(11-10-2004 version)
(A presentation made to the Geological Society of So. Nevada, UNLV, Nov. 18, 2004)
Title: Meteorites, Cosmic Dust & Their Parent Bodies
by:
Maxwell Blanchard, Planetary Scientist
Retired from: NASA - Ames Research Center & Johnson Space Center
Currently at: Astronomy Instructor, CCSN
Summary
Mineralogical & geochemical analyses of meteorites & cosmic dust when combined with
information gained from the many unmanned missions to asteroids & comets are yielding
significant information about the early formation of our solar system. Most, if not all, of the
meteorites found on Earth originate from asteroids, comets, moons & planets in our solar system.
In contrast, the origin of cosmic dust includes: pre-solar nebula, stars beyond our own sun,
comets, & C type asteroids.
Meteorites
Meteorites are fragments from asteroids, comets, planets & moons in our solar system. As such,
by analyzing their mineralogy & geochemistry, the conditions under which they were formed can
be deduced.
On the basis of their mineralogy & geochemistry meteorites can be divided into 3 general groups:
Iron, Stony-Iron, & Stony.
Meteorite compositions and parent bodies:
Iron meteorites are composed typically of Ni-Fe metal. Evidence indicates they originated from
cores of parent asteroids that were destroyed when the parent asteroid was impacted by another
asteroid, or comet. Ages of Iron meteorites have been reported to be over 4.5 billion years.
Stony-Iron meteorites are sub-divided into two sub-groups: pallasites & mesosiderites. The
pallasites are composed typically of Ni-Fe metal & olivine. The mesosiderites are composed
typically of Ni-Fe metal & fragments of achondrite meteorites (i.e. basalts). Evidence indicates
these meteorites originated from fragments from a core & a basaltic crust that was released from
the parent body when it was impacted by another asteroid, or comet. Ages of mesosiderites have
been reported to be 3.9 - 4.5 billion years.
Stony meteorites can be sub-divided into: achondrite & chondrites.

Achondrite meteorites are typically composed of common basaltic minerals (e.g.
plagioclase & pyroxene). Evidence indicates these meteorites are fragments from a
parent body that experienced crystallization from a magma (i.e. differentiation). Later,
they became mixtures of fragments of the crust as they were ejected from the parent
body when it was impacted by another asteroid, or comet. Some of these meteorites
have been confirmed as basaltic rocks from Mars & the Moon, while others have been
confirmed as originating from asteroids (e.g. Vesta). All achondrite meteorites have
experienced shock metamorphism sometimes transforming the plagioclase into
maskelynite, a shock formed glass. Ages of some achondrites from Mars have been
reported in the range of 0.3 - 1.3 billion years while those from some asteroids have been
reported to be 4.5 billion years.
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
Chondrite meteorites are typically composed of olivine, pyroxene & Ni-Fe metal. Their
textures are aggregational & consist of mixtures of various proportions of the following
components:
o Chondrules which are millimeter size , spheroidal, olivine grains (often with
pyroxene & Ni-Fe metal grains) & make up about 70% of the volume,
o High temperature refractory inclusions, irregular in shape & range in size from
millimeter to the limit of microscopic visibility containing nonvolatile elements
(e.g. Al, Ca, & Ti) & make up about 10% of the volume,
o Matrix material, sub-micron porous mineral material that fills the remaining space
between the chondrules & refractory inclusions.
Evidence indicates that all three of these components of the chondrites formed from
undifferentiated cosmic matter while dispersed in space & condensed from the gases & dust in
the pre-solar nebula. The chondrite meteorites, therefore, formed planetesimals that eventually
grew into the planets, moons, asteroids & comets currently found in our solar system. As a group,
the chondrites are among the oldest matter in our solar system. About 87% of the meteorites that
fall to Earth are chondrites. No chondrites exhibit properties of ever having experienced the
process of differentiation. Ages of chondrite meteorites are over 4.5 billion years.
A fascinating sub-group within the chondrite meteorites is the carbonaceous chondrites. These
are composed typically of the same mineral constituents as the ordinary chondrites plus: water,
hydrated silicates, & carbon (e.g. amino acids). Analyses of material in the matrix of the
carbonaceous chondrites have identified grains with isotopes that indicate some are from stars
other than our own sun.
Cosmic Dust
Cosmic dust consists of sub-micron to micron sized aggregates that have been collected: in
space by spacecraft; in the upper atmosphere by high altitude aircraft, rockets, & balloons; from
glacial ice cores. These aggregates do not resemble any terrestrial material. Many exhibit an
unusual porosity suggesting that at one time they were heterogeneous mixtures of dust & ice.
The mineralogy is complex. Often olivine, pyroxene, Ni-Fe, FeS, hydrated silicates, & carbon coexist in the same aggregate. Many contain deuterium which is interpreted as a signature of the
interstellar medium. Recently, aggregates have yielded anomalous oxygen isotopes that
demonstrate some components are from stars beyond our Sun. The evidence indicates the
origins of cosmic dust include: the pre-solar nebula, stars other than our own sun, comets & the C
type asteroids.
Comets
In 1986, the Giotto spacecraft recorded the first close up image of a comet (i.e. Halley). In 2004,
the Stardust spacecraft obtained much better close up images of the comet Wild 2. These images
confirmed that comets are basically "dirty snowballs". It is expected that the Stardust mission will
return samples of comet Wild 2 & the Japanese mission, Hayabusa, plans to return samples from
the comet Itokawa.
Typically, cometary masses are about the size of small asteroids (i.e. 1012 - 1016 kg). Comets
orbit our Sun beyond the orbit of Neptune (i.e. > 30 AU) and their numbers increase in the Kuiper
belt with most residing in the Oort cloud (50 - 50,000 AU). As a comet's orbit nears the Sun, the
ice sublimates releasing a brilliant colored tail of gases & dust particles that may be as long as
100 x 106 km. As a result of multiple passages around the Sun, eventually, the comet
disintegrates. The debris remaining in orbit from the comet results in meteor streams (e.g. Leonid
meteor showers that occur every year in Nov. & were released from a comet named Temple Tuttle that disintegrated after multiple orbits around the Sun).
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That comets are weak was further indicated in 1994 when the comet Shoemaker-Levy 9 was torn
apart into many pieces by the gravitational pull of Jupiter as the comet was in orbit around the
Sun. As a result, all fragments went into orbit around Jupiter. Galileo, Voyager 2, & Hubble Space
Telescope imaged these fragments impacting into Jupiter's atmosphere.
Spectra taken of the tails of comets indicate they are: elements, molecules & compounds having
low atomic numbers (e.g. H, OH, CH, NH, C, N, O) that were driven away from the comet's
surface by solar radiation as the frozen gases sublimed. The dust tails of comets are material that
has been described above in the section entitled cosmic dust.
Evidence indicates that comets are aggregates of pre-solar ices & dust that accumulated in the
far reaches of our solar system during the time that the planets, moons & asteroids were forming.
Asteroids
Asteroids are objects that orbit the Sun & most occupy a zone between Mars & Jupiter. There are
different groups of asteroids named for the zones in which they orbit (e.g. Apollo asteroids have
Earth crossing orbits, Trojan asteroids orbit in Lagrange points where they are locked in orbit with
Jupiter because the gravitational forces from Jupiter equals that from the Sun).
The Earth crossing asteroids, of which about 300 are currently known, are potentially hazardous
to earth. In 1968, Icarus missed Earth by only 6 x 106 km. In 1994, another asteroid came within
105,000 km of Earth. Statistics gathered from previous impacts on Earth indicate that real
hazards exist. By comparing impact intervals with diameter of impacting body, estimates show
that a 1 km diameter asteroid strikes the Earth about every 100,000 years, while larger bodies
impact less frequently and smaller bodies impact more frequently. A 1 km dia. asteroid would
cause damage on earth equivalent to an explosion of 100,000 megatons of TNT (note: the world
war II H bombs produced explosions of only 20 kilotons of TNT). Within the asteroid belt, there
are about 5,000 asteroids whose orbits are well characterized, and another 13,000 smaller
asteroids are known but the orbits have not been well determined. There may be as many as a
million asteroids in this zone including the smaller objects.
Asteroids vary in size with the largest being 940 km in dia., Ceres. Few are larger than 300 km
diameter. The cumulative mass of all asteroids has been estimated to be less than 1/10 of the
Moon.
Pre-impact orbits, established on the basis of impact trajectories, have been determined for some
meteorites that have also been analyzed in the laboratory. The mineralogy & geochemistry of
these meteorites has been correlated with spectral signatures of the reflected sunlight for specific
asteroids, obtained by spectrometers on spacecraft & ground based observatories. By comparing
the spectral signatures of an asteroid with the mineralogy & geochemistry of a meteorite, it has
been possible to identify the actual parent body for some meteorites.
Asteroids have been divided into types using their spectral signatures & mineralogy. Among
others, the most abundant spectral types are: S (15%), C (75%).
o
The S type include: Gaspra (20 km in dia.) & Ida (60 km in dia.), imaged by Galileo
spacecraft. They are covered with impact craters ranging in size 100 m to 2 km in
diameter & are covered with a layer of dust on the surface. They are thought to be
fragments of much larger asteroids that were destroyed by collisions in the past. This
type shows a close link to ordinary chondrite meteorites yielding spectra with the
presence of olivine, pyroxene, & Ni-Fe metal.
o
The C type include: Mathilde, imaged in 1997, by the NEAR spacecraft. Mathilda is 60
km in diameter. Its mass was measured at 1017 kg & its bulk density was calculated to be
1.4 g/cm3 indicating that the object is quite porous. The asteroid, EROS, was also visited
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by the NEAR spacecraft. Its mass was measured to be 1015 kg & its bulk density
calculated to be 2.7 g/cm 3 indicating the object was more rock like. The C types are very
dark, reflecting only 5% of the sunlight (about 2x darker than a charcoal briquette). This
type shows a close link to carbonaceous chondrite meteorites with the presence of
hydrated clay minerals & organic matter.
Two other interesting types are the M (10%) & A types. The M type, e.g. Psyche, exhibits spectra
similar to Ni-Fe metal meteorites. The A type, e.g. Asporina, exhibits spectra similar to the
pallasite meteorites. These two types of asteroids suggest the parent body may be another
asteroid that experienced differentiation &, as such, the M type (i.e. Ni-Fe metal) was once a part
of the core of an asteroid, & the A type (i.e. pallasite) was a fragment of the mantle of an asteroid.
Current models about the origin of asteroids have been improved considerably with the images &
spectra taken by passing spacecraft. Spectra obtained by spectrometers on-board spacecraft &
ground based observatories, when combined with the analysis of meteorites, whose orbits were
measured & determined to originate from the asteroid belt, suggest that asteroids condensed out
of the same matter that formed our planets & moons. However, in the asteroid belt, no planet
ever formed. The matter in this zone grew as it aggregated, and, at the same time, collisions were
destroying the same bodies. The model of the internal structure of asteroids ranges from a "pile
of rubble" to an object "layered like an onion".
References for further Reading
Hutchison & Graham (1993) Meteorites, Sterling Publ. 60p.
McSween (1999) Meteorite & Their Parent Bodies, Cambridge Univ. Press, 2nd ed., 310p.
McSween (1999) Meteorites, pgs 351-364, in The New Solar System, 4th ed., by Beatty,
Petersen & Chaikin, Sky Publ. & Cambridge Univ. Press.
Morrison & Owen (1996) The Planetary System, Addison-Wesley Publ., 2nd ed., 570p.
Wood (1988) Chondritic Meteorites & the Solar Nebula, An. Rev. Earth Planet. Sci., vol. 16, p 5372.
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