Evolution asteroids

Extraterrestrial materials consist of samples from the Moon, Mars, and a variety of smaller bodies such as asteroids and comets. These planetary samples have been used to deduce the evolution of our solar system. A major difference between extraterrestrial and terrestrial materials is the existence of primordial isotopic heterogeneities in the early solar system. These heterogeneities are not observed on the Earth or on the Moon, because they have become obliterated during high-temperature processes over geologic time. In primitive meteorites, however, components that acquired their isotopic compositions through interaction with constituents of the solar nebula have remained unchanged since that time. 

Kleine, T., Touboul, M., Bourdon, B. et al. (2009) Hf-W chronology of the accretion and early evolution of asteroids and terrestrial planets. Geochimica et Cosmochimica Acta,... 

The chondrite parent bodies obviously could not have accreted before their constituent chondrules formed. Based on the formation times of chondrules, accretion of the ordinary chondrite parent bodies began 2.5-3 Myr after CAIs (4565.7—4565.2 Ma). The end of accretion can be inferred from the metamorphic history of the chondrite parent bodies. Isotopic data from metamorphic assemblages, coupled with thermal modeling of the chondrite parent bodies, suggest that the time of peak metamorphism for the H chondrite parent body was at-4563 Ma. As will be discussed in Chapter 11, it is likely that the source of heat for metamorphism on chondrite parent bodies was the decay of26 Al, perhaps with a contribution from 60Fe. Thermal evolution models indicate that accretion of chondritic asteroids could not have occurred earlier than -2 Myr after CAI formation, or they would have melted. 

Many asteroids are dry, as evidenced by meteorites in which water is virtually absent. These samples include many classes of chondrites, as well as melted chunks of the crusts, mantles, and cores of differentiated objects. Anhydrous bodies were important building blocks of the rocky terrestrial planets, and their chemical compositions reveal details of processes that occurred within our own planet on a larger scale. The distributions of these asteroids within the solar system also provide insights into their formation and evolution. 

Clark, B.E., Hapke, B., Pieters, C. and Britt, D. (2002) Asteroid space weathering and regolith evolution. In Asteroids HI, eds. Bottke, W. E, Cellino, A., Paolicchi, P. and Binzel, R. P. Tucson University of Arizona Press, pp. 585-602. 

With the success of the Stardust mission, tests of our models for solar nebula, and thus protoplanetary disk evolution, are no longer limited to asteroidal bodies (meteorites), but now can be applied to cometary bodies as well. Stardust returned dust grains that were ejected from the surface of comet Wild 2, a Jupiter-family cometthatis thought to have formed at distances of >20 AU from the Sun (Brownlee et al. 2006). Thus, we now have samples of materials from the outer solar nebula that can be studied in detail. 

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. 

Several types of the early Solar System materials are available for laboratory analysis (see Chapter 1 and Table 1.1 and Fig. 1.1). Each material has unique characteristics and provides specific constraints on the chemistry of the solar nebula. Major components of this sample are meteorites, fragments of asteroids, that serve as an excellent archive of the early Solar System conditions. Primitive chondritic meteorites contain glassy spherical inclusions termed chondrules, some of the oldest solids in the Solar System. Most chondrites were modified by aqueous alteration or metamorphic processes in parent bodies but there are some chondrites that are minimally altered (un-equilibrated chondrites, UCs). They have yielded a wealth of information on the chemistry, physics, and evolution of the young Solar System. 

There is evidence from chondrites that the solar nebula was well mixed between 0.1 and 10 AU during its first several million years of the evolution, as shown by the homogeneity in concentrations of many isotopes of refractory elements (Boss 2004 Chapter 9). This is likely caused by the evaporation and recondensation of solids in the very hot inner nebula, followed by outward transport due to turbulent diffusion and angular momentum removal. Materials out of which terrestrial planets and asteroids are built have been heated to temperatures above 1300 K and are thus depleted in volatile elements. The inner solar nebula, with some exceptions, does not retain memories of the pristine interstellar medium (ISM) chemical composition (Palme 2001 Trieloff Palme 2006). 

In this chapter we compare the evolution of protoplanetary disks to that of the proto-solar nebula. We start by summarizing the observational constraints on the lifetime of protoplanetary disks and discuss four major disk-dispersal mechanisms. Then, we seek constraints on the clearing of gas and dust in the proto-solar nebula from the properties of meteorites, asteroids, and planets. Finally, we try to anchor the evolution of protoplanetary disks to the Solar System chronology and discuss what observations and experiments are needed to understand how common is the history of the Solar System. 

Figure 9.2 Fraction of stars with excess emission at IRAC wavelengths (between 3.6 and 8 um) as a function of the age of the stellar group. In addition to the data presented in Hernandez et al. (2008) and references therein, we have included the disk frequencies in the TW Hya association (Weinberger et al. 2004), and from the FEPS sample of Sun-like stars (Silverstone et al. 2006). The dot-dashed line is the least-squares fit to the L-band data from Haisch et al. (2001b). Above the plot we show a comparison to the formation timescale of CAIs, chondrules, and the asteroid Vesta in the Solar System. As we discuss in Section 9.4 there is evidence that CAIs formed early, in the first Myr of disk evolution.

Figure 9.5 Summary of the timescales for the formation of chondrules, asteroids, and planets in the Solar System compared to the lifetime of disks around young stars. The Solar System chronology is based on the dating of the CAIs, which, we assume, formed within the first Myr of disk evolution. The inner-disk frequency is from infrared excess measurements of stars in different stellar groups (see Section 9.1.1). The timescale for the outer-disk dispersal is discussed in Sections 9.1.1 and 9.1.2. The Solar System chronology is summarized in Section 9.3. For the formation timescales of giant planets, we used those in Desch (2007) with the assumption that outer-disk planetesimals formed 2 Myr after CAIs.

Meteorites provide perhaps the best record of the chemical evolution of small bodies in the Solar System, and this record is supplemented by asteroidal spectroscopy. Meteorites show progressive degrees of thermal processing on their parent asteroids, from primitive carbonaceous chondrites that contain percent-level quantities of water, through ordinary chondrites that show a wide range of degree of thermal metamorphism, to the achondrites that have been melted and differentiated. 

Earth and Mars clearly contain H2O. Venus s atmosphere is very dry, and composed mainly of CO2, but the high D/H ratio of the small amount of water present suggests Venus was once much wetter than today (Zahnle 1998). Mercury is perhaps too small and too close to the Sun to have acquired and retained water. Water may have been present in much of the material that accreted to form the Earth. Small amounts of water may have been adsorbed onto dust grains at 1 AU by physisorp-tion or chemisorption (Drake 2005). Once Jupiter formed, substantial amounts of water could have been delivered to the growing Earth in the form of planetesimals and planetary embryos from the Asteroid Belt (Morbidelli et al. 2000). It is also possible that Earth lay beyond the snowline at some point during the evolution of the solar nebula (Chiang et al. 2001) so that local planetesimals contained ice. 

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). 

Bottke et al (2005a,b) found that the current asteroid size distribution arose early in its history, when the total mass and collision rate were much higher than today. Once the Asteroid Belt was dynamically depleted and reached roughly its current mass (via the processes described above), there was little further evolution of the size distribution, and hence it has been referred to as a fossil size distribution. Collisions still occur, albeit at a reduced rate, and large collisions lead to the formation of asteroid families, which are groups of asteroids that are clustered in orbital-element (a, e, i) space. Numerous asteroid families can be seen in Fig. 10.6. 

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