[meteorite-list] (no subject)

From: countdeiro at earthlink.net <countdeiro_at_meteoritecentral.com>
Date: Sun, 4 Apr 2010 11:43:15 -0400 (EDT)
Message-ID: <17644039.1270395795801.JavaMail.root_at_wamui-hunyo.atl.sa.earthlink.net>

Thanks Shawn,

Excellent post. If accepted...these definitions will bring about a standardization in description that was sorely needed in some quarters. Particularly in the trading of micro-meteorites and smaller material.

Count Deiro
IMCA 3536

-----Original Message-----
>From: Shawn Alan <photophlow at yahoo.com>
>Sent: Apr 4, 2010 3:14 AM
>To: meteorite-list at meteoritecentral.com
>Subject: [meteorite-list] "Meteorite and meteoroid: New comprehensive definitions" second part of the artical
>Hello List
>Here is the second part of the artical
>Meteorite and meteoroid: New comprehensive definitions
>Alan E. RUBIN1* and Jeffrey N. GROSSMAN2
>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)
>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:
>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.
>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.
>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).
>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:
>Meteorites are created by the impacts of meteoroids
>or larger natural bodies.
>Additional Complications
>Fragments of Meteorites
>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.
>Degraded Meteorites
>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:
>An object is a meteorite as long as there is something
>recognizable remaining either of the original minerals
>or the original structure.
>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
>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.
>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
>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.
>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).
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>Shawn Alan
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Received on Sun 04 Apr 2010 11:43:15 AM PDT

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