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

From: Shawn Alan <photophlow_at_meteoritecentral.com>
Date: Sun, 4 Apr 2010 00:10:58 -0700 (PDT)
Message-ID: <590809.63327.qm_at_web113604.mail.gq1.yahoo.com>

Hello Larry, Dirk and List

Here is the first part of the artical and if I had posted this twice already I am sorry for some reason it wastn post after I emailed and if it didnt post twice then here is the first 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)


INTRODUCTION

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.

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.

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.

PROBLEMS WITH THE DEFINITIONS OF
METEORITE AND METEOROID

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.

Meteoroids may Hit Spacecraft and Other Artificial
Targets

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.

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:

Meteorites can arrive on man-made objects or other
artifacts.

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?)

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.

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:

Clasts within meteorites should not be called
meteorites.

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.

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.

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.

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?

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.

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:

Meteorites are natural solid objects that spent time in
interplanetary space.

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:

Meteorites must be transported by natural means.

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.

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.

Shawn Alan
Received on Sun 04 Apr 2010 03:10:58 AM PDT