Part 1 Composition of Earth Composition of solar system Origin of the elements Part 2...
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Formation of Earth, Moon, and planets (1) Part 1 Composition of Earth Composition of solar system Origin of the elements Part 2 Geochronometry: Age of Earth Formation of Earth and Moon. Differentiation of core and mantle. Isotope tracing: sequence of events.
Part 1 Composition of Earth Composition of solar system Origin of the elements Part 2 Geochronometry: Age of Earth Formation of Earth and Moon. Differentiation
Part 1 Composition of Earth Composition of solar system Origin
of the elements Part 2 Geochronometry: Age of Earth Formation of
Earth and Moon. Differentiation of core and mantle. Isotope
tracing: sequence of events.
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Internal structure known from seismology. Radial distribution
of seismic wave velocity and density. Spherically symmetric
reference model (PREM)
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Composition of Earth? Crust and mantle: mostly silicates Core:
Fe Ni Distribution of elements in Earth
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Abundance of elements in solar system is quite similar (except
for H, He) Abundance measured in Suns photosphere and in
meteorites.
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Where do the elements come from? How did Universe evolve?
Expanding universe Olbers paradox: Why is the sky dark at night?
Hubbles expanding universe Gamows Big bang: only H and He were
formed in Big bang Penzias & Wilson: Cosmic background
radiation Short history of the Universe
Slide 8
Olbers paradox: why is the sky dark at night? Assume universe
is and density of stars and galaxies is uniform Energy received
from distant star ~ r -2 Number of stars between r and r +dr ~ 4 r
2 dr Energy received from universe of radius r ~ r If universe is ,
energy received is
Slide 9
Hubbles expanding universe Universe finite or infinite? Is
Universe in steady state? Einsteins general relativity framework
for cosmology. Steady state solutions. Friedmann-Lemaitre dynamic
universe. Hubble discovers that spectrum of distant stars is
shifted toward the red, i.e. toward lower frequencies. Doppler
shift frequency obs = source /(1+v/c) (v velocity away from source)
Interpretation: Doppler shift. Stars are moving away. The further
they are, the faster they are moving away!
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Age and radius of the Universe Hubble constant = v / r
(velocity / distance) Assuming v constant, star at distance r has
traveled away from us at velocity v for time r/v = 1/H If H = 50
(km/s)/Mparsec (1 parsec = 3 LY = 3 x 3 10 7 x 3 10 5 km = 3 10 13
km) Age = 1/H ~ 20 Gyr Velocity of light c = limit Radius of
Universe when c is reached R = H c (Overestimated if expansion
slows down)
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Hubble constant and age of the Universe Note: It was very
difficult to determine distance. In 1920, the Hubble constant was
over-estimated and the age of the Universe was thought to be 2 Gyr
(i.e. < age of Earth). Present estimate is 13.5Gyr
Slide 13
Alpher, Bethe, Gamow: -- Gamow suggested that synthesis of
elements from elementary particles occurred following the Big Bang.
Using nuclear physics, Gamow et al. predicted that only H and He
could have been synthesized and that the Universe is made of 76 % H
and 24 % He This is roughly what is observed. Question: Where do
the other elements come from? They are synthesized by nuclear
reactions in stars. (Bethe cycle). Other consequence: when
electromagnetic radiation and matter decouple: atoms become stable.
Cosmic background radiation (radiation from decoupling time) must
fill the Universe.
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4 Fundamental forces in physics. Gravity Weak (holds neutron
together) Note that free neutron is not stable n -> p + e + e
Electromagnetic (holds atoms together) Strong (holds nuclei) When
temperature and energy density in Universe decrease, nuclei become
stable. Then as Universe gets colder atoms become stable and
electromagnetic radiation does not interact with matter any more.
Remnant electromagnetic radiation from time of decoupling is cosmic
background radiation
Slide 15
Elements abundance The term nucleosynthesis refers to the
formation of heavier elements, atomic nuclei with many protons and
neutrons, from the fusion of lighter elements. The Big Bang theory
predicts that the early universe was a very hot place. One second
after the Big Bang, the temperature of the universe was roughly 10
billion degrees and was filled with a sea of neutrons, protons,
electrons, anti- electrons (positrons), photons and neutrinos. As
the universe cooled, the neutrons either decayed into protons and
electrons or combined with protons to make deuterium (an isotope of
hydrogen). During the first three minutes of the universe, most of
the deuterium combined to make helium. Trace amounts of lithium
were also produced at this time. This process of light element
formation in the early universe is called Big Bang nucleosynthesis
(BBN).Big Bangisotope The predicted abundance of deuterium, helium
and lithium depends on the density of ordinary matter in the early
universe, as shown in the figure at left. These results indicate
that the yield of helium is relatively insensitive to the abundance
of ordinary matter, above a certain threshold. We generically
expect about 24% of the ordinary matter in the universe to be
helium produced in the Big Bang. This is in very good agreement
with observations and is another major triumph for the Big Bang
theory.density of ordinary matter
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Blackbody radiation Stefans law Total Power radiated ~ T 4 (5.6
10 -8 W m -2 K -4 ) Distribution of energy / frequency (wavelength)
of radiation depends on temperature. By determining power spectrum
of radiation, we can determine temperature.
Slide 18
Radiation and the expansion of the Universe Electromagnetic
radiation in expanding universe. Energy inversely proportional to
wavelength (E=h=hc/) Wavelength of radiation increases in expanding
universe. Energy density decreases (Total energy conserved)
Temperature decreases: Present temperature ~3K
Slide 19
Summary CMB radiation The existence of the CMB radiation was
first predicted by George Gamow in 1948, and by Ralph Alpher and
Robert Herman in 1950. It was first observed inadvertently in 1965
by Arno Penzias and Robert Wilson at the Bell Telephone
Laboratories in Murray Hill, New Jersey. The radiation was acting
as a source of excess noise in a radio receiver they were building.
Coincidentally, researchers at nearby Princeton University, led by
Robert Dicke and including Dave Wilkinson of the WMAP science team,
were devising an experiment to find the CMB. When they heard about
the Bell Labs result they immediately realized that the CMB had
been found. The result was a pair of papers in the Physical Review:
one by Penzias and Wilson detailing the observations, and one by
Dicke, Peebles, Roll, and Wilkinson giving the cosmological
interpretation. Penzias and Wilson shared the 1978 Nobel prize in
physics for their discovery. Today, the CMB radiation is very cold,
only 2.725 above absolute zero, thus this radiation shines
primarily in the microwave portion of the electromagnetic spectrum,
and is invisible to the naked eye. However, it fills the universe
and can be detected everywhere we look. In fact, if we could see
microwaves, the entire sky would glow with a brightness that was
astonishingly uniform in every direction. The temperature is
uniform to better than one part in a thousand! This uniformity is
one compelling reason to interpret the radiation as remnant heat
from the Big Bang; it would be very difficult to imagine a local
source of radiation that was this uniform. absolute
zeroelectromagnetic spectrum
Slide 20
Evolution of early universe (first 3 minutes) Universe expands:
it gets less dense and colder Particles become stable (p+ p- ) (e+
e- ) (p++ e- n + ) Free neutrons are unstable Nuclei form: neutrons
fixed and stable in nuclei At 3000K, atoms become stable. No more
interaction between electromagnetic radiation and matter (atoms)
Radiation cools down in expanding universe
Slide 21
Element abundance in solar system Note peak of Fe
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Star formation and evolution Gravitational collapse yields
energy (~3GM 2 /5R) When pressure and temperature increase in the
collapsing star, there is enough energy to start nuclear fusion
reactions which yield more energy Balance between pressure and
gravity maintains the interior of the star in (non-equilibrium)
steady-state. At the end of the life of star, fuel is burned, star
collapses, with several possible scenarios depending on mass of
star: it will collapse and end as white dwarf, neutron star, black
hole, or explode as nova or super nova) Nova explosion allows
elements heavier than Fe to be removed from reactions and
preserved.
Slide 24
Origin of elements: Stardust. Elements other than H and He do
not come from Big Bang. (Sun is a second generation star!)
Nucleosynthesis in stars. (reactions H + H -> D (H 2 ) D+H >
He 3 He 3 + He 3 -> He 4 + H + H etc. liberate energy) Note the
peak of Fe It corresponds to minimum energy /nucleon Synthesizing
elements heavier than Fe requires that energy is provided Available
in stars, but if heavy elements are not removed, they will react to
return to minimum energy 2 ways to remove heavy elements. Reaction
in star atmosphere and expulsion in space. Explosion of the star
(Nova, Super nova)
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Hypotheses Solar system formation Constraints Sun = 99% of mass
Planets = 99 % of angular momentum Bodes law Distribution of
Elements Recent cosmochemical data (isotopes, etc.) Planets
extracted from sun by passing star (Jeans-Jeffreys) Sun formed then
captured planets from cloud Sun and planets formed together
(Laplace)
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Clues to the Formation of the Solar System Inner planets are
small and dense Outer planets are large and have low density
Satellites of the outer planets are made mostly of ices Cratered
surfaces are everywhere in the Solar System Saturn has such a low
density that it can't be solid anywhere Formation of the Earth by
accretion: Initial solar nebula consists of mixtures of grains
(rock) and ices. The initial ratio is about 90% ices and 10% grains
The sun is on so there is a temperature gradient in this mixture
:
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Take home message?
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The data brings into high resolution the seeds that generated
the cosmic structure we see today. These patterns are tiny
temperature differences within an extraordinarily evenly dispersed
microwave light bathing the Universe, which now averages a frigid
2.73 degrees above absolute zero temperature. WMAP resolves slight
temperature fluctuations, which vary by only millionths of a
degree. Anisotropy in CMB very weak
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The Origin of the Cosmic Microwave Background One of the basic
predictions of the Big Bang theory is that the universe is
expanding. This expansion indicates the universe was smaller,
denser and hotter in the distant past. When the visible universe
was half its present size, the density of matter was eight times
higher and the cosmic microwave background was twice as hot. When
the visible universe was one hundredth of its present size, the
cosmic microwave background was a hundred times hotter (273 degrees
above absolute zero, the temperature at which water freezes to form
ice on the Earth's surface). In addition to this cosmic microwave
background radiation, the early universe was filled with hot
hydrogen gas with a density of about 1000 atoms per cubic
centimeter. When the visible universe was only one hundred
millionth its present size, its temperature was 273 million degrees
above absolute zero and the density of matter was comparable to the
density of air at the Earth's surface. At these high temperatures,
the hydrogen was completely ionized into free protons and
electrons.Big Bangexpanding Since the universe was so very hot
through most of its early history, there were no atoms in the early
universe, only free electrons and nuclei. (Nuclei are made of
neutrons and protons). The cosmic microwave background photons
easily scatter off of electrons. Thus, photons wandered through the
early universe, just as optical light wanders through a dense fog.
This process of multiple scattering produces what is called a
thermal or blackbody spectrum of photons. According to the Big Bang
theory, the frequency spectrum of the CMB should have this
blackbody form. This was indeed measured with tremendous accuracy
by the FIRAS experiment on NASA's COBE satellite.
Slide 41
Nucleosynthesis The term nucleosynthesis refers to the
formation of heavier elements, atomic nuclei with many protons and
neutrons, from the fusion of lighter elements. The Big Bang theory
predicts that the early universe was a very hot place. One second
after the Big Bang, the temperature of the universe was roughly 10
billion degrees and was filled with a sea of neutrons, protons,
electrons, anti-electrons (positrons), photons and neutrinos. As
the universe cooled, the neutrons either decayed into protons and
electrons or combined with protons to make deuterium (an isotope of
hydrogen). During the first three minutes of the universe, most of
the deuterium combined to make helium. Trace amounts of lithium
were also produced at this time. This process of light element
formation in the early universe is called Big Bang nucleosynthesis
(BBN). The quantity of light elements predicted for a given
universe density serves as a double check on density
observationsThe predicted abundance of deuterium, helium and
lithium depends on the density of ordinary matter in the early
universe, as shown in the figure at left. These results indicate
that the yield of helium is relatively insensitive to the abundance
of ordinary matter, above a certain threshold. We generically
expect about 24% of the ordinary matter in the universe to be
helium produced in the Big Bang. This is in very good agreement
with observations and is another major triumph for the Big Bang
theory. However, the Big Bang model can be tested further. In order
for the predicted yields of the other light elements to come out in
agreement with observations, the overall density of the ordinary
matter must be roughly 4% of the critical density. The WMAP
satellite should be able to directly measure the ordinary matter
density and compare the observed value to the predictions of Big
Bang nucleosynthesis. This will be an important and stringent test
of the model. If the results agree, it will be a further evidence
in support of the Big Bang theory. If the results are in conflict,
it will either point to 1) errors in the data, 2) an incomplete
understanding of the process of Big Bang nucleosynthesis, 3) a
misunderstanding of the mechanisms that produce fluctuations in the
microwave background radiation, or 4) a more fundamental problem
with the Big Bang theory. Nucleosynthesis in Stars Elements heavier
than lithium are all synthesized in stars. During the late stages
of stellar evolution, massive stars burn helium to carbon, oxygen,
silicon, sulfur, and iron. Elements heavier than iron are produced
in two ways: in the outer envelopes of super-giant stars and in the
explosion of a supernovae. All carbon-based life on Earth is
literally composed of stardust.
Slide 42
The Big Bang theory predicts that the early universe was a very
hot place and that as it expands, the gas within it cools. Thus the
universe should be filled with radiation that is literally the
remnant heat left over from the Big Bang, called the cosmic
microwave background radiation, or CMB. Noble winners Penzias and
Wilson with the 1965 CMB detector.The existence of the CMB
radiation was first predicted by George Gamow in 1948, and by Ralph
Alpher and Robert Herman in 1950. It was first observed
inadvertently in 1965 by Arno Penzias and Robert Wilson at the Bell
Telephone Laboratories in Murray Hill, New Jersey. The radiation
was acting as a source of excess noise in a radio receiver they
were building. Coincidentally, researchers at nearby Princeton
University, led by Robert Dicke and including Dave Wilkinson of the
WMAP science team, were devising an experiment to find the CMB.
When they heard about the Bell Labs result they immediately
realized that the CMB had been found. The result was a pair of
papers in the Physical Review: one by Penzias and Wilson detailing
the observations, and one by Dicke, Peebles, Roll, and Wilkinson
giving the cosmological interpretation. Penzias and Wilson shared
the 1978 Nobel prize in physics for their discovery. Uniform color
oval representing the temperature variation across the sky of the
CMB.Today, the CMB radiation is very cold, only 2.725 above
absolute zero, thus this radiation shines primarily in the
microwave portion of the electromagnetic spectrum, and is invisible
to the naked eye. However, it fills the universe and can be
detected everywhere we look. In fact, if we could see microwaves,
the entire sky would glow with a brightness that was astonishingly
uniform in every direction. The picture at left shows a false color
depiction of the temperature (brightness) of the CMB over the full
sky (projected onto an oval, similar to a map of the Earth). The
temperature is uniform to better than one part in a thousand! This
uniformity is one compelling reason to interpret the radiation as
remnant heat from the Big Bang; it would be very difficult to
imagine a local source of radiation that was this uniform. In fact,
many scientists have tried to devise alternative explanations for
the source of this radiation but none have succeeded. Since light
travels at a finite speed, astronomers observing distant objects
are looking into the past. Most of the stars that are visible to
the naked eye in the night sky are 10 to 100 light years away.
Thus, we see them as they were 10 to 100 years ago. We observe
Andromeda, the nearest big galaxy, as it was three million years
ago. Astronomers observing distant galaxies with the Hubble Space
Telescope can see them as they were only a few billion years after
the Big Bang. (Most cosmologists believe that the universe is
between 12 and 14 billion years old.) One of the basic predictions
of the Big Bang theory is that the universe is expanding. This
expansion indicates the universe was smaller, denser and hotter in
the distant past. When the visible universe was half its present
size, the density of matter was eight times higher and the cosmic
microwave background was twice as hot. When the visible universe
was one hundredth of its present size, the cosmic microwave
background was a hundred times hotter (273 degrees above absolute
zero or 32 degrees Fahrenheit, the temperature at which water
freezes to form ice on the Earth's surface). In addition to this
cosmic microwave background radiation, the early universe was
filled with hot hydrogen gas with a density of about 1000 atoms per
cubic centimeter. When the visible universe was only one hundred
millionth its present size, its temperature was 273 million degrees
above absolute zero and the density of matter was comparable to the
density of air at the Earth's surface. At these high temperatures,
the hydrogen was completely ionized into free protons and
electrons. Since the universe was so very hot through most of its
early history, there were no atoms in the early universe, only free
electrons and nuclei. (Nuclei are made of neutrons and protons).
The cosmic microwave background photons easily scatter off of
electrons. Thus, photons wandered through the early universe, just
as optical light wanders through a dense fog. This process of
multiple scattering produces what is called a thermal or blackbody
spectrum of photons. According to the Big Bang theory, the
frequency spectrum of the CMB should have this blackbody form. This
was indeed measured with tremendous accuracy by the FIRAS
experiment on NASA's COBE satellite. FIRAS SpectrumThis figure
shows the prediction of the Big Bang theory for the energy spectrum
of the cosmic microwave background radiation compared to the
observed energy spectrum. The FIRAS experiment measured the
spectrum at 34 equally spaced points along the blackbody curve. The
error bars on the data points are so small that they can not be
seen under the predicted curve in the figure! There is no
alternative theory yet proposed that predicts this energy spectrum.
The accurate measurement of its shape was another important test of
the Big Bang theory. Surface of Last Scattering Eventually, the
universe cooled sufficiently that protons and electrons could
combine to form neutral hydrogen. This was thought to occur roughly
400,000 years after the Big Bang when the universe was about one
eleven hundredth its present size. Cosmic microwave background
photons interact very weakly with neutral hydrogen. CMB Surface of
Last Scattering compared to looking up at a cloud surface.The
behavior of CMB photons moving through the early universe is
analogous to the propagation of optical light through the Earth's
atmosphere. Water droplets in a cloud are very effective at
scattering light, while optical light moves freely through clear
air. Thus, on a cloudy day, we can look through the air out towards
the clouds, but can not see through the opaque clouds. Cosmologists
studying the cosmic microwave background radiation can look through
much of the universe back to when it was opaque: a view back to
400,000 years after the Big Bang. This wall of light is called the
surface of last scattering since it was the last time most of the
CMB photons directly scattered off of matter. When we make maps of
the temperature of the CMB, we are mapping this surface of last
scattering. As shown above, one of the most striking features about
the cosmic microwave background is its uniformity. Only with very
sensitive instruments, such as COBE and WMAP, can cosmologists
detect fluctuations in the cosmic microwave background temperature.
By studying these fluctuations, cosmologists can learn about the
origin of galaxies and large scale structures of galaxies and they
can measure the basic parameters of the Big Bang theory. Additional
information for reading
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Steps in the accretion process Step 1: accretion of cm sized
particles Step 1: Step 2: Physical Collision on km scale Step 2:
Step 3: Gravitational accretion on 10-100 km scale Step 3: Step 4:
Molten protoplanet from the heat of accretion Step 4: Final step is
differentiation of the earth: Light objects float; heavy objects
sink. Iron-Nickel Core (magnetic field) and oxygen-silicon crust In
the outer part of the solar system, the same 4 step process of
accretion occurred but it was accretion of ices (cometisemals)
instead of grains.
Slide 53
Planetary accretion: energy aspects(1). When planet starts to
form. Nucleus collides with other bodies (planetisimals).
Collisions give energy. Kinetic energy is converted into heat. How
much energy is available. Assume that accretion brings together
particles from infinite distance. Gravitational potential energy
converted to kinetic energy which is converted to heat. Energy
avalailable: self potential energy of a sphere E = 3 G M 2 / 5R
Energy / unit mass = 3 G M / 5 R (This is a big number!!!) (G
gravitational ~ 6.6 10 -11 N m 2 /kg 2 (m 3 /s 2 /kg), R radius, M
mass of Earth)
Slide 54
Planetary accretion: energy aspects (2). When planet becomes
hot, it radiates energy. Black body radiation Total energy radiated
= (4 R 2 T 4 ) But impacts cause dense cloud of dust How much
energy can be radiated depends on how long it takes for the planet
to accrete. What happens when core forms?
Slide 55
Things to note about the formation of planets via accretion
There is a lot of heat dissipated in the final accretion process
resulting in initially molten objects Any molten object of size
greater than about 500 km has sufficient gravity to cause
gravitational separation of light and heavy elements thus producing
a differentiated body The accretion process is inefficient, there
is lots of left over debris. In the inner part of the solar system,
the leftover rocky debris cratered the surfaces of the newly formed
planets. In the outer part of the solar system, much of the
leftover rocky debris was ejected from the solar system due to the
large masses of the planets which formed there. Some of this
material was ejected into a large "Comet Cloud" which has a
distance of about 100,000 AU from the Sun and some of the leftover
debris ( beyond Pluto) could not be ejected (as it was far away
from Uranus and Neptune) and hence remained there. This material is
known as the Kuiper Belt and it was recently discovered by the
Hubble Space Telescopediscovered by the Hubble Space Telescope