Asteroid belt, Solar System

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. 

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. 

Anhydrous planetesimals formed within the inner solar system, unlike the ice-bearing bodies discussed in the next chapter. These objects, composed of rock and metal, were the primary building blocks of the terrestrial planets. Relics of that population may survive today as asteroids that dominate the inner portions of the main 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. 

Planets, satellites, and small bodies provide a wide range of dynamical and chemical constraints on the building of the Solar System from planetesimals. In addition to the primary parameters of planets, the planet mass and semi-major axis distributions, the relative masses of the cores (exceptionally large for Mercury and low for the Moon) provide further constraints. In addition, the Asteroid Belt seems to be depleted in mass by three to four orders of magnitude and its medium- to small-sized... 

The astrophysical models of protoplanetary disks based on optical observations and laboratory experiments and meteoritic measurements provide the basis for theories of nebular evolution. The best and most precise relevant measurements are from meteoritic analysis. Meteorites from the Asteroid Belt of our Solar System are the best record of the evolution of the solar nebula from a gas-dust mixture to an organized planetary system. The addition of cometary and solar-wind sample analysis complement these data. Combination of fundamental laboratory-based experiments and modeling efforts has led to a highly resolved understanding of the chemical conditions and processes in the primordial solar nebula (see Chapter 6). In this chapter an overview of recent advances in our understanding of the chemical and isotopic evolution of the early Solar System and protoplanetary disks is presented. 

Sample returns from additional comets, outer main belt and Trojan asteroids, and Kuiper Belt objects representing distinct regions of the early Solar System. 

The giant planets, especially Jupiter and Saturn, significantly influenced accretion in the inner Solar System, with important consequences for the properties of the terrestrial planets, described in Section 10.4.1. The influence of the giant planets is especially strong in the Asteroid Belt. Given that meteorites are our primary samples of primitive Solar System material, understanding the role of dynamical and collisional processes in the formation and evolution of the Asteroid Belt is of fundamental importance for theories of planet formation (Section 10.4.2). 

Figure 10.7 Snapshots from a simulation of accretion in the inner Solar System, from O Brien et al. (2006). Jupiter and Saturn are present at t = 0 on their current orbits, black particles are embryos and gray particles are planetesimals. By 30 Myr all embryos and many of the planetesimals are cleared from the Asteroid Belt region (shown as the black line). The remaining asteroids are dynamically excited and have experienced significant radial displacement from their initial locations.

Planetary-mass bodies probably formed in the Asteroid Belt and were responsible for its dynamical excitation, radial mixing, and mass depletion. The orbits of these bodies became unstable once Jupiter and Saturn formed. These objects and most remaining planetesimals fell into the Sun or were ejected from the Solar System. The Asteroid Belt may have been further depleted when the giant planets passed through a resonance before reaching their current orbits. The Asteroid Belt has lost relatively little mass due to collisional erosion, and most asteroids >100 km in diameter are probably primordial. 

These particles probably come from the asteroid belt (Elynn, 1994). They are brought to Earth by the action of the Poynting-Robertson effect. Perhaps they are derived from sources that contain uncondensed volatiles from the inner part of the nebula. This would be the only example of a clear enhancement of moderately volatile elements in solar system material. 

The primary feature of the main asteroid belt is its great depletion in mass relative to other regions of the planetary system. The present mass of the main belt is 5 X 10 m , which represents 0.1 -0.01 % of the solid mass that existed at the time planetesimals were forming. There are several ways the main asteroid belt could have lost most of its primordial mass. Substantial loss by collisional erosion appears to be mled out by the preservation of asteroid Vesta s basaltic cmst, which formed in the first few million years of the solar system (Davis et al., 1994). More plausible models are based on the existence of orbital resonances associated with the giant planets. 

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