[meteorite-list] "Meteorite and meteoroid: New comprehensive definitions" second part of the artical

From: Shawn Alan <photophlow_at_meteoritecentral.com>
Date: Sun, 4 Apr 2010 00:14:36 -0700 (PDT)
Message-ID: <810229.4183.qm_at_web113612.mail.gq1.yahoo.com>

Hello List

Here is the second part of the artical

Meteorite and meteoroid: New comprehensive definitions

by
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
structure.

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
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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Shawn Alan
Received on Sun 04 Apr 2010 03:14:36 AM PDT