Introduction to MeteoritesVishnu Reddy

Basic TerminologyMeteoroid – 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)

Very bright due to high entry velocity and/or large mass

Hannover, 1995 FireballLeonid Shower, 1999

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 MeteorsMeteors 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 FallMeteoroid 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 InteriorsDuring entry, surface material is During entry, surface material is removed faster than heat is removed faster than heat is conducted into the interiorconducted into the interior

Thin (a few millimeters or less) fusion Thin (a few millimeters or less) fusion crust (glassy melted layer) coats the crust (glassy melted layer) coats the outside of meteoriteoutside of meteorite

Interior does not see the heat of Interior does not see the heat of atmospheric entryatmospheric entry

Meteorites are Meteorites are not hotnot hot when they when they reach the ground!reach the ground!

Interiors are typically cold, reflecting Interiors are typically cold, reflecting their temperature in space, which their temperature in space, which preserves delicate materialspreserves 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” versus “Falls”

““Finds” are meteorites discovered at some Finds” are meteorites discovered at some time after they fell to Earthtime after they fell to Earth

Their terrestrial age may range from a few Their terrestrial age may range from a few days to about a million years (Antarctic days to about a million years (Antarctic finds)finds)

Some fossil meteorites have been found in Some fossil meteorites have been found in limestones of Ordovician age (~470 Myr)limestones of Ordovician age (~470 Myr)

With increasing terrestrial age, the meteorite With increasing terrestrial age, the meteorite progressively deteriorates until it disappearsprogressively deteriorates until it disappears

FindsFindsFinds are much more abundant than falls.Finds are much more abundant than falls.

Among the non-Antarctic finds, iron Among the non-Antarctic finds, iron meteorites are very abundant.meteorites are very abundant.

To find a meteorite, one must recognize that To find a meteorite, one must recognize that it is an it is an unusual rockunusual rock or a or a rock in an unusual rock in an unusual placeplace::

Stony meteorites often look like common rocks.Stony meteorites often look like common rocks.

Iron meteorites are often quite distinctive (very Iron meteorites are often quite distinctive (very dense, very hard).dense, very hard).

An iron meteorite is much more likely to be An iron meteorite is much more likely to be recognized as an oddity.recognized as an oddity.

Rocks in an unusual placeAntarctic 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 Most meteorites have been recovered from the “blue recovered from the “blue ice” regions of Antarctica.ice” regions of Antarctica.

These are areas where These are areas where moving ice stagnates moving ice stagnates against barriers and against barriers and evaporates.evaporates.

Meteorites in the ice are Meteorites in the ice are transported to these transported to these regions.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 TypesThe 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 MeteoriteWidmanstätten 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 MeteoritesTwo major types

PallasitesOlivine in a NiFe metal matrix

Formed at a core-mantle boundary in a differentiated parent body

MesosideritesBasaltic 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 MeteoritesPallasite

Olivine & NiFe Metal

MesosideriteBasalt 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 rocksWide 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 bodyTypes 3 to 6

Type 3 (not metamorphosed) Type 6 (most metamorphosed)[e.g., LL3, L6, H5, EL5, CV4, etc.]

Aqueous Alteration - within parent bodyTypes 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, HowarditesExcept 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 AchondritesNearly 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]

Anom. Fe-richAnom. Si-rich

IronsPallasites

MesosideritesHEDs

UreilitesAubrites

CI Chon.CM Chon.

CO Chon.CV Chon.

LL Chon.L Chon.

H Chon.E Chon.

H, L, and LL-chondrites make up >75% of falls

Planetary Meteorites

Lunar Meteorites Martian Meteorites

~27-30 different lunar 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 > terrestrial volcanic glasses.– Silica >> meteorites or lunar samples.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

Layered (Muong-Nong type)

Round

Disk

Teardrop

Oval

Barbell

Button (Flanged)

Cigar

Layered

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

~300 mile / ~ 500 km

Between Button (Australia) and Layered Tektites (SE Asia)

Splash-form Tektites

Distribution of Tektite Forms in S.E. Asia and Australia

MNSF

BFMicro

Formation of TektitesTektites 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

Thailand

Philippines

AcknowledgementAcknowledgement

Thanks to Dr Mike Gaffey for Thanks to Dr Mike Gaffey for the slides and pictures used in the slides and pictures used in

this talk.this talk.

Questions?Questions?