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.
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. High-energy 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 μm to 10 m. Their logic was that objects smaller than 100 μm 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.
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 μm to 1 mm, although in practice the term is most often applied to objects smaller than approximately 100 μm. 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 μm 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 μm, 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 μm, following Love and Brownlee (1991). Beech and Steel (1995) chose 100 μm 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 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.
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 μm. 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 μm. 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 μm are dust particles.
By this definition, IDPs are particles smaller than 10 μm. 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 purposes of the present paper, we adopt 1 m as 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 μm; 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.