Transcript document

Introduction to Meteorites
Vishnu Reddy
Basic Terminology
Meteoroid – A small natural rock or metallic
object in space.
– “Small” is somewhat arbitrary (e.g., <10 meters)
– Larger bodies would be designated as “asteroids”
or “comets”
Meteor – The visual phenomena (fireball,
light trail, etc.) produced by a meteoroid
entering the atmosphere at high velocity
(“shooting star”).
Meteorite – A natural rock or metallic object
from space which has fallen to the Earth’s
surface.
A METEOROID approaching
the Earth.
A METEOR formed by a
meteoroid entering the
atmosphere at high
velocity.
A METEORITE which is a part
of the meteoroid that survived
atmospheric entry and reached
the ground.
Fireballs (Bolides)
Leonid Shower, 1999
Hannover, 1995 Fireball
Very bright due to high entry velocity and/or large mass
How does a meteoroid (or a portion)
survive atmospheric entry?
Meteoroid survival depends of dissipating its
kinetic energy without vaporizing the entire
body
– Low angle of entry  long flight path allows
dissipation of energy over longer period
– Low entry velocity  minimizes the amount of
kinetic energy which must be dissipated
All tracked and recovered meteorites had
Ventry < 20 km/sec
Meteor Showers
Short intervals of intense
meteor activity.
Most meteor showers are
associated with debris
streams along the orbits of
comets.
No known meteorite fall has
been associated with a
meteor shower (high entry
velocity).
1833 Leonid Shower
Sporadic Meteors
Meteors which occur throughout the year and are
not part of recognized showers
Appear to derive from both asteroids and comets
High velocity meteors (brightest) are primarily of
cometary origin
Lower velocity meteors (most likely to survive
atmospheric entry) are mostly of asteroidal origin
All known meteorite falls are from sporadic meteors
Typical Meteorite Fall
Meteoroid first encounters atmospheric resistance at
~110 km elevation.
Resistance increases as the meteoroid penetrates
deeper into the atmosphere.
At ~100 km, the friction of hypervelocity passage
through the atmosphere begins to produce strong
heating and a meteor becomes visible.
If the meteoroid is sufficiently strong, deceleration
continues down to ~30 km altitude, at which point
the meteorite is in free fall.
The meteoroid is now just a falling rock which takes
~5 minutes to reach the ground (meteorite).
Meteorite Interiors
During entry, surface material is
removed faster than heat is
conducted into the interior
Thin (a few millimeters or less)
fusion crust (glassy melted layer)
coats the outside of meteorite
Interior does not see the heat of
atmospheric entry
Meteorites are not hot when they
reach the ground!
Interiors are typically cold, reflecting
their temperature in space, which
preserves delicate materials
Peekskill Meteorite (12.4 kg)
and 1980 Chevrolet Malibu
When police arrived on the scene,
they filed a report for criminal
mischief by a very strong male.
The smell of gas from the
punctured gas tank prompted the
fire department to investigate, at
which time they found the
meteorite.
If the meteorite had been “burning
hot” as sometimes described, it
would have set the car on fire.
Meteor associated with the October 9,
1992 fall of the Peekskill (NY) Meteorite
“Falls” versus “Finds”
Meteorites which are observed to fall (or
which are detected immediately after the
event) are designated “Falls”
Falls are important because:
– They are fresh (not altered by terrestrial
environment)
– There is little bias in identifying them
Falls provide pristine samples, especially
important for the detection of extraterrestrial
organic compounds in meteorites
Falls give good statistics on the relative
abundances of meteorite types reaching the
Earth in recent times
“Finds” versus “Falls”
“Finds” are meteorites discovered at some
time after they fell to Earth
Their terrestrial age may range from a few
days to about a million years (Antarctic
finds)
Some fossil meteorites have been found in
limestones of Ordovician age (~470 Myr)
With increasing terrestrial age, the meteorite
progressively deteriorates until it disappears
Finds
Finds are much more abundant than falls.
Among the non-Antarctic finds, iron
meteorites are very abundant.
To find a meteorite, one must recognize that
it is an unusual rock or a rock in an unusual
place:
Stony meteorites often look like common rocks.
Iron meteorites are often quite distinctive (very
dense, very hard).
 An iron meteorite is much more likely to be
recognized as an oddity.
Rocks in an unusual place
Antarctic Meteorites
Since the discovery of a concentration of meteorites
near the Yamato Mountains by a Japanese team in 1969,
more than 20,000 meteorites have been recovered from
Antarctica.
Locations of Antarctic
Finds
Most meteorites have been
recovered from the “blue
ice” regions of Antarctica.
These are areas where
moving ice stagnates
against barriers and
evaporates.
Meteorites in the ice are
transported to these
regions.
Significance of the Antarctic Finds
These meteorites fell
up to a million years
ago.
They have less
sampling bias than
other finds.
The large sample
allows discovery of
rare types.
Meteorite Types
The different types of meteorites are
distinguished based on:
– Mineralogy (type and abundance of minerals
present)
– Petrology (textures of mineral assemblage)
– Genesis or formation processes
Iron meteorites
Stony-iron meteorites
Stones
– Chondrites
– Achondrites
Iron meteorites (“Siderites”)
“sider-” from Latin for “iron” or “star”
Composed primarily of NiFe metal with
accessory troilite (FeS) and silicates
Older structural classification based on:
– Ni content in metal
– Widths of the crystal layers in the Widmanstätten
pattern if present
Newer chemical classification based on:
– Ni, Ge & Ga contents
– ~80 different types
Etched Surface of Iron Meteorite
Widmanstätten Pattern
Generally wide bands of
kamacite separated by
narrow bands of taenite.
Most very large meteorites are irons
Cape York Iron – 31 tonnes
American Museum of Natural History
Iron Meteorites
Formed by melting of metal-bearing parent
material (~chondrites).
Dense liquid metal segregated to the core of
parent body.
The known iron meteorites represent at least
80 different parent bodies.
Iron meteorites have long (up to several
billion years) cosmic ray exposure ages.
– High strength allows long lifetime as meteoroid in
space
Stony-Iron Meteorites
Two major types
Pallasites
Olivine in a NiFe metal matrix
Formed at a core-mantle boundary in a
differentiated parent body
Mesosiderites
Basaltic clasts in NiFe metal matrix
Mixture of crust and core lithologies from
differentiated parent body(ies)
Newer or rare stony-iron-like meteorites
Stony-Iron Meteorites
Pallasite
Mesosiderite
Olivine & NiFe Metal
Basalt clasts & NiFe Metal
Stony Meteorites
Chondrites
– Limited range of compositions
– ~“Solar” composition (for non-volatile
elements)
– Sedimentary and meta-sedimentary rocks
Achondrites (“Not-chondrites”)
Igneous rocks
Wide range of compositions produced by
differentiation in magmatic systems
Chondrites
The name “chondrites” - from “chondrules” - is
an anachronism.
Chondrules are small spherical inclusions,
apparently crystallized melt droplets.
However, not all chondrites have chondrules.
Later definition was that “chondrites” had
undifferentiated solar compositions.
Current definition of “chondrite” is an
undifferentiated sample of inner solar nebular
materials (grains, etc.).
Chondrites: Chemical Subtypes
Chemical Groups – formed and/or accreted
in different parts of the solar nebula
Distinguished by different elemental
abundance ratios (e.g., Mg/Si, Fe/Si, etc.)
Ordinary chondrites - H, L, LL, (HH, L/LL)
Carbonaceous chondrites - CI, CM, CV, CO,
CR, CK, CB
Enstatite chondrites - EH, EL
Other chondrites - R, K, (F)
Chondrites
Metamorphic / Alteration Subtypes
Thermal Metamorphism - within parent body
Types 3 to 6
Type 3 (not metamorphosed)
Type 6 (most metamorphosed)
[e.g., LL3, L6, H5, EL5, CV4, etc.]
Aqueous Alteration - within parent body
Types 3 to 1
Type 3 (unaltered)
Type 1 (most altered)
[e.g., CI1, CM2, CV3, etc.]
Achondrites - I
Stony meteorites with “non-solar”
compositions
Igneous rocks formed by melting and
density-controlled phase segregation within
a parent body
Basaltic Achondrites (HEDs)
Eucrites, Diogenites, Howardites
Except for one sample, all appear to come from a
single parent body, asteroid 4 Vesta
Ureilites
Carbon-rich, ultramafic (olivine) assemblage
Achondrites - II
Aubrites / Enstatite Achondrites
Nearly pure enstatite (iron-free pyroxene)
Angrites [Angra dos Reis / ADOR]
Assemblages dominated by high Al, Ti & Ca
pyroxenes
Primitive Achondrites
– Chondrite-like compositions but have undergone
small (~1% to >10%) degrees of partial melting
– Lodranites/Acapulcoites
– Winonites / Silicates in IAB Iron meteorites
Meteorite Type Fall Frequency
Meteorite Fall Frequencies [Wasson, 1974]
E Chon.
H Chon.
L Chon.
LL Chon.
CV Chon.
CO Chon.
CM Chon.
CI Chon.
Aubrites
Ureilites
HEDs
M esosiderites
Pallasites
Irons
Anom. Si-rich
Anom. Fe-rich
H, L, and LL-chondrites make up >75% of falls
Planetary Meteorites
Lunar Meteorites
~27-30 different
lunar meteorites
Martian Meteorites
~26 different
Martian meteorites
Current Directions in Meteoritics
Identification of new meteorite types,
especially among the large Antarctic sample
collection
– R-type chondrites (Rumurutites)
– CB-type chondrites (Bencubbenites)
Identification of specific asteroidal parent
bodies of meteorites
Better understanding of meteorite origins
Tektites
Tektites are small glassy objects, usually
black or green in color.
Compositionally restricted, high silica,
natural glasses.
– Silica > terrestrial volcanic glasses.
– Silica >> meteorites or lunar samples.
Most tektite shapes were produced by
aerodynamic forces in the atmosphere.
Deposits of microtektites are found on
the sea-floor around the major strewn
fields.
Tektite Shapes
Round
Disk
Teardrop
Oval
Barbell
Button (Flanged)
Cigar
Layered
Layered (Muong-Nong type)
Australian Button or Flanged Tektite
Flange on Button Tektite
Formation:
Molten glob blown out of the atmosphere.
Solidifies as sphere.
Re-enters atmosphere.
Front melts and flows forming flange.
Aerodynamic Sculpting of a
Button Tektite
Button Tektite
Muong Nong (layered) Tektites
“Shape-less" tektites originally discovered at
Muong Nong in Laos.
These tektites were blocky and fragmental in shape
and had conspicuous layering.
Very much larger than other tektites.
Muong Nong, Laos
Layered tektites now known from Thailand,
Laos, Cambodia, Vietnam and S. China
Between Button (Australia) and Layered
Tektites (SE Asia)
Splash-form Tektites
Distribution of Tektite Forms in S.E.
Asia and Australia
MN
SF
Micro
BF
Formation of Tektites
Tektites are not from the Moon.
Large explosion at or near Earth’s
surface
– Cometary or weak asteroidal body breaking
up in atmosphere?
Flash melting of surface soil
Ejection of melt
Proximal melt forms “puddles”
Mid-range ejecta forms “splash-form”
tektites
Distal ejecta forms button tektites
Distribution of Tektite Forms
Global Distribution of Tektites
Tektites are not found randomly on the
Earth.
The major tektite fields are located in:
–
–
–
–
Austral-Asia
Ivory Coast
North American
Eastern Europe (Czech Republic)
Their ages range from 770,000 years old
(Australasian) to 35 million (North
America)
Major Tektite Strewn Fields
Ages of Tektite Events
Strewnfield
Age
Australasian
0.77 Myr
Ivory Coast
1.1 Myr
Argentine
3.3 Myr
Czech and Slovak
14.7 Myr
North American
35.7 Myr
Libyan
< 35 Myr
Significance of Tektites
Tektites were formed during large impact
events (comets or low density asteroids).
There have been at least five tektite
forming events in the past 35 Myr.
The ~770,000 yrbp Australasian event
would have effected early humans in that
region, and perhaps world-wide.
If an event of the Australasian scale were
to occur today, it would probably kill a
large fraction of the human race.
During the Australasian event, molten
glass was falling back to the surface
more than a 1000 km from the impact site
Philippines
Thailand
Acknowledgement
Thanks to Dr Mike Gaffey for
the slides and pictures used in
this talk.
Questions?