4. Piecing asteroids back together

Olivine chondrule from the H3 Brownfield chondrite (photo J M Derochette)Asteroids number in the millions and range from 930 km across to metre-sized. The bigger ones are classified according to their mineral content, which can be inferred from from analysing their spectra. In the most widely used classification scheme, the bulk of the population fall into three groups. The most numerous are the C-group asteroids, about 75% of the total; they have dark surfaces 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 enstatite) and P-types (‘pseudo-M’). The spectra of the M-types suggest a predominantly iron content and thus a link with iron meteorites. Asteroids do not compose an orderly compositional gradient across the main belt but are jumbled together, especially the smaller ones. At sizes over 50 km S-types dominate the inner belt, C-types the middle belt, and others, chiefly M- and D-types, the outer belt.

Vesta is a rare example of a large parent body. It is thought to have 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 enormous impact craters 500 and 400 km wide and 13 km deep near its south pole. Although 23,000 times smaller than Earth by mass, 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 the heat is unclear, for although the asteroid formed only 2–4 million years after CAIs, the heat generated by radioactivity seems to have been insufficient to cause large-scale melting (Kunihiro et al. 2004, Sokol et al. 2007). That it is no younger than most chondrules is challenging for the nebula hypothesis. Possibly Vesta derived from the explosion of a still larger body and was already molten at that point.

Whatever its origin, Vesta is exceptional. It is the second largest asteroid in the main belt and, being rockier than Ceres, the only one known to have differentiated. M-type and basaltic V-type asteroids, along with corresponding meteorites, show that other bodies once existed large enough to have differentiated, but these have long since perished and 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 the 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.

Within the various types, asteroids can also be grouped into families, each representing a body that broke up into multitudinous smaller pieces. There are around 64 known families in the asteroid belt. The biggest is the Flora family. Comprising over 800 S-type asteroids, it originated from the break-up of a body around 200 km across. About 50% of the family-forming collisions may have occurred in the first 500 million years of the solar system (Marzari et al. 1999).

The 80-ton erstwhile asteroid 2008 TC3 is thought by some to have resulted from the catastrophic disintegration of a planet the size of Mercury or Mars (Bischoff et al. 2010, Nabiei et al. 2018). It exploded 37 km above the Nubian Desert in Sudan and showered the surface with a surprising assortment of meteorites. Some were a mixture of enstatite chondrites and ordinary chondrites. Some were a carbonaceous type of achondrite known as ureilite, representing mantle material. The asteroid itself was only loosely bound, so was most probably a second-generation body, comprising fine dust and aggregated fragments that were originally not closely associated.

With the minor exception of recent sample-return missions, only meteorites give us direct access to the composition of the asteroids, so in the interests of objectivity meteorites 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 stony-irons, irons and the HED family). As above [section 3], chondrites are aggregations of relatively fine material that issued from one or more exploded planets. The immediate forbears of the achondrites – Vesta being the only extant example – were probably coherent masses of expelled molten material. Vesta’s differentiated composition indicates that the planet from which it originated had not differentiated, at least not fully; otherwise Vesta would not itself have had the wherewithal to differentiate into crust, mantle and core.

Basic scheme of meteorite classification

Assuming that terrestrial planets all started out 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%. Thus, 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 asteroids is called ‘the great dunite shortage’ (dunite being rock composed of olivine).

Diagram from Palme and ONeillThe iron content of chondrites can be compared by quantifying it as a proportion of silicon, the dominant element after oxygen. The proportion varies enormously depending on type (see diagram). By far the most common group (over 90%) are the ordinary chondrites. These are characterised by abundant chondrules, relatively sparse matrix, few refractory inclusions and 1.5–8% iron. Relative to the bulk composition of Earth, most chondrites are depleted in iron

Analysis has confirmed that the olivine grains within chondrules derive from mantle material (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, predominantly 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 therefore chondrites (comprising CAIs, chondrules and matrix) + iron meteorites. The academic debate over which of the chondrite groups contributed 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 cooled remains of molten rock, for example basalt, but it cannot be assumed that the rock before it melted contained chondrules. Those that derive from Vesta are as old as most chondrules. So 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 that chondrules were forming. Polymict ureilites are thought to represent material from the daughter bodies that reassembled after the catastrophic disruption of the parent ureilite body (Goodrich et al. 2015).

Ages can be expected to vary somewhat, since isotope dating dates the point at which a mineral cools and crystallises to become a closed system, and both cooling rates and crystallisation rates vary. A km-sized lump of molten iron will take longer to cool than a mm-sized chondrule, for instance. That aside, tungsten isotope data suggest that 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, Burckhardt et al. 2008, Connelly et al. 2019). The different apparent ages within the 0.3–3.0 Ma bracket reflect distinct degrees of melting and melt segregation within a single body, with Fe-FeS melt forming at ~1000° C and pure Fe melt forming at ~1700° C (Kruijer et al. 2014). As with other achondrites, that irons should have formed this early is surprising, since they originated from the core of a differentiated body, whereas CAIs and chondrules supposedly were primitive. Once the insulating rocky mantle was stripped away, cooling was rapid. Almost all the iron core remnants examined in one study had been exposed simultaneously, and within 7.8–11.7 Ma of CAI formation (Hunt et al. 2022).

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 did they derive from bodies that had a large mantle round their cores? We can distinguish between the possibilities by reference to inferred cooling rates. These turn out to be incompatible with cooling in a core insulated 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 done on irons of the IVA variety, but the findings apply to a range of 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” he says. “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.”

Once regarded as the stuff of fringe speculation, exploding planets have now 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 siliate-iron meteorites record … an impact-related disruption of a molten and differentiated ca. 1000 km diameter planetary embryo’ (Connelly et al. 2019). One problem with interpreting chondrules as the products of brief catastrophic events is that chondrules from the same chondrite group can yield ages spread over at least 3 Ma (Connelly et al. 2012). The scenario intrinsically implies a shorter timescale, as if the conventional dating inflated true time.

Explosions no longer being outlawed, the starting scenario could equally be a single large planet exploding to produce many globes of molten iron from its core. The high abundance of iron meteorites itself favours the idea. The maximum size of the original metal-rich body or bodies is not well constrained, and estimates have increased over the years. 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 which 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). 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 was already 20 GPa there, the planet could have been considerably bigger than Mars. In the context of the solar nebula, it is of course difficult to postulate vanished bodies even the size of Mars, let alone planets bigger than Earth.