by: Charles O’Dale
- METEORITE TYPES
- DATING METEORITES
- METEORITE GLOSSARY
Incoming meteoroids enter the earth’s atmosphere at 11 km/sec to 72 km/sec. Ram pressure between the air and the object create a very high temperature plasma at the front of the meteor. This plasma becomes visible at between about 120 km and 75 km above the earth. Energy goes into melting and vaporizing stone and metal. Energy is shed as material ablates. In a couple of seconds most meteors are have been consumed. The left-over debris is called meteoric dust or just meteor dust.
If a meteoroid’s size, composition, speed and entry angle allow it to survive the “meteor” phase of entry, it will slow to about 4 km/sec and enter “dark flight” at 20 km to 15 km above earth. Light emission from incandescence and ion recombination ceases. The meteor will arch into a more vertical trajectory, slow to terminal velocity of about 0.1 km/sec and fall as a meteorite.
If the meteoroid is of sufficient size to keep it’s hyper-velocity >12 km/sec through the atmosphere becoming an IMPACTOR, it will impact the ground and explode. The kinetic energy of an object of mass m traveling at a speed v is = mv2/2, provided v is much less than the speed of light.
2. METEORITE TYPES
IRONS (siderites) — composed chiefly of iron-nickel alloys kamacite and taenite. May also contain plesite, troilite, schreibersite and graphite .
Octohedrites — 6-16% nickel. Polished and etched specimens show Widmanstatten figures. Divided into fine, medium and coarse. Fine: kamacite bands 0.05 – 0.5 mm wide. Medium: kamacite bands 0.5 – 2.0 mm wide. Coarse: kamacite bands wider than 2.0 mm.
Hexahedrites and nickel poor ataxites — contain less than 6% nickel and consist of kamacite only.
Nickel-rich ataxites — contain more than 9% nickel and consist of pure taenite or an irregular mixture of kamacite and taenite.
STONES (aerolites) — composed mostly of ferromagnesian (iron and magnesium) silicate minerals, usually with some metal in the form of grains.
- ACHONDRITE: does not contain chondrules and are poor in metal.
- CHONDRITES: contain generally rounded masses called chondrules embedded in a ground mass. Also 5-25% nickel-iron and often up to 5% troilite.
- EUCRITE: common class of achondrite meteorites composed of pigeonite and plagioclase. These meteorites formed as basaltic flows on a parent body, probably asteroid 4 Vesta. (Data collected by NASA’s Dawn Mission, in orbit around Vesta from 2011-2012, strengthed the association between Vesta and eucrite meteorites.)
- ULTRAMAFIC ACHONDRITE: A meteorite that contains interstitial carbon in the form of graphite or diamond.
STONY-IRONS (siderolites) — contain approximately equal amounts of stony and metal materials.
- MESOSIDERITES: pyrexene-plagioclase stony-irons.
- PALASITES: olivine stony-irons.
A common class of achondrite meteorites composed of pigeonite and plagioclase. These meteorites formed as basaltic flows on a parent body, probably asteroid 4 Vesta. (Data collected by NASA’s Dawn Mission, in orbit around Vesta from 2011-2012, strengthed the association between Vesta and eucrite meteorites.)
3. DATING METEORITES
The Ages of Meteorites
The lead-lead isochron age of chondrules in the CR chondrite Acfer 059 is 4564.7 ± 0.6 million years ago (Ma), whereas the lead isotopic age of calcium-aluminum–rich inclusions (CAIs) in the CV chondrite Efremovka is 4567.2 ± 0.6 Ma. (Amelin Y. 2002)
Meteorites, most of which are fragments of asteroids, are very interesting objects to study because they provide important evidence about the age, composition, and history of the early solar system. There are many types of meteorites. Some are from primitive asteroids whose material is little modified since they formed from the early solar nebula. Others are from larger asteroids that got hot enough to melt and send lava flows to the surface. A few are even from the Moon and Mars. The most primitive type of meteorites are called chondrites, because they contain little spheres of olivine crystals known as chondrules. Because of their importance, meteorites have been extensively dated radiometrically; the vast majority appear to be 4.4–4.6 Ga (billion years) old. Some meteorites, because of their mineralogy, can be dated by more than one radiometric dating technique, which provides scientists with a powerful check of the validity of the results. The results from three meteorites are shown in Table 1. Many more, plus a discussion of the different types of meteorites and their origins, can be found in Dalrymple (1991).
There are 3 important things to know about the ages in Table 1. The first is that each meteorite was dated by more than one laboratory — Allende by 2 laboratories, Guarena by 2 laboratories, and St Severin by four laboratories. This pretty much eliminates any significant laboratory biases or any major analytical mistakes. The second thing is that some of the results have been repeated using the same technique, which is another check against analytical errors. The third is that all three meteorites were dated by more than one method — two methods each for Allende and Guarena, and four methods for St Severin. This is extremely powerful verification of the validity of both the theory and practice of radiometric dating. In the case of St Severin, for example, we have 4 different natural clocks (actually 5, for the Pb-Pb method involves 2 different radioactive uranium isotopes), each running at a different rate and each using elements that respond to chemical and physical conditions in much different ways. And yet, they all give the same result to within a few percent.
Yuri Amelin, Alexander N. Krot Ian D. Hutcheon Alexander A. Ulyanov Lead Isotopic Ages of Chondrules and Calcium-Aluminum-Rich Inclusions Science 2002
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Krot A. Dating the Earliest Solids in our Solar System PSRD Discoveries 2002
4. METEORITE GLOSSARY
ablation: Removal of material from a solid object through vaporization.
accretion: The process of building solids from the accumulation of material.
achondrite: Type of stony meteorite characteristic of most stony meteorites that crystallized from magmas. The term means without chondrules.
aerolite: Another name for stony meteorite.
Albite: Silicate mineral of the plagioclase series – NaAl2Si3O8.
Anorthite: Silicate mineral of the plagioclase series – CaAl2Si2O8.
asteroid: Any of the numerous small rocky bodies in orbit around the Sun. Most asteroids reside in the “main belt” between Mars and Jupiter, but some have Earth crossing orbits.
ataxite: An iron meteorite consisting of either pure kamacite, an irregular mixture of kamacite and taenite, or pure taenite.
bolide: Exploding fireball – “Fire-ball”; meteor exploding in passing through the Earth’s atmosphere.
Bronzite: Pyroxene with 10-20% FeSiO3.
CAI; Calcium-aluminum-rich inclusions (CAIs) are found in chondritic meteorites. CAIs are primitive objects that formed in the solar nebula before the planets formed. CAIs are light-colored objects rich in refractory elements (that condense at a high temperature). Besides calcium and aluminum, this includes magnesium, titanium, and rare earth elements (REE). All share a high temperature origin. Some might be condensates from the solar nebula. Other CAIs might be evaporation residues. G. J. MacPherson et al: Calcium-Aluminum-rich Inclusions: Major Unanswered Questions Chondrites and the Protoplanetary Disk 2005
chondrite: Stony meteorite containing chondrules. Chondrite meteorites are aggregates of preplanetary grains and dust, accreted and compacted into rocks at the beginning of the solar system but still preserving their aggregate character as well as the distinctive individual characteristics of all the diverse particles composing them. Among their constituents are calcium-aluminumrich inclusions (CAIs), which are minor in mass fraction (< 5%) but major in significance: CAIs preserve direct clues to the processes and environments that existed during the nebula phase of our solar system, during its first few million years, prior to the formation of planets. Intensive research over more than 30 years has demonstrated that CAIs are the oldest-known objects that formed in the infant solar nebula, that they formed in an environment that was hot and extremely reducing (consistent with hot hydrogen gas), that their elemental compositions are the result of volatility-controlled processes (evaporation-condensation), that their isotopic compositions retain a component of presolar nucleosynthetic origin, and that they record >1–2 million years of complex post-formation history that included repeated melting and secondary alteration both in the nebula and on asteroidal parent bodies.
Chondrite is an abundant class of stony meteorites with chemical compositions similar to that of the Sun and characterized by the presence of chondrules (see definition below). Chondrites come from asteroids that did not melt when formed and are designated as H, L, LL, E, or C depending on chemical compositions. The H, L, and LL types are called ordinary chondrites. The L chondrites are composed of silicate minerals (mostly olivine and pyroxene, but feldspar as well), metallic nickel-iron, and iron sulfide (called troilite). Most L chondrites are severely shocked-damaged, probably by a large impact on the asteroid in which they formed. The E type are called enstatite chondrites, a rare type that formed under very reducing conditions and are composed primarily of a magnesium silicate called enstatite. They are subdivided into the low-iron (EL) chemical group and the high-iron (HL) group. The C –carbonaceous chondrites– contain water-bearing minerals and carbon compounds including a variety of organic molecules such as amino acids. Carbonaceous chondrites are the most primitive meteorites–primitive in a chemical way. For example, the CI group of carbonaceous chondrites are closest in composition to the photosphere (visible surface) of the Sun.
chondrule: Small nearly spherical aggregate of the minerals olivine and/or pyroxene found in large numbers in most stony meteorites (chondrites). Most chondrules are 0.5 to 2 millimeters in size and are composed of olivine and pyroxene, with smaller amounts of glass and iron-nickel metal. Two main chondrule types have been identified. Type I contain only small amounts of oxidized iron (FeO); olivine crystals in them contain only about 2 mole percent of the iron-rich-olivine fayalite (Fe2SiO4) end member. Type II chondrules contain much more FeO; olivine crystals in them typically contain 10-30 mole percent fayalite. The shapes of the mineral grains in them indicate that chondrules were once molten droplets exceeding 1,027° C and floating freely in space.
Daubreelite: Mineral found in iron meteorites. Usually associated with troilite. Soluble in nitric acid but not in hydrochloric acid. Chemical formula FeCr2S4.
differation: A process in which denser metal sinks to form an iron-rich core, leaving behind a mantle and/or crust of lighter, silicate-rich material.
end-point: The point where a fireball disappears, often in a shower of “sparks”.
Enstatite: Pyroxene with less than 10% FeSiO3.
etch: To corrode a prepared surface with acid for the purpose of revealing structural details.
FCAI – rare type of CAI. Fractionation, UNidentified nuclear isotope properties. FUN CAIs are characterized by 26Al/27Al ratios much lower than the canonical value of ~5×10-5(at the time of our Solar System’s formation); they also can have large isotopic anomalies in many elements.
fireball: Very bright meteor.
fusion crust: The outer covering of a meteorite produced by solidification of melted surface materials formed as a meteorite passes through the atmosphere.
Graphite: A mineral form of the element carbon.
HED: The three linked stony meteorite groups known as the HEDs are howardites, eucrites, and diogenites. They come from asteroid Vesta. (Data collected by NASA’s Dawn Mission, in orbit around Vesta from 2011-2012, strengthed the association between Vesta and HED meteorites.)
- howardites; polymict breccias containing both eucrite and diogenite fragments plus CM, CI and CR chondrite material;
- eucrites; contain equal proportions of Ca-rich plagioclase (An96-75) and pigeonite with minor olivine, chromian spinel, silica minerals, ilmenite, FeNi metal, troilite and phosphates;
- diogenites; orthopyroxene-rich cumulates subdivided into orthopyroxenites and harzburgites containing orthopyroxene (Wo1-3En71-77Fs22-24), olivine, chromite, phosphates, metal and occasional plagioclase.
hexahedrite: Iron meteorite consisting of large cubic crystals of kamacite.
Hypersthene: Silicate mineral of the pyroxene group with the formula (Mg, Fe)SiO3.
iron: Meteorite: Iron meteorites are made, almost completely, of iron and nickel metal. They are chemically distinguished and grouped according to the abundances of the trace elements such as gallium and germanium, as well as nickel. Initially, irons were classified into four groups and were given Roman numerals I, II, III, and IV. Today 13 groups are recognized and designated further by letters A through G according to concentrations of siderophile (“iron-loving”) trace elements. Iron meteorites that do not fit into the groups are called ungrouped. The two iron-nickel alloys in iron meteorites are called kamacite (low-nickel content, usually up to 7.5 wt% nickel) and taenite (high-nickel content, ~20 to 50 wt% nickel). These alloys are rare in terrestrial rocks.
isotope: A combination of similar chemical elements that have different atomic numbers and physical properties.
kamacite: is an alloy of iron and nickel, which is found on earth only in meteorites. The proportion iron:nickel is between 90:10 to 95:5; small quantities of other elements, such as cobalt or carbon may also be present.
Limonite: Hydrous iron oxide having variable composition.
Maghemite: Magnetic variation of the mineral magnetite.
Magnetite: An oxide of iron, Fe3O4.
mesosiderite: Stony-iron meteorite, the stony portion of which consists of the pyroxene and plagioclase minerals.
meteor: The light phenomenon produced by a solid body moving very rapidly through the atmosphere. Popularly called a “shooting star”.
meteorite: A natural solid object from space that retains its identity after having landed on earth. The term is also used to refer to the object in space before colliding with the earth.
meteoriticist: Scientist who is a specialist in the study of meteors and meteorites.
Moldavite: SiO2(+Al2O3) A silica-rich glass (tektite) formed in a burst of heat and energy produced by the impact of a large hypervelocity object with a planet. The rock in the impact area was instantly vaporized by the heat of the impact, then immediately condensed into a molten liquid, and then almost immediately solidified into an amorphous glass. The condensation and solidification were so fast that mineral crystals did not form, and gases were trapped in the moldavite glass.
octahedrite: Most common type of iron meteorite. Contains bands of taenite and kamacite referred to as Widmanstatten structure.
octahedron: A solid geometric form having eight faces.
olivine: The mineral olivine is a magnesium iron silicate abundant in meteorites; (Mg²⁺, Fe²⁺)₂SiO₄. A type of nesosilicate or orthosilicate. It is a common mineral in the Earth’s subsurface but weathers quickly on the surface.
pallasite: A stony-iron meteorite that is a mixture of isolated silicate crystals (usually olivine) surrounded by metal.
path: The projection of the trajectory of a meteor or fireball against the sky as seen by an observer.
Plagioclase: Group of silicate minerals ranging in composition from pure albite to pure anorthite.
planetesimal: Small solid bodies, also protoplanets, within a solar nebula that grow as matter accretes.
Plessite: Nickel-iron mixture of kamacite and taenite.
Pyroxene: General name given to a large group of ferromagnesian silicate minerals abundant in meteorites. The general formula is (Mg, Fe)SiO3.
refractory inclusion: Residues and condensates enriched in elements like calcium, aluminum and titanium found in meteorites. These inclusions are often referred to as Ca-, Al-rich inclusions, or “CAIs.” Most refractory inclusions contain the minerals spinel and melilite and/or hibonite. Refactory inclusions may have formed up to 4 million years before chondrules.
regmaglypts: Shallow depressions resembling thumbprints found on the surface of many meteorites produced by ablation as the meteorite passed through the atmosphere.
Rhabdite: Thin plates of schreibersite appearing as needles on an etched surface of an iron meteorite.
Rubinite (chemical formula: Ca3Ti3+2Si3O12)
Schreibersite: Hard, metallic white, nickel-iron mineral found only in meteorites. Difficult to dissolve in acids and therefore, brilliant on the etched surface of an iron meteorite. Resembles taenite in appearance. Its chemical formula is (Fe, Ni, Co)3P.
siderite: Another name for an iron meteorite.
siderolite: Another name for a stony-iron meteorite.
solar nebula: The sun’s accretion disk.
specific gravity: Ratio of the density of a material to water.
Spinel: MgAl2O4, magnesium aluminum oxide mineral, with Fe+2 able to substitute for Mg and with Cr or Fe+3 able to substitute for Al.
Taenite (Fe,Ni): : A metal found in most meteorites consisting of iron with 27 to 65% nickel. More acid resistant than kamacite, and therefore, more brilliant upon the etched surface of an iron meteorite. Often occurs as thin bands bordering kamacite bands in octahedrites.
train: Anything remaining along the trajectory of a meteor or fireball after the head of the meteor has passed. May be light, dust, vapor, ionization.
trajectory: True line of flight of a meteor or fireball relative to the earth.
Troilite: Bronze-yellow mineral (FeS) found in meteorites. Occurs as rounded nodules or thin plates in iron meteorites and as grains in stony meteorites.
Ureilite: An ultramafic achondrite meteorite that contains interstitial carbon in the form of graphite or diamond.
Widmanstätten Pattern: Figures appearing on an etched surface of an octahedrite. The result of an intergrowth of kamacite and taenite produced as the meteorite parent body cooled in space. A pattern found in iron meteorites due to a change in crystal structure of iron-nickel metal grains during cooling. This structural change produces a pattern of crystallographically oriented kamacite (low-nickel content, usually up to 7.5 wt% nickel) plates in taenite (high-nickel content, ~20 to 50 wt% nickel).
G. J. MacPherson et al: Calcium-Aluminum-rich Inclusions: Major Unanswered Questions Chondrites and the Protoplanetary Disk 2005
Direct evidence of complex prebiotic chemistry from a water-rich world in the outer solar system is provided by the 4.5-billion-year-old halite crystals hosted in the Zag and Monahans (1998) meteorites. This study offers the first comprehensive organic analysis of the soluble and insoluble organic compounds found in the millimeter-sized halite crystals containing brine inclusions and sheds light on the nature and activity of aqueous fluids on a primitive parent body. Associated with these trapped brines are organic compounds exhibiting wide chemical variations representing organic precursors, intermediates, and reaction products that make up life’s precursor molecules such as amino acids. The organic compounds also contain a mixture of C-, O-, and N-bearing macromolecular carbon materials exhibiting a wide range of structural order, as well as aromatic, ketone, imine, and/or imidazole compounds. The enrichment in 15N is comparable to the organic matter in pristine Renazzo-type carbonaceous chondrites, which reflects the sources of interstellar 15N, such as ammonia and amino acids. The amino acid content of the Zag halite deviates from the meteorite matrix, supporting an exogenic origin of the halite, and therefore, the Zag meteorite contains organics synthesized on two distinct parent bodies. Our study suggests that the asteroidal parent body where the halite precipitated, potentially asteroid 1 Ceres, shows evidence for a complex combination of biologically and prebiologically relevant molecules.