Asteroid differentiated

Taylor G. J., Keil K., McCoy T., Haack H., and Scott E. R. D. (1993) Asteroid differentiation—pyroclastic volcanism to Magma Oceans. Meteoritics 28, 34-52. 

Stony Irons. The stony iron meteorites are composed of substantial iron and siUcate components. The paHasites contain cm-sized ohvine crystals embedded ia a soHd FeNi metal matrix and have properties consistent with formation at the core mantle boundary of differentiated asteroids. The mesosiderites are composed of metal and siUcates that were fractured and remixed, presumably ia the near-surface regions of their parent bodies. 

Differentiated meteorites they come from asteroids which have been through a fusion process which led to a more or less clear separation into nucleus, mantle and crust. 

Doualla-Bell F, BonneauMJ, Labrie F, Fortier MA (1995) Differential effects of asteroidal antiestrogen, EM-139, on prostaglandin and cyclic adenosine 3, 5 -monophosphate production in the circular and longitudinal layers of bovine myometrium. Biol Reprod 52 1358-1363... 

Evolved extraterrestrial materials are generally igneous rocks, which according to their thermal history can be discnssed analogonsly to terrestrial samples. To this category belong planetary bodies, differentiated asteroids, and achondritic meteorites. 

The interiors of planets, moons, and many asteroids either are, or have been in the past, molten. The behavior of molten silicates and metal is important in understanding how a planet or moon evolved from an undifferentiated collection of presolar materials into the differentiated object we see today. Basaltic volcanism is ubiquitous on the terrestrial planets and many asteroids. A knowledge of atomic structure and chemical bonding is necessary to understand how basaltic melts are generated and how they crystallize. Melting and crystallization are also important processes in the formation of chondrules, tiny millimeter-sized spherical obj ects that give chondritic meteorites their name. The melting, crystallization, and sublimation of ices are dominant processes in the histories of the moons of the outer planets, comets, asteroids, and probably of the Earth. 

Some meteorites, and all planetary samples, have undergone melting and differentiation at some stage. Hence, the compositions of differentiated materials do not resemble solar system abundances. These samples can, however, tell us about various geochemical processes within asteroids and planets. 

Iron meteorites offer the unique opportunity to examine metallic cores from deep within differentiated bodies. Most of these samples were exposed and dislodged when asteroids collided and fragmented. Although irons constitute only about 6% of meteorite falls, they are well represented in museum collections. Most iron meteorites show wide variations in siderophile-element abundances, which can be explained by processes like fractional crystallization in cores that mimic those in achondrites. However, some show perplexing chemical trends that may be inconsistent with their formation as asteroid cores. 

Mesosiderites are a highly enigmatic group of differentiated meteorites. They are breccias composed of iron-nickel metal and silicate in roughly equal proportions. The metal represents molten material from the deep interior of an asteroid, whereas the silicate fraction consists of basalts and pyroxene cumulates similar to HED meteorites that formed near the surface. It is difficult to construct models that allow mixing of such diverse materials, but these disparate materials are generally thought to have been violently mixed by impact. 

In this chapter, we review what is known about the chronology of the solar system, based on the radioisotope systems described in Chapter 8. We start by discussing the age of materials that formed the solar system. Short-lived radionuclides also provide information about the galactic environment in which the solar system formed. We then consider how the age of the solar system is estimated from its oldest surviving materials - the refractory inclusions in chondrites. We discuss constraints on the accretion of chondritic asteroids and their subsequent metamorphism and alteration. Next, we discuss the chronology of differentiated asteroids, and of the Earth, Moon, and Mars. Finally, we consider the impact histories of the solar system bodies, the timescales for the transport of meteorites from their parent bodies to the Earth, and the residence time of meteorites on the Earth s surface before they disintegrate due to weathering. 

Give two lines of reasoning to support the idea that differentiated asteroids accreted before chondritic asteroids. 

Nyquist, L. E., Kleine, T., Shih, C.-Y. and Reese, Y. D. (2009a) The distribution of shortlived radioisotopes in the early solar system and the chronology of asteroid accretion, differentiation, and secondary mineralization. Geochimica et Cosmochimica Acta, 73, 5115-5136. 

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. 

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. 

It is very difficult to reconstruct parent body compositions from differentiated meteorites. In this section, we will describe the chemical characteristics of the meteorites themselves, but the compositions of the bulk asteroids from which they were derived can only be inferred in the broadest terms and are usually assumed to have been chondritic. 

The heliocentric pattern of asteroid types, with thermally processed objects closer to the Sun (as inferred from spectra), persists despite subsequent dynamical stirring of asteroid orbits and ejection of bodies from the main belt. Differentiated objects appear to have formed earlier than chondritic bodies, and dynamical modeling suggests they may have accreted... 

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. 

When asteroid collisions are especially violent, sufficient kinetic energy may be imparted to launch fragments at greater than escape velocities. In that case, separate asteroids are formed. These fragments share similar orbital characteristics and are referred to as families. The members of most asteroid families share the same spectral characteristics, further linking them together. Families composed of fragments of differentiated asteroids can potentially provide important information on their internal compositions. 

Anhydrous planetesimals, and especially the meteorites derived from them, provide crucial cosmochemical data. Spectroscopic studies of asteroids do not provide chemical analyses, but the spectral similarities of several asteroid classes to known meteorite types provide indirect evidence of their compositions. The few chemical analyses of asteroids by spacecraft are consistent with ordinary chondrite or primitive achondrite compositions. Laboratory analyses of anhydrous meteorites - chondrites, achondrites, irons, and stony irons - allow us to study important chemical fractionations in early solar system bodies. Fractionations among chondrites occur mostly in elements with higher volatility, reflecting the accretion of various components whose compositions were determined by high- and low-temperature processes such as condensation and evaporation. Fractionations among achondrites and irons are more complex and involve partitioning of elements between melts and crystals during differentiation. 

Thermal metamorphism in chondrites and melting in differentiated asteroids are driven by heat produced by the decay of short-lived radionuclides (especially 26A1). Thermal models can reproduce the peak temperatures and cooling rates estimated for meteorites, as well as... 

Ghosh, A. and McSween, H. Y. (1998) A thermal model for the differentiation of asteroid 4 Vesta, based on radiogenic heating. Icarus, 134, 187-206. 

Haack, H., Rasmussen, K. L. and Warren, P. H. (1990) Effects of regolith/megaregolith insulation on the cooling histories of differentiated asteroids. Journal of Geophysical Research, 95, 5111—5124. 

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