[meteorite-list] Re : (no subject)

From: Galactic Stone & Ironworks <meteoritemike_at_meteoritecentral.com>
Date: Sun, 4 Apr 2010 12:19:10 -0400
Message-ID: <j2xe51421551004040919j55dd18bat5872c14c7bb3f0ba_at_mail.gmail.com>

Hi List,

Several list members contacted me privately to remind me that Dr.
Rubin and Dr. Grossman should be addressed as "Dr." and not the more
casual "Mr." - that was an oversight on my part and was not
intentional. Both of these men deserve much respect in their field.

Best regards and Happy Easter,


On 4/4/10, countdeiro at earthlink.net <countdeiro at earthlink.net> wrote:
> 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
>>Visit the Archives at
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Mike Gilmer - Galactic Stone & Ironworks Meteorites
Mike Gilmer - Galactic Stone & Ironworks Meteorites
Received on Sun 04 Apr 2010 12:19:10 PM PDT

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