Asteroids number in the millions and range from 930 km across to metre-sized. The mineral content of the bigger ones can be inferred remotely from analysing their spectra. In the most widely used scheme, the bulk of the population fall into three groups. The most numerous are the C-group asteroids, some 75% of the total; their surfaces are dark and are similar in composition to carbonaceous chondrites. C-types (‘c’ for carbon) and D-types (‘d’ for dark) are subdivisions of the group. The S-group (‘s’ for stone) accounts for about 15%; they contain various silicate compounds and a certain amount of nickel and iron, and are the source of ordinary chondrites. S-types and V-types (‘v’ for Vesta) are subdivisions. The remaining 10% comprise the X-group, subdivided into M-types (‘m’ for metal), E-types (‘e’ for the silicate mineral enstatite) and P-types (‘pseudo-M’). The spectra of the M-types suggest a predominantly iron content and thus a relationship with iron meteorites. Asteroids are somewhat jumbled across the main belt, especially the smaller ones, but at sizes over 50 km S-types are most common in the inner belt, C-types in the middle belt, others, chiefly M- and D-types, in the outer belt.
Vesta is a rare example of a large parent body. It spawned the V-type asteroids and the ‘HED’ group of achondritic meteorites (meteorites lacking chondrules), most if not all of which can be attributed to the two impact craters 500 and 400 km wide and 13 km deep near its south pole. Although its mass is 23,000 times smaller than Earth’s, Vesta was still large enough to have differentiated into mantle and core: internal melting caused light minerals (silicates) to float towards the surface and caused heavy minerals (rich in iron) to sink towards the centre. The source of its internal heat is unclear. Although it formed only 2–4 million years after CAIs, its radioactivity then seems to have been insufficient to cause large-scale melting (Kunihiro et al. 2004, Sokol et al. 2007). That Vesta is no younger than most chondrules is also challenging. Possibly it derived from the explosion of a still larger body and was already molten at that juncture.
Other bodies existed large enough to have differentiated have long since perished. What remains are the products of disaggregation rather than aggregation. Some asteroids, such as Itokawa, the near-Earth S-type body visited in 2005 by Japan’s Hyabusa probe, are loosely bound ‘rubble piles’. They grew from dust and boulders produced by the break-up of older, more coherent bodies, so that they too are not first-generation asteroids on the way to becoming planetesimals. Itokawa is 40% porous.
When the 80-ton former asteroid 2008 TC3 exploded in the stratosphere above Sudan’s Nubian Desert, it produced a surprising assortment of meteorites (Bischoff et al. 2010, Nabiei et al. 2018). Some were a mixture of enstatite and ordinary chondrites, some a type of achondrite known as ureilite. Only loosely bound, the asteroid comprised dust and fragments that were not originally associated. It is thought by some to have resulted from the catastrophic disintegration of a planet the size of Mercury or Mars.
With the exception of recent sample-return missions, only meteorites give us direct access to the composition of the asteroids. In the interest of objectivity they have to be classified separately. There are two main types: those whose parents underwent heating but not melting (chondrites) and those whose parents underwent melting all the way to differentiation (achondrites, such as irons, stony-irons and the HED family). As above [section 3], chondrites are aggregations of relatively fine material that derived from one or more exploded planets. The immediate forbears of the achondrites were probably coherent masses of expelled molten material. Vesta is the only extant example. The body from which it originated cannot have differentiated fully, otherwise it would not itself have had the wherewithal to differentiate into crust, mantle and core.
Assuming that terrestrial planets all started with the same composition as the Earth’s, olivine-rich mantle and thin plagioclase-rich crust should make up around 85% of their volume, metal cores the other 15%. In other words, asteroids of mantle silicate composition should be 5–6 times more plentiful than those composed of iron. However, they are ‘astonishingly rare’ (Haack & McCoy 2005). The problem posed by the scarcity of olivine-rich asteroids compared to the abundance of metal-rich ones is called ‘the great dunite shortage’ (dunite being rock composed of olivine).
The iron content of chondrites can be quantified by normalising it to silicon. The proportion varies almost continuously from achondrites, which are low in Fe, to ordinary chondrites, the most common chondrite group (over 90%), which are high in Fe. The higher the Fe, the greater the differentiation, as a result of Fe sinking towards the core. The Fe/Si to Fe/O ratio is constant, as would be expected if the meteorites were all remnants of a single body. The achondrites are consistent with a crustal origin, chondrites with a mantle origin. Being rich in volatiles, chondrites have higher O content (lower Fe/O) than ordinary chondrites but the same average Fe content.
Analysis has confirmed that the olivine grains within chondrules have a mantle composition (Libourel & Krot 2007). In terms of the nebula hypothesis, ‘Chondrules are not as pristine as conventionally viewed; instead, they consist of nebular and asteroidal materials and must have postdated accretion … and [also have postdated] differentiation of some early generation planetesimals.’ If the asteroids sampled by chondrites were to be completely melted and differentiated, the resultant body would comprise a small metallic core (since they have some iron and nickel), a thick olivine-dominated mantle and a thin, dominantly basaltic crust (Greenwood et al. 2015). Thus the problem posed by the shortage of differentiated, olivine-rich asteroids is solved by recognising that olivine and other such silicates are the main ingredients of chondritic asteroids.
The recipe for a complete planet is chondrites (comprising CAIs, chondrules and matrix) + iron meteorites. The academic debate over which of the chondrite groups gave rise to Earth’s bulk composition is misconceived. Collectively, chondrites derive from the same starting material from which every rocky planet was made. Achondrites lack chondrules because they are the remains of igneous rock, such as basalt, but one cannot assume that the rock before it melted contained chondrules. The achondrites that derive from Vesta are as old as most chondrules. So too are the ureilites, dated to 4566.7 ± 1.5 Ma (Zhu et al. 2020). Various other achondrites date between 4566.5 ± 0.20 Ma and 4565.47 ± 0.20 Ma (Baker et al. 2005, Wadhwa et al. 2009, Reger et al. 2023). Evidently a body large enough to have undergone at least partial differentiation existed at the same time as chondrules were forming. The chronological evidence by itself disproves the nebula hypothesis.
Ages can be expected to vary somewhat, since isotope decay series date the point at which crystallisation of the mineral seals off the system, and rates of cooling and crystallisation vary. A km-sized lump of molten iron would have taken longer to cool than a mm-sized chondrule. That aside, metal-silicate differentiation in the parents of iron meteorites occurred extremely early, within 0.3–3.0 Ma of CAI formation and about the same time as chondrules (Schersten et al. 2006, Qin et al. 2008, Burkhardt et al. 2008, Connelly et al. 2019, Anand et al. 2021, Spitzer et al. 2021). The different ages within the 0.3–3.0 Ma bracket reflect different degrees of melting and melt segregation within single bodies, with Fe-FeS melt forming at ~1000° C and pure Fe melt forming at ~1700° C (Kruijer et al. 2014). As with other achondrites, formation of iron meteorites this early is surprising, since they originated from differentiated bodies, whereas CAIs and chondrules were supposedly primitive. Once the rocky mantle was stripped away, cooling was rapid.
Another major issue concerns the size and nature of the bodies from which the iron meteorites derived. Were the meteorites chipped off predominantly metallic bodies or from bodies that had a mantle round their cores? The former seems more likely, since cooling rates are incompatible with insulation by a thick mantle. The parents must have been ‘much larger than previously inferred and cooled with little or no mantle’ (Yang et al. 2007, 2008). Most of the research has been on irons of the IVA variety, but the findings apply equally to other meteorites. The preferred scenario involves protoplanets the size of the Moon or larger being stripped of their mantle in collisions:
If the initial mass [of the asteroid belt] was ~103 times higher [than now] and many Moon-sized or larger protoplanets were present, collision rates would have been greatly enhanced while the differentiated bodies were still hot. As mantles are not efficiently stripped from cores by catastrophic impacts between asteroid-sized bodies, the IVA metallic body may have formed as a result of a grazing collision between protoplanets that disrupted the core of a Moon-sized body creating a string of metal-rich bodies.
The scenario of collisions leaving strings of metal-rich bodies is as elaborated by Asphaug et al., quoted at the beginning of this series. Colliding protoplanets do not simply merge. In some cases the result can be explosive, especially for the smaller body. As one author put it in the press release:
“As two massive objects pass near each other, gravitational forces induce dramatic physical changes – decompressing, melting, stripping material away, and even annihilating the smaller object” Williams says. “You can do a lot of physics and chemistry on objects in the Solar System without even touching them.” …
A planet exerts enormous pressure on itself through self-gravity, but the gravitational pull of a larger object passing close by can cause that pressure to drop precipitously. The effects of this depressurization can be explosive, Williams says. “It’s like uncorking the world’s most carbonated beverage. What happens when a planet gets decompressed by 50% is something we don’t understand very well, but it can shift the chemistry and physics all over the place, producing a complexity of materials that could very well account for the heterogeneity we see in meteorites.”
No longer the stuff of fringe speculation, exploding planets have entered the mainstream. ‘Chondrites do not represent the precursor material from which asteroids accreted and then differentiated’; instead they ‘derive from asteroids that accreted late … [possibly] by re-accretion of debris produced during collisional disruption of older asteroids’ (Kleine et al. 2005). ‘Group IVA iron and silicate-iron meteorites record … an impact-related disruption of a molten and differentiated ca. 1000 km diameter planetary embryo’ (Connelly et al. 2019).
Explosions no longer being outlawed, the starting scenario could equally be a single large planet exploding to produce numerous globes of molten iron from its core, as the abundance of iron meteorites might indicate. The maximum size of the original metal-rich body or bodies is not well constrained, and estimates have increased. The conclusion that the ureilites of 2008 TC3 (the asteroid that disintegrated above the Nubian Desert) came from a parent somewhere between Mercury and Mars in size is based on diamonds in them that could only have formed under pressures above 20 GPa, equivalent to the pressure at the centre of Mercury and the core-mantle boundary of Mars (Nabiei et al. 2018). In the context of the solar nebula, postulating vanished bodies the size of Mars is daring. However, 20 GPa is the minimum required, and ureilites are mantle rocks. If the diamonds originated in the middle or upper mantle and the pressure there was already 20 GPa, the planet could have been considerably bigger than Mars – bigger than Earth.
