Chondrites, the most common type of meteorite, offer geologists important clues. Dated (on the basis of modern radioisotope decay rates) to around 4.56 billion years ago, they are thought to be the oldest objects in the solar system. Nonetheless, they have not proved easy to interpret. They have up to four constituents:
- Millimetre-sized melt droplets, or ‘chondrules’, which were flash-heated to 1,500-1,800° C and then cooled. Their composition varies, from silicate-rich to metal-sulfide-rich, and the proportion of chondrules to matrix varies, from 0% to an extraordinary 80% of total volume.
- Millimetre-sized inclusions rich in oxides and silicates of calcium, aluminium and magnesium, known as CAIs (calcium-aluminium-rich inclusions). These formed in similarly high temperatures, if not higher, and they also have very variable composition. They are much less common than chondrules, ranging from 0 to 10% of the total.
- Metal grains, composed of diamond and graphite carbon, SiC and aluminium oxide.
- A mostly fine-grained matrix of variable composition, from the same source as the chondrules, although the matrix formed in rather cooler conditions.
The variability of these constituents, not only from one chondrite to another but also within the same chondrite, is no trivial detail. An originating nebula of dust and gas would be expected to have been homogeneous over medium-scale distances, so, assuming that the solar system must have come from such a nebula, one has to excuse the differences in composition by supposing that, for example, ‘the separate classes of chondrules were derived from separate regions and that mixing subsequent to chondrule formation was not thorough’ (Taylor 2001). That is, the chondrules accreted very quickly, before differences in their composition could be smoothed out.
CAIs and chondrules 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, having one less neutron than the normal isotope), and it is these that enable a remarkably precise chronology for the early solar system to be determined. The very oldest constituents are the CAIs, dating to 4,568 million years ago (Burckhardt et al. 2008), the date which officially marks the birth of the solar system. Although some chondrules have the same age as the CAIs, the majority yield ages 1-2 million years younger.
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 had already supposedly been in existence for 8–9 billion years. They must have come into existence therefore some time after the elements that make up the bulk of the chondrites, as products of some natural process. But what was that 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 would not have been able to generate 60Fe. Iron-60 is an isotope that can only form, it is thought, in the extreme conditions that occur just before a massive star explodes into a supernova. So an alternative scenario is that a nearby massive star happened to explode about this time and seeded the nebula with heavy elements. The blast might even have precipitated the nebula’s collapse into a disc, after which the disc condensed into the sun and myriads of smaller bodies.
Or perhaps 60Fe did not enter the solar system until 1 million years after 26Al, as another team have argued (Bizzarro et al. 2007)? 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 itself. However, this is to downplay evidence that 60Fe was already present in CAIs (e.g. Quitté et al. 2007). There is also the problem that two of the other short-lived isotopes that were present in CAIs, 10Be and 36Cl, cannot be formed through stellar nucleosynthesis (Hsu et al. 2006). All things considered, it seems best not to disassociate the origin of 60Fe from that of the other short-lived isotopes.
The existence of CAIs and chondrules was not predicted by standard models for the formation of the solar system (Connolly et al. 2006). They cause problems for the models. CAIs are rich in refractory elements (those with high melting and vaporisation temperatures) and poor in volatiles, so they are interpreted as being the first solids to condense from the gaseous nebula as it cooled. Condensation is consistent with their irregular shapes and fluffy textures. Alternatively, some of them could represent the residue of a high-temperature melt after the evaporation of less refractory elements. Although all CAIs are interpreted as condensates, many underwent melting and in those cases a previous episode of condensation cannot be directly inferred (MacPherson et al. 2005). Whatever process was involved, the preceding solids had a composition similar to that of chondrites as a whole, and the bulk composition of chondrites, in turn, was very similar to the Earth’s, and presumably that of all rocky planets. CAIs appear to have originated from within the solar system.
How long CAI formation went on for is difficult to determine. Some research suggests a period of less than 20,000 years (Thrane et al. 2006), about the smallest interval that high-precision radioisotope dating can resolve. Another study, analysing a different group of chondrites, indicates at least two episodes of CAI formation, one producing 26Al-poor condensates and another 26Al-rich (Krot et al. 2008). A third analysis indicates a period of 300,000 years, possibly relating to the time when CAIs were subject to repeated reheating rather than continually being formed (Young et al. 2005).
Chondrules consist of iron-magnesium silicates, minerals characteristic of rocks. Many were flash-heated to temperatures of up to 1,800° C, following which they cooled, some rapidly (1,000° C/hour or more), others not (down to 2-10° C/hour). Since, in a cold environment, it would have taken only tens of seconds for millimetre-sized droplets to radiate away their heat, the rates point to a hot environment that quickly dissipated, such as an exploding fireball. As with CAIs, some chondrules may have formed by condensation from a vapour rather than simply as melt droplets, but this is uncertain and the circumstances iof formation continue to be debated. Prior to aggregation with other chondrite material the solidified droplets floated in space. Among the many explanations for the flash-heating, one that is increasingly being favoured is vapour-melt produced in large-scale collisions.
Chondrite research presumes that the solar system emerged from the remains of a stellar supernova, because only supernovas can produce the isotopes found in meteorites. The CAIs and chondrules themselves, however, indicate that the isotopes were generated within a pre-existing planet, and that it was the planet, in the course of rapid heating, which exploded. Material rich in calcium and aluminium reached the highest temperatures because it included the most abundant radioisotopes, 41Ca and 26Al, and these generated the most heat. Vaporisation was instantaneous. Thereafter, calcium- and aluminium-rich material typically condensed out before silicate-rich material because it had a higher melting temperature. Inmixed with the vapour was fine-grained material from the shattered, unmelted outer parts of the planet that re-aggregated to make up the matrix of the chondrites. Some of that free-floating material still exists as ‘interplanetary dust particles’. In short, the nebula was that of a planetary explosion, not a solar system being born. At the point where its history becomes traceable, a fully formed solar system could already have been existence.