Most meteoroids, which end up as meteorites if they land on Earth, are fragments of asteroids, and most asteroids are fragments of larger bodies that broke up as they collided with each other. Those over 1 km in size number more than a million, the largest being Ceres (945 km), Vesta (525 km), Pallas (512 km) and Hygiea (434 km), but these account for about 44% of the main asteroid belt and its total mass remains miniscule. Ignoring the rocky material inside Jupiter, it is estimated to have been initially 500–1000 times greater, around that of an Earth-sized planet. Traditionally asteroids are believed to be the more solid remnants of a primeval cloud of dust and gas that, over a few million years, condensed and aggregated into the various bodies of the solar system. Most ended up between Mars and Jupiter, where they were prevented from growing further by the disruptive influence of Jupiter’s gravity. Some drifted into orbits closer to Earth.
Some researchers have been exploring possibilities that to some extent challenge this view. The Galaxy is a violent place. Jets of plasma spurt from newborn stars, massive old stars explode into supernovae, a gravitational sink at the centre gobbles up everything within reach. In its early days the solar system itself was a violent place, crowded with planetesimals rising through the ranks and erratic ‘oligarchs’ slugging it out for a limited number of permanent positions. Computer simulations re-enact the battle:
In many cases, the smaller planet escapes from the collision highly deformed, spun up, depressurized from equilibrium, stripped of its outer layers, and sometimes pulled apart into a chain of diverse objects. Remnants of these ‘hit-and-run’ collisions are predicted to be common among remnant planet-forming populations.
Erik Asphaug et al., Nature 439:155-60 (2006).
Earth itself collided with a planet. The latter vaporised in the encounter, then partially re-constituted itself as the Moon. A glancing blow by a giant asteroid may have been what flattened Mars’s northern hemisphere (Andrews-Hanna et al. 2008); certainly something major must have happened to generate its crustal dichotomy. Impact craters disfigure every solid surface that has not been recently resurfaced.
According to the standard chronology, it was not until hundreds of millions of years after the planets formed that the solar system quietened down. Impacts gouged out basins up to 3000 km across (Mars’s elliptical depression is more than 8500 km across), in a storm or series of storms that rocked the entire solar system. Thereafter colliding asteroids rapidly dwindled in size and frequency.
- Melt droplets or chondrules (from the Greek word chondros meaning ‘grain’), up to 1 mm in size and varying in composition from silicate- to metal-sulfide-rich. The proportion of chondrules varies from 0% to an extraordinary 80%.
- Micron- to centimetre-sized aggregations of tiny spheroids rich in silicates of calcium and aluminium, known as ‘CAIs’ (calcium-aluminium-rich inclusions). They are much less abundant than chondrules, ranging from 0 to 10% of the total.
- Similar-sized ‘amoeboid olivine aggregates’ (AOAs) made up dominantly of (Mg,Fe)2SiO4, and calcium-aluminium-rich minerals. Though not well understood, they appear to be chemically intermediate between CAIs and chondrules and are almost as common as CAIs.
- Tiny 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 not trivial. Their origins were diverse, and since an originating nebula of dust and gas is expected to have been homogeneous over medium-scale distances, one has to suppose that ‘separate classes of chondrules [etc] 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 is not predicted by models of how the solar system originated (Connolly et al. 2006). CAIs are rich in refractory elements, i.e. those with high melting and vaporisation temperatures, around 1800° C, and conversely poor in volatiles. They are therefore interpreted to be the first solids to have condensed. Condensation from a gaseous state accords with their irregular shapes and often fluffy textures. On the other hand, temperatures this high are not observed in protoplanetary discs even close to the nascent star (Li et al. 2021), and CAIs are common only in chondrites that formed in the cooler outer solar system – the opposite of what is expected (Dunham et al. 2023).
The iron-magnesium silicates that preponderate in chondrules formed by flash-heating to temperatures of 1300–1700 °C, following which some cooled rapidly (1000° C per hour or more), others less rapidly (down to 2–10° C per hour). Since in a cold environment the droplets would have taken only minutes to radiate their heat away, the rates point to a hot environment that expanded and dissipated, such as an exploding fireball. Prior to aggregation with the other chondrite constituents the droplets floated in space. Among the 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 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 are thought to be the CAIs. Dating to 4568.2 ±22 Ma (Bouvier & Wadhwa 2010), they mark the official birth of the solar system and formed all about the same time, at most within 0.4 Ma of each other. So did the AOAs. Chondrules also began to form then, but the majority are 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 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 was already 9 billion years old. They must have originated after the elements making up the bulk of the chondrites. But by what process?
One idea is that most of the isotopes were synthesised by exceptionally high-energy solar irradiation of the dust particles. But this cannot have been the whole story, since such reactions could not have generated 60Fe. Iron-60 can only form today in the extreme conditions that occur before a massive star goes supernova. Just at this moment, a nearby massive star must have chanced to explode and seed 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 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 arriving when the star later went supernova. However, this is to downplay evidence that 60Fe was already present in CAIs (e.g. Quitté et al. 2007). Whatever the case, such scenarios mean that the nebula hypothesis is no longer a simple one.
On grounds of parsimony, it seems better not to disassociate the origin of 60Fe from from that of the other short-lived isotopes. The presence of short-lived isotopes in the CAIs and chondrules suggest that the CAIs and chondrules arose as a direct result of whatever event generated the isotopes. Chondrite constituents arose together. The event would then be a silicate-rich body – a rocky planet – exploding as a result of the heat produced by the isotopes as they decayed. The fine-grained matrix came from the shattered, unmelted outermost part of the planet. Material rich in calcium and aluminium (chondrules as well as CAIs) originated from deeper in the planet. They reached the highest temperatures because the radioisotopes 41Ca and 26Al generated the most heat. The outer layers exploded before the inner layers.
What has been reconstructed as a solar nebula, impregnated with the short-lived elements of an exploding star, was in reality a cloud of shattered and vaporised rock. 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 against each other. In the one, asteroids are examples of the building blocks of planets yet to form, in the other, the debris of erstwhile planets. The nebula hypothesis being the only game in town, however, inconsistencies in its disfavour are ignored.
Perhaps the most glaring inconsistency is the presence of rock fragments in the chondrites. In some instances their chemistry implies silica-saturated magmas produced by large-scale melting and differentiation. ‘The increasing number of planetary igneous fragments observed within (primitive) chondrites strongly suggests that differentiated meteorites come from bodies that accreted and differentiated before the chondrite parent bodies.’ (Sokol et al. 2007) Some of the fragments had undergone thermal metamorphism, i.e. high-temperature modification after the rock had crystallised. Since there is no evidence of shock, impacts are unlikely to have been the cause.
