Association of Meteorites with Asteroids

If meteoritic material comes from specific regions of the asteroid belt, then the asteroids in such regions should have the chemical and mineralogical composition observed in the meteorites. The surface mineralogical composition of asteroids, in principle, can be determined directly by observations from Earth of the fraction of sunlight they reflect (albedo) and the spectrum of the reflected light (reflectance spectrum). A number of processes conspire, however, to make the association of certain asteroids with the various meteorite groups much more difficult than might be expected.

Although no two asteroidal reflectance spectra are exactly alike in detail, most asteroids fall into one of two general groups, the S class and the C class. S class asteroids (e.g., Gaspra and Ida, observed by the Galileo spacecraft, and Eros, visited by the NEAR Shoemaker spacecraft) have moderate albedos and contain mixtures of olivine, pyroxene, and metallic iron. These are the same minerals found in ordinary chondrites, but they also are present in a number of other meteorite types. The C class asteroids (e.g., Mathilde, observed by NEAR Shoemaker) have low albedos, and their more featureless spectra indicate the presence of light-absorbing materials, although at least half have a spectral feature associated with iron-bearing hydrous silicates. It is plausible to consider the C class asteroids as candidate sources for certain groups of carbonaceous chondrite meteorites. Their low albedos and spectral evidence of hydrous silicates, however, make them unlikely sources of ordinary chondrites.

When the S class asteroids are considered in more detail, there are difficulties in identifying them all as sources of ordinary chondrites. Largely because of their apparent range of mineralogies—specifically their ratios of olivine to pyroxene—the S class asteroids have been divided into seven subclasses. In light of this, it is possible that the S class actually represents a number of unrelated groups of asteroids. In addition, some research has linked the S class asteroids to several groups of achondrites. On the other hand, if most S type asteroids are not related to the ordinary chondrites, scientists would be challenged to explain how an uncommon and unidentified class of asteroid is supplying most of the meteorites to Earth.

The asteroids in the S(IV) subclass seem to have mineralogies that best match those of the ordinary chondrites. This is supported by measurements made by NEAR Shoemaker of the elemental composition of the surface of Eros, which is classified as an S(IV) asteroid. With the notable exception of a low sulfur content, the composition of Eros was found to be consistent with that of an ordinary chondrite.

Scientists have come to recognize relatively recently that the surfaces of asteroids and other solid bodies are not necessarily representative of what lies just a short distance beneath those surfaces.

Both Eros's low-sulfur measurement and the fact that, overall, the spectra of S class asteroids do not exactly match those of ordinary chondrites may be due, at least partially, to the effects of a poorly understood set of processes collectively called space weathering. Important component processes of space weathering are thought to be the impacts of meteorites and micrometeorites and the impingement of energetic solar wind particles and solar radiation on surface materials. Over time these processes act to modify the chemical and physical surface properties of airless bodies such as Mercury, the Moon and some other planetary satellites, and asteroids and comets.

Comparisons of younger surfaces around craters with older terrains on Eros by NEAR Shoemaker, and on Gaspra and Ida by Galileo, support the idea that space weathering occurs on asteroids.

Asteroids are thought to be covered by a layer of pulverized rock, called rego-lith, produced by bombardment with meteorites of all sizes over millions to billions of years. The regolith need only be as thin as a few sheets of paper to completely mask the underlying material from reflectance spectroscopy, although on most asteroids it is probably much thicker. Unfortunately, because it is so loosely bound together, this regolith material does not survive entry into Earth's atmosphere in pieces that are large enough to identify as meteorites and analyze. Consequently, scientists do not have samples of regolith that can be compared with meteorites or asteroids directly. On the Moon, however, systematic changes are observed in the mineralogy and reflectance properties of the surface material as a result of this col-lisional grinding and other space weathering processes. Thus, although it seems likely that ordinary chondrites do come from S class asteroids, space weathering may be making it difficult to determine with certainty which S class asteroids are the parent bodies of these meteorites and which are unrelated but have a grossly similar mineralogy.

Space weathering must also affect the spectra of the asteroidal sources of the other meteorite groups. Nevertheless, a number of more-or-less convincing associations between groups of meteorites and types of asteroids have been made. It has been proposed that the CV and CO groups of carbonaceous chondrites come from the K class asteroids. As mentioned above, a number of lines of evidence, including spectral measurements, point to the asteroid Vesta being the source of the howardite-eucrite-diogenite association and the mesosiderites. The most likely source of the iron meteorites is the M class of asteroids, but enstatite chon-drites and mesosiderites have also been linked to them. The pallasites may come from A class asteroids.

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