Extraterrestrial materials are samples from other bodies in the solar system that can be studied in Earth-bound laboratories. Sensitive and ever-improving analytical techniques are used to provide information at levels of detail and sophistication that cannot bematched by telescopic or spacecraft investigations (see Analytical methods). Much of the knowledge of early solar system bodies, processes, environments, and chronology has come from the study of these samples. Extraterrestrial materials that are available for laboratory study include meteoritic materials that fall naturally to the Earth, some meteoritic material that has been captured in space,and lunar samples that were recovered by the Apollo and Luna sample-return missions ﬂown to the Moon during the years 1969 to 1972 (1). Missions to return samples from Mars, asteroids, and comets have been studied but have never been successfully implemented. The meteoritic materials in existing collections include samples from asteroids, comets, the Moon, and probably Mars. The comet and asteroidsamples are the best preserved solids from the early solar system and are the oldest and most cosmochemically primitive samples available for direct study. Because of the primitive and unfractionated nature, these samples provide the best determination of the composition of the Sun and the solar system as a whole. It has been shown that many meteorites contain preserved interstellar grains,particles older than the Sun that formed around other stars and served as the initial building blocks of the solar system.
Meteorites by deﬁnition are extraterrestrial materials that fall from the sky and actually hit the surface of the Earth. In space they are considered to be meteoroids and during their luminous entry into the atmosphere they are called meteors. Meteorite strictlyapplies only after impacting the Earth. Conventional meteorites are rocks ranging in size from a centimeter to a few meters. The largest known meteorite is the 70-ton Hoba that resides at its discovery site in South Africa. Larger meteoroids do not sufﬁciently decelerate from cosmic velocity in the atmosphere and are destroyed upon impact, forming an explosion crater. Meteorites fall randomly toEarth but are not found randomly distributed on the Earth’s surface. The highest general concentrations of meteorites occur in Antarctica where long exposure time and the combined effects of ice movement and sublimation concentrate meteorites on top of blue ice ﬁelds. Because of the scarcity of country rocks, it has been possible to collect over 10,000 meteorites from Antarctica since the early1970s. In Antarctica and elsewhere, meteorites are often found in clusters created by the breakup of a larger body during hypervelocity entry into the atmosphere. When a meteor breaks up at high altitude, the resulting fragments impact over an elliptical region several kilometers across the ground, forming a strewn ﬁeld where sometimes thousands of individual specimens are found. Because ofatmospheric breakup, the number of individual meteorite specimens that are collected is much larger than the actual number of meteoroids that produced them. Meteoroids are themselves fragments of bodies that broke up in space, and the actual lineage of meteoritic samples may trace back to a relatively limited number of initial parent bodies.
Table 1. MeteoriteClassiﬁcation
Meteorite type chondrites Class CI CM CV CO CR H L LL E aubrites urelites howardites diogenites eucrites lunar all types pallasites mesosiderites of meteorites seen to fall. Frequency of occurrence,a % 0.7 2.0 1.1 0.8
75% of the mass of many meteorites. It is possible that chondrules were the primary building blocks of the Earth and the terrestrial planets. The material between...