[meteorite-list] "Meteorite and meteoroid: New comprehensive definitions" the whole artical

From: Shawn Alan <photophlow_at_meteoritecentral.com>
Date: Sat, 3 Apr 2010 23:57:05 -0700 (PDT)
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Hello Larry, Dirk and List
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Here is the whole artical
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Meteorite and meteoroid: New comprehensive definitions
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by
Alan E. RUBIN1* and Jeffrey N. GROSSMAN2
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1Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095?1567, USA
2U.S. Geological Survey, 954 National Center, Reston, Virginia 20192, USA
*Corresponding author. E-mail: aerubin at ucla.edu
(Received 05 May 2009; revision accepted 14 September 2009)
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INTRODUCTION
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Since Chladni (1794) published On the Origin of the
Pallas Iron and Others Similar to it, and on Some
Associated Natural Phenomena and made plausible the
hypothesis that rocks could fall from the sky, the
definition of the word meteorite has remained essentially
unchanged, as reflected in the ten quotations given
above. Nearly all modern reference works use a similar
definition. Meteorites are almost always defined to be
solid bodies that have fallen through the Earth?s
atmosphere and landed on the Earth?s surface.
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Nineteenth-century definitions tend to leave open the
origin of the falling material, whereas later definitions
specify that the material came from space.
Many recent definitions of meteorite, including the
one adopted by the International Astronomical Union
(IAU), specify that meteorites originated as meteoroids.
The latter term was defined by the IAU as ??a solid
object moving in interplanetary space, of a size
considerably smaller than an asteroid and considerably
larger than an atom or molecule?? (Millman 1961). Beech
and Steel (1995) suggested modifying this definition to
include only natural objects in the size range 100 lm to
10 m. Because modern usage frequently ties these two
terms together, with meteoroids forming the pre-impact
precursors of meteorites, it is imperative that the
definitions be consistent.
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With the advent of the Space Age and the discovery
of new sources of extraterrestrial material, it is clear
that most existing definitions of the term meteorite are
too restrictive. Indeed, there are already three objects
recognized by the Meteoritical Society?s Committee on
Meteorite Nomenclature (NomCom) that violate most
traditional definitions of meteorite (with the exception
of the one given in Gomes and Keil 1980) because they
were not found on Earth?s surface. Two millimeter-size
chondrites discovered among samples returned from the
Moon during the Apollo missions have been described
and named as meteorites: Bench Crater (McSween 1976;
Zolensky et al. 1996) and Hadley Rille (Haggerty 1972;
Grossman 1997; Rubin 1997). A IAB-complex iron
identified on the surface of Mars by the Opportunity
rover was recently given a formal meteorite name:
Meridiani Planum (Connolly et al. 2006; Schro? der et al.
2008). The existence of these objects, combined with
other probable meteorites from the Moon and Mars
that have not yet been formally named (as well as other
conceivable examples), has led us to re-examine the
term meteorite and the related term meteoroid in a
search for precise, comprehensive definitions.
The NomCom is responsible for approving a unique
name for every properly described meteorite. Meteorites
are traditionally named for a geographic feature in the
vicinity of the place where they were found. Thus, any
change in the definition of meteorite will have practical
consequences for how they are named.
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PROBLEMS WITH THE DEFINITIONS OF
METEORITE AND METEOROID
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Where Do Meteorites Occur?
Meteorites are Not Restricted to Earth
The discoveries of the Bench Crater carbonaceous
chondrite and Hadley Rille enstatite chondrite among
returned lunar samples and the identification of the
Meridiani Planum iron on Mars demonstrate that
foreign objects, analogous to meteorites found on
Earth, can arrive intact on the surfaces of other
planetary bodies. The literature designations of these
objects as meteorites have been widely accepted in the
meteorite research community. The two meteorites
found on the Moon were not derived from objects that
produced meteors, a phenomenon that requires the
presence of an atmosphere.1 Although the words meteor
and meteorite share a common Greek root meaning
??high in the air,?? there is no reason to link these terms
in a modern definition by requiring meteorites to have
produced meteors during an atmospheric transit.
If the chondrites found on the Moon or irons found
on Mars are considered meteorites, then it is reasonable
that a comprehensive definition of meteorite would
allow for their presence on other planets as well as
airless bodies such as asteroids and comets, or the
natural satellites of any of these bodies. Thus, the first
refinement needed for a comprehensive definition of
meteorite is:
Meteorites can occur on any celestial body, not just
Earth.
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Meteoroids may Hit Spacecraft and Other Artificial
Targets
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Another difficult situation arises when considering
projectiles that strike a spacecraft. For example,
publications reporting on the Long Duration Exposure
Facility (LDEF), which was exposed to interplanetary
space in low Earth orbit for 5.75 years, generally used
the term meteoroid (not meteorite) to describe both the
small impactors and the resulting particulate debris that
was collected (e.g., Clark 1984). However, as pointed
out by Beech and Youngblood (1994), according to
existing definitions, meteoroids are defined as objects
moving in interplanetary space and meteorites are
defined as objects that have reached Earth. Neither
definition seems to apply to material that has struck a
spacecraft: that material is no longer in interplanetary
space as an independent body, nor has it reached Earth
or any other celestial body. One could quibble over
whether a platform in orbit around the Earth is simply
an extension of Earth?s surface, but it is also easy to
imagine a situation where an object hits a spacecraft in
orbit around the Sun or traveling with sufficient velocity
to escape the solar system altogether. Beech and
Youngblood (1994) indicated that either a new
definition is needed for the term meteorite or a new
term needs to be created to cover material that hits a
spacecraft.
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The essential characteristic of a meteorite is that it
represents material that comes from one place and
survives an accretionary impact someplace else. In
addition, the essential characteristic of a meteoroid is its
independent existence as a solid object in interplanetary
space. The most straightforward way to retain these
characteristics is to allow the definition of meteorite to
cover material that accretes to man-made objects.
Returning to the LDEF example, we would prefer to
say that meteoroids impacted the facility and that
some of this material survived as small meteorites,
which further refines the definition:
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Meteorites can arrive on man-made objects or other
artifacts.
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We note that this revision to the definition of
meteorite also covers other situations that could be
considered gray areas, where an object never actually
hits the Earth?s surface. This potentially includes
impacts into cars, airplanes, boats, buildings, and other
man-made structures. (Such impactors have recently
been termed ??hammers?? by meteorite dealers and
collectors.) Non-man-made structures (e.g., beaver
dams, termite mounds, bird bowers) and even alien
spacecraft would also be covered by this revision.
The Problem of Meteorites within Meteorites (within
Meteorites?)
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Foreign clasts found in ordinary-chondrite regolith
breccias and howardites almost certainly originated as
projectiles that collided with the parent asteroids of their
hosts. Prominent examples include H-chondrite clasts in
the LL chondrite, St. Mesmin (Dodd 1974), an LL5 clast
in the H chondrite, Dimmitt (Rubin et al. 1983), and
CM clasts in the Kapoeta howardite (e.g., Zolensky
et al. 1996). Although we know of no precedent for
using the term meteorite to describe individual foreign
clasts inside chondrite and achondrite breccias, it seems
clear that some of these clasts could once have been
properly called ??asteroidal meteorites.??2 However, we do
not recommend using this term for describing xenoliths
in specimens from individual meteorites. Complex
breccias such as the Kaidun meteorite are known in
which the bulk of the specimen is composed of
millimeter-size clasts of diverse asteroidal and,
conceivably, planetary origins (Zolensky and Ivanov
2003). In Kaidun and other meteorite breccias, the clasts
themselves may be breccias containing material derived
from diverse sources. Brecciation is common among
chondrites and achondrites and it is not always easy to
determine which clasts may be locally derived and which
may be foreign (i.e., meteoritic) (Scott et al. 1985). These
facts would make it difficult to decide which clasts are
worthy of the name meteorite. There would also be a
nightmare of nomenclature if one tried to give each
potential meteorite in a complex, polymict breccia a
unique name.
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Consequently, we recommend that the term
meteorite be reserved for objects that have experienced
an accretion event, not for any of the constituents or
clasts within those objects. In other words, an object
should lose its nomenclatural status as a meteorite when
it and the material into which it has been incorporated
together become a projectile and accrete as a meteorite
to another body. For example, the CM chondritic clasts
in the Kapoeta achondrite should not be considered
meteorites because they occur within a meteorite that
hit the Earth. However, if a spacecraft were to go to
asteroid 4 Vesta (if that is, in fact, the parent body of
HED achondrites like Kapoeta) and collect CM
chondrite fragments from the regolith, these could be
considered asteroidal meteorites. Although samples
returned from a future mission to the Kaidun breccia?s
parent body would pose the same issues of classification
and nomenclature that were described above, the
situation would be analogous to samples recovered from
the Moon; each foreign projectile fragment would
deserve to be called a meteorite. We leave this as a
nomenclature problem for the future.
Another refinement needed for a comprehensive
definition of meteorite is therefore:
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Clasts within meteorites should not be called
meteorites.
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The Nature of Meteoritic Material
Existing definitions vary in their descriptions of
what types of material meteorites represent. Three terms
used commonly in literature definitions to define
meteoritic material are solids, extraterrestrial materials,
and meteoroids.
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However, object 2008 TC3, which dropped
fragments of the anomalous ureilite Almahata Sitta in
northern Sudan on October 7, 2008, was considered to
be an asteroid (Jenniskens et al. 2009) despite the fact
that its diameter was 4.1 ? 0.3 m. The term
micrometeoroid has also been used for decades (e.g.,
Shapiro 1963); Love and Brownlee (1991) applied it to
meteoroids in the size range of 10 lm to 1 mm,
although in practice the term is most often applied to
objects smaller than approximately 100 lm. These size
ranges need to be modified.
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Similar terms are used to describe meteoritic
material in different size ranges. The largest known
meteorite is the 60 metric-ton Hoba iron, which has
dimensions of approximately 3 ? 3 ? 1 m (Grady
2000). The smallest object named as a meteorite by the
NomCom is Yamato 8333; this weighs 12 mg (Yanai
and Kojima 1995) and corresponds to a particle
diameter of approximately 2 mm. There are several
unclassified objects in the Yamato collection that are
even smaller. The term micrometeorites has been
applied to tiny meteorites that have been found on
Earth; these are typically smaller than 500 lm in
diameter (e.g., Engrand and Maurette 1998), but
recent collections in Antarctica have produced
micrometeorites as large as 2 mm in diameter
(Rochette et al. 2008). Very small particles of
meteoritic material, frequently ?1 lm, are usually
called cosmic dust or interplanetary dust particles
(IDPs). Micrometeorites and particles of dust can be
quite numerous in many terrestrial collections and are
therefore not individually named by the NomCom.
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Thus, a similar portfolio of terms is used to
describe both meteorites and meteoroids. Interplanetary
dust is used to describe tiny particles, regardless of
whether they have accreted to a larger body or still exist
as independent particles in space. The prefix micro- is
applied to objects coarser than dust but below
approximately 0.1?1 mm in size. The unmodified words
meteorites and meteoroids are used to describe objects
up to several meters in diameter. These terms are useful
and suggest that the same size ranges should be used
whether one is referring to objects in interplanetary
space or objects that have accreted as meteorites. But
what size ranges are the most appropriate for both
meteorites and meteoroids?
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For the purposes of this paper, we define the upper
limit of particle size that should be considered dust as
10 lm, following Love and Brownlee (1991). Beech and
Steel (1995) chose 100 lm as the upper limit on
micrometeorite and micrometeoroid size because, as
stated above, particles smaller that this were considered
unlikely to cause a meteor during passage through the
Earth?s atmosphere. We reject this value for several
Most definitions of meteorite state that the material
must be a solid or a meteoroid, which are equivalent if
one uses the simple IAU definition of meteoroid as ??a
solid object moving in interplanetary space.?? The word
solid, if unaccompanied by a modifier, is problematic
because it allows for the existence of man-made
meteorites. Once Sputnik 1 was launched on October 4,
1957, it became inevitable that man-made solid objects
would one day fall to Earth. Two spectacular examples
of this were the debris from the U.S. Skylab space
station, which fell across the southeastern Indian Ocean
on July 11, 1979, and the nuclear reactor of the Soviet
Cosmos-1402 satellite, which fell in the South Atlantic
Ocean on February 7, 1983. Most researchers and
collectors would probably not accept surviving
fragments of these artificial satellites as genuine
meteorites. Thus, the word solid is not sufficient to
define what kinds of materials can be meteorites, nor is
the word meteoroid as defined by the IAU.
The Krot et al. (2003) definition of meteorite
specifies that the material must have an extraterrestrial
origin. Although this succeeds in limiting meteorites to
non-anthropogenic material, it is too restrictive. First of
all, it allows the rather unlikely, but conceivable,
situation where a crashed alien spacecraft would be
considered a meteorite. (In the novel The war of the
worlds [Wells 1898], the crash-landed Martian spacecraft
were first thought to be meteorites.) More importantly,
however, there is a plausible situation in which the
word extraterrestrial clearly fails as part of a
comprehensive definition. This concerns the potential
existence of terrestrial (or terran) meteorites. Highenergy
impacts on the Earth could propel some ejecta
to velocities greater than that necessary for escape. If
such a rock were to land on the Moon, for example, it
should properly be considered a terrestrial meteorite
(e.g., Armstrong et al. 2002; Crawford et al. 2008).
Because of this possibility, meteorites cannot be limited
to extraterrestrial material.
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It follows that the definition of meteorite must
include only natural materials, including (but not
necessarily limited to) silicate and non-silicate minerals,
mineraloids, organic matter, amorphous material, metal
and ice, without regard to whether this material is
asteroidal, planetary, cometary, derived from a natural
satellite, or originating outside the solar system. Use of
the term meteoroid in the sense of Beech and Steel (1995)
to describe the precursors of meteorites is acceptable
because these workers restricted the definition of
meteoroid to include only natural solid objects. Beech
and Steel discussed the possibility that objects termed
meteoroids could be derived from comets as well as
asteroids; meteoroids simply represent the collection of
objects too small to be easily detected from Earth. We
would extend their discussion to acknowledge the
possibility that meteoroids could be derived from any of
the natural bodies of the solar system, and that some
could conceivably be from natural bodies originating
outside our solar system. This usage bars artificial
objects from being called meteorites and allows for the
possible existence of terrestrial meteorites on other
astronomical bodies. Thus, the revised definition of
meteorite should have the constraint:
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Meteorites are natural solid objects that spent time in
interplanetary space.
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The Transport of Meteorites
One potential situation that could complicate our
definition of meteorite is one in which ??meteorites??
might be created intentionally. In the novel, The Moon
is a harsh mistress (Heinlein 1966), revolutionary
??loonies?? use an electronic catapult to hurl moon rocks
at Earth. It is also conceivable that astronauts traveling
back home from Mars with a collection of martian
rocks could jettison a large boulder from their
spacecraft along a trajectory that would cause it to fall
to Earth. Interesting as it might be to examine the
fusion crust of such a rock or prized as the boulder?s
remnants might be to collectors, these materials would
probably be regarded by researchers as artificial
meteorites, not the genuine article. This thought
experiment suggests another restriction in a new
comprehensive definition of meteorite:
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Meteorites must be transported by natural means.
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There are a number of conceivable, natural
transport processes that can lead to the formation of
natural solid objects in interplanetary space, and
ultimately to meteorites. Meteorite precursor objects
may be primary bodies that were never part of larger
objects and thus were never launched from a larger
body. Alternatively, they may have been ejected from
larger parent bodies by collisions, or been derived from
landslides on low-gravity bodies or by shedding of
material from the equator of a rapidly spinning object.
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The Sizes of Meteorites and Meteoroids
Meteoroids in interplanetary space and meteorites
found on Earth and other bodies span a wide size
range. The IAU definition of meteoroid vaguely limits
these objects to those smaller than asteroids but larger
than atoms or molecules. Beech and Steel (1995)
suggested modifying this definition to include only
objects in the range 100 lm to 10 m. Their logic was
that objects smaller than 100 lm were unlikely to
produce meteors during atmospheric passage and
should be considered dust, whereas 10 m was close to
the minimum size of astronomically detectable objects
that could be called asteroids.
reasons. We have already argued that meteorites can
accrete to airless bodies, which suggests that there is no
reason to limit meteorites to objects that once produced
a meteor. Moreover, because meteorites can fall
through atmospheres around other celestial bodies (e.g.,
Mars, Venus, Titan), the size of the smallest accreting
meteoroids that cause meteors will probably vary with
atmospheric density and composition and the celestial
body?s escape velocity.
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There are more practical reasons that can be used
to select the best upper size cutoff for micrometeorites
and micrometeoroids. Meteorites have long been
recognized as rare, special kinds of rocks. The practice
of naming individual meteorites after the places where
they were found is based on this special status.
Generally, to receive a name, a meteorite must be well
classified and large enough to provide material for
curation and research. Much of the material that
forms meteorites in the inner solar system is relatively
coarse grained. Many chondrites and nearly all
achondrites and iron-rich meteorites have mineral grain
sizes that exceed 100 lm. Although in many cases it is
possible to classify small particles of meteoritic
material at least tentatively, this process is greatly
hindered when the particle size is significantly smaller
than the parental rock?s grain size. To allow for
proper classification, 2 mm is a more useful size cutoff
than 100 lm. In addition, the number of objects that
accrete to the Earth (and other bodies) varies
exponentially with the inverse of mass (e.g., Brown
1960, 1961; Huss 1990; Bland et al. 1996). Single
expeditions to recover micrometeorites have found
thousands of particles in the sub-millimeter size range
(Rochette et al. 2008), but very few that exceed 2 mm.
The 2 mm divide also seems to form an approximate
break between the smallest objects that have
historically been called meteorites and the largest
objects called micrometeorites. This leads to additional
refinements to our definitions:
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Micrometeorites are meteorites smaller than 2 mm in
diameter; micrometeoroids are meteoroids smaller
than 2 mm in diameter; objects smaller than 10 lm
are dust particles.
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By this definition, IDPs are particles smaller than
10 lm. We are not proposing a lower size limit for IDPs.
Before it impacted the Earth, object 2008 TC3 was
approximately 4 m across and was officially classified as
an asteroid (Jenniskens et al. 2009). It is likely that
when smaller interplanetary objects are observed
telescopically, they will also be called asteroids, even if
they are of sub-meter size. Thus, the boundary between
meteoroids and asteroids is soft and will only shrink
with improved observational capabilities. For the
minimum asteroid size. We thus differ from Beech and
Steel (1995) who suggested a 10 m cutoff between
meteoroids and asteroids.
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The Relationship between Meteorites and Meteoroids
It is tempting to include in our definition of
meteorite a statement that meteorites originate as
meteoroids, which, using our modified definition are
natural solid objects moving in space, with a size less that
1 m, but larger than 10 lm; this was done in previous
definitions such as that of McSween (1987). However,
because the Hoba iron meteorite is larger than 1 m
across, it represents a fragment of an asteroid, not a
meteoroid, under our definition of meteoroid. If a mass
of iron 12 m in diameter deriving from an asteroidal
core were to be found on Earth or another celestial
body, it would almost certainly be called a meteorite,
despite the fact that it was too large to have originated
as a meteoroid even under the Beech and Steel (1995)
definition. In addition, the Canyon Diablo iron
meteorites associated with the Barringer (Meteor)
Crater in Arizona, are fragments of an impacting
asteroid that was several tens of meters in diameter
(e.g., Roddy et al. 1980); the Morokweng chondrite may
be a fragment of a kilometer-size asteroid that created
the >70 km Morokweng crater in South Africa (Maier
et al. 2006).
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Comets, particularly Jupiter-family comets (JFCs),
could also produce meteorites. A small fraction of JFCs
evolve into near-Earth objects (Levison and Duncan
1997) and could impact main-belt asteroids at relatively
low velocities (approximately 5 km s)1) (Campins and
Swindle 1998). Meteorites could also be derived from
moons around planetary bodies. Lunar meteorites are
well known on Earth, and meteorites derived from
Phobos may impact Mars, especially after the orbit of
Phobos decays sufficiently (e.g., Bills et al. 2005).
We see no simple way out of this semantic
dilemma, so we add the refinement:
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Meteorites are created by the impacts of meteoroids
or larger natural bodies.
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Additional Complications
Fragments of Meteorites
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Meteorite showers result from the fragmentation of
a meteoroid (or larger body) in the atmosphere. In the
case of the L6 chondrite Holbrook, about 14,000
individual stones fell (Grady 2000). Each of these stones
is considered a meteorite, paired with the others that
fell at the same time. A meteorite can break apart when
it collides with the surface of a body or it can fragment
at a later time due to mechanical and chemical
weathering. Each fragment of a meteorite is itself
considered a meteorite, paired with the other objects
that fell during the same event.
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Degraded Meteorites
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Weathering and other secondary processes on the
body to which a meteorite accretes can greatly alter
meteoritic material. Chondritic material has been
found embedded in terrestrial sedimentary rocks in
Sweden (e.g., Thorslund and Wickman 1981; Schmitz
et al. 2001). Other than the minor phase chromite (and
tiny inclusions within chromite), the primary minerals
in these extraterrestrial objects have been replaced by
secondary phases. Despite this extensive alteration,
some of these rocks (e.g., Brunflo) contain wellpreserved
chondrule pseudomorphs. Iron meteorites
can be severely weathered at the Earth?s surface,
forming a substance known as meteorite shale
(Leonard 1951) in which the original metal has been
completely oxidized; nevertheless, this material can still
preserve remnants of a Widmansta? tten structure. The
NomCom considers these types of materials to be
relict meteorites, defined as ??highly altered materials
that may have a meteoritic origin. . .which are
dominantly (>95%) composed of secondary minerals
formed on the body on which the object was found??
(Meteoritical Society, 2006). Many relict meteorites
have received formal meteorite names in recent years.
We support the use of this terminology and would
further revise our definition as follows:
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An object is a meteorite as long as there is something
recognizable remaining either of the original minerals
or the original structure.
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We assert that objects that are completely melted
during atmospheric transit or weathered to the point
of complete destruction of all minerals and structures
should not be called meteorites. This would include
cosmic spherules (reviewed by Taylor and Brownlee
1991), ice meteorites that melted, and bits of what
appear to be separated fusion crust from larger
meteorites (eight of which have received formal
meteorite names from the NomCom as relict
meteorites, incorrectly in our opinion). A report of
possibly meteoritic material in sediments near the
Cretaceous ? Tertiary boundary (Kyte 1998) presents a
borderline case. No primary minerals remain in this
object although the textures of secondary minerals are
suggestive of some kind of primary chondritic
structure.
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Meteorites accreted by their own parent body
We now consider whether it is possible for an
object to become a meteorite on the same celestial
body from which it was derived. For example, if
ejecta from a terrestrial impact crater lands back on
Earth, can it be considered a meteorite? Tektites are
widely held to be glass objects produced by large
impacts on Earth. Australite buttons were launched
on sub-orbital ballistic trajectories from their parent
crater and quenched into glass; they were partly
remelted on the way down when they encountered
denser portions of the atmosphere (e.g., Taylor 1961
and references therein). Most researchers would likely
agree that these objects should not be considered
meteorites. However, if terrestrial ejecta reached the
Moon, we have argued that it should be considered a
terrestrial meteorite. The critical difference is that the
hypothetical material in the latter case escaped the
dominant gravitational influence of Earth, whereas
tektites did not.
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Material launched from a celestial body that
achieves an independent orbit around the Sun or some
other celestial body, and which eventually is re-accreted
by the original body, should be considered a meteorite.
The difficulty, of course, would be in proving that this
had happened, but a terrestrial rock that had been
exposed to cosmic rays and had a well-developed fusion
crust should be considered a possible terrestrial
meteorite. In a related context, Gladman and Coffey
(2009) calculated that large fractions of material ejected
from Mercury by impacts achieve independent orbits
around the Sun and are re-accreted by Mercury only
after several million years. Any of this material that
survived re-accretion could be considered a meteorite.
The next refinement of the definition of meteorite is
then:
?
An object that lands on its own parent body is a
meteorite if it previously escaped the dominant
gravitational influence of that body.
?
Relative sizes
As a final clarification, we suggest that:
?
An object should be considered a meteorite only if it
accretes to a body larger than itself.
?
REVISED DEFINITIONS OF METEORITE AND
METEOROID
?
>From the discussion above, new definitions of
meteorite and meteoroid are proposed:
Meteoroid: A 10 lm to 1-meter-size natural solid
object moving in interplanetary space. Meteoroids may
be primary objects or derived by the fragmentation of
larger celestial bodies, not limited to asteroids.
Micrometeoroid: A meteoroid between 10 lm and
2 mm in size.
Meteorite: A natural solid object larger than 10 lm
in size, derived from a celestial body, that was
transported by natural means from the body on which
it formed to a region outside the dominant gravitational
influence of that body, and that later collided with a
natural or artificial body larger than itself (even if it is
the same body from which it was launched). Weathering
processes do not affect an object?s status as a meteorite
as long as something recognizable remains of its
original minerals or structure. An object loses its status
as a meteorite if it is incorporated into a larger rock
that becomes a meteorite itself.
Micrometeorite: A meteorite between 10 lm and
2 mm in size.
?
Interplanetary dust particle (IDP): A particle
smaller than 10 lm in size moving in interplanetary
space. If such particles subsequently accrete to larger
natural or artificial bodies, they are still called IDPs.
Acknowledgments?We thank our colleagues for useful
discussions and C. R. Chapman, P. Schweitzer, and
J. Mars for useful reviews.
?
This work was supported in
part by NASA Cosmochemistry Grants NNG06GF95G
(A. E. Rubin) and NNH08AI80I (J. N. Grossman).
Editorial Handling?Dr. A. J. Timothy Jull
?
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Received on Sun 04 Apr 2010 02:57:05 AM PDT