Planetesimals, accretion

Wadhwa, M., Amelin, Y., Bogdanovski, O. et al. (2009) Ancient relative and absolute ages for a basaltic meteorite implications for timescales of planetesimal accretion and differentiation. Geochimica et Cosmochimica Acta, 73, 5189-5201. 

Kerridge J. F. (1993) What can meteorites tell us about nebular conditions and processes during planetesimal accretion Icarus 106, 135-150. 

Timescales of Planetesimal Accretion and Early Chemical Differentiation... 

The thermal history of planetesimals accreted in the early history of the solar system depends on the timescale of accretion and the amount of incorporated short-lived radioactive nuclei, primarily A1 with a half-life of 7.1 X 10 years. In some cases there was sufficient heat to completely... 

Unaltered chondrites are characterized by extreme mineralogical parsimony, with no more than about twenty different mineral species (4). As planetesimal accretion progressed and chondrite parent bodies became larger, aqueous and thermal alteration led to new suites of minerals (5,6). Yet the total mineralogical... 

After planetary accretion was complete there remained two groups of surviving planetesimals, the comets and asteroids. These populations still exist and play an important role in the Earth s history. Asteroids from the belt between Mars and Jupiter and comets from reservoirs beyond the outer planets are stochastically perturbed into Earth-crossing orbits and they have collided with Earth throughout its entire history. The impact rate for 1 km diameter bodies is approximately three per million years and impacts of 10 km size bodies occur on a... 

The paramount importance of carbon in the cosmos is shown by the fact that more than 75% of the approximately 120 interstellar and circumstellar molecules so far identified are carbon containing (Henning and Salama, 1998). Molecules apparently travel from the ISM via protoplanetary discs to the planetesimals and from there, via accretion, to the heavenly bodies formed. The molecules so far identified in ISM come from quite different types of compounds ... 

The formation of the planets around the proto-sun initially started as a simple accretion process, aggregating small particles to form larger particles. This process was common to all planets, even the gas giants Jupiter and Saturn and to a lesser extent Neptune and Uranus. The planetesimals form at different rates and as soon as Jupiter and Saturn had reached a critical mass they were able to trap large amounts of hydrogen and helium from the solar nebula. The centres of Jupiter... 

Cosmochemistry is the study of the chemical composition of the universe and the processes that produced those compositions. This is a tall order, to be sure. Understandably, cosmochemistry focuses primarily on the objects in our own solar system, because that is where we have direct access to the most chemical information. That part of cosmochemistry encompasses the compositions of the Sun, its retinue of planets and their satellites, the almost innumerable asteroids and comets, and the smaller samples (meteorites, interplanetary dust particles or IDPs, returned lunar samples) derived from them. From their chemistry, determined by laboratory measurements of samples or by various remote-sensing techniques, cosmochemists try to unravel the processes that formed or affected them and to fix the chronology of these events. Meteorites offer a unique window on the solar nebula - the disk-shaped cocoon of gas and dust that enveloped the early Sun some 4.57 billion years ago, and from which planetesimals and planets accreted (Fig. 1.1). 

Cosmochemistry is the study of the chemical compositions of various solar system materials. Chondrites are the most abundant primitive samples. They are essentially sedimentary rocks composed of mechanical mixtures of materials with different origins (chondrules, refractory inclusions, metal, sulfide, matrix), which we will call components. Chondrites formed by the accretion of solid particles within the solar nebula or onto the surfaces of growing planetesimals. They are very old (>4.5 billion years, as measured by radioactive chronometers) and contain some of the earliest formed objects in the solar system. Chondrites have bulk chemical compositions very similar to the solar photosphere, except... 

Several kinds of physical processes have been hypothesized to have affected planetesimals prior to their accretion into planets. These processes could conceivably explain some chemical fractionations observed among asteroids and planets. 

Bizzarre, M., Baker, J. A., Haack, H. and Lundgaard, K. L. (2005) Rapid timescales for accretion and melting of differentiated planetesimals inferred from 26Al-26Mg chrono-metry. Astrophysical Journal, 632, L41-L44. 

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. 

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. 

Pluto and some moons of the giant planets contain considerable amounts of ices and deserve special mention. In some cases they even exhibit active or recent processes that liberate liquids and gases derived from ices, suggesting a tentative link with cometary activity. These bodies, which are too large to be called planetesimals, include former KBOs now relocated into the planetary region, as well as objects that probably accreted in the giant planet region. 

Mars is more volatile-rich than Earth, reflecting a higher proportion of accreted volatilebearing planetesimals. It is also more highly oxidized than Earth, so that twice as much of its iron has remained as Fe2+ in the mantle rather than in the metallic core. Wanke and Dreibus (1988) suggested that oxidation occurred during accretion, as water in accreted planetesimals reacted with iron metal. 

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