Planetary Differentiation on Earth and Its Implications: From The Solar Nebula to Today Guest Scient

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Planetary Differentiation on Earth and Its
Implications From The Solar Nebula to
TodayGuest Scientist Kevin Wheeler

• Originally Presented
• 10 Feb 2007

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Culmination of a Quest

• The next few months mark the culmination of
  Kevin Wheelers five year quest to discover more
  about Earths interior than anyone has ever
  knownin short, he will soon defend his Doctoral
  dissertation!
• Today, Kevin shares with use some of his
  investigations during this period.
• We wish him Good Luck!

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Todays Program

• We first consider some theories about the origin
  of the Earth, especially those that explain
  possible origins and changes into the complex,
  multi-layered planet it is today.
• Next, well consider in general terms what we
  know about Earths interior
• Finally, well review some aspects of
  Radioactivity, key to heating the interior

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How, putting it briefly, did the Solar System
come into existence?

• The Solar System formed when a cold,
  slowly-rotating cloud of gas and dust collapsed
  because of its own gravity about 4.5 billion
  years ago. As the Sun grew hot enough to ignite
  the nuclear reactions which sustain it today, it
  vaporized the cold ices and frozen gasses in the
  inner solar system, leaving behind the rocky dust
  and metals which form the inner planets. The
  outer Solar System remained cold, and the ices
  and gas there collected into the giant outer
  planets.
• The problem with this scenario is that we now
  have observations of planetary systems around
  other stars -- and few if any of them resemble
  our Solar System.
• http//www.ifa.hawaii.edu/faculty/barnes/a
  st110_01/fotss.html

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This videoclip summarizes current theories about
our origins

• Origin of the Solar System (3.4 Mbyte avi
  animation)http//www.ifa.hawaii.edu/faculty/barn
  es/ast110_01/fotss.html

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Planetary formation from the solar nebula

• One generally accepted theory is that the
  nebula from which our Solar System formed was
  composed of gas and dust. Somehow in that cloud,
  the Sun formed in the center and the planets
  formed around it. The inner, rocky planets formed
  by accretion--they accumulated dust and rocks to
  become planets. Studying the physics of planet
  formation and countless computer simulations
  reveal three stages in the accretion of the
  planets. During the first stage, dust
  grains stuck to each other until objects were
  large enough to begin to attract material with
  their gravity fields, producing objects the size
  of asteroids (up to a few hundred kilometers in
  diameter).

This discussion comes from http//www.psrd.hawaii
.edu/Dec98/OriginEarthMoon.html
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• During the second stage, a period of
  runaway growth took place, leading to tens of
  objects much larger than the Moon. Most of the
  mass of the inner Solar System was contained
  within these planetary embryos. It may have taken
  only about a million years from the end of stage
  1 to the end of stage 2.

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• During the final stage, these huge objects
  whacked into each other, creating larger planets,
  but a smaller number of them. The entire process
  was dominated by large impacts, making the
  formation of the Moon by a giant impact a natural
  consequence of planet formation. Simulations
  indicate that the third stage took 100 to 200
  million years, about the time estimated from
  isotopic data on rocks from Earth, the Moon, and
  meteorites.

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• Impact events, like the ones that formed
  Meteor Crater about 50,000 years ago in Arizona
  and the Manicouagan impact structure about 210
  million years ago in Quebec, represent the
  dominant process of planetary accretion (growth)
  and surface restructuring. Planets without
  significant tectonic reworking, weathering or
  erosion of their surfaces have old surfaces that
  reflect numerous impacts during their early
  growth stages. Although the rate of impacting has
  diminished over the past 4.5 billion years, these
  events still happen periodically, occasionally
  with enough energy to cause massive destruction.

http//wapi.isu.edu/Geo_Pgt/Mod03_PlanetaryEvo/mod
3_pt1.htm
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Another View of the Creation of Our Planet

• Earth passed through four distinct phases
  from its formation to the present. They are
  fairly typical phases for all terrestrial planets
  to have gone through. The better we understand
  Earth, the more we can infer to other planets
  such as Mars.

http//www.windows.ucar.edu/tour/link/earth/earth
.html
Much of the following discussion comes from
http//starryskies.com/solar_system/Earth_html/un
der_the_surface.html
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First stage Differentiation

• As the materials that became Earth gathered
  together, they underwent a separation according
  to density. The most dense (iron and nickel, for
  example) settled downward towards the center of
  the planet, and lighter materials (such as
  oxygen, hydrogen gas, argon) stayed near the
  surface and in the atmosphere. The densest
  minerals formed the core of the Earth, while the
  lighter silicate minerals formed the crust.

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Second stage Heavy Cratering

• As Earth solidified, impacts from
  objects left the typical cratering marks we can
  still see on the Moon, Mars, and Mercury. During
  the early history of the solar system, there much
  have been a great deal of debris left over from
  planet-making which floated around the planets.
  The early Earth's surface much have resembled the
  Moon's as we see it today, heavily cratered with
  craters on top of craters. As the debris began to
  clear, cratering slowed down.

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Third stage Flooding

• As radioactive decay of some elements
  heated up the Earth's interior, lava began
  upwelling through fissures in the Earth's crust.
  Lava flooded crater impacts and other basins.
  Outgassing of water and cooling of the atmosphere
  caused rain to condense. These first flooding
  rains began to fill the primordial oceans and
  lakes.

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Fourth/current stage Surface Evolution

• This process began perhaps 3.5 billion years
  ago soon after rain began to fall. Crustal
  movements result in uplifting to produce folded
  mountains in some areas, fault-block and volcanic
  mountains in others. Running water, wind, and ice
  continually weather and erode the surface. Over
  most continents, a thin veneer of sediment covers
  igneous and metamorphic rocks The Rock Cycle.

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Differentiation from a homogenous body into a
heterogeneous one

• Heat buildup inside Earth reached a maximum
  early in the Earth's history and declined
  significantly since. Greater heat due to
• greater abundance of radioactive elements,
• greater number of impacts, and
• early gravitational crowding.

This and the next slide are based on
http//www.geology.sdsu.edu/how_volcanoes_work/Hea
t.html
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• Initial accretion of particles resulted in a
  rather homogeneous sphere composed of a loose
  amalgam of metallic fragments (iron meteorites),
  rocky fragments (stony meteorites), and icy
  fragments (comets).
• Increased heat content of the early Earth
  resulted in melting of the Earth's interior, so
  that the young planet became density stratified
  with the heavier (metallic) materials sinking to
  the center of the earth, and the lighter (rocky)
  materials floating upward toward the surface of
  the earth.
• Very lightest volatile materials (derived from
  comets) were easily melted or vaporized, rising
  beyond the earth's rocky surface to form the
  early oceans and the atmosphere.

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How Can We Interpret the Interior?

• Technology does not allow us to dig a hole
  to China. So how can we understand Earths
  interior? One of the first clues came from
  working out Earths magnitude. Earths average
  density is 5.52 g/cm3, but most surface rocks
  range from 2.7 3.0 g/cm3. The best explanation
  was a core composed of dense elements, such as Fe
  and Ni.

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Evidence from Outer Space

• Meteorites generally belong to one of three
  types Stony, similar to surficial rocks
  Stony-Iron and Metallic, mostly Fe with some
  Ni and other heavy elements. We now speculate
  that many meteorites come from the Asteroid Belt
  between Mars and Jupiter, and are the remains of
  a planet that went through differentiation and
  then broke apart.

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Evidence from Seismic Waves

• Studying records of P- and S-waves moving
  through Earths interior led to theories thaton
  a very broad levelthere are three layers beneath
  the crust mantle, outer core, and inner core.

http//www.seismo.unr.edu/ftp/pub/louie/class/100/
interior.html
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Our usual image of Earth

• The crust, on which we live, is very thin
• The mantle has denser materials interacting with
  the crust in plate tectonics
• The outer core is liquid Fe and other elements
• The inner core is solid Fe and Ni

http//www.seismo.unr.edu/ftp/pub/louie/class/100/
interior.html
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We now know that Earths interior is subdivided
into many zones. Kevin will present some of the
evidence to support this understanding.
http//www.windows.ucar.edu/tour/link/earth/image
s/earthint_image.htmledumid
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Todays models are much more detailed and
sophisticated

• Red blobs are warmer plumes of less dense
  material, rising principally into the ocean-ridge
  spreading centers. A huge plume seems to be
  feeding spreading at the East Pacific Rise
  directly from the core. Most of the heat being
  released from the Earth's interior emerges at the
  fast-spreading East Pacific Rise

http//www.seismo.unr.edu/ftp/pub/louie/class/100/
interior.html
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What Are Conditions within Earth Interior?

• Density, pressure, and temperature within Earths
  interior will obviously have major impacts on
  how our planet behaves and even the materials at
  various depths.
• In general, tremendous pressures and very high
  temperatures control what minerals or other
  substances will exist.

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NYS Earth Science students and teachers are
familiar with this image representing conditions
within Earths Interior
http//emsc32.nysed.gov/osa/reftable/esp10-16.pdf
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Radioactive Decay

• One final concept we must consider in our
  introductory overview is Radioactivity.
• As most of you know, broadly speaking, this
  involves the transformation of an atom of one
  chemical element into an atom of another, with
  the release of energy.
• This energy release has played a major role in
  the interior heating of our planet.

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Two Basic Types of Radioactive Decay

• Alpha Decay
• Parent Atom emits an alpha particle (like He
  nucleus 2 P and 2 N)
• Energy released
• Atomic Number drops by 2
• Mass Number drops by 4
• U-238 ? Th-234 a energy

• Beta Decay
• Parent element emits a beta particle (like an
  electron)
• Energy released
• Atomic Number increases by 1
• Mass Number remains the sameTh-234 ? Pa-234 ß
  energy
• C-14 ? N-14 ß energy

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• With this general background, we may be ready
  to greet Kevin Wheeler and listen to his
  discussion about
• Planetary Differentiation on Earth and Its
  Implications From The Solar Nebula to Today