7. Impacts or explosions from within?

An exploding cloud of aluminium-silica-rich vapour could have been the result of a planetary body suffering one of two processes: rapid de-compression following shock compression in a collision, or rapid heating from within.

Within a multimillion-year timescale for solar system formation, the current approach is to imagine a sea of planetesimals slugging it out in repeated impacts until a few attained the rank of planet. We know from the lunar record that around 4.2 Ga, and to a lesser extent in the immediately preceding period, impacts were common. Iron meteorites, however, demonstrate that erstwhile bodies of at least dwarf planet rank already existed 360 Ma earlier. Out of their disintegration, whether or not from collisions, smaller bodies arose. The larger the bodies originally, the fewer there were, and thus the less likely collisions were between them, even if somehow they could have been disturbed from their orbits. An impact scenario commends itself only because the heat generated by radioactivity, although much greater than now (because there was much more radioactive matter and the half-lives of now extinct radioisotopes were much shorter), would not have been high enough to cause whole planets to disintegrate.

But suppose that rates of radioactivity were themselves higher. If heat deep within a terrestrial planet were to have increased more quickly than it could have been conducted to the surface and radiated away, the outer part would have remained solid. Before long, thermal expansion would have ruptured the outer part and magma surging through the fractures would have overwhelmed the planet. This may have been what happened. The oldest rocks of every extant terrestrial planet are igneous: Mercury, Venus, Mars and the Moon were once blanketed with magma to a depth of several kilometers. The reason this did not happen on Earth is that the lithosphere was buffered from the melting interior by a subterranean ocean.

Mercury is now the smallest terrestrial planet, but its disproportionately massive core –around three quarters of its total mass – suggests that at some point early in its history it shed part of its mantle. It underwent incomplete disruption because it was originally intermediate in size, bigger than Earth but smaller than the planets which completely disintegrated. If so, Mercury thus gives us an important clue to why some planets exploded and others did not.

However, Earth’s core still has a mass more than seven times greater than Mercury’s core, so the story cannot be quite so simple. At the point that Mercury shed most of its bulk, we must infer that it was only partially differentiated. Most of the iron that might otherwise have melted and sank to the centre was still in its proto-mantle and was lost at the same time. The remaining iron-rich stump of a body then continued to differentiate towards its present proportions, in the same way as we inferred Vesta continued to differentiate after detaching from its parent body.

At this juncture we need to delve more fundamentally into the origin of radioisotopes. Some were so short-lived that they can have come into existence only shortly before the chondrules and CAIs in which they were trapped. One of these was the radioisotope calcium-41. We know of its existence because meteorites contain measurable amounts of its decay product, potassium-41. The half-life of a radioactive element (radioisotope) is the time taken for half of it to decay into another element, in this case a mere 99,400 years in present-day terms. After 10 half-lives all but 0.1% will have decayed, and if the quantity was small to begin with, the amount remaining will be immeasurable. (For comparison, uranium-238, with a current half-life of 4.47 billion years, is barely radioactive.) Thus the former presence of 41Ca in the oldest meteorites means that the radioisotope cannot have come into existence much more than a million years earlier. Evidently a creation origin for the 41Ca can be ruled out. But in that case, long-lived isotopes must also have had a natural origin, since whether a radioisotope is considered short-lived or long-lived is just a matter of degree.

The usual explanation for the presence of radioisotopes in the solar system is that they were synthesised in supernovas. Long-lived isotopes were injected into the nebula well before it collapsed into a star-forming disc, while the most ephemeral came from a nearby supernova that happened to occur just as the solar system was forming. In either case, the injected isotopes should have been homogeneous, a scenario that the data do not support (Krestianinov et al. 2023). The alternative view is that the coming into being of unstable isotopes immediately before year zero of the solar system is no coincidence. They were synthesised, not outside the solar system, but by high-temperature reactions deep within the planets themselves. Some elements, such as uranium, are radioactive in all their isotopic forms, so we may suppose that uranium-238, by far the most stable form, existed from the beginning. Paradoxically, although very dense, it has an affinity for silicate minerals. Given that its distribution through the Earth was initially homogeneous, uranium as the planet melted would have become concentrated in the upper mantle and granites of the middle and upper crust.

Thermonuclear fusion is mainly what generates the energy of all stars, including the Sun. In their cores, temperatures are high enough to overcome the electrostatic forces that keep atoms apart, and the pressure high enough to fuse the atoms into new elements. In the process, excess binding energy is converted into heat energy. The Sun is a relatively small star, so the temperature at its core – around 15 million degrees – is sufficient only to fuse hydrogen, the simplest and lightest element. More massive stars can go further and fuse the initial product, helium, into carbon, nitrogen and oxygen. The temperature required here is around 100 million degrees.

Rates of radioactivity are related to the speed of light. The higher the speed, the faster isotopes decay, because a faster speed of light results in a lower atomic mass and lower binding energy. The natural disintegration of atoms by decay and the natural synthesis of atoms by fusion are controlled by that energy. Thus, if the speed of light had once been faster, the threshold above which nuclear fusion began to take place would have dropped and the temperatures enabling the Sun to fuse only hydrogen atoms would also have facilitated the synthesis of carbon, nitrogen and oxygen. The present Sun derives about 1% of its energy from the C-N-O cycle. Theoretically its mass is too small for such reactions, but if carbon already exists in the core, it can act as a catalyst for the cycle.

Since there is no way of identifying directly what elements lie at the centre of the Sun, we have to infer them from the abundance of elements in its atmosphere. Depending on the degree of convection, these may or may not be representative of what lies within. If only helium nucleosynthesis had gone on, we would expect elements after hydrogen and helium to reflect chondritic abundances, as a consequence of swallowing up the debris of exploded planets. That is not the case. Three elements in the atmosphere far exceed chondritic abundances (Lodders 2020): nitrogen (49 times more abundant), carbon (9.7 times) and oxygen (2.2 times) – the very elements that today are the product of nuclear fusion in more massive stars, where they occur in much the same proportions. Thus we have evidence that significant C-N-O nucleosynthesis did occur in the past and that, since the Sun is too small to have ever reached 100 million degrees, thresholds for thermonuclear fusion were once much lower than now. Conceivably, heavier elements were also synthesised in processes requiring still higher energy levels, but their greater density put them beyond the power of convective currents to dredge them up, in which case the excess abundance of those elements would be beyond detection. The Sun’s atmosphere gives no hint of nucleosynthesis beyond oxygen. Elements heavier than fluorine, a volatile, are all consistently about the same as in chondrites, strong evidence that they originated from meteorites and hence from former planets.

With electrostatic forces being so much lower, early on, nucleosynthesis will also have taken place in the planets, the type of reaction depending on whether the planet was terrestrial or gaseous. Terrestrial planets already had the full range of stable elements, so a radioactive element might have been generated simply by capturing or knocking out a neutron.Aluminium, for instance, is the seventh most abundant element in planets, and turning the natural isotope 27Al into 26Al was but a small step. Turning 40Ca, the eighth most abundant, into radioactive 41Ca was also a small step. Stable variants too were synthesised, such as 18O and 17O, both of them permutations of the original isotope 16O. As with stars, the thresholds for nuclear reactions would have been proportional to size, with larger planets more capable of nucleosynthesis than smaller ones. Owing to the heat produced by the radioisotopes, it was the biggest planets that exploded.

Jupiter with its moon Io in foregroundIn the creation model, the four non-rocky planets started out as pure hydrogen, like the Sun. Being much more massive than the extant rocky planets, they too would have fused helium in their interiors but probably not heavier elements. Jupiter’s temperature at the core is around 20,000° C and its composition approximately 75% hydrogen and 24% helium by mass. Saturn at its core is cooler, around 12,000° C, and would have fused a smaller proportion of helium. The minor elements C, N, S, P, Ar, Kr and Xe are three times more abundant than in the Sun (Cavalié et al. 2023). Why these should be in excess is not well understood; they may have come from volatile-rich asteroids and comets.

In terms of mass Uranus and Neptune are 15–18% smaller than Saturn and consequently contain a smaller proportion of helium. Their global ice-to-rock-ratio is unknown and depends on modelling assumptions, including which elements are chosen to represent the heavier elements (Helled & Fortney 2020). Whether their rock is concentrated in the core or diffuse is also unknown.

One of the great mysteries of solar system physics is why Jupiter, Saturn and Neptune should radiate around twice as much energy as they receive from the Sun. One suggestion is that the excess heat arises from the fusion of deuterium (Fukuhara 2020), but this does not account for Neptune’s high heat flux. More probably the excess is residual heat from the fusion of helium, no longer occurring. Uranus, a relatively cool planet, is the odd man out. Possibly another large body crashed into Uranus (hence the tilt of its axis) and turned much of its deep interior inside out, so that the heat radiated away.

That chondrules originated from mantle silicates that underwent catastrophic heating is no longer controversial, whatever the cause of the heating. Consequently, chondrites do not represent the primordial material from which asteroids accreted and then differentiated. ‘Most chondrules formed by impacts’ (Hutchison et al. 2005). ‘Chondrules and refractory inclusions are impact-melt objects’ (Sears 2005). ‘Chondrules and metal grains in the CB chondrites formed from a vapour-melt plume produced by a giant impact between planetary embryos’ (Krot et al. 2005). ‘Entire planetesimals were melted internally by the decay of 26Al, and collisions led to the splashing of their liquid contents. Cascades of molten droplets would thereby have been lofted into the nebula where they would have cooled to become chondrules.’ (Sanders & Taylor 2005) The feasibility of an explosive origin has grown as it has become clear that differentiated bodies existed already before the formation of chondrules (Sokol et al. 2007, Burckhardt et al. 2008, Kleine et al. 2009). CAIs may also derive from planetary explosions (Sanders 2009).

Recent work, moreover, has shown that chondrules and matrix are complementary in composition. The chondrules of carbonaceous chondrites, for example, have a low Si/Mg and Fe/Mg ratio compared to the bulk ratio, whereas the matrix has a high Si/Mg and Fe/Mg ratio. The components must have had a common source (Palme et al. 2015). The most natural interpretation is that chondrules represent intermediate-depth planetary material that became depleted as a result of silicon-rich minerals rising towards the surface and iron sinking towards the core. The same complementarity is seen in isotope ratios. For example, chondrules are enriched in tungsten-183 relative to tungsten-186 whereas the matrix is depleted; bulk (whole-rock) values fall in the middle. Systematic variation of this kind rules out an impact origin; the complementarity must be the outcome of nucleosynthesis (Budde et al. 2016), and specifically within a planetary body.

Radionuclides began to be synthesised in the cores of rocky planets from the start, generating excess heat through radioactive decay. The rise in temperature accelerated nucleosynthesis faster than the rapidly falling speed of light slowed it down, until the consequent rise in pressure caused the mantle to burst. Temperatures rose most rapidly in the lower mantle, where Al and Ca were most abundant and 26Al and 41Ca most abundantly synthesised, albeit in small quantities. At the moment of explosion, the melt vaporised, cooled and either turned into droplets or irregularly filled the interstices of the newly forming chondrites, depending on the temperature of the vaporised melt and how quickly the droplets and pulverised material from the outermost layer coalesced. Shock-heated rock fragments (‘metamorphic clasts’) also entered the mix. The depressurised core broke up mostly into bodies of dwarf-planet size.

The explosions were of course violent, but not so violent that the fragments dispersed huge distances from their point of origin. The gravity of the exploding body put a brake on their dispersion. Some of the fragments remained in the vicinity of their progenitor, some crashed into other planets, some went into orbit around them. The retention of volatile sodium in the chondrules (but not in the CAIs) shows that they formed in an environment dense enough to prevent evaporation, and certainly denser than the supposed nebula (Alexander et al. 2008). Radiogenic heating continued for a time within the new bodies.