AstronomyGeology 330 Seminar on Asteroids Tuesdays 47 pm Kendade Hall 203 Tom Burbine tburbinemtholy

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Astronomy/Geology 330Seminar on
AsteroidsTuesdays 4-7 pmKendade Hall 203Tom
Burbinetburbine_at_mtholyoke.edu
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In Two Weeks

• Kendade 305
• Bill Irvine will talk about Comets, the
  Interstellar Medium, and Life
• Then pizza
• Then a discussion about getting into graduate
  school

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For credit for coming to class those weeks we
have speakers

• You need to write down a question to ask the
  speaker and hand it in to me

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Also

• I have adjusted the speaking schedule for student
  presentations since one speaker had to move dates

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How old is the universe?
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How old is the universe?

• 13.7 billion years
• Estimates today are usually between 10 and 15
  billion years

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How old is the solar system?
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How old is the solar system?

• 4.6 billion years
• All meteorites tend to have these ages
• Except

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How old is the solar system?

• 4.6 billion years
• All meteorites tend to have these ages
• Except
• Martian meteorites
• Lunar meteorites

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Ages

• Ages

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How do you determine this age?
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Radioactive Dating

• Basic principles
• -dn ?n
• dt
• n is the number of atoms
• ? is the decay constant
• rearrange
• dn -?n
• dt

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Remember

• Number of original atoms (parent atoms)
• number of daughter atoms today number of
  parent atoms today

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Then

• ? dn -? ? dt from t 0 to t
• n
• from no to n
• no atoms present at starting time (t 0)
• n atoms present at time t
• ln(n/no) -?t
• n noe-?t
• Half life ln 2/? 0.693/?

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What are the assumptions to get an age?
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What are the assumptions?

• No loss of parent atoms
• Loss will increase the apparent age of the
  sample.
• No loss of daughter atoms
• Loss will decrease the apparent age of the
  sample.
• No addition of daughter atoms or if daughter
  atoms was present when the sample formed
• If there was, the age of the sample will be
  inflated
• These can all be corrected for

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Basic Formula

• Number of daughter atoms formed number of
  parent atoms consumed
• If there were daughter atoms originally there
• D Do no - n
• Remember n noe-?t so no n e?t
• D- Do n e?t n
• D Do n (e?t 1)

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Commonly Used Long-Lived Isotopes in
Geochronology
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How do you determine isotopic values?
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How do you determine isotopic values?

• Mass Spectrometer

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It is easier

• To determine ratios of isotopic values than
  actual abundances

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Example

• 87Rb ? 87Sr electron antineutrino energy
• Half-life is 48.8 billion years
• 87Sr 87Srinitial 87Rb (e?t 1)
• 87Sr 87Srinitial 87Rb (e?t 1)
• 86Sr 86Sr 86Sr

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Example

• Formula for line
• 87Sr 87Srinitial (e?t 1) 87Rb
• 86Sr 86Sr 86Sr
• y b m x

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(e?t 1)
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Problem Set 4 Due Next Week

• Taken from an example from Hyperphysics at
  Georgia State University

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Solar Nebula

• Gaseous cloud from which the solar system is
  believed to have formed from

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How did the Solar System form

• Began as a cloud of interstellar gas
• Began to collapse
• Raised the interior temperature higher and higher

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Tends to form a Disk
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How does the Sun turn on

• The temperature and density of the center of the
  cloud increased until nuclear fusion could take
  place
• Temperatures over 10 million Kelvin are needed
  for fusion

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Condensation Sequence

• Temperature (K) Mineral
• 1758 (1513) Corundum, Al2O3
• 1647 (1393) Perovskite, CaTiO3
• 1625 (1450) Melilite, Ca2Al2SiO7-Ca2MgSi2O7
• 1513 (1362) Spinel, MgAl2O4
• 1471 FeNi metal
• 1450 Diopside, CaMgSi2O6
• 1444 Forsterite, Mg2SiO4
• 1362 Anorthite, CaAl2Si2O8
• 1349 Enstatite, Mg2Si2O6
• lt 1000 Alkali-bearing feldspar,
  (Na,K)AlSi3O8-CaAl2Si2O8
• lt 1000 Ferrous olivines, (Mg,Fe)2SiO4 Ferrous
  pyroxenes, (Mg,Fe)2Si2O6
• 700 Troilite, FeS
• 490 Sepentine, (Fe.Mg)3Si2O5(OH)
• 405 Magnetite, Fe3O4
• 400 Talc, Mg4Si4O10(OH)4

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Condensation Sequence

• Temperature (K) Mineral
• 1758 (1513) Corundum, Al2O3
• 1647 (1393) Perovskite, CaTiO3
  CAIs
• 1625 (1450) Melilite, Ca2Al2SiO7-Ca2MgSi2O7
• 1513 (1362) Spinel, MgAl2O4
• 1471 FeNi metal
• 1450 Diopside, CaMgSi2O6
• 1444 Forsterite, Mg2SiO4
• 1362 Anorthite, CaAl2Si2O8
• 1349 Enstatite, Mg2Si2O6
  enstatite chondrites
• lt 1000 Alkali-bearing feldspar,
  (Na,K)AlSi3O8-CaAl2Si2O8
• lt 1000 Ferrous olivines, (Mg,Fe)2SiO4 Ferrous
  pyroxenes, (Mg,Fe)2Si2O6
  ordinary chondrites
• 700 Troilite, FeS
• 490 Sepentine, (Fe.Mg)3Si2O5(OH) CM
  chondrites
• 405 Magnetite, Fe3O4 CI chondrites
• 400 Talc, Mg4Si4O10(OH)4
• lt 273 Ice Comets

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Which meteorite type has the composition most
similar to the solar nebula?
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Does not include very volatile elements
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Volatile Elements

• Volatile Elements elements that condense at low
  temperatures
• For example, some very volatile elements
• H
• He
• C
• O
• N

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Would expect

• That these very volatile elements would be
  abundant in comets
• Comets may have compositions more similar to the
  solar nebula

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How do planets form?

• Dust particles condense
• Collide
• Form larger particles
• Form asteroidal-sized bodies (planetesimals)
• As the planetesimals get bigger, they form larger
  bodies
• These become planets

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Why

• Do we have 9 planets?

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• The colors indicate the
• fraction of the planets mass
• consisting of water, going
• from gray (no water), through
• red, yellow, green, light blue,
• dark blue, and white. Each
• progressive color implies
• 5 times more water.

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What heats the asteroids?
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Radioactive Decay

• Generally thought to be due to 26Al
• 26Al ? 26Mg electron energy
• Half-life of only 700,000 years
• Releases lots of energy
• If 0.005 of all the aluminum in a chondrite was
  26Al, (most is aluminum-27, which is not
  radioactive), it would release enough energy to
  melt asteroids a few kilometers across and larger
  

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Any questions?