Chapter 6: Meteor ages and origins
History of Meteorite Studies
• Read interesting history in textbook, p. 131
• “Stones from the sky” observed by ancient Chinese, Greeks,
Romans
• Only became accepted in early 1800s
• Huge collections of meteorites found in Antarctic ice fields in
1969
 Falling on ice less damaging then on rock
 Easier to find (contrast with surrounding terrain)
 Less subject to erosion
Classification
• Differentiation forms the main basis of classification
• Many are breccias: formed of broken fragments cemented
together
 Breccias and fractures suggest violent collisions
(a) A stony meteorite often has a dark crust, created when its surface is melted by
the tremendous heat generated during passage through the atmosphere. (b) Iron
meteorites usually contain some nickel as well. Most show characteristic crystalline
patterns when their surfaces are cut, polished, and etched with acid.
Ices
• solids that contain C,N,O – which are gaseous at T≥200K
• CNO overall more abundant than Fe,Mg,Ca,Al,Na…
 but Fe,Mg,…condense into grains at much higher T
• ices are more abundant in outer SS objects – i.e. satellites,
comets, some asteroids, Kuiper Belt…
• commonest ices: CH4 (methane), NH3 (ammonia), H2O (water)
• vapourous “at the least excuse”
Core of Halley’s comet
shown outgassing as
Sun heats the ice
Stone meteorites
• Stony meteorites are the most common (95%)
• Include the least differentiated meteorites
• Most numerous are the chondrites: a class that contain small
spherules called chondrules
Ordinary chondrites
are the most common
type of stone
meteorite,
Carbonaceous (C)
chondrites are some
of the most complex
of all meteorites.
Enstatite (E)
chondrites are a
rare and unusual
type of meteorite.
Achondrite meteorites are
very similar in appearance
to terrestrial igneous rocks.
Chondrules
• Glassy spherules embedded in meteorites
• Composed primarily of olivine and pyroxene, with moderately high
melting temperatures.
• Form during rapid cooling of droplets of molten material
• Formation?
 Possibly formed in
shock waves within
original solar nebula
 Or during impacts on
surfaces of
planetesimals.
Chondrites
• Chondritic meteors have
nearly solar abundances
of elements – except for
volatiles
 the oldest SS
samples
 formed from early
solar nebula
 most are
unchanged since
formation
Carbonaceous Chondrites
• High abundance of carbon, mostly in the form of graphite grains,
silicon carbide and mixtures of organic molecules
• Lowest temperature condensates – formed at low temperature and
have not been altered since.
• Only account for 5% of falls, but
they are more common in space:
 Easily broken up in atmosphere
 Dominant type of meteorite in
lunar soil
• Probably come from outer
asteroid belt or comets (>2.5 AU)
based on spectral analysis of
distant bodies
Carbonaceous Chondrites
•
•
•
•
•
CI
Closest to solar composition
Higher volatile content (up
to 22% water bound to
other minerals)
Low density, only 2200
kg/m3
No heating, but high
brecciation
Actually: have no
chondrules!
CM and CV
• 2-16% bound water
• Breccias are common
CO
• Only about 1%
bound water
• Breccias are
rare
Ordinary Chondrites
•
•
•
•
•
Most numerous meteorites
Similar chemical composition to CCs; also have not been melted
Do not have the carbon and water-bearing matrix
Slightly more processed than CCs
Probably formed among terrestrial planets
 Spectra of inner asteroids indicate they have similar compositions
 Same proportions of O isotopes in Earth, Moon and Mars rocks
Parent bodies of chondrites
• Minor heating did affect some chondrites
 Cooling rate can be deduced from the properties of individual
crystals in the metal particles
 In a large body, the outer layers act as insulation: the deeper inside
you go, the longer the cooling time.
• Chondritic material was typically insulated by overlying material up
to about 50 km thick.
 Fits the theory that chondrites are fragments of asteroids
Achondrites
• Coarse crystal structure, which indicates slow cooling in insulated
surroundings
• Most similar to terrestrial igneous rocks.
• Non-solar chemical compositions
• Iron and other metals is purely metallic
• Produced when parent
material (probably a
chondrite) melted
 Melting would destroy
chondrules
 Iron would drain away
leaving the silicate
material typical of
achondrites
Eucrites
• A type of achondrite that is
lavalike, basaltic igneous rock
 Formed from the solidification of
molten material
 Probably from the asteroid 4
Vesta, which is the only asteroid
whose spectrum shows it has a
eucritelike lava surface
• HST found evidence for a huge
crater on 4Vesta, supporting this
theory
Iron meteorites
• Iron meteorites commonly
present large-sized crystals,
being compounds of two ironnickel alloy varieties.
• The large-sized crystals
indicate that they cooled
more slowly – probably deep
inside a larger, parent body.
Falls and Finds
Falls are meteoroids seen in their flight through the atmosphere
and located on Earth by following that trajectory.
Finds are meteorites discovered serendipitously.
Meteorite
Type
Falls
%Falls
Finds
%Finds
Total
%Total
42
5%
681
27%
723
22%
9
1%
59
2%
68
2%
Stones
781
94%
1741
71%
2522
76%
Total
832
100%
2481
100%
3313
100%
Chondrites
712
91%
1667
96%
2379
94%
Achondrites
69
9%
74
4%
143
6%
Irons
Stony-Irons
Of the Stones:
Note that stones represent a much larger % of falls than finds;
presumably this is because stones are more likely to be eroded by
wind, water etc. and they are also more similar in appearance to
normal Earth rocks.
Break
Impact Rates
•
Estimate of total meteoroid flux range from
107-109 kg/yr.

•
•
•
•
i.e. At least ~1 kg every second !
Most are very small, micrometeorites that do
not hit the ground
Objects large enough to hit the ground
subsonically and form craters occur about once
per year
Giant explosions about once per century
Most (6/7) falls occur over oceans or poles so
go unnoticed.
Impact Rate
Primeval impact rates
• Analysis of lunar surface: compare dates of surface rock to the
number of craters to determine how impact rate changes with
time.
• Little information about conditions >4 Gyr ago, before the oldest
surfaces were formed.
Radioactive decay age measurement
• Many elements have several isotopic forms, some of which are
unstable and decay into other elements.
• Radioactive decay obeys a simple law: the probability that a given
isotope will decay into its “daughter” isotope is constant,
independent of time and the original number of atoms.
• Mathematically: dn/dt = -λn where λ is the decay constant
(units=#/sec).
 Integrating this from t=0 to t=t gives a classical exponential relation:
n(t) = n(0)e-λt.
 In a given sample we can measure n(t) and we know λ for a given
decay process; if we can somehow determine n(0) we can find t.
Radioactive decay age measurement
• Consider two isotopes r (the radiogenic/unstable) and s (the
stable decay product).
• Initially (t=0) the sample will start with some atoms of the
unstable isotope, r0, and some of the stable, s0.
• When we measure the sample at some later time (t) it contains
fewer atoms of the unstable isotope:
r (t )  r0e
and more of the stable: s (t )
 t


 s0  r (t ) et  1
Radioactive decay age measurement
r (t )  r0e
 t


s(t )  s0  r (t ) et  1
• If we measure s and r for different pieces of a given meteorite,
we could make a plot which has (hopefully) a linear slope given by
et-1
 However, we cannot be sure that r0 and s0 were the same throughout
the sample.
 So compare the abundances to a stable isotope of the daughter (s),
call it s.
t
0


s
s(t )
r (t )


e 1
s(t ) s(t ) s(t )
Rubidium-Strontium System
• One common method uses
isotopes of Rubidium and
Strontium
• 87Rb is a radioactive
element that decays into
87Sr with a half-life of
48.8 Gyr
 Measure abundances
relative to the stable
isotope 86Sr
Half-life
Decay constants are usually given in terms of the half-life, the time
it takes for the sample to decay to half its initial mass. What is
the relationship between half-life t and the decay constant ?
Example
1993 observations of chondrules in the Allende meteorite, which
fell as a 2 ton fireball in Mexico, 1969.
Analysis of the whole
rock indicates an
age of 4.5 Gyr.
Suggests in this case
the chondrules
were disturbed by
a later event.
Ages
• Ages for the oldest meteorites are found to be 4.566±0.002 Gyr.
• There is a significant difference between the oldest ordinary
chondrites at 4.563±0.001 Gyr and the oldest achondrites at
4.558±0.001 Gyr.
• Thus the formation of planetesimals began within a few million
years of the earliest grains condensing out of the protosolar
nebula and the planetesimals themselves formed over a period of
~10Myr.
Where do they come from?
• There are few places in the Solar System where small bodies
could have survived for so long.
• Even on circular orbits, in between large planets, most small
bodies will be perturbed onto planet-crossing orbits.
Where do they come from?
• There are few places in
the Solar System where
small bodies could have
survived for so long.
1. Asteroid belt
 Between Mars and
Jupiter
2. Trojans
 Lagrangian points along
Jupiter’s orbit
3. Kuiper belt
 Outside Neptune’s orbit
4. Oort cloud
 Huge reservoir of
comets outside
heliosphere
Origins of meteorites
• Orbits of Earth-approaching meteors have been measured for
some using networks of automatic cameras
 All are found to have aphelia in or near asteroid belt
Orbit reconstruction
•
•
•
•
The fireball producing the Tagish Lake meteorite on 18 Jan 2000 was
witnessed at dawn in the Yukon and NWT.
70 eyewitnesses interviewed
24 still photos and 5 videos were obtained; a subset of these had
sufficient foreground structure to permit angular measurements
From a synthesis of various data, the orbital parameters could be
measured:
semimajor axis
2.1 AU
eccentricity
0.57
perihelion distance
0.891
aphelion distance
3.3 AU
inclination
1.4 degrees
orbital period
1072 days
entry velocity
15.8 km/s
Collisions in the asteroid belt
•
How do meteors get out of the asteroid belt?
•
What is the typical collision time between asteroids?
•
Requires 3 stages
1. Initial collision ejects fragments
2. Fragments 0.1-10 m diameter would drift due to Yarkovsky effect



Sunlight warms one side of a larger body.
The warm side rotates away from the Sun and radiates thermal energy
as photons which provide a “thrust”
This can move particles either in or out
3. Hit an orbital resonance which send them into orbits intersecting
Earth’s
Lunar meteorites
• Found in Antarctica,
just following the
American and Russian
trips to the moon
 Subsequently found
in hot deserts:
Australia, Africa,
Oman
• Mostly originate from
the far side of the
moon, and other
regions we have not
directly sampled
Martian meteorites
•
•
•
•
•
•
Lavalike types: shergotites, nakhlites, chassignites (SNC)
Most examples are basaltic and only 1.3 Gyr old
Oxygen isotopes ratios show they are not from Earth or moon.
Asteroids cooled too early to produce such young lava.
Contain N and noble gases matching those found on Mars by the
Viking lander
Crater counts on Mars indicate widespread basaltic lava flows
1.3 Gyr ago
Simulations show 8% of particles knocked off Mars in a big
collision would impact Earth.