-5 Bya, formation of Earth 4.6 Bya,
formation of Moon in Big Whack 4.5 Bya, Large impact
basins on Moon 3.8 Bya, Origin of life 3.8 Bya.
For next time
• Read chap. 4 of Jakosky about the earliest life
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and read “Vital Dust", pages 1-23 in course
packet about origin of life.
Think about your course project. Read sample
project “The Movie 2001 and the Evolution of
Man”. Proposal (1-2 pages) due Feb. 14, 2003.
Next lecture, Friday. Feb 7, 2003, “Origin of Life”.