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 and non-carbonaceous meteorites. The difference is considered more fundamental than the division into chondrites, achondrites and irons. In principle, carbonaceous chondrites are so called because they are enriched in carbon. Much of the excess inheres in the minerals calcite, CaCO3, and dolomite, CaMg(CO3)2, which precipitated when carbonic acid – CO2 dissolved in water – exsolved calcium and magnesium from the existing minerals.
Frozen carbon dioxide is found throughout the solar system, from the Kuiper Belt to Mercury (De Prá et al. 2024). The constituent elements presumably 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 as the wind pressure increased decreased in volume.
It is generally agreed that carbonaceous meteorites 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 belt (Warren 2011). Although mixing and drifting have obscured the zonation of the main asteroid belt, carbonaceous asteroids are more frequent in the middle to outer belt and non-carbonaceous asteroids (S-types) in the inner belt. Examples of inward migration include the carbonaceous asteroid Ryugu, which orbits eccentrically between Mars and Earth, 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 (Warren 2011, Dauphas & Schauble 2016). A graph of 95Mo plotted against 94Mo, for example, reveals one ratio for carbonaceous chondrites, achondrites and irons and a lower degree of enrichment 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.
What accounts for the differences? That they reflect nucleosynthetic processes is generally agreed. However, according to standard 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 discrete 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 than either.
CAIs are typically many times more abundant in carbonaceous chondrites (0.5–3% by volume) than in the non-carbonaceous enstatite and ordinary chondrites (less than 0.1%) (Desch et al. 2018). As they required high temperatures to form, the more massive of the planets that exploded must have been the one between Saturn and Uranus, consistent with the higher proportions of neutron-rich isotopes in carbonaceous chondrites.
