Dust Accretion

Dust Accretion - Aeolian Material Derived Soil... 

First came dust accretion to form the protoearth, a hot, dry rock. A grazing collision between the Earth and a Mars-sized body formed the moon. All volatile substances, including water, were lost. There is mounting evidence that comets could have provided the young Earth with its water, atmosphere, and carbon compounds that seeded prebiotic life (Delsemme, 2001 de Duve, 1995). But prebiotic Earth would have been much different from present-day Earth. 

Love, S. G. and Brownlee, D. E. (1993). A direct measurement of the terrestrial mass accretion rate of cosmic dust. Science, 262, 550-3. 

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). 

In recent years, a new source of information about stellar nucleosynthesis and the history of the elements between their ejection from stars and their incorporation into the solar system has become available. This source is the tiny dust grains that condensed from gas ejected from stars at the end of their lives and that survived unaltered to be incorporated into solar system materials. These presolar grains (Fig. 5.1) originated before the solar system formed and were part of the raw materials for the Sun, the planets, and other solar-system objects. They survived the collapse of the Sun s parent molecular cloud and the formation of the accretion disk and were incorporated essentially unchanged into the parent bodies of the chondritic meteorites. They are found in the fine-grained matrix of the least metamorphosed chondrites and in interplanetary dust particles (IDPs), materials that were not processed by high-temperature events in the solar system. 

Schmitz, B., Peucker-Ehrenbrink, B., Linstrom, M. and Tassinari, M. (1997) Accretion rates of meteorites and cosmic dust in the early Ordovician. Science, 278, 88-90. 

It is also possible that neither of these mechanisms for providing water to the inner planets is correct. Another hypothesis is that absorption of water onto dust particles in the accretion disk might account for the Earth s oceans (Drake, 2005). As already mentioned, the amount of water required to explain Earth s water is not large on a per-gram basis. Regardless of whether comets, asteroids, or nebular particles were the source of our planet s oceans, the water likely came from more distant regions of the nebular disk. 

When the supersaturation ratio S becomes greater than unit, the small liquid droplets (i.e. molecular clusters) commence to appear. Almost all the droplets are immediately destroyed due to evaporation and only small fraction of the droplets (critical clusters) with radii greater than a critical radius r have a chance to survive and grow by accretion of vapor molecules (monomers) onto their surface. It is assumed that macroscopic thermodynamics is applied to the critical clusters that are considered as liquid droplets containing the large number of monomers, that is nx>>i. The number of the critical clusters formed per unit time per unit volume is the nucleation rate J so that the number density of dust grains is Nd = JJdt. Expressions for calculation of the nucleation rate and other quantities can be found in the review paper by Draine (1981). 

What is important in the context of chondrite chemistry is the fact that the probable presence in the protosolar nebula of dust particles and complex organic molecules is evidenced. This does not mean that all the organic matter detected in carbonaceous chondrites is necessarily molecules still present in the protosolar nebula. Readers interested in details of the formation of the solar system and the accretion phenomena will find a lot of information in the papers by Larimer 14) and Cameron 1S). 

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