3. Chondritic meteorites

The most common type of meteorite are chondrites. Dated by present rates of radioisotope decay to around 4.56 billion years ago, they are thought to be the oldest objects in the solar system, but they are not easy to interpret. Far from homogeneous, they have up to six constituents:
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  • Melt droplets or chondrules (from the Greek word chondros meaning ‘grain’), up to 1 mm in size and varying in composition from silicate-rich to metal-sulfide-rich. Among chondrites the proportion of chondrules varies from 0% to an extraordinary 80%.
  • Micron- to centimetre-sized aggregations of tiny spheroids rich in silicates of refractory calcium and aluminium, known as ‘CAIs’ (calcium-aluminium-rich inclusions). These are much less abundant than chondrules, ranging from 0 to 10% of the total.
  • Similar-sized ‘amoeboid olivine aggregates’ (AOAs) made up of crystals of various silicates, dominantly olivine, and calcium-aluminium-rich inclusions. Though not well understood, they appear to be chemically intermediate between CAIs and chondrules. They are somewhat less common than CAIs.
  • Lithic (rock) fragments.
  • Nano-sized grains of corundum (Al2O3), diamond, graphite carbon and carborundum (SiC).
  • A matrix of volatile-rich, mostly very fine, initially amorphous grains that are broadly complementary in composition to the chondrules (Palme et al. 2015, Patzer et al. 2022).

The variability of these constituents, not only from one chondrite to another but also within the same chondrite, is no trivial detail. Evidently they did not all form in the rocks where they are found. They must have accreted from a variety of sources. Since an originating nebula of dust and gas would be expected to have been homogeneous over medium-scale distances, one has to suppose that ‘separate classes of chondrules [for example] were derived from separate regions and that mixing subsequent to chondrule formation was not thorough’ (Taylor 2001); that is, they accreted very quickly, before differences in their composition could be smoothed out.

The existence of CAIs and chondrules was not predicted by models of how the solar system originated (Connolly et al. 2006). CAIs are rich in refractory elements (with high melting and vaporisation temperatures) and poor in volatiles. They are interpreted as the first solids to condense from a gaseous nebula of around 1800 °C. Condensation is consistent with their irregular shapes and fluffy textures. However, the nebula would have been hot enough only within 0.5 AU of the protosun, requiring that the CAIs condensed there and subsequently migrated. Inconsistently, CAIs are rare in chondrites that formed in the inner solar system (out to 5 AU) but common in those that formed in the outer solar system (Dunham et al. 2023). An alternative interpretation might be that they represent planetary mantle material that briefly vaporised and then condensed.

Chondrules consist of iron-magnesium silicates, They formed by flash-heating to temperatures of 1300–1700 °C, following which some cooled rapidly (1000 °C per hour or more), others more slowly (down to 2–10 °C per hour). Since it would have taken only minutes in a cold environment for the droplets to radiate away their heat, the rates point to a hot environment that expanded and dissipated, such as an exploding fireball, but the circumstances are debated. Prior to aggregation with other chondrite material the droplets floated in space. Among the many explanations for the flash-heating, one increasingly favoured has been the production of melt in large-scale collisions.

Large-scale collisions, however, should not have been occurring at the very beginning of the solar system, before the aggregation of dust particles into large bodies. Chondrules and CAIs commonly include the decay products of several extinct, very short-lived radioactive elements such as iron-60 (written as 60Fe, a neutron-rich isotope of iron) and aluminium-26 (26Al), and these enable the determination of a remarkably precise chronology for the early solar system. The oldest constituents seem to be the CAIs. Dating to 4568.2 ±22 Ma (Bouvier & Wadhwa 2010), they mark the official birth of the solar system and formed about the same time, at most within 0.4 Ma of each other. So did the AOAs. likewise began from this point but went on for much longer, the majority of chondrules being 1.8–3 Ma younger; a few are younger still (Krot 2019, Pape et al. 2019).

The origin of the short-lived isotopes, and their presence just at the moment when the solar system was coming into being, is an enigma. Some had half-lives as short as 100,000 years, in a context where the Milky Way (most of it) was already 9 billion years old. They must have come into existence some time after the elements making up the bulk of the chondrites. But by what process?

One idea is that most of the radioisotopes were synthesised by exceptionally high-energy solar irradiation of dust particles in the nebula. But this cannot have been the whole story, since such reactions could not have generated 60Fe. Iron-60 can only form, it is thought, in the extreme conditions that occur just before a massive star goes supernova. Thus a nearby massive star must have happened to explode just at this time and seeded the nebula with heavy elements. The blast might even have precipitated the nebula’s collapse into a disc, if it did not rip it apart.

Another team (Bizzarro et al. 2007) have argued that 60Fe perhaps did not enter the solar system until 1 million years after 26Al. Their supernova scenario postulates that 26Al was expelled by stellar winds during the penultimate stage of a still more massive star, with 60Fe being ejected into the solar system later by a shock wave from the supernova. However, this is to downplay evidence that 60Fe was already present in CAIs (e.g. Quitté et al. 2007). Furthermore, the ratio of 60Fe to 26Al is much lower than would be expected if both isotopes were produced by a supernova. All things considered, it seems better not to disassociate the origin of 60Fe from that of the other short-lived isotopes.

Prima facie, the presence of short-lived isotopes in the CAIs and chondrules is evidence that the CAIs and chondrules arose as a direct consequence of the generation of those isotopes, and that all chondrite constituents had the same origin. The scenario would then be that the particles originated from a planet that exploded as a result of the heat produced by the decaying isotopes. The fine-grained matrix of the chondrites came from the shattered, unmelted outermost part of the planet. Some of the remains still exist as free-floating interplanetary dust. Material rich in calcium and aluminium (chondrules as well as CAIs) originated from lower down. They reached the highest temperatures because the radioisotopes 41Ca and 26Al generated the most heat. Possibly outer layers of the planet exploded before inner layers.

In such a view, what has been reconstructed as a solar nebula, impregnated with the short-lived elements of an exploding star, was in reality the cloud of shattered and vaporised rock produced by a planetary explosion. Because the supposed nebula was initially extremely hot and had the same composition as the planets, the two scenarios are not dissimilar and need to be weighed one against the other. If the nebula hypothesis is treated as the only game in town, it will be assumed to be true regardless of whether the evidence supports it and inconsistencies, potentially arguments in its disfavour, will be shelved.