Meteorites reveal the early history of the Solar System

Such temperatures are expected near the young Sun. However, CAIs are most abundant in a category of meteorites called carbonaceous chondrites, which formed far from the Sun and even incorporated ice. This “meteoritic hot-cold” paradox is a stumbling block for most models of the formation of the Solar System.

“Classical” models generally consider an isolated protoplanetary disc. However, the CAIs formed in such a short time (~200,000 years) that the protoplanetary disc must still have been in the process of forming, from a contracting interstellar cloud. In this primordial phase, the disc in formation was anything but isolated: it was sprinkled over its entire surface by cold material from the cloud.

In this series of articles, the authors have modelled this period and followed the history and transport of various components in the early Solar System. In this way, they were able to determine the traces of the first hundreds of millennia of the Solar System’s existence. In these simulations, IACs are indeed produced near the young Sun (within the Earth’s current orbit) by the vaporisation of interstellar material. But the forming disc, which is compact at first, rapidly spreads out, carrying some of the CAI towards the cold exterior of the primitive Solar System. The IACs that were unable to ride this wave were quickly swallowed up by the young Sun. It is thus possible to explain the overabundance of CAIs in (distant) carbonaceous chondrites compared with their counterparts formed in the inner Solar System.

The authors also sought to understand isotopic variations in the Solar System. Most chemical elements exist in several variants of different masses: isotopes. Oxygen atoms, for example, can be oxygen 16 or 17 or 18 (the number being proportional to their mass). The proportions of these different isotopes are more or less constant on Earth. But they are not the same in the other stars of the solar system, as meteorites show, and the origin of these “isotopic signatures” is not understood. Yet these variations are the most significant in CAIs. Hence the idea that they were inherited from the original cloud. Since this cloud received contributions from different stars with different isotopic proportions, it is conceivable that they were only imperfectly mixed. The authors were able to show that CAIs could have ‘fossilised’ this isotopic heterogeneity in the cloud as it fed the protoplanetary disc. Once the disc had formed, these variations would have diminished, notably as a result of mixing in the disc, in line with observations.

In conclusion, chondrites are astonishing time capsules, filled with coded histories dating back several billion years. These studies are helping to decipher them and put their mysterious components back into their astrophysical context, opening up new perspectives in the study of the origin of the diversity of composition of meteorites.