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2. Asteroids, Comets, and Planet Formation

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2. Asteroids, Comets, and Planet Formation ASTEROIDS, COMETS, AND PLANET FORMATION Michael H. Carr INTRODUCTION Asteroids have been classified in two ways. The first is according to their orbits, which is of interest to those The solar system includes numerous small objects, attempting to estimate the rate of impact on the generally classifiable as asteroids, comets, or inter- various planets. The Amor asteroids, for example, planetary dust. Asteroids and comets are of consider- have perihelia (the point in the orbit closest to the Sun) able importance in the study of the terrestrial planets. inside Mars' orbit but outside Earth's. Clearly, ob- The impacts of these bodies with the planets have jects of this class can impact Mars but not Earth. In repeatedly reshaped the planets' surfaces, redistributed contrast, the Apollo asteroids have perihelia inside the the surface materials and changed their chemistry and Earth's orbit and so can impact both Earth and Mars. mineralogy. In addition, impacts leave scars which Alternatively, the asteroids are also grouped accord- provide a means of assessing ages. Asteroids, and ing to their albedo and spectral reflectivity. This classi- possibly old comet nuclei, continually rain on the fication is useful in that the reflectivity gives some indi- Earth, and many thousands are preserved and are cation of composition, and tentative correlations have displayed in our museums as meteorites. In this been made between the asteroid groupings and various chapter the nature of comets and asteroids is briefly types of meteorites. reviewed.The formation of planets is also examined Comets have also impacted the planets throughout here because most of the evidence as to how the planets their history and affected their geology much in the formed comes from meteorites. The discussion is short same way as asteroids. Comets appear to be agglom- and general. The chapter is included mainly to pro- erations of ice and dust that have spent most of their vide background for the subsequent more detailed lifetime far beyond the orbits of the planets (Whipple, treatment of the planets. 1974). They are believed to reside in large numbers (1011-1012)in the most distant reaches of the solar ASTEROIDS AND COMETS system, in the so-called Oort cloud, which is estimated to be about 100 000 AU across. The cloud is believed Over 2000 asteroids are known (Chapman, 1975); to have formed early in the history of the solar system the largest being Ceres, about 1000 km in diameter. by ejection of material from the region of the planets, They appear star-like on telescopic photographs, and outward, almost to the edge of the Sun's influence. are recognized as a result of their motions relative to Ejection resulted from orbit perturbations induced by the stars. Their shapes are not known, but variations close passes by a planet. Distances from the Sun in in brightness suggest that they are irregular. Most the Oort cloud are so great, and motions of the comets asteroids are in orbits with relatively low eccentricities in the cloud so slow, that perturbation of their orbits (C0.3) as compared with comets, and most are in the by passing stars are occasionally sufficient to cause asteroid belt between Mars and Jupiter. a comet to plunge into the inner solar system on a THE GEOLOGY OF THE TERRESTRIAL PLANETS near-parabolic orbit around the Sun. As the ice-dust mm across, consisting mainly of olivine and ortho- ball approaches the Sun, its surface warms and the phyroxene, but possibly with minor amounts of clino- ices start to sublime. The comet then becomes visi- pyroxene, iron sulfide and iron-nickel. The chondritic ble and develops its characteristic shape. The gases meteorites formed very early in the history of the solar given off are ionized and swept away by the solar system, while the planets were forming, and so pro- wind, along with any dust that is released. At this stage vide direct evidence of conditions at that time. three parts can generally be distinguished: a bright The most primitive meteorites are the carbonaceous head (coma) roughly spherical in shape, a tail of ion- chondrites, in which the chondrules are embedded in ized gas which streams out from the head in a straight a fine-grained, earthy matrix. This matrix consists of line radial to the Sun, and a tail of dust particles which hydrated, platey minerals such as serpentine and extend in a slightly curved arc, also roughly radial to montmorillonite, together with small amounts of mag- the Sun. netite, iron-nickel sulfide, sulfates and carbonates. Comets are of two major kinds, periodic and non- Small amounts of organic compounds are also gen- periodic. The nonperiodic comets constitute the vast erally present. The matrix is thus a low-temperature, majority. They move in parabolic orbits and make volatile-rich assembly, in contrast to the high- only one pass through the center of the solar system. temperature assemblage of the chondrules. The chon- Their inclinations are random, with prograde and drules and matrix are in disequilibrium and clearly retrograde orbits equal in number. From these char- formed under very different conditions. acteristics we can conclude that the Oort cloud, their Some carbonaceous chondrites also have small, source, is spherically disposed around the solar system irregularly-shaped inclusions composed largely of Ca- and not restricted to the plane of the planets. In con- Al-Ti-rich minerals. The minerals in the inclusions trast, the periodic comets are all in low-inclination, are those which are expected to have condensed first prograde orbits. They are believed to be former para- from the nebular gas in the early stages of cooling of bolic comets whose orbits have been perturbed by the the primitive solar nebula. Their formation had been planets, mainly Jupiter. Such perturbations are possi- predicted from thermodynamical considerations ble only for those comets which initially moved in pro- (Grossman, 1972) before they were discovered in the grade orbits with low inclinations. Periodic comets Allende meteorite. The Ca-Al-rich inclusions are have limited lifetimes in that they lose mass each time therefore samples of the most primitive matter of the they pass the Sun, indicating some asteroids may be solar system. Their irregular shape indicates that they burned out comet remnants. have never gone through a melting stage but formed directly from the nebular gas. Carbonaceous chondrites resemble the Sun in com- METEORITES position. The only significant differences are lesser amounts of hydrogen, carbon, nitrogen, and noble Meteorites provide us with samples of asteroids and gases in the carbonaceous chondrites.These elements possibly burned out comets, and are of fundamental are unlikely to have condensed in the inner solar importance for understanding the formation of the system because of their high volatility, so their lack planets. They provide the best evidence of the nature of incorporation into the carbonaceous chondrites is of the materials that accumulated to form the planets, not surprising. Because of (1) the compositional cor- as well as the processes of accumulation. Meteorites respondence between the Sun and the carbonaceous have a wide range of composition and mineralogy chondrites, (2) the presence of the Ca-Al-rich inclu- (Wood, 1979; Wasson, 1974). Some, termed differen- sions, and (3) the disequilibrium mixture represented tiated meteorites, resemble terrestrial igneous rocks. by the chondrules and matrix, the carbonaceous chon- They appear to have been part of a larger body that drites are believed to be indicative of the primitive broke up at some intermediate stage in the history of solar nebula composition, depleted somewhat in the the solar system, after having gone through a stage more volatile elements. In models of planet forma- of heating and magmatic differentiation. The other tion, therefore, carbonaceous chondrites are com- main class of meteorites, the chondrites, are distinc- monly used to represent the materials from which the tively different from any terrestrial rock. They are planets formed. AU carbonaceous chondrites have ages mostly agglomerates of fragmental debris and typically comparable to the age of the solar system (4.6 billion contain chondrules-spherical mineral aggregates, 1-2 years). ASTEROIDS, COMETS, AND PLANET FORMATION Carbonaceous chondrites represent only a small formed at depths of 30-50 km. These depths are prob- fraction of all chondrites; ordinary chondrites are much ably representative of the radius of the parent body more common. These also contain olivine-pyroxene from which the meteorite formed. chondrules but their chondrules are embedded in a In recent years the question of how bodies as small matrix of the same composition as the chondrules as a few tens of kilometers across could have heated themselves. Many of the ordinary chondrites have up and created the metamorphic and igneous textures been heated so that the boundaries between chondrules seen in meteorites has been resolved. It now appears and matrix have become diffuse, and the constituent that radioactive heating by the decay of short-lived minerals have been recrystallized to form granulitic radioisotopes, mainly 26Al(half-life of lo6years), 244P~ textures suggestive of high temperatures. Exposure (82 x lo6years) and 1291(16 x lo6years), was respon- ages for the ordinary chondrites, that is, the time the sible, since the decay products of these nuclides have meteorite has been exposed to cosmic rays, are mostly been found in the meteorites. Since these short-lived in the lo6-lo8 year range-far shorter than the age nuclides were incorporated into meteorites before they of the solar system. had decayed, the time interval between element for- The carbonaceous chondrites are agglomerations mation and meteorite formation must have been rela- of chondrules, the more primitive Ca-Al-rich inclu- tively short-no more than a few million years.
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