Astronomy 330

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Astronomy 330

• Lecture 24

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Astronomy 330 Meteorites

• Meteorites are defined to be extraterrestrial
  fragments which reach the surface of Earth.
• Meteors while in the atmosphere, meteroids while
  in space
• Meteorites reach the Earth frequently and their
  stones are recovered for study.
• These fragments have a stony or a metallic
  composition and weigh a few kg.
• Large falls of meteorites can also occur with 100
  to 1000 kg making it to Earth (most often these
  are broken up into many smaller pieces.

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Astronomy 330

• Others, as we have seen are much larger and
  produce impact craters or explode in the
  atmosphere.
• There are many different types of meteorites.
• This suggests the parent bodies (asteroids) are
  also varied.
• Types are divided into stony, iron, and
  stony-iron.
• Stony-iron meteorites are rare.

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Astronomy 330

• Iron meteorites are nearly pure metallic
  nickel-iron and have a high density of 7 g/cm3.
• They are easily recognizable as extraterrestrial
  since iron and other metals on Earth are
  oxidized.
• Stony meteorites resemble Earths rocks and
  stony-metallic meteorites are a mixture of the
  two.

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Astronomy 330

• A 2nd, more useful categorization is also used
  primitive or differentiated and is based on
  chemical properties.
• If the meteorites chemistry resembles the early
  solar system materials it is called primitive.
• Primitive meteorites have been altered little
  since the formation of the solar system.
• All primitive meteorites are stony.
• Meteorites which have experienced chemical change
  since their formation are called differentiated.

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Astronomy 330

• Differentiated meteorites solidified out of a
  molten state and appear to be pieces of larger,
  differentiated bodies that lost their volatiles
  and experienced much heating.
• All iron, stony-iron and many of the stony
  meteorites are differentiated.
• Some meteorites have uniform compositions, others
  seem to have been broken and mixed by impact
  processesthese are brecciasremember what
  breccias are?

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Astronomy 330 Origins of Meteorites

• Sky monitoring (continuous observation of the sky
  in a systematic way) has only seen 3 meteorites
  as they entered and traveled through the Earths
  atmosphere.
• Doing this has allowed reconstruction of their
  orbits around the sun and shown them to have come
  from the asteroid belt.

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Astronomy 330 Ages

• Using radioactive isotope dating, all primitive
  meteorites have solidification ages of 4.5 - 4.6
  billion yearsthe age of the solar system.

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Astronomy 330 Primitive Meteorites

• Have chemical compositions that have remained
  unchanged and are similar to the Suns (except
  that they have no H, He, Ar, C, and O).
• These meteorites are used to define solar
  abundances of rare elements which cannot be
  directly detected by observing the Sun.
• Another name for primitive meteorites is
  chondritesthey contain chodrules.

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Astronomy 330

• Chondrules are about 1 mm in diameter and appear
  to be frozen droplets.
• Not all Chondrites contain chondrules.
• Appear as light gray rocks.
• Many are breccias and their density is about 3
  g/cm3, similar to Earths rocks.
• Many show evidence of having been heated in the
  past or the effects of liquid waterthese are
  partially modified meteorites and not completely
  primitive.

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Astronomy 330

• Many primitive meteorites contain grains of
  metallic iron which makes up about 10-30 of the
  total weight.
• Primitive meteorites are therefore further
  classed by their metal content.
• H (high iron), L (Low iron) and LL (very low
  iron).
• Also contain silicon, oxygen, magnesium and
  sulfur.
• Since all the primitive meteorites have the same
  age they all formed together in the solar system.

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Astronomy 330 Carbonaceous meteorites

• The most primitive of the primitive meteorites.
• Rich in C (a few percent by weight) and also
  volatile compounds such as water.
• Almost no metallic iron.
• Dark gray to black in color.
• Less dense, 2.5 g/cm3.
• Probably formed in the cooler regions of solar
  nebula to have retained volatiles.

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Astronomy 330

• Carbonaceous meteorites also contain complex
  organic compounds (up to 3 of their
  composition).
• This was the first evidence that complex carbon
  chemistry can occur outside the Earth and not be
  associated with life.
• Also, this indicates that meteorites may have
  played a role supplying the early Earth with the
  primitive carbon compounds from which life arose.

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Astronomy 330

• Some of these meteorites also have been shown to
  have amino acidsthe Murchison meteorite.
• These meteorites contain left-handed and
  right-handed forms of these organic molecules.
  Life on Earth uses exclusively the left-handed
  form. This indicates the meteorites have not
  been contaminated.
• Also the C12/C13 ratio is different in the
  meteorites when compared to that on the Earth.

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Astronomy 330 Differentiated Meteorites

• Composed of materials that have been melted AFTER
  they condensed from the solar nebula.
• Their parent bodies underwent some process of
  heating differentiated.
• Also called achondrites.
• Iron meteorites are the best example of this type
  and consistent of almost pure metallic
  nickel-iron (trace quantities of sulfur, carbon,
  and other metals such as platinum are also
  detected).

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Astronomy 330

• Nickel makes up about 10 by weight of such
  meteorites.
• Iron meteorites make up of 2 of all meteorites
  falling to Earth.
• Such meteorites are fragments of the metallic
  cores of their parent bodies.
• These parent bodies must have been on the order
  of the size of todays planets to have contained
  metallic cores.
• Chemical analysis indicates that there must have
  been at least 12 different sources for these iron
  meteorites.

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Astronomy 330

• The stony-metal meteorites are thought to have
  originated from the location where the parent
  body transitions from its metallic core to a
  silicate mantle.
• The pure iron meteorites cannot be dated. The
  stony-irons can and again are 4.5-4.6 billion
  years old. Hence, their fully differentiated
  parent bodies must have been around at this time.

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Astronomy 330

• These is a group of differentiated stony
  meteorites as well which seem to have come from
  the crusts of these differentiated parent bodies.
• They are composed of basalts, crystallized lavas
  (like on Earth or the Moon).
• Also, they are breccias so they must have been
  present on the surfaces of these parent bodies
  for a long time.
• Several of these type of meteorites are thought
  to have come from specific objectsVesta and Mars.

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Astronomy 330 Source of Primitive Meteorites

• Must have been small (less than a few hundred km)
  and condensed directly out of the solar nebula.
• Large bodies would have retained too much heat
  and altered the meteorites from their primitive
  state.
• Must have included organic compounds and liquid
  water.
• Further, the variety of the chemical composition
  of the primitive meteorites indicates that their
  must have been many different parent bodies.

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Astronomy 330 Source of Differentiated Meteorites

• Parent bodies must have been no larger than 100
  km in diameterroughly the same size as the
  undifferentiated parent bodies.
• Why did one set of bodies differentiate and the
  other did not? We dont know!

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Astronomy 330 Reading

• Read Chapter 4 of Morrison and Owen