Annu. Rev. Astron. Astrophys. 1990. 28: 37-70
Copyright © 1990 by . All rights reserved

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6.3 Solar System Dust

Primitive meteorites, interplanetary dust particles (IDPs), and cometary dust provide some information regarding interstellar grains, although all solar-system dust has been significantly processed, both chemically and physically, since having been in the pre-solar system molecular cloud. For reviews on the various types of objects, see (14, 15, 82).

Meteorites are almost entirely asteroidal in origin, since cometary meteoroids cannot survive entry into the Earth's atmosphere. Most meteorites have undergone obvious metamorphism, with carbonaceous chondrites being most primitive. One of this class, the Murray meteorite, has tiny SiC inclusions showing isotopic anomalies (8, 186) proving an interstellar origin. The small amount of carbon in primitive meteorites is mostly poorly ordered, not graphitic, but tiny (ca. 25 Å) diamonds are present (97). It is difficult to imagine a solar system origin for these, so diamond bonding in interstellar carbon is strongly indicated. Meteoritic silicates are much more crystalline than the interstellar varieties, showing that meteorites must have been much warmer than interstellar temperatures. Graphite, if originally present, could have been lost by chemical reactions with water and hydrogen.

IDPs are both cometary and asteroidal in origin, with the cometary being more primitive and, therefore, relevant to interstellar dust. Some cometary IDPs, collected from high-flying aircraft, are a few microns in size, with a very fragile, open structure consisting of submicron mineral grains, stuck together into an open matrix (see e.g. (14, 15). Their silicates have been annealed to crystalline forms (143), with the types of minerals similar to those observed in cometary dust. Some of the carbon, poorly ordered but aromatic in nature, occurs as submicron grains. There is also carbon coated onto the silicate materials. Some IDPs have relatively large D/H ratios (185), suggestive of molecular clouds.

The crystalline structure of cometary silicates shows that even cometary dust has been fairly heavily modified from the original interstellar dust. There are whole particles consisting of volatile material containing C, H, O, and N (``CHON''), with little refractory material within; there are also low-density silicate and carbonaceous grains. Meteoroids (mostly cometary) entering the upper atmosphere have densities of 0.01-1 gm cm-3, so the fluffy refractory particles are common in comets and are probably present in interstellar dust at least deep within molecular clouds.

In summary, solar system material shows that (a) small particles can survive all of the rigors of the ISM after formation in a supernova; (b) grains form large structures deep inside molecular clouds, with voids possibly packed with ices; and (c) there was heating and associated chemical processing which took place before the formation of comets, possibly in the molecular cloud material.

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