8. How many exploded planets this side of Neptune?

The discussion concerning the diffuse rocky cores of Jupiter and Saturn has already hinted that there were two: one between Mars and Jupiter and 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 high-temperature melt droplets) but, given that the two bodies would not have had the same mass, on isotopic differences. Nucleosynthesis took place in all the planets and the extent to which that happened depended on how massive they were.

The predicted dichotomy is seen in the fundamental distinction between carbonaceous and non-carbonaceous meteorites. This dichotomy cuts across the division of meteorites into chondrites, achondrites or irons. Analyses of certain low-abundance elements such as Ti, Cr, Ni and Mo show that carbonaceous meteorites tend to have higher proportions of the neutron-rich isotopes (Warren 2011, Dauphas & Schauble 2016). For example, a graph of 95Mo plotted against 94Mo, will show one ratio for carbonaceous chondrites, achondrites or irons and another, lower ratio for non-carbonaceous chondrites, achondrites and irons. (Note that iron meteorites are carbon-poor but are classified as carbonaceous or non-carbonaceous on the basis of their molybdenum isotope ratios. In this sense most are non-carbonaceous.) Intermediate compositions are not found, nor is there any change in the ratio over time. Since chondrules and matrix and the corresponding irons all show the same ratios, the components must have had the same ancestry.

So what accounts for the dichotomy? That the isotopic ratios reflect nucleosynthetic processes is generally agreed. Thus one seems forced to suppose that the nebula originated from several distinct stellar sources and that the isotopes retained their distinctive ratios in two discrete ‘reservoirs’, though the bulk composition of the reservoirs was identical. This is most 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 2 Ma? The alternative explanation is that it was the planets that provided the boundaries for distinct isotopic evolution. The degree to which neutron-heavy isotopes were synthesised depended on planetary size.

The carbonaceous chondrites and achondrites are so-called because, compared to the bulk Earth and the ordinary chondrites (which make up the majority), they are enriched in carbon. Much of the excess inheres in the minerals calcite and dolomite. These are secondary minerals, in that they precipitated from a solution of water and CO2 gas and of Ca and Mg ions exsolved from the primary rock minerals. The carbon and oxygen must have originated from the Sun, at a time when it was generating and blasting out significant amounts of C, N and O. The water, on the other hand, must have come from the primeval Oort Cloud, diffusing in the opposite direction towards the Sun, but impeded by the solar wind. At the relevant time the front where they met and beyond which C, N and O were able to interact with the water lay somewhere beyond Jupiter. We may deduce therefore that carbonaceous meteorites represent the debris from an exploded planet in that region, where they acquired their carbon secondarily. Non-carbonaceous meteorites represent the debris from the nearer planet, because their components – chondrules and matrix particles – accreted after the Sun had ceased to synthesise C, N and O and its wind had become less energetic. Interestingly, the isotopic dichotomy is also seen in nitrogen itself. Relative to the terrestrial standard, carbonaceous irons and comets are enriched in 15N, which has one more neutron than the main isotope, whereas non-carbonaceous irons are depleted (Füri & Marty). The source of 15N was the Sun. Earth was protected from the solar wind to some extent by its magnetic shield.

The idea that carbonaceous chondrites formed beyond Jupiter and the other chondrites this side of Jupiter was first proposed by Paul Warren (2011). The outer nebula was where the giant gas and ice planets formed, and coarse-grained carbonaceous chondrites are rich in water and volatiles, even if the abundance of carbon, water and volatiles itself is not well explained. Later the gravitational influence of Jupiter drew some of the chondrites into the main asteroid belt. Because of this mixing and drifting, the main belt is not sharply zoned by asteroid type. Nonetheless, carbonaceous asteroids (C-types, P-types and D-types, in that order) are more frequent in the middle to outer belt, and non-carbonaceous asteroids (S-types) on this side of the belt. The asteroids that struck the Moon, consistently, were non-carbonaceous in isotopic composition (Worsham & Kleine 2021).

CAIs are typically much 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 CAIs require the highest melting 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.