The Chondrites as Clues on Planetary Formation

Those primitive meteorites called chondrites provide empirical evidence on the way planetary bodies were formed. The chondrites come from primitive bodies because, with the exception of a few very volatile elements, most of their elements have remained accurately in the same abundance ratios as in the Sun. This establishes not only that they derived from the same primeval reservoir as the Sun's, but also that they have never been through any process of differentiation, such as those that have separated the cores from the mantles of the different planets.

The chondritic meteorites come from the asteroid belt, that is, roughly from 2 AU to 4 AU. This was established from accurate triangulation of three orbits of chondrite meteorites observed as meteors during their entry in the atmosphere and recovered on the ground later. The results were entirely confirmed by the orbits of about 30 bright meteors identified as chondritic but unrecovered later (Wetherill and Chapman, 1988). Chondrites are assumed to be the fragments of asteroidal collisions. Their parent bodies had a radius of the order of 100 km. This size is implied by their concordant radiogenic ages of 4 billion years or more, implying a rather fast cooling. This small size explains why they had no differentiation induced by gravitation, since their gravity was never larger than 10~2 g.

Chondrites are stony meteorites (made mostly of silicates) classified as carbonaceous, ordinary and enstatite chondrites according to their diminishing degree of oxidation. The enstatite chondrites are completely reduced, and the carbonaceous chondrites are completely (CI, CM) or almost completely (CO, CV) oxidized. The most oxidized carbonaceous chondrites are those that contain the most volatile elements and, in particular, very large amounts of organic compounds (there is typically 6% carbon in the CI type). The chon-drite classes seem to sample different regions of the accretion disk across the asteroid belt. Although the evidence is indirect, the infrared spectra of asteroids seem to imply that the dark C asteroids have carbonaceous chondritic surfaces, whereas the light S asteroids look more like ordinary chondrites. See, however, the controversy about the identification of the S asteroids in Wetherill and Chapman (1988). Another important clue comes from the fact that the C asteroids begin to outnumber the S asteroids at distances beyond 2.6 AU (Morrison, 1977).

The silicate matrix of chondrites shows that it was made by a moderate compression of fine dust grains of different origins. In spite of their close contact, these often submicrometer-sized grains are chemically unequilibrated. For instance, oxidized grains touch reduced grains, some have been altered by liquid water and some have not, and some refractory grains are in close contact with volatile grains. The matrix also imprisons larger objects, such as millimeter-sized chondrules or CAI (calcium-aluminum inclusions). The chondrules show signs of transient (and often partial) melting (1,500-1,600 K). The CAI are refractory grains probably made at temperatures higher than 1,600 K.

This heterogeneous composition seems to imply a process entirely comparable to a sedimentation. In our rivers, when water turbulence subsides, sand sediments to the bottom, bringing together grains of quartz, feldspar, mica, calcite, or silicates of widely different origins. Since chondrites have never felt the gravity of a large planet, the sedimentation must have taken place in the solar accretion disk.

This is exactly what the models of the viscous accretion disks predict. During the gravitational collapse of the interstellar nodule into the disk, the gravitational energy of the infall kept a high rate of turbulence in the gas. This turbulence kept the dust in suspension perhaps for some 105 years. However, when the collapse rate subsided, before completely stopping, there was a time when the dust was not supported any more by turbulent eddies in the gas. In the quasi-quiescent disk, it fell down to the midplane: this is a sedimentation that is going to make thin equatorial rings of dust around the Sun, much wider than but otherwise entirely comparable to Saturn's rings.

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