In the discussion about the diffuse rocky cores 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 – not, of course, based on whether they included chondrules (since both explosions would have produced melt droplets) but on isotopic differences. Nucleosynthesis took place in all the planets and the extent to which that happened would have depended on how massive they were.
Just such a dichotomy is seen in the distinction between carbonaceous and non-carbonaceous meteorites, a difference regarded as more fundamental than that which divides meteorites into chondrites, achondrites and irons.
Carbonaceous chondrites and achondrites were initially so called because, compared to ordinary chondrites (the majority) and to the bulk composition of Earth, most of them are enriched in carbon. Much of the excess carbon inheres in the secondary minerals calcite and dolomite. The minerals are secondary because they are precipitates. A solution consisting of water and CO2 gas, i.e. carbonic acid, exsolved calcium and magnesium ions from the existing rock minerals to produce new minerals, CaCO3 and CaMg(CO3)2. The carbon and oxygen in the acid originated from the Sun, at a time when it was generating and blasting out significant amounts of carbon, nitrogen and oxygen. The water can only have come from the outer solar system. As it diffused through the solar system inward, the water was impeded by the solar wind. The front where they met and beyond which C, N and O were able to interact with the water lay somewhere beyond Jupiter. The interaction must have occurred after the parent bodies had already fragmented, so as pervasively to expose their mineral surfaces. We may infer that meteorites containing calcite and dolomite originated from an exploded planet beyond Jupiter, while non-carbonaceous meteorites represent debris from a nearer planet.
The idea that carbonaceous chondrites formed in the outer solar system and the other chondrites this side of Jupiter was first proposed by Paul Warren. However, he showed that the dichotomy was primarily reflected in isotopic differences rather than carbon content. In the case of minor elements such as Ti, Cr, Ni and Mo, carbonaceous meteorites tended to be enriched in the isotopes that were neutron-heavy (Warren 2011, Dauphas & Schauble 2016). Isotopic ratios quantify the extent of enrichment. A graph of 95Mo plotted against 94Mo, for example, shows one ratio for carbonaceous chondrites, achondrites and irons and another, lower ratio for non-carbonaceous chondrites, achondrites and irons. Intermediate compositions do not occur, there is no change in the ratios over time, and sometimes the isotopic dichotomy cuts across differences in carbon content. Thus ureilites are carbon-rich but isotopically non-carbonaceous; most iron meteorites are carbon-poor, but isotopically some group among the carbonaceous meteorites. The key point is that, because chondrules, matrix and the corresponding irons all show the ratios characteristic of either carbonaceous or non-carbonaceous chondrites, these diverse components within the two groups all must have had the same ancestry.
So what accounts for the isotopic differences? That they reflect nucleosynthetic processes is generally agreed. In orthodox cosmology, however, nucleosynthesis takes place only in stars. One is forced to conclude that two supernovae contributed to the nebula and that the isotopes retained their distinctive ratios in discrete ‘reservoirs’. This is implausible. The same problem arises when trying to account for compositionally distinct chondrule groups: if chondrules are condensates from the nebula, how can different regions of the nebula have remained compositionally distinct for two million years? The alternative explanation is that the discrete reservoirs were in fact planets, and degree to which neutron-heavy isotopes were synthesised depended on planetary size.
Nitrogen also exhibits the dichotomy – an unusual case because, unlike titanium and so on, the source of nitrogen had to be the Sun. Relative to the terrestrial standard, carbonaceous irons and comets are enriched in 15N, which has one more neutron than the main isotope, 14N, whereas non-carbonaceous irons are depleted (Füri & Marty 2015). The spatial pattern is the same – enrichment this side of Jupiter, depletion further out – but in this instance the enrichment must have been taking place in the Sun rather than the planets.
CAIs are typically much 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 CAIs require the highest temperatures, the bigger of the two planets that exploded must have been the one between Saturn and Uranus, consistent with the higher proportions of neutron-rich isotopes in carbonaceous chondrites.
Although the main asteroid belt is not sharply zoned because of mixing and drifting, carbonaceous asteroids are more frequent in the middle to outer belt and non-carbonaceous asteroids (S-types) in the inner belt. Later Jupiter’s gravitational influence drew some of the carbonaceous chondrites into the main asteroid belt (Warren 2011). A particular example of inward migration is the comet Wild 2. As we have seen (p. 174), frozen volatiles co-exist with rock-derived material that was subjected to melting and rapid cooling.
