Meteorites and their record of
formation of our Solar System
EAS 302 Lecture 3
How can we know how the Solar
System and the Earth formed?
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Meteorites: preserve a record of early
events in our Solar System (at least the
inner part)
Observing star formation elsewhere in
the universe
Laws of physics; understanding chemical
behavior of the elements
Observing our own solar system:
distribution of the elements, mass,
angular momentum, etc. in our own solar
system.
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Meteorites
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Much of the evidence
for how the solar
system formed comes
from meteorites.
Meteorites are simply
stones that fall from
the sky.
Most come from
asteroids of the
asteroid belt (judging
from reconstructed
orbits).
A few from Mars and
the Moon.
Meteorites
 Meteorites
were once
parts of larger
bodies:
asteroids and
planetesimals.
 Broken up as
orbits are
perturbed by
Jupiter,
causing
collisions
among
asteroids
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Meteorites are usually
divided into three
classes:
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Irons
 iron-nickel alloy
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Stones
 mostly silicates (like
Earth’s mantle & crust)
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Stony-Irons
 Mixture of the above
A better classification is:
 Primitive Meteorites
 Chondrites
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These meteorites contain
information about the
formation of the Solar
System
Differentiated
Meteorites
 Irons, stony-irons and nonchondrule-bearing stones
called achondrites.
 Achondrites are similar to
igneous rocks
 These meteorites provide
information about the
formation of planets
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Some ‘Cosmochemical’ Definitions:
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Volatile: elements with low boiling points
or forming compounds with low boiling
points
Refractory: Elements with high boiling
points or forming compounds (oxides)
with high boiling points.
Chondrules and
Chondrites
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Chondrites are socalled because they
contain chondrules.
Chondrules are mmsized spheres that
were once molten.
Show evidence of
rapid cooling.
Many different
mineralogical kinds,
but individuals are
mostly monomineralic.
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Chondrites
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Classes
 Carbonaceous (C)
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compositions close to that of the Sun for non-volatiles
 Ordinary (H, L, LL)
 Enstatite (E)
 (plus several newly discovered classes)
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Classes related by variation in:
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metal content
volatile element content
state of oxidation
Chemical variations between classes thought to
be due to processes in solar nebula
Many chondrites contain decay products of
radioactive elements synthesized shortly before
they formed.
Constituents of Chondrites
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Chondrules
 once molten droplets: give evidence of high temperatures
for short times
 Likely formed in shock waves.
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Refractory Inclusions
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highly refractory compounds such as Al, Ti, Ca oxides
rich in Platinum group metals and refractory trace elements
gas condensates or refractory residues
give evidence of very high temperatures
Matrix
 More volatile material, metal; rich in C, H (e.g., carbonates,
hydrated silicates, and organic molecules (in carbonaceous
chondrites)
 Ameboidal Olivine (condesnate from solar nebula).
 Includes, in some cases, presolar grains
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Composition of Carbonaceous
Chondrites
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The most “primitive” of
the chondrites are the
carbonaceous
chondrites, particularly
those of class CI.
Except for the most
gaseous elements, their
composition matches
that of the Sun.
CI meteorites consist
only of “matrix”; no
chondrules.
Considered to be
collection of variably
processed dust from the
solar nebula
So why the STARDUST
Mission?
Differentiated Meteorites
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Achondrites
 igneous or brecciated
(broken-up) textures
 similar to igneous rocks
from Earth and Moon
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Irons
 Appear to have cooled
from molten iron
 textures give evidence of
slow cooling: interiors of
asteroid size bodies (10
to 1000 km diameter)
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Chemical variations
reflect melting and
segregation on parent
bodies
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Meteorite Ages
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Most meteorites give ages close to 4.556 Ga by
most dating methods. (Ga = billion years)
Primitive meteorites generally slightly older than
differentiated ones, but not by much
Virtually all events recorded in meteorites
occurred within few 10’s of millions of years of
4.56 Ga.
A small group of achondrites gives much
younger ages (e.g., 1.3Ga). These are the SNC
meteorites, thought to be from Mars; there are
also a few rare meteorites from the Moon.
Cosmic ray exposure ages much younger:
meteorites came from larger bodies.
Isotopic “Anomalies” in Meteorites
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Some parts of some
meteorites contain
elements whose
isotopic composition is
different from that of
other materials from
solar system.
Most noteworthy in
meteorites of classes
CV and CM.
Anomalies found in
 Calcium-Aluminum
Inclusions (CAI’s)
 Chondrules
 Matrix
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Kinds of Isotopic Anomalies
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Decay products of short-lived radioactive
nuclides:
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26Mg
(from 26Al – t1/2 = 0.7 Ma)
(from 129I – t1/2 = 16 Ma)
53Cr (from 53Mn – t
1/2 = 3.7 Ma)
129Xe
Variation in isotopic composition of many
elements, particularly C, N, Si, in minute
diamonds, SiC, and graphite of groundmass.
Variation of O isotopic composition among all
meteorite classes
Variation in isotopic composition of all elements
in a few rare CAI’s
However, most elements in most meteorites
have uniform isotopic composition - so the solar
nebula was mostly well mixed.
Meaning of Isotopic Anomalies?
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One or more (likely the latter) episodes of
heavy element nucleosynthesis shortly
before the solar system formed.
 Red Giant(s) and/or Supernovae
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Some radiogenic nuclides (particularly
10Be and possibly 26Al) may have been
produced by spallation resulting from
collisions with energetic atoms
accelerated by the magnetic fields of the
young Sun.
Short-lived radionuclides can be used to
deduce the chronology of events in the
early Solar System.
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Summary: what do meteorites tell us?
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The solar system formed 4.56 billion years ago.
Features of the most primitive meteorites
(chondrites) are consistent with formation from a
cloud of gas and dust.
At least locally, the solar nebula was very hot – hot
enough to melt some material and vaporize other
material.
Planetesimals formed and differentiated very
quickly (segregation of iron to form cores).
 Time scales of a few million to a few tens of millions of years.
 Planetesimals were hot enough to melt extensively.
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The solar nebula was, for the most part, well mixed.
One of more episodes of nucleosynthesis occurred
very shortly before the solar system formed - hence
there were red giants and supernova explosions in
the neighborhood.
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