In the discussion about the rock content of Jupiter and Saturn it was suggested that there were two ex-planets: one between Mars and Jupiter, the other between Saturn and Uranus. If so, then meteorites should fall into two primary groups – based not on whether they included chondrules (since both explosions would have produced melt droplets) but on some factor related either to their position (how near the Sun they were) or to their mass (the extent to which they synthesised new isotopes).
One such dichotomy is seen in the distinction between carbonaceous meteorites and non-carbonaceous meteorites. The difference is deemed more fundamental than the division into chondrites, achondrites and irons. In principle, carbonaceous chondrites are so called because they are enriched in carbon-bearing minerals such as calcite, CaCO3, and dolomite, CaMg(CO3)2. These are instances of ‘aqueous alteration’ and are thus secondary minerals. They precipitated when carbonic acid – CO2 dissolved in water – exsolved calcium and magnesium from existing minerals.
Frozen carbon dioxide is found throughout the solar system, from the Kuiper Belt to Mercury (De Prá et al. 2024). Presumably the constituent elements emanated from the Sun at a time when it was generating and blasting out significant amounts of carbon, nitrogen and oxygen. The water, diffusing inward through the solar system, came from beyond Neptune and decreased in volume sunwards as the wind pressure increased.
Carbonaceous meteorites are believed to have originated from the outer system, beyond Jupiter, and non-carbonaceous meteorites from the solar system this side of Jupiter. Later Jupiter’s gravitational influence drew some of the carbonaceous chondrites into the main asteroid belt (Warren 2011). Although drifting and mixing have obscured the zonation of the main asteroid belt, carbonaceous asteroids are more frequent in the middle to outer belt, non-carbonaceous asteroids in the inner belt, consistent with the hypothesis. Examples of inward migration include the carbonaceous asteroid Ryugu and the comet Wild 2.
The dichotomy is most clearly reflected in isotopic differences. Isotopes in carbonaceous meteorites tend to be neutron-heavy, particularly in the case of minor elements such as Ti, Cr, Ni and Mo. A graph of 95Mo plotted against 94Mo, for example, reveals one ratio for carbonaceous chondrites, achondrites and irons and another for non-carbonaceous chondrites, achondrites and irons. Sometimes the dichotomy cuts across differences in carbon content. For example, ureilites are carbon-rich (hence the diamonds) but isotopically non-carbonaceous. Most iron meteorites are carbon-poor, but some isotopically line up with the carbonaceous meteorites. Intermediate compositions do not occur. The key point is that, because chondrules, matrix and irons all show the ratios characteristic of one group or the other, each group must have had the same ancestry.
So what accounts for the differences? That they reflect nucleosynthetic processes is generally agreed. However, according to conventional cosmology, nucleosynthesis takes place only in stars. This entails, implausibly, that two supernovae contributed to the nebula and that the injected isotopes retained their distinctive ratios in discrete ‘reservoirs’. The alternative explanation is that the reservoirs were in fact planets, and that the degree to which neutron-heavy isotopes were synthesised depended on planetary mass. Carbonaceous chondrites have more 95Mo and 94Mo proportionally because the planet from which they originated was the more massive of the two. Earth and Mars have lower ratios than either because these still extant planets were less massive.
CAIs are typically many times more abundant in carbonaceous chondrites (0.5–3% by volume) than in enstatite and ordinary chondrites (less than 0.1%) (Desch et al. 2018). As they required high temperatures to form, the more massive of the missing planets must have been the one between Saturn and Uranus, consistent with the higher proportions of neutron-rich isotopes in carbonaceous chondrites.
