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Flgure 1. Chemical classification of meteorites Metmite classes (IeW are based on proportions of metallic F b N i (red). FeS (orange), and silicate (white). Each class is divided imo a number of groups (right) by major chemical ConstiluBnls. i.e.. chondritic groups are delineated by amounts 01 metallic Fe-Ni and iron in ferromagnesian silicates
eteorite studies constitute the major, but not the sole, part of cosmochemistry, the discipline initiated by H. C. Urey in the mid-1950s to determine chemical and physical processes important in the Solar System’s formation and evolution. Meteorites are of essential interest because they contain the oldest Solar System materials available for research and sample a wide range of parent bodies-exteriors and interion-some primitive, some highly evolved. Meteorites carry decipherable records of certain solar and galactic effects and yield otherwise unobtainable data about the genesis, evolution, and composition of the Earth and other major planets, satellites, asteroids, and the Sun. Some contain iuclusions tracing events from before the Solar System formed; others contain organic matter derived from giant molecular clouds in the interstellar medium. Meteorites also provide an important body of “ground truth,” in a chemical and physical sense, which is critical to interpreting planetary data obtained by remote sensing. It is especially advantageous that meteorites occur on the Earth’s surface, where the full spectrum of laboratory analytical techniques can he applied, ranging from the simplest~tothe most sophis-
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: ticated. As the recently released report of the U S . National Commission on Space put it: If one picture is worth 10,000 words, then one sample is worth 10,ooO pictures ( I ) . Because of the interdisciplinary nature of meteorite studies--overlapping chemistry, physics, geology, and astronomy-no brief article can summarize the full scope of current research. (Several recent general books of varying complexity [ 2 4 ] cover specific aspects and are highly recommended; many other books and references are cited in them.) After introducing some basic cosmochemical facts and approaches, this REPORT will illustrate the nature of questions that cosmochemists ask and how they go about answering them. For reasons to be described, we will focus on certain trace elements-especially Ag, Au, Bi, Cd, Co, Cs, In, Rh, Se, Te, TI, and Zn-that are particularly responsive to relatively low temperature processes and that yield important genetic information.
orbital plane is the ecliptic plane and whose mean distance to the Sun, 1.5 X 108 km, is 1astronomical unit (AU). There are, in addition, two belts of smaller objects: the putative Oort cloud a t 50,000 AU-the comet source-and the asteroid belt, mainly located a t 2.2-3.6 AU, i.e., between the orbits of Mars and Jupiter. The asteroid belt contains more than 3000 numbered minor planets detected from Earth The short-lived (May 1983 to March 1984) infrared astronomy satellite, IRAS, ohserved 15,000. About 30
ble 1. NowAntarctic meteor-
I
The genetic framewak A major goal of meteorite studies is to establish the origin and early evolutionary history of the Solar System, which consists of the Sun and nine major planets including Earth, whose
ANALYTICAL CHEMISTRY. VOL. 58, NO. 9, AUGUST 1986
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objects, the Apollo asteroids, have orbits that cross the Earth‘s and are, therefore, inherently unstable. To attain Earth-crossing orbits, an object or its fragments must he acted on by an external force, usually a massive collision. Fragments from a large object, i.e., the Moon or Mars, must reach escape velocity-2.4 and 5.0 kmls, respectively. Ring asteroids must acquire an impulse of 1-3 km/s to attain a Mars-crossing orbit, following which Mars’s gravitational field can perturb objects into more elliptical, Earth-crossing (Apollo-like) orbits. Mars may he the largest object from which we can expect meteorites because shock velocity and shock heating are related. Velocities greatly exceeding 5 kmls imply shocks large enough to vaporize planetary materials. Whether special conditions can be found permitting such high velocities without substantial shock loading is currently being investigated. The asteroid-meteorite connection is well established ( 2 , 3 , 5 ) Three . ordinary meteorite falls recovered in Czechoslovakia (19591, Oklahoma (1970), and Canada (1977) were each photographed simultaneously from two or more points during atmospheric passage so that their orbits could he calculated. These orbits resembled
those of Apollo asteroids, having perihelia (closest solar approach) of 51 AU and aphelia of 2-4 AU. Plots of reflectance (or albedo) vs. wavelength from 0.4 to 2.2 fim demonstrate matches between specific meteorite types and asteroid surfaces, suggesting a linkage. For example, many asteroids, including the largest-1 Ceres with a diameter of 1025 km-are of the C type; that is, they have spectral reflectances like those of carbonaceous chondrites, a primitive meteorite type that can contain up to 5% organic matter of considerable complexity ( 6 ) . Meteorites are classified as stones (Chondrites and achondrites), stony irons, and irons based on their relative proportions of Fe-Ni, silicate, and FeS (Figure 1). Chondrites contain spherical millimeter- to centimeter-sized chondrules or their fragments, silicates that were rapidly melted and cooled in a few minutes early in the Solar System’s history. Such rapid heating and cooling are easy to perform on the laboratory scale but difficult to achieve on the Solar System scale (7).Yet, large volumes of chondrules must have been present in the Solar System because the number of chondrites is large (Table I). Chondrites date hack to the Solar System’s formation-indeed provide chronome-
ters for it-and represent accumulated primary nebular condensate and accretionary products. A portion of this condensate formed from the hot nebula as millimeter-sized Ca- and Alrich inclusions (CAI) that are aggregates of minerals predicted as vapor deposition products by thermodyuamic calculations. These CAI, found mainly in carbonaceous chondrites, exhibit many isotopic anomalies and may contain presolar material (2,6,8). Other condensates formed at much lower temperatures. Heating by a variety of sources (short- or long-lived radioactivities, gravitational energy release, etc.), mainly in asteroidal-sized parent bodies early in the Solar System’s history, caused partial or complete melting (Figure 2) that transformed some chondrites into differentiated meteorites (igneous achondrites, irons, and stony irons). These transformations involved chemical and physical fractionations that are being studied and that are not yet well understood. Similar events must also have occurred early in the history of the terrestrial planets and Moon. Thus, establishing the genetic linkage between specific chondritic and differentiated meteoritic types should provide a model system for understanding the Earth’s
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Flgure 2. Stepwise formation of meteorites (bottom) from the nebula (top) Spscific processes in parentheses(at right) do not Chemically hamionate meteaitic material; the omera do. b is not clear Whether primary wbuW podUctD were small planetesimalsu asteroidBI-sIzedprlmhive bodies. The &IcBI line is not lo sale
Figure 5. A meteoritic impact breccia The whhe hwt is an achOndrite (aubre): blaCk i w l u s i m are collision debrb tom the primnlvefasterite chondrite parent body (phdo munesy of SmimsanianInstitution)
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early evolution. The idea that the Earth's core consists of metallic Fe-Ni was originally suggested by the existence of iron meteorites; subsequent data support this. Using compositional information, chondrites and differentiated meteorites can he further classified into various chemical groups (Figure 1) deriving from different starting materials or parts of the chemically inhomogeneous nebula (Figure 2). Each meteoritic group has a characteristic stable oxygen isotopic composition ('6OPO/ lSO) trend that seems characteristic of the batch of nebular material from which it condensed (9).(Isotopic anomalies in meteorites yield many sorts of genetic information [SI.)Because isotopic homogenization is much easier than chemical homogenization, nebular chemical heterogeneity is assumed. Chondrites are classified into six or more groups, based on proportions of iron as metal and silicate and on total iron (as Fe, FeO, and FeS) content, that is, high, low, or very low (H, L, or LL, respectively) Fe concentration. Some parts of primitive chondritic parent bodies were heated less than others, and in these parts, solid-state processes occurred that can he recognized petrolpgically (e.g., in making chondrules less distinct) or chemically (e.g., in homogenizing Fez+ contents of olivine (Fe,Mg),SiO,, pyroxene (Fe,Mg)SiOs),or ferromagnesian silicates. Ten such criteria permit each chondritic chemical group to he classified into one of seven petrologic types (2,3),the higher numbers indicating the greater degree of secondary metamorphism. Some chemical-petrologic types are unknown (e.g., H, L, LL or E l or 2); others are especially abundant (e.g., H5 or L6). Which, if any, group (e.g., E3-7, LL3-7) was metamorphosed under open-system conditions, so that mobile elements and compounds could have been lost, is being debated. Metamorphism occurred shortly after the Solar System formed 4.6 X l o 9 yr ago and involved temperatures of about 400 "C for type 3 to almost 1100 "C for type 7. Because no internal energy source known is sufficient to partly or totally disrupt an asteroid, collisions must have produced meteoroids (which hecame meteorites upon landing on Earth), each of which, in principle, records shock loading. In practice, different meteorites experienced different shock intensities: Very low pressures will not register, whereas high shock pressures induce marked changes observable petrographically or by thermoluminescence. Effects in chondrites permit estimation of shock pressures or facies ranging from a (
