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.

Very bright due to high entry velocity and/or large mass 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 Olivine & NiFe Metal Mesosiderite Basalt clasts & NiFe Metal

Stony Meteorites Chondrites Achondrites (“Not-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 H, L, and LL-chondrites make up >75% of falls

~27-30 different lunar meteorites ~26 different Martian meteorites 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 >> 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.

Layered (Muong-Nong type) 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

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 ~300 mile / ~ 500 km 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?