Asteroids differentiation

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. 

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. 

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. 

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. 

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... 

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. 

The terrestrial planets and the Moon are differentiated, with dense iron-rich cores and rocky mantles. The uncompressed densities of Earth and Venus are similar. Mercury has a high density which suggests it has relatively large core. Conversely, the Moon has a low density, indicating a very small core. There is little observational evidence that asteroids are differentiated except for Vesta and Ceres (Thomas et al. 2005). However, iron meteorites from the cores of differentiated asteroids are quite common, and the irons found to date come from several dozen different parent bodies (Meibom Clark 1999). Most meteorites come from asteroids that never differentiated. These chondritic meteorites consist of intimate mixtures of heterogeneous material millimeter-sized rounded particles that were once molten, called chondrules, similarly sized calcium-aluminum-rich inclusions (CAIs), and micrometer-sized matrix grains. 

Chemical evidence indicates that magmatic iron meteorite groups formed by fractional crystallization, most likely in the cores of differentiated asteroids. 

The iron metal and troilite of mesosiderites are presumed to represent core materials of an asteroid. Mixing of this with crustal silicates requires an unusual formation process. Some have suggested that a naked molten core (a core with the sihcate crust and mantle largely stripped off) impacted a differentiated asteroid at low velocity (Wasson and Rubin, 1985). Others have suggested that an impact disrupted the differentiated, mesosiderite parent body, which reaccreted. This process mixed materials from different portions of the parent body, with mesosiderites representing a location where the core and crust were mixed together (Haack et al., 1996 Scott et al., 2001). 

Mittlefehldt D. W., Bogard D. D., Berkley J. L., and Garrison D. H. (2003) Brachinites—igneous rocks from a differentiated asteroid. Meteorit. Planet. Sci. (submitted). 

Without iron and stony-iron meteorites, our chances of ever sampling the deep interior of a differentiated planetary object would be next to nil. Although we live on a planet with a very substantial core, we will never be able to sample it. Fortunately, asteroid collisions provide us with a rich sampling of the deep interiors of differentiated asteroids. 

Iron and stony-iron meteorites constitute —6% of meteorite falls (Grady, 2000). Despite their scarcity among falls, iron meteorites are our only samples of 75 of the 135 asteroids from which meteorites originate (Keil et ai, 1994 Scott, 1979 Meibom and Clark, 1999 see also Chapter 1.05), suggesting that both differentiated asteroids and the geologic processes that produced them were common. 

Can we be sure that the iron meteorites are indeed fragments of cores Since no differentiated asteroid has yet been visited by a spacecraft, we rely on circumstantial evidence. Some M-type asteroids have spectral characteristics expected from exposed metallic cores (Tholen, 1989), while others exhibit basaltic surfaces, a hallmark of global differentiation. Although olivine-rich mantles should dominate the volume of differentiated asteroids, there is an enigmatic lack of olivine-rich asteroids (and meteorites) that could represent mantle material (Burbine et al., 1996). Until we visit an asteroid with parts of a core-mantle boundary exposed, our best evidence supporting a core origin is detailed smdies of iron meteorites. 

Iron-nickel alloys are expected in the cores of differentiated asteroids, but what other evidence supports the notion that iron meteorites sample the metallic cores of differentiated asteroids What suggests that these asteroids were sufficiently heated to trigger core formation, and that iron meteorites sample cores rather than isolated pods of once molten metal First and foremost. 

If the iron and stony-iron meteorites came from fully differentiated asteroids, how did these asteroids heat to the point of partial melting and how did the metal segregate from the silicates Unlike large planets, where potential energy release triggers core formation, small asteroids require an additional heat source. The heat source(s) for asteroidal melting produced a wide range of products, from unmetamorphosed chondrites to fully molten asteroids, as well as partially melted asteroids. Samples from these latter asteroids provide us with a rare opportunity to observe core formation—frozen in place. 

After the cores of the differentiated asteroids had crystallized, a slow cooling period commenced. During this period the most prominent feature of iron meteorites evolved—the Widmanstatten pattern (Figure 8). Several characteristics of the Widmanstatten pattern may be used to constrain the thermal evolution and the sizes of the iron meteorite parent bodies. 

Keil K. and Wilson L. (1993) Explosive volcanism and the compositions of cores of differentiated asteroids. Earth Planet. Sci. Lett. 117, 111-124. 

Scott E. R. D., Haack H., and Love S. G. (2001) Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid. Meteorit. Planet. Sci. 36, 869-881. 

Many CAIs, together with some chondrules and samples of differentiated asteroids, contained short-lived radioactive isotopes at the time they formed. This is deduced from the abundances of the daughter isotopes seen in modern meteorites. The short-lived isotopes include " Ca, A1, °Be, e, Mn, and ° Pd, with half-lives (in units of Myr) 0.13, 0.7, 1.5, 1.5, 3.7, and 6.5, respectively. Many of these isotopes could have been produced from stable ones by absorption of neutrons in a supernova or the outer layers of a giant star. In particular, °Fe can only be produced efficiently by stellar nucleosynthesis and so must have come from an external source (Shukolyukov and Lugmair, 1993). Conversely, some isotopes such as Be almost certainly formed in the protoplanetary nebula when material was bombarded by solar cosmic rays (McKeegan et al., 2000). Multiple sources are possible for some short-lived isotopes. The abundances of the decay products of... 

Magmatic iron meteorites are thought to be samples from the metal cores of differentiated asteroids (18) and as such are ideally suited for application of Hf-W chronometry. Iron meteorites contain virtually no Hf (i.e., HfrW 0), such that the timing of core formation in their parent bodies can be calculated from their alone. Tungsten isotope data are now available for a vast number... 

This result is inconsistent with the standard model for asteroid formation, in which chondrites represent the precursor material from which asteroids accreted and then differentiated. The Hf-W ages for iron meteorites reveal that the opposite is the case, i.e., differentiated asteroids are the oldest planetesimals and undifferentiated asteroids (the chondrite parent bodies) formed later. This most likely reflects different abundances of Al at the time of parent body accretion. Early formed bodies were heated by the decay of abundant Al and could melt and differentiate into core and mantle, whereas later formed bodies remained undifferentiated because they contained too little Al to cause melting and differentiation. 

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