Protoplanetary disk dynamics and dust evolution

Thus far we have introduced and provided brief overviews of the dynamical effects believed to be responsible for shaping the environments in which dust would be processed within protoplanetary disks. In the remaining part of this chapter we discuss how these processes may have left their fingerprints on the properties of dust, as revealed from telescopic observations and from laboratory analysis of primitive samples. This should not be taken as a definitive discussion, but rather as an examination of how current models of protoplanetary disk evolution can be used to make sense of the properties of meteorites, comets, and the dust that is present around young stars. 

Observations of protoplanetary disks indicate that these objects remain optically thick for timescales of millions of years, meaning that a population of dust is sustained for that period of time (see Chapter 9 for a detailed discussion of disk lifetimes and dispersal mechanisms). As will be discussed in Chapter 10, the timescale for dust growth and incorporation into planetesimals is less than this time period. Additionally, the timescale for dust settling is much less than the age of these disks. However, the apparent contradictions between these timescales and the observations can be explained within the context of the processes described thus far. 

In terms of aggregating into larger bodies, dust grains are expected to grow through collisions and sticking to other particles. In a laminar disk, the rate at which growth occurs is determined by the differential motions of the particles due to the 

The most common type of refractory inclusions are the calcium-aluminum-rich inclusions (CAIs) whose mineralogy is consistent with the minerals that are 

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. 

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