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The ultimate goal of meteorite classification is to group all meteorite specimens that share a common origin on a single, identifiable parent body. This could be a planet, asteroid, Moon, or other current Solar System object, or one that existed some time in the past (e.g. a shattered asteroid). However, with a few exceptions, this goal is beyond the reach of current science, mostly because there is inadequate information about the nature of most Solar System bodies (especially asteroids and comets) to achieve such a classification. Instead, modern meteorite classification relies on placing specimens into "groups" in which all members share certain key physical, chemical, isotopic, and mineralogical properties consistent with a common origin on a single parent body, even if that body is unidentified. Several meteorite groups classified this way may come from a single, heterogeneous parent body or a single group may contain members that came from a variety of very similar but distinct parent bodies. As such information comes to light, the classification system will most likely evolve. ==Terminology== Beyond the assignment of meteorites into groups (see above), which is essentially universally accepted, there is no consensus among researchers as to what hierarchy of classification terms is most appropriate. For chondrites, groups may be divided into subgroups where there are features that distinguish certain meteorites from the others in the group, but it is thought that all still come from a single parent body. It is also fairly common for groups that seem to be closely related to each other to be referred to as clans. In turn, groups or clans that appear to be loosely related are often referred to as chondrite classes (e.g., carbonaceous chondrites, enstatite chondrites, and ordinary chondrites). But higher order terms for aggregating groups of meteorites tend to be somewhat chaotic in the scientific and popular literature. There is little agreement on how to fit nonchondritic meteorite groups into an overall scheme. Several other classification terms are in widespread use: * Type, a historic top level of classification (see below) that grouped all meteorites into one of four types; chondrite, achondrite, iron or stony-iron. * Anomalous, meteorites that are members of well-established groups that are different enough in some important property to merit distinction from the other members. * Grouplet, a provisional group with less than 5 members. * Duo, a provisional group with only 2 members. * Ungrouped, meteorites that do not fit any known group, though they may fit into a clan or class (e.g., the meteorite Acfer 094 is in an ungrouped member of the CM-CO clan of carbonaceous chondrites). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Meteorite classification」の詳細全文を読む スポンサード リンク
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