Asteroid belt

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

The two rare earth elements niobium (Nb) and tantalum (Ta) were the main subject of study in the investigation referred to. Both elements have very similar properties and almost always occur together in our solar system. However, the silicate crust of the Earth contains around 30% less niobium (compared to its sister tantalum). Where are the missing 30% of niobium They must be in the Earth s FeNi core. It is known that the metallic core can only take up niobium under huge pressures, and the conditions necessary for this may have been present on Earth. Analyses of meteorites from the asteroid belt and from Mars show that these do not have a niobium deficit. 

New computer simulations of the accretion process of the protoearth indicate that only a few large bodies with a high water concentration collided with the Earth during the later bombardment. They apparently came from the same region of the asteroid belt as the carbonaceous chondrites. 

Hapke B (2001) Space weathering from Mercury to die asteroid belt. J Geophys Res 106 10039-10073 Hapke B, Cassidy W, Wells E (1994) Vqior deposits in die lunar regolidi Technical comment. Science 264 1779... 

Aluminum-26 is an important nuclide for investigating the cosmic-ray exposure history of meteorites on their way to Earth from the asteroid belt. It can also be used to estimate the terrestrial age of a meteorite. In both of these applications, the 26 A1 is alive in the samples, having been produced by cosmic-ray interactions with elements heavier than aluminum, primarily silicon. Cosmic-ray-exposure dating will be discussed in Chapter 9. 

Eugster, O., Herzog, G. F., Marti, K. and Caffee, M. W. (2006) Irradiation records, cosmic-ray exposure ages, and transfer time of meteorites. In Meteorites and the Early Solar System II, eds. Lauretta, D. S. and McSween, H. Y., Jr. Tucson University of Arizona Press, pp. 829-851. A good summary of what is known about cosmic-ray exposure ages and the transfer of meteorites from the asteroid belt to Earth. 

S-complex asteroids, which include the older E, S, and M groups, dominate the inner and middle belt out to 2.95 AU, C-complex asteroids are most common in the outer belt, and X-complex bodies, which include the P and D classes, are most common at about 3 AU (Fig. 11.7b). Note that this distribution represents only a part of the main asteroid belt shown in Figure 11.7a. Because of uncertainties in the interpretation of S-complex objects as either ordinary chondrites or achondrites, we can no longer say that the innermost asteroids are differentiated but we can infer that S-complex bodies were at least heated (recall from Chapter 6 that ordinary chondrites are mostly metamorphosed). Cl and 2 chondrites have suffered extensive aqueous alteration, suggesting they formed beyond a snowline marking the condensation of ice that later melted that snowhne likely marks the transition to C-complex objects at about 3 AU. 

Contours of peak temperature (K) in asteroids as functions of size (diameter) and heliocentric distance. Accretion times corresponding to various solar distances are given at the top of the figure. Shaded vertical bars mark major divisions in the asteroid belt based on spectral interpretations of asteroid thermal histories. Modified from Grimm and McSween (1993). 

These models provide an explanation for the thermal structure of the asteroid belt that is probably correct in principle but not in its details. The recognition that differentiated asteroids formed earlier than chondrites, perhaps within the terrestrial planet region, requires models in which asteroid accretion was initiated earlier than 2 Myr after CAI formation. 

Collisions among asteroids are commonplace, and impact craters occur on all asteroid surfaces (Fig. 11.1) (Chapman, 2002). Cratering rates within the main asteroid belt are a... 

Describe how various classes of asteroids are arrayed within the asteroid belt, and offer an explanation for this distribution. 

Grimm, R. E. and McSween, H. Y. (1993) Heliocentric zoning of the asteroid belt by aluminum-26 heating. Science, 259, 653-655. 

Several classes of asteroids are also thought to contain ices presently, or contained them at some earlier time. The D- and F-class asteroids occur in the outmost main belt, and the C-, G-, B-, and F-class asteroids are concentrated within the central part of the belt. These asteroids probably formed near their present locations, in which case they represent icebearing planetesimals that accreted inside the orbit of Jupiter. A few asteroids exhibiting cometary activity also occur within the asteroid belt. 

Although planetesimals that formed beyond the snowline are composed of relatively primitive materials (chondritic solids and ices), their compositions are variable. That should not be surprising, because objects now in the asteroid belt, the Kuiper belt, and the Oort cloud formed in different parts of the outer solar system and were assembled at different temperatures. In a systematic study of the spectra of 41 comets, A Heam el al. (1995) recognized two compositional groups, one depleted in carbon-chain (C2 and C3) compounds and the other undepleted (Fig. 12.18). NH compounds in the same comets show no discemable trend. The depleted group represents comets derived from the Kuiper belt, whereas the undepleted group consists of Oort cloud comets. 

Asteroids in the outer asteroid belt show considerable spectral variability, due in part to differences in the degree of aqueous alteration. However, alteration alone is not sufficient to explain all the compositional variability observed in meteorites derived from these objects. Laboratory studies of carbonaceous chondrites show significant differences in the compositions and proportions of the various primary components, demonstrating that accreted materials in the asteroid belt were not uniform. 

At the end of this section, two final points merit some attention. The first one concerns the location of the parent bodies of the chondrites. The asteroid belt between Mars and Jupiter is one possibility favoured by many authors, but some other possibilities exist (such as the families of asteroids crossing the earth s orbit). Asteroids themselves are of many different types and some of them are probably extinct comets (after too many passages at the perihelion). Some chondrites could be fragments of these extinct comets, but this hypothesis is not the most probable one. People interested in this problem will find information in Refs. 2, 5 and 9. 

Like the periodic table of the elements (Chapter 4) and gaps in the asteroid belt, the spacing of Saturn s rings fits a numerical pattern based on the golden ratio [72]. 

Figure 8.2 Logarithmic spiral with superimposed mean planetary orbits. The circles in blue define the orbits of inner planets on a larger (self-similarj scale. The divergence angle of 108° causes those planets at angles of 5 x 108° apart to lie on opposite sides of the spiral origin. These pairs are Neptune-Mars, Uranus-Earth, Saturn-Venus and Jupiter-Mercury. The hypothetical antipode of the asteroid belt, a second, unobserved group of unagglomerated fragments, has been swallowed up by the sun...

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