meteorite

Introduction
any interplanetary particle or chunk of stony or metallic matter known as a meteoroid that survives its passage through the Earth's atmosphere and strikes the ground. The term is often applied to similar objects that reach the surfaces of other planets or satellites.
Any source that can eject such material into interplanetary space should, at least in principle, be thought of as a candidate source of meteorites. There is no fundamental reason all meteorites must come from similar sources. It turns out, however, that there are some regions of the solar system that are much more effective than others in introducing material of substantial strength into Earth-crossing orbits.
Laboratory and theoretical studies fully confirm the older belief that most meteorites are fragments of asteroids. These same studies show that a small fraction, less than 1 percent of meteorites, come from nonasteroidal sources. The lunar origin of several meteorites is well established, and it is probable that at least eight others came from Mars. There is evidence from fireball data that a small part of the material in cometary orbits (i.e., with aphelia beyond Jupiter) may possess sufficient strength to successfully penetrate the atmosphere. It is not known if any of this material is present in existing meteorite collections. If it is, the best candidate material would be carbonaceous stony meteorites, probably those of type CI (see below), of which five separate falls have been recovered.
With these few exceptions, it is safe to regard all meteorites as samples broken from outcrops of rock or metal, which until fairly recently in solar-system history were part of asteroidal bodies, mostly in the inner region of the asteroid belt (between about 2.2 and 2.6 AU). Like rocks from the Moon, the Earth, or any other similar planetary body, their present state is determined by the total effect of events that occurred on the body throughout the entire history of the solar system. There is no a priori reason why such samples must be pristine samples of a primordial solar nebula from which the present solar system evolved. On the other hand, the principal driving force behind asteroid studies has been the plausible belief that small “primitive” bodies such as asteroids and comets are those most likely to preserve evidence of events that took place in the early solar system. Insofar as this belief is correct, meteorites, samples of these bodies, share this property. Evidence derived from the study of meteorites themselves supports this conclusion.

Types of meteorites
The most fundamental distinction between the various meteorites—no two of which are exactly alike—is the division between chemically undifferentiated and differentiated meteorites. This concept arises from the fact that there is an average chemical composition of the solar system. This average composition must be very close to the composition of the Sun, because the Sun contains most of the mass of the solar system. Spectroscopic comparison of the Sun's chemical makeup with those of other stars shows that its composition in turn is closely related to a cosmic average of the relative abundances of the elements. Important deviations from such average abundances are observed, but these do not invalidate the view that they are deviations from normal abundance ratios determined by the processes by which the chemical elements are formed in stars at various stages of their evolution, returned to the interstellar medium where they are mixed, and then incorporated into new stars and their planetary systems when they are formed.
Since the late 1940s, important advances have been made in the chemical analysis of both meteorites and the Sun. A remarkable result has emerged from this work. Although at one time there appeared to be major differences between the Sun and typical meteorites in the ratios of elements to one another (e.g., iron to silicon), these differences tended to disappear as the accuracy of the measurements improved. It turned out that, for most meteorites and most elements, the solar and meteoritic values of the element ratios relative to silicon (taken as a standard) agreed to within better than a factor of two.
Two kinds of exceptions to this rule were found. Relative to the Sun, the meteorites were deficient in the more volatile elements. For the most volatile elements, hydrogen and the noble gases, the deficiencies were gross—more than a factor of 10,000. For less volatile elements, the deficiencies were smaller; and for the nonvolatile elements, or “refractory” elements, such as iron, magnesium, aluminum, and calcium, the meteoritic and solar abundance ratios were identical within the accuracy of the data.
The other kind of exception to the rule relates to differences between meteorites. For some meteorites, as, for example, those consisting primarily of metallic iron, the similarity between meteoritic and solar abundance ratios fails completely. This also is true for basaltic meteorites, those that appear to have been at one time volcanic magmas and have undergone chemical fractionation of the sort observed in terrestrial igneous rocks.

Undifferentiated meteorites
The meteorites that do obey the rule prove to be of a kind that had already been grouped together on textural grounds—namely, the chondrites. From observed fall rates, this is the most abundant type of meteorite (Table). The designation chondrite is based on the occurrence in these meteorites of small (about one millimetre in diameter) spherules called chondrules. In many chondrites, the composition of the chondrules is quite heterogeneous, and the space between the chondrules is filled with a fine-grained matrix material that is richer in volatile elements. In terms of terrestrial rocks, these meteorites seem more akin to sedimentary conglomerates composed of a mechanical mixture of a jumble of components rather than to rocks formed by a process of igneous differentiation.
Not all chondrites contain chondrules of heterogeneous composition. More often, the mixture of heterogeneous chondrules and matrix appears to have undergone a thermal metamorphism that caused the chemical components of the chondrules to come into equilibrium with one another and with the matrix material. This metamorphism was accompanied by the loss of relatively volatile elements. All of these chondrites share one common property. They do not appear to have ever experienced chemical differentiation associated with igneous melting. This shared property is the basis for grouping them all as undifferentiated meteorites even though they clearly differ from one another in the degree to which they have retained volatiles and in various other ways.

Undifferentiated meteorites are classified in two complementary ways. Based on their major element concentrations (Fe, Mg, O), carbon content, and abundance of chondrules, these meteorites naturally cluster into the distinct classes shown in the firstTable. In addition, within each of these classes, the meteorites differ according to the degree that they have been thermally metamorphosed or experienced loss of volatile elements. This difference is referred to as the petrologic type (second Table). For example, the Allende carbonaceous chondrite is classified CV3, indicating that it belongs to group CV (first Table) and petrologic type 3 (second Table).
Chondrites obviously differ from one another in several important respects. As has been pointed out, they vary in the extent to which they have undergone thermal metamorphism. Another important distinction is between the more abundant ordinary chondrites (of which there are three principal kinds) and the rarer chondritic meteorites that exhibit significant chemical differences. One of these types is the enstatite chondrite, which is, among other things, chemically more reduced than the ordinary chondrites. Almost all the iron in these meteorites, for example, is in metallic form. As a result, most of the abundant silicate mineral, pyroxene, is present as nearly pure enstatite (MgSiO3) rather than in magnesium-iron solid solution minerals, such as bronzite and hypersthene, found in the ordinary chondrites. In enstatite chondrites, the readily oxidized element silicon is even found in the reduced state, and calcium occurs as the sulfide mineral oldhamite (CaS) rather than in its more usual silicate forms.
Other very important varieties of chondrites are grouped together as the carbonaceous chondrites. As their name implies, they characteristically contain more carbon (0.5 to 5 percent) than the ordinary chondrites (only about 0.1 percent). The mineral constituents of the carbonaceous chondrites are less chemically equilibrated with one another than even the unequilibrated ordinary chondrites. In many cases, one finds in the same meteorite carbonaceous material that formed at low temperatures and inclusions of the most refractory minerals—perovskite (CaTiO3), hibonite (CaAl12O19), and melilite (solid solutions of Ca2Al2SiO7 and Ca2MgSi2O7).

Perhaps the most interesting type of meteorite is the CI carbonaceous chondrite. Strictly speaking, one could legitimately question why such meteorites are called chondrites at all inasmuch as they do not contain chondrules. When compared with solar abundances (Figure 1), however, it turns out that they are the least differentiated meteorites of all, and in making a classification scheme it certainly makes sense to group them with the other undifferentiated meteorites. In accordance with the correlation already observed between chemical undifferentiation and chemical disequilibrium, the constituents of CI carbonaceous chondrites are far from equilibrium. The iron in these meteorites, for example, is highly oxidized, most of it occurring in the ferric iron-bearing mineral magnetite (Fe3O4). At the same time, carbon is present as highly reduced complex hydrocarbons. In equilibrium, the carbon would be oxidized to carbon monoxide and carbon dioxide, and the iron would be reduced to metallic iron.
Because CI chondrites are the most undifferentiated meteorites known, it has been speculated that, unlike most meteorites, they are of cometary rather than of asteroidal origin, since comets are believed to represent the most unaltered material in the solar system. There are difficulties in accepting this speculation as being correct. For example, detailed study of these meteorites shows that, in spite of their chemically undifferentiated and unequilibrated nature, they have had a complex chemical and physical history and are not simply a collection of interstellar dust. On the other hand, scientific knowledge about the nature and origin of comets is still limited, so that it would be unwise to dismiss this intriguing hypothesis prematurely.

Differentiated meteorites
Differentiated meteorites exhibit the type of chemical fractionation one would expect to occur on a planetary body that underwent core formation and magmatic differentiation similar to that observed in terrestrial and lunar volcanic rocks. Indeed, at one time it was thought likely that the most abundant type of differentiated stony meteorite, the basaltic achondrites, were actually lunar mare basalts. Their similarities to the lunar basalts subsequently returned by the Apollo missions showed that this was by no means a farfetched idea, but detailed considerations such as oxygen isotope ratios showed that it was not correct.

In classifying differentiated meteorites, the major division is made between the iron meteorites and the stony differentiated meteorites, the achondrites (“not chondrites”). In addition, there are a number of stony-iron meteorites that contain mixtures of large masses of nickel-iron metal and differentiated silicate rock. See Tables for the usual classification scheme for iron meteoritesandsilicate-rich differentiated meteorites.
There are compelling reasons for believing that, like the chondrites, all differentiated meteorites, with the exception of those few from the Moon and possibly Mars, are asteroidal fragments. Because asteroids are too small to have experienced the accretional and long-lived radioactive heating that powers igneous processes on the inner planets, this may seem surprising. A partial solution to this enigma comes from radiometric dating, which shows that this differentiation took place very early in the history of the solar system, about 4.6 billion years ago. At that time other heating mechanisms may have been available—e.g., heating by short-lived radioactive isotopes such as aluminum-26 (which has a half-life of about 700,000 years) or inductive heating by intense solar activity. It is even possible that asteroids from which differentiated meteorites seem to be derived are fragments of much larger differentiated planetesimals originally present in the inner planet region during the development of the Earth and Venus and which were stored in quasi-stable orbits in the innermost asteroid belt (2.2 AU) of the early solar system.

Specific asteroidal source regions for recovered meteorites
There is compelling, even if circumstantial, evidence that nearly all meteorites are derived from the asteroid belt. For scientific understanding of this matter to be complete, it is necessary to know which asteroids are the sources of particular types of meteorites and the mechanisms by which meteorites are transported from the asteroid belt to the Earth. It is possible that the ultimate answer will not be found until asteroids are explored by spacecraft. Nevertheless, considerable information relevant to this question is already available.
For the most abundant meteorite type, the ordinary chondrites, there is a fairly large body of evidence that indicates that most of these meteorites strike the Earth while traveling in a rather special type of orbit. Such an orbit has its perihelion just inside the Earth's orbit, as well as low inclination (less than about 10°) and fairly high eccentricity (about 0.6). This has been determined on the basis of the time of day during which meteorites are observed to fall. It has been noted that about twice as many ordinary chondrites fall during the daylight afternoon hours as during daylight hours before noon. This fact requires that the meteorite must be moving faster than the Earth at the time of impact and, therefore, must be near its perihelion according to Keplerian dynamics. Quantitative consideration of the implications of time of fall data is entirely consistent with evidence obtained by visual observations of meteorite radiants and with about 30 fireballs identified by the Prairie Network as being, at least physically, equivalent to stony meteorites.
This distribution of orbits places strong constraints on the source region in the asteroid belt from which ordinary chondrites can be derived. If meteorites in the kilogram mass range are to be derived directly from the asteroid belt, the mechanism by which they are extracted from the belt and transferred to an Earth-crossing orbit must operate quite rapidly on a time scale of a few million years. Only then can a meteoroid escape destruction by collision while near its aphelion in the asteroid belt. This collisional lifetime is not simply a theoretical result but is measured directly by the cosmic-ray exposure ages of meteorites, as discussed below.
High-energy galactic cosmic rays—primarily protons—have a range of penetration on the order of a few metres in meteoritic material. Any meteoroid of smaller dimensions will be radiated throughout by this proton bombardment. The high-energy protons cause spallation reactions (nuclear interactions that result in the release of many nucleons) on the abundant elements in the meteoroidal target. As a consequence, a large number of otherwise rare isotopic species, both stable and radioactive, are produced. These include the stable noble gas isotopes helium-3, neon-21, and argon-36, as well as the rare isotope potassium-40 and various short- and moderately long-lived radioactive isotopes, including hydrogen-3 (with a half-life of 12.26 years), beryllium-7 (53.29 days), beryllium-10 (1.6 × 106 years), aluminum-26 (7.2 × 105 years), manganese-53 (3.7 × 106 years), and cobalt-60 (5.272 years). The concentration of the shorter-lived radioactive isotopes can be used to monitor the cosmic-ray bombardment rate, and the accumulation of the stable species (e.g., neon-21) measures the time in the past that this bombardment began—i.e., the time that the meteoroidal fragment was separated from a larger object that was large enough to have shielded it from cosmic-ray bombardment. For chondritic meteorites, the distribution of cosmic-ray exposure age falls off quite quickly with age. This is to some extent a consequence of the dynamic evolution of the meteoroid orbits but for the most part should be attributed to a “collision half-life” of 5 to 10 million years.
There are only two processes known that can accelerate meteoroidal fragments into Earth-crossing orbits on this short time scale given by cosmic-ray exposure ages. These processes are direct collisional ejection at velocities of about five kilometres per second and gravitational acceleration by dynamic resonances in the asteroid belt, of which the 3:1 resonance at 2.5 AU is of dominant importance. In a hypervelocity collision, some material is ejected at the required high velocity, but the quantity of this material is small and most of it is pulverized by the associated shock pressures. High-velocity ejection is likely to be responsible for the occurrence of meteorites from Mars or the Moon, but it completely fails to provide the observed quantity of meteoroids from the asteroid belt. The resonant mechanisms are therefore of much greater importance. Bodies orbiting the Sun with a semimajor axis near 2.5 AU will complete three revolutions about the Sun in the time that Jupiter, a strong source of gravitational perturbations, executes one revolution. The resulting resonant acceleration will cause the orbit of the body to become “chaotic,” and its perihelion will become Earth-crossing in about one million years.
The calculated quantity of asteroidal material in the meteorite size range delivered to the Earth from the 3:1 resonance agrees well with the 108 gram per year terrestrial impact rate of ordinary chondritic meteoroids. Moreover, this resonant acceleration presents a natural mechanism for concentrating meteoroid perihelia near the Earth's orbit, thereby explaining the special distribution of orbits observed for ordinary chondrites. In fact, as it turns out, meteoroids derived by this resonance acceleration mechanism should be overly concentrated near 1 AU; the ratio of afternoon to morning falls should be about 3:1 instead of the observed 2:1. This discrepancy is removed when one takes into account the fact that larger asteroidal fragments (those reaching the size of Apollo objects) will also be accelerated into an Earth-crossing orbit by the same resonant mechanism. Meteorite-size fragments will be produced as collision debris from these larger bodies, but they will not have the special distribution of orbits exhibited by the smaller asteroidal fragments introduced more directly into Earth-crossing orbits. When the contribution from resonant asteroidal ejecta over the entire size range is averaged, the predicted and observed orbital distribution match rather well.
Orbital statistics are not very well known for other meteorite types. It does appear that the differentiated basaltic achondrites fail to share the special orbital distribution observed for the ordinary chondrites. Basaltic achondrites are most likely the collision debris of Apollo objects that were extracted by another resonance mechanism known to operate in the innermost belt; this mechanism can be shown to be effective only in providing larger Earth-crossing bodies. Although more work remains to be done on this problem, it seems likely that known resonant mechanisms are adequate to explain the dynamical processes by which all classes of meteorites are delivered from their asteroidal source regions into Earth-crossing orbits.
Derivation of meteoritic material from these designated regions of the inner asteroid belt implies that asteroids in such regions have the chemical and mineralogical composition observed in the meteorites. The surface mineralogical composition of asteroids can be determined directly by Earth-based reflectance spectrometry. These measurements have been made for most of the larger asteroids. Although no two reflectance spectra are exactly alike in detail, most asteroids fall into one of two general groups, the S class and the C class. The S-class asteroids have moderate albedos (though comparatively high overall) and contain mixtures of olivine (magnesium, iron silicates), pyroxene (silicates containing magnesium, iron, calcium, and aluminum), and metallic iron. These are the same minerals found in ordinary chondrites and in basaltic achondrites. The C-class asteroids have low reflectance, and their more featureless spectra indicate the presence of light-absorbing opaque minerals. It is plausible to consider these asteroids as candidate sources for carbonaceous meteorites.
When considered in more detail, there are certain difficulties in identifying the required number of S-class asteroids as ordinary chondrite sources. Although qualitatively the mineralogy of these asteroids agrees with that of ordinary chondrites, the proportions of the constituent minerals to one another does not match as well. In particular, the S-class asteroids appear to have, at least on their surfaces, about twice as much metallic iron as the ordinary chondrites. The solution to this discrepancy is not known at present. It may be that the surfaces of the asteroids are not truly representative of their interiors, having been altered by collisional bombardment or exposure to solar radiation and particles. Another possibility is that a systematic change in mineralogical composition occurs during the process of collisional grinding of asteroids into ever-smaller bodies, ultimately to those of meteorite size. There is some indication that the discrepancy is less serious for small asteroidal fragments observed as Apollo objects, which is consistent with this hypothesis.

Meteorites, asteroids, and the early solar system
A prime long-range objective of exploring the Moon and planets by spacecraft is to collect samples of these bodies for detailed laboratory study. With the few exceptions noted earlier, meteorites are samples of asteroids delivered to Earth by fairly well-understood natural mechanisms. It also is believed that samples collected from asteroids will prove especially valuable, because such small bodies are most likely to be “primitive” and thus retain the record of events that occurred during their own formation and that of the solar system in general. The study of meteorites is already providing scientists with much valuable information directly related to this matter.

Isotopic records
Meteorites indicate to investigators that the asteroid belt must always have been a relatively tranquil region of the solar system. Some unequilibrated chondrites have inherited and preserved without complete mixing the remnants of presolar interstellar grains. This is demonstrated by variations of the ratios of the oxygen isotopes oxygen-16, oxygen-17, and oxygen-18 within a single meteorite and between different meteorites. It is well known that the oxygen isotopes are fractionated by natural chemical processes. Variations in the ratio of oxygen-18 to oxygen-16, for example, form the basis for paleotemperature studies of ancient terrestrial sedimentary rocks by making use of the temperature dependence of the isotopic chemical equilibrium between seawater and the calcium carbonate that forms the shells of marine organisms. It is characteristic of this natural fractionation dependent on isotopic mass that the degree of fractionation is proportional to the difference of the masses of the isotopes. Thus, in these marine sediments, the variation in the ratio oxygen-18:oxygen-16 is twice that of oxygen-17 to oxygen-16. In some unequilibrated chondrites, however, the variations in the oxygen-18 to oxygen-16 ratios are equal to those in oxygen-17:oxygen-16. It is possible to devise special chemical processes that could produce fractionation of this kind, but it is much more likely that the variations found in the meteorites are caused by nuclear processes that predated the formation of the Sun and solar system. The interstellar grains that were the carriers of these isotope anomalies were probably formed in stellar atmospheres and preserved the signatures of the isotope-formation processes of stars of particular types. These isotopic variations of nuclear origin are found not only in oxygen but also in other less abundant elements, including neon, xenon, titanium, and chromium.

Probable early evolution of the solar system based on meteoritic evidence
The need to provide an environment sufficiently tranquil to preserve this isotopic record, as well as other fragile relics of early solar system events observed in meteorites, places important constraints on the formation of the solar system. If one examines the distribution of matter in the present solar system, it is seen that the density is high both in the region of the inner planets and in the region of the giant planets in the outer solar system but very low in the wide space between Mars and Jupiter. This fact itself is surprising: Why should the solar nebula from which the solar system formed have had a great hole in it? The answer is that it probably did not. In order for asteroids to have formed and developed at all on the time scale of a few million years indicated by the radiometric dating of meteorites, densities more like those in the regions occupied by the giant planets would have been required, as shown by theoretical calculations. It is difficult to escape the conclusion that the quantity of matter in what is now the asteroid belt must have been much greater, perhaps by as much as 10,000 times the quantity observed there today. Therefore some natural process has to have removed almost all the material in this region of the solar system after the formation of asteroidal bodies, but in a sufficiently gentle way to have preserved the relics of presolar and early solar events found in meteorites.
Although the details are not yet understood, it seems most likely that the formation of the giant planets, particularly Jupiter, quickly resulted in the evacuation of most of the matter from this region of the solar system. This means that Jupiter formed rapidly, before bodies in the asteroid belt had grown to become full-fledged planets. The mineralogical and chemical record of the undifferentiated meteorites is not compatible with their once having been part of a planet even as large as the Moon. Also, the very energetic collisional events that would be associated with the dispersal of large planets in the asteroid belt would preclude preservation of the observed relics.
These constraints, for the most part based on meteoritic evidence, define a conceivable chain of events for the early evolution of the solar system beyond the region of the inner planets. Even though the density of matter was at least as great in the asteroid belt as in the region of Jupiter, planetary growth must have been more rapid in Jupiter's vicinity than in the asteroid belt. (Some suggestions have been made as to how this may be possible, but it remains to be seen if they are satisfactory.) Within about one million years, proto-Jupiter(s) began to capture the massive quantities of hydrogen and helium from the solar nebula that constitute most of the giant planet today. At the same time, thousands of large asteroids greater than 100 kilometres in diameter, including some as big as the largest present-day asteroids but not much bigger, had formed. Shortly thereafter, as Jupiter approached its present mass, most of the residual nebular gas was removed by the intense solar ultraviolet radiation characteristic of young stars.
Up until the time Jupiter approached its present mass, the asteroids moved in nearly circular orbits in accordance with the weak mutual gravitational perturbations expected for small bodies. During the final formation of Jupiter and Saturn and the removal of the nebular gas, the changing mass distribution in the outer solar system caused waves of resonant perturbations to sweep through the asteroid belt, increasing the eccentricities and inclinations of the asteroids to the moderate values observed today. Because the asteroids had not grown larger than the asteroids of today, collisions at the relative velocities of about five kilometres per second, associated with their present-day eccentricities and inclinations, would begin to grind down the smaller bodies. This would occur without the shock effects associated with disruption of larger planetary bodies at higher impact velocities.
The foregoing scenario of planetary growth is certain to be wrong in detail and may well be wrong altogether. Nevertheless, without the samples of asteroids provided by recovered meteorites, there would be little observational basis at all for formulating models of this kind. All the qualitative statements made above should be considered not as established facts but rather as statements of theoretical problems that need to be thoroughly worked out. Their results must be compared with old and new observational data and reformulated and reworked until a satisfactory understanding of planetary formation is achieved.

Thermal evolution of the solar nebula and planetesimals
The available meteoritic evidence is relevant to many other unsolved central questions concerning the early solar system, including some that bear on the way the Sun was formed and its early history. One of these is the question of the thermal evolution of the solar nebula and the planetesimals that formed and grew within it.
As discussed above, the environment of the early asteroid belt must have been a rather “tranquil” one. By tranquil, one actually means thermally, rather than physically, uneventful. The collisions that led to the removal of most of the material from the asteroid belt were highly disruptive, and there is ample evidence of extensive physical disruption in the textures of even the most primitive meteorites. Yet, the preservation of primordial relics, such as the isotope anomalies, argues against widespread heating of the asteroidal region to temperatures as high as 1,000 K.
A relatively cool solar nebula at distances this far from the Sun is in agreement with most current theoretical calculations of the formation of the Sun. In addition to the evidence for an overall low-temperature origin for this region of the solar system, however, the meteoritic record clearly shows the imprint of high-temperature events of major importance, which are not well understood at present. The most apparent of these is the very abundant presence of chondrules in all undifferentiated meteorites except the CI chondrites. Chondrules appear to have once been melted droplets primarily of silicate composition and would have required temperatures of about 1,500 K to have formed.
If chondrules were a relatively rare meteoritic curiosity, one could legitimately consider them an interesting detail to be explained someday but not a matter of central importance. Yet, the fact that chondrules (or their broken fragments) make up most of the mass of the most abundant class of meteorites, the ordinary chondrites, and a major portion of other chondrites, indicates that their formation must have been of major importance in the early solar system. Even if ordinary chondrites formed only within a restricted region of the asteroid belt adjacent to the 3∶1 Kirkwood gap (i.e., between 2.44 and 2.56 AU), this is still about 10 percent of the entire asteroid belt. It also is likely that other chondrule-bearing meteorites formed outside this region, even though their asteroidal sources may be in that region today.
It seems impossible that chondrules are the product of igneous differentiation because they are of nearly undifferentiated solar composition except for the most volatile elements. Nor does it seem likely that they are impact droplets, such as those found in the lunar soil. The expected low impact velocities of asteroidal planetesimals argue against a ubiquitous high-velocity impact environment. Thus, there seems to be no way by which the chondrules could have formed in or on planetesimals. In all likelihood, they were formed in the solar nebula. At the same time, the evidence for rapid cooling of chondrules argues against their formation by large-scale condensation from a very hot solar nebula. Local, transient heating events appear to have been important on a wide scale in the solar nebula, but the nature and cause of these events remain unknown.

A problem of similar difficulty is that of the origin of the large (up to more than one centimetre in diameter), highly refractory inclusions found in the CV meteorites (see Table), especially prominent in the Allende carbonaceous chondrites. Though not as ubiquitous as chondrules, these inclusions are by no means of negligible abundance. Unlike chondrules, they are highly fractionated chemically, apparently as a result of more prolonged heating to about 1,500 K. Because many of the isotopic anomalies are associated with these refractory inclusions, interpretation of this important evidence is limited by a poor understanding of their origin.
The foregoing thermal events most likely occurred in the solar nebula rather than on growing asteroidal bodies. In addition, thermal effects are observed in meteorites associated with internal heating. The most apparent of these are the differentiated meteorites, which probably represent about 10 percent of the asteroidal region sampled. The asteroids from which these meteorites were fragmented, though probably formed from a relatively cool solar nebula, experienced internal heating, core formation, and igneous differentiation within a few million years of the formation of the solar nebula itself. Clear and detailed evidence for this is based on (1) radiometric dating of the minerals formed by this igneous differentiation, (2) the mineral assemblages of the resulting igneous rocks, and (3) the slow cooling that produced large crystals of differentiated minerals (e.g., the so-called Widmanstätten structure observed in many nickel-iron meteorites). The actual process responsible for this heating is yet unknown, but several good possibilities are being evaluated. These include heating by the short-lived radioactive isotope aluminum-26, heating by electric currents induced by early solar activity, and accretional heating of planetesimals in the terrestrial planetary region, followed by fragmentation and transfer to the innermost asteroid belt.
The ordinary chondrites also experienced heating after the formation of chondritic planetesimals, but not enough to produce melting. As a result, chondritic material, presumably once resembling the unequilibrated ordinary chondrites, was metamorphosed to produce the more abundant equilibrated ordinary chondrites. The time scale for this metamorphism is, within uncertainties, the same as that which produced the parent asteroids of the differentiated meteorites. It is plausible that some of the same heat sources (e.g., aluminum-26) may have been responsible.

George W. Wetherill
Additional Reading
Introductory information can be found in Harry Y. McSween, Jr., Meteorites and Their Parent Planets (1987); Robert T. Dodd, Thunderstones and Shooting Stars: The Meaning of Meteorites (1986); John G. Burke, Cosmic Debris: Meteorites in History (1986); Robert Hutchison, The Search for Our Beginning: An Enquiry, Based on Meteorite Research, into the Origin of Our Planet and of Life (1983); and John A. Wood, Meteorites and the Origin of Planets (1968). More advanced treatments are John T. Wasson, Meteorites: Their Record of Early Solar-System History (1985), and Meteorites: Classification and Properties (1974); V.A. Bronshten, Physics of Meteoric Phenomena (1983; originally published in Russian, 1981); and Robert T. Dodd, Meteorites: A Petrologic-Chemical Synthesis (1981). A descriptive and historical treatment of iron meteorites, including beautiful photographs, is Vagn F. Buchwald, Handbook of Iron Meteorites, Their Distribution, Composition, and Structure, 3 vol. (1975). H.H. Nininger, Out of the Sky: An Introduction to Meteorites (1952, reprinted 1959), provides firsthand experiences of fall phenomena on a nontechnical level. See also D.E. Brownlee, “Cosmic Dust: Collection and Research,” Annual Reviews of Earth and Planetary Sciences, 13:147–173 (1985). A catalog of known meteorites, including data regarding their fall, is A.L. Graham, A.W.R. Bevan, and R. Hutchison (eds.), Catalogue of Meteorites, 4th ed. rev. and enlarged (1985). There are two journals devoted to papers on meteorites and related bodies: Meteoritika (annual), published in Russia; and Meteoritics (quarterly). Many papers on meteorites are published in Geochimica et Cosmochimica Acta (monthly).