Most meteoroids (which end up as meteorites if they land on Earth) are fragments of asteroids, and most asteroids are fragments of larger asteroids, breaking up as they collide with each other. The largest are hundreds of kilometres across – the size of some moons. Whatever their size, in the orthodox view asteroids are remnants of a primeval cloud of dust and gas that, over a few million years, aggregated and condensed into the various bodies of the solar system. Most of the remnants occur in a belt between Mars and Jupiter, where they were prevented from reaching larger sizes by the disruptive influence of Jupiter’s gravity. Their total mass at the start is calculated to have been 200 times greater than the present mass, possibly much more. Thus the process of collisional attrition that has gone on ever since is the reverse of what, with much less certainty, is thought originally to have happened.

That is the text-book account. However, several leading researchers are now beginning to explore possibilities that to some extent challenge this view. As has become ever more apparent, the galaxy in which the solar system resides is a violent place. Jets of plasma spew from newly born stars, massive old stars explode into supernovae, a gravitational sink at the centre of the galaxy gobbles up everything within its reach. In its early days the solar system itself was a violent arena, crowded with planetesimals rising through the ranks and erratic ‘oligarchs’ slugging it out for a limited number of permanent positions. Computer simulations now 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, and thus to be relevant to asteroid formation and meteorite petrogenesis.

Erik Asphaug et al., Nature 439:155-60 (2006).

In an effort to explain the unEarth-like composition of the Moon cosmologists attribute its genesis to a collision between the Earth and another juvenile planet, which vaporised in the encounter and then re-formed. Others suggest that a glancing blow by a giant asteroid was the event that flattened Mars’s northern hemisphere by several kilometres – for certainly something remarkable must have happened to produce the feature. With impacts occurring with this frequency and on this scale, it seems that at least some asteroids originated from bodies larger than themselves, from planets rather than primordial grains of dust.

According to the orthodox chronology, it was not until long after the present planets had emerged that the solar system quietened down. Mars’s cataclysm occurred during its ‘Noachian’ period, very early in its history. Subsequent impacts gouged out basins up to 3,300 km across, in an asteroid storm that rocked the entire inner solar system, even as far as Mercury. Meteorite ALH 84001 – a chunk of Martian crust presented, like the Black Stone of Mecca, as proof that Mars once hosted life (a claim now largely discredited) – records that bombardment in its own fabric. The surface of every terrestrial planet was pulverised. While the chronology is not well constrained, the lunar evidence indicates that the bombardment was at its most intense only shortly before it stopped, more than 700 million years after the origin of the solar system.

What was going on? Planets supposedly grew in inverse proportion to the amount of material remaining. So why was the solar system not long since swept clean of leftovers from the formation process? Why was the cessation of the bombardment so abrupt?

Chondritic meteorites

Chondrites, the most common type of meteorite, offer geologists important clues. These date to around 4.567 billion years ago and they are thought to be the oldest and most primitive objects in the solar system. Their main components are:

• Millimetre-sized melt droplets, or “chondrules”, of varying composition, which were flash-heated to around 1,700° C and then rapidly cooled. Chondrules can make up to 80% of the total volume.
• Centimetre-sized condensates rich in calcium and aluminium that also appear to have cooled rapidly. These are usually separate inclusions in the meteorite but occasionally occur within the chondrules.
• A usually fine-grained matrix, which has recently been shown to have had the same source as the chondrules.

The calcium-aluminium-rich inclusions (CAIs) tend to yield ages 2-3 million years older than the chondrules, and contain certain exotic short-lived radioactive elements (¹⁸Ca, ²⁶Al and the iron isotope ⁶⁰Fe) which suggest an origin in a nearby supernova. How the chondrules formed is hotly debated, but their composition suggests evaporation of material initially rich in iron and silica and implies some kind of planetary origin. Explanations for the mysterious flash-heating include shock waves and vapour-melt produced in a collision between asteroids or proto-planets. Whatever the explanation, the different age and provenance of the CAIs show that the meteoroids were not simply fragments broken off a much larger body. Vaporised matter from the solar system was mixing with matter injected from beyond the solar system. Only subsequently did the mixture agglomerate to form heterogeneous larger bodies.

Water in the heavens

Surprisingly, many meteorites – of every type – contain minerals that must have formed through reaction with liquid water, such as clays and carbonates. Studies have shown that the minerals were hydrated before the whole body came together but after melting of the chondrules. Similar minerals have been found in the short-period comet Tempel 1, as investigated by NASA’s Deep Impact probe. Chondrites with very low proportions of chondrules can contain up to 20% water.

Water, or evidence of water in the distant past, occurs in nearly all parts of the solar system, even as far as the Kuiper Belt, beyond Neptune. Martian land forms indicate that, apart from impact cratering, ‘rainfall and surface runoff were the most important geologic processes in the early history of Mars’ (Craddock & Howard 2002). Water is also thought to have once been abundant on Venus, notwithstanding its present fiery temperature.

The ubiquity of these traces suggests that water may have been a constituent of the matter injected from beyond the solar system. After hydrogen and carbon monoxide, water is one of the most common molecules in the clouds of dust and gas that pervade our galaxy. Hence it is not strange to find it abundant in the outer solar system itself. Much of the Kuiper Belt consists of water in the form of ice, generally interpreted as as a remnant of the protoplanetary disc. It could also, however, be interpreted as a remnant of the aqueous cocoon that formed around the solar system after creation.

Planets smashed to pieces

There are two locations where former planets might once have existed. One is the asteroid belt between Mars and Jupiter. The other is the diffuse Kuiper Belt on the perimeter of the solar system. In either case the small numerous bodies in these regions could be the remains of two planets with similar orbits but different orbital periods that collided with each other. The fragments that shot out in directions roughly parallel to the orbits of their parents would be those continuing to orbit at that distance from the Sun, heavily battered following impacts with each other. Fragments that shot out in other directions would have (i) crashed into other planets, (ii) been captured by other planets to become their satellites, or (iii) attained, eventually, solar orbits closer or further away from the Sun, including the asteroids classified as long-period comets. The rocky material at the centre of Jupiter and Saturn, their rocky moons, and some of the Trojan asteroids that lie in the same orbit as Jupiter are all likely to have derived from the inner solar system collision. The icy moons of Neptune and Uranus, the methane that gives Neptune its blue appearance, and the Centaurs that orbit the Sun between Neptune and Jupiter, by contrast, are all likely to have derived from the outer solar system collision. There would also have been some interchange between the two regions, whereby the asteroid belt received volatile material from the Kuiper Belt and the Kuiper Belt received rocky material from the asteroid belt. Only a tiny fraction of the mass of the original planets would remain in their original locations.

How, then, does this scenario compare with the known nature of the small solar system bodies? Asteroids are grouped into different classes according to their mineral content (inferred from their spectra). There are three main classes: C-types, comprising about 75% of the total, S-types, comprising about 17%, and M-types, comprising the remaining 8%. How meteorites fit into this scheme is not always clear, for it can be difficult to link them with a particular asteroid or class of asteroid in outer space. One example of successful identification is 4 Vesta, thought to be the source of a family of non-chrondritic meteorites. Some asteroids are not solid enough to be the potential parents of meteorites, being just loosely bound ‘rubble piles’ that would completely dissipate on entering Earth’s atmosphere. An example is Itokawa, shown in the top picture.

C-type asteroids (‘c’ for ‘carbon’) consist primarily of carbonaceous material. S-types (‘s’ for ‘stone’) contain various silicate compounds and some nickel-iron. M-types (‘m’ for ‘metal’) consist primarily of nickel-iron. Since many asteroids appear to have been chipped off larger bodies, it is the larger asteroids that would have differentiated into the outer layer, mantle and dense metallic core suggested by these three types. Vesta, at 530 km in diameter, is a still extant example. Differentiation is dated to within a few million years of the CAIs, and it is assumed that this occurred through melting of a previously cold body: light minerals floated towards the surface and heavy minerals sank towards the centre. However, a heat source for this cannot always be found in the orthodox scenario. The gravitational energy released by contraction would have been too small, and although radioactive decay produces heat, the short-lived radioisotopes within the asteroids would by then mostly have decayed into their stable daughter elements (Kunihiro et al. 2004, Sokol et al. 2007). It may be better to suppose that the asteroids that differentiated were already in a molten state when they formed.

Could the asteroid types have originated from a single planetary body with their bulk composition? The trend of scientific speculation is certainly in that direction. There are at least 100 distinct types of iron meteorite, and as Jeffrey Taylor has pointed out, it seems improbable that this material was all the product of impacts chipping away to the core of small, differentiated asteroids. Some iron meteorites may derive directly from the core of a protoplanet (Kleine et al. 2005). Others, such as the group IVA meteorites, are proposed to derive from an asteroid 300 km in diameter that had been stripped of its silicate mantle. Prior to being disembowelled the preceding body could have been 1,000 km or more across (Yang et al. 2007), and it, in turn, could have been the product of a collision between still larger bodies:

If the initial mass [of the asteroid belt] was ~10³ times higher 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.

Given that the bulk of the asteroid belt formed from the same region of the nebula and therefore must have had approximately the same composition, it is as reasonable to suppose that there were just two protoplanets at this time as to suppose that there were a multitude.

The scenario of a collision leaving a string of metal-rich bodies draws on the work of Asphaug et al. already quoted from. Colliding planets do not, as is commonly assumed, simply merge. In some cases the result can be explosive, especially for the smaller body. As one of the authors 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 annih¬ilating 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 at this stage, 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.”

Chondrites may be understood as fragments of asteroids that were too small to undergo differentiation. They originated from planetary silicates that vaporised in the impact, the vapour condensing into droplets and immediately cohering with material from beyond the solar system to form highly compacted aggregates. By contrast, most of the debris from the larger of the two bodies in the encounter did not vaporise but was expelled far beyond the present asteroid belt. A few chrondrites incorporated metamorphic clasts (rock fragments) from the shattered body.

Other writers have also concluded that chondrules were formed by impact vaporisation. Within a multimillion-year timescale for solar system formation, one naturally thinks of numerous impacts between bodies of various sizes, but the basic message is the same. ‘Most chondrules formed by impacts’ (Hutchison et al. 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).

Though as yet poorly known, the composition of the Kuiper Belt seems to be the end-product of a mixture of events and influences. Some objects consist mainly of water-ice, interpreted here as the remnants of a collapsed water envelope that surrounded the solar system. Other objects, including the largest bodies, seem to consist mainly of methane. The interstellar medium and polymer-producing cosmic rays have added to the mix. Short-period comets, which are also complex chemically, are believed to be Kuiper Belt Objects (KBOs) that migrated into the inner solar system.

Evidence supporting the hypothesis that the Kuiper Belt arose from a collision between two gaseous planets includes:

• its fragmentary nature – it is estimated to contain more than 100,000 objects over 50 km in size and, wildly contrary to computer models, quadrillions of objects 10-100 m in size (Cooray 2006);
• its low overall density – this is not satisfactorily explained by the solar nebula hypothesis and is known as the ‘missing mass problem’, though the problem seems now to be largely counteracted by the quantity of the 10-100 metre-size objects;
• the ‘surprisingly high level of dynamical excitation’ of the objects;
• the existence of ‘scattered disc objects’ that extend in erratic orbits beyond the Kuiper Belt.

Dwarf planets such as Pluto, Sedna and Eris, all of them, like the scattered disc objects, on highly elliptical orbits at an angle to the plane of the ecliptic, would also be the frozen reconstituted remains of a former planet.

Asteroids, comets and moons

Asteroids have proved to be much more complex bodies than was expected on the basis of the nebula hypothesis. Some, like Vesta, are large and have substantial mass, or are smaller fragments of such bodies. Others, small and large, are weakly bound rubble piles. In neither case can they be understood as the product of arrested dust accretion, just one step on from the primordial dust itself. The rubble piles appear to be agglomerations of dust and boulders produced from asteroids colliding with each other, possibly from very large colliding asteroids (Asphaug et al.), while the Vesta-like asteroids are differentiated bodies and, as such, also have a history that obscures their origin. The parents of chrondritic meteorites are a third category, since they never underwent differentiation. In short, the oldest datable objects in the solar system cannot be interpreted as the immediate leftovers from its formation.

Short-period comets, it now appears, are much the same as asteroids, but rich in volatiles. Some have solid, cratered nuclei; others are low-density rubble piles that disintegrate under the tidal forces of the Sun and Jupiter. They become comets when their elliptical orbits bring them close to the Sun, and they cease to be comets, actual or potential, once they have lost their volatiles. Each year several asteroids are reclassified as comets on being found to exhibit a coma and/or tail. There also exist comets with nuclei consisting primarily of frozen volatiles (ices); they are thought to have been drawn into the inner solar system from the Kuiper Belt. Some comets have been established to lie completely within the asteroid belt.

‘Long-period’ comets are those with elongated orbits that extend far beyond the observable solar system. Their life-times are long in comparison to short-period comets but short in comparison to the presumed age of the solar system. Another characteristic is that, collectively, they can orbit the Sun at any inclination. All this has prompted the hypothesis that the solar system is enclosed by a vast replenishing ‘cloud’ of such comets, known as the ‘Oort cloud’. The comets could, however, simply be collision fragments that shot out at angles oblique to the ecliptic plane. Aside from the undetected Oort cloud, their orbital periods suggest a collision that took place less than 100,000 years ago.

They also seem not to be fundamentally different from asteroids in composition, though they are richer in silicates. Prior to the Deep Impact mission, scientists expected the silicate particles of comets to be entirely amorphous, like those in interstellar space, on the basis that comets formed in the outermost parts of the solar nebula where there was not enough heat for crystals to form. It now seems likely that all comets contain crystalline silicates, indicative of having been subjected to temperatures up to 1,000 ºC (Wooden et al. 2005). The co-existence of high and low-temperature materials in these bodies is thus problematic, but is consistent with the idea that they originated as fragments of terrestrial planets in the inner solar system, between Mars and Jupiter, and subsequently mixed with water vapour further out.

Except for the Earth’s moon, none of the satellites orbiting planets are likely to have formed in situ. Mercury and Venus have no satellites. Mars has two and these are clearly asteroids. Jupiter has at least 63, Saturn at least 56, the majority less than 10 km in diameter. Uranus has 27, Neptune 13. In the scenario proposed here satellites are (i) expelled main belt asteroids, (ii) expelled Kuiper Belt Objects, or (iii) combinations of the two as a result of one type slamming into the other. Spherical shape is not evidence of a particular origin, since above a certain mass self-gravity will naturally pull a body into a sphere.

Astronomers classify the moons into regular satellites, which trace close, circular orbits about the host’s equator, and irregular satellites, which trace more distant and elliptical orbits at an angle to the equator. Usually the regular satellites orbit in the same (‘prograde’) direction as the planet’s rotation, and they tend to be bigger. For these reasons, in the nebula hypothesis, they are thought to have formed from the same rotating disc of gas that gathered round the future planet’s rocky core – an explanation that is of course entirely dependent on the nebula hypothesis. The fact that Jupiter’s moons get rockier closer to the planet is prima facie evidence against a disc origin.

Irregular moons tend to have a retrograde motion because elliptical orbits are more stable when retrograde. However, Saturn’s irregulars, although similarly elliptical, are a much more even mix of prograde and retrograde, and it is possible to model the capture of both (Astakhov et al. 2003). Size is also not necessarily an identifying factor. Triton is the biggest of Neptune’s satellites, has an inclined but circular orbit, and lies between a group of small inner prograde satellites and a number of exterior satellites with both prograde and retrograde orbits. It may be that large bodies captured by a planet tend to adopt an orbit around the equator because that is the angle at which they increasingly tend to approach if moving Sun-ward, i.e. from the Kuiper Belt. Low-mass bodies flying out from a single point will be less likely to approach along the ecliptic plane. In any case, the present solar system offers no mechanism through which moons could be captured, and it is therefore thought that capture must have taken place early in its history, when there was a much greater likelihood of collisions and near misses (Jewitt et al. 2006).

Not a supernova

In Big Bang cosmology galaxies, like solar systems, form from a rotating cloud of gas, consisting of hydrogen and helium. Massive stars condense out of the cloud, go through a comparatively rapid life-cycle in the course of which they synthesise elements heavier than hydrogen and helium and finally explode. In the nuclear fusion chains leading up to the explosions still heavier elements arise, along with numerous short-lived radionuclides. The dispersing gas then becomes the raw material for a generation of less massive stars. In this context it may not seem unreasonable to attribute the short-lived radionuclides and water vapour in the early solar system to a supernova.

Creation theory takes a rather different view. Here the starting-point is not the ‘Big Bang’, but the massive ultraluminous power-houses called quasars (the ‘light’ of Gen 1:3 being the quasar originally at the centre of our own galaxy). These developed into galaxies not as a result of gaseous matter being sucked gravitationally inwards but through the ejection of streams of plasma outwards, which subsequently condensed into stars. In this view the clouds of dust and gas permeating the arms of galaxies are the sparse remains, after star formation, of what was ejected from the centre. They are rich in ‘metals’ (elements heavier than helium) because it was the galactic nuclei that first produced metals. Supernovae contributed only later, and in a relatively minor way, to the mix.

As this is not the place to elaborate on the question of how galaxies form, suffice to say that the Milky Way galaxy must also have originated in this manner. In the beginning there were only quasars, not stars in the modern sense. Thus the violent blast of hot gaseous matter that injected the solar system with exotic radioisotopes had probably to do with the formation of the Milky Way’s spiral arms rather than the death of a massive star.

Count-down to disaster

To sum up, our present knowledge of the early solar system allows us to reconstruct the following sequence of events:

1. An ejection of hot metal-rich gas from the Milky Way’s nucleus reaches as far as the solar system and suffuses it with water vapour and short-lived radioisotopes.
2. Two terrestrial planets between the orbits of Mars and Jupiter collide and explode. Asteroids form in the collision.
3. Shock waves, perhaps from a nearby supernova, drive the asteroids through the solar system.
4. Asteroids bombard the sun, the moon and the planets, leaving craters on Mercury, Mars and the Moon that are still visible.
5. Also as a result of the shock waves, the cocoon of water vapour around the solar system collapses and likewise drenches the planets.

This is the Hadean cataclysm that forms the starting point of recolonisation theory. Despite its links with the Genesis tradition, the theory attributes more history to the solar system than the nebula theory does, since it does not interpret asteroids and KBOs as the most primeval bodies in the solar system. The Earth, the Moon and the Sun are substantially older. In this view the oldest radioisotope dates, coming out at 4.568 billion years ago, do not necessarily measure the age of the solar system. Indeed, if, as is commonly surmised, short-lived isotopes originated from nucleosynthesis beyond the solar system, there would be no reason at all to associate them with the origin of the solar system. Long-lived isotopes such as Uranium-235 and -238, the ones that provide us with an ‘absolute’ chronology, might have been in existence for much longer.

The oldest radioisotope dates mark only the earliest date when the cooling of a hot fluid mix of minerals sealed the isotopes, preventing daughter isotopes from being exchanged with their surroundings. Without the initial isolation of daughter and parent, so that a daughter-parent ratio greater than the original ratio can be attributed to subsequent decay of the parent, radioisotope dating of a rock would not be possible. Isotopic closure does not therefore represent year zero. Chondrules in particular had a prior history, a time when the minerals were part of another body, here argued to be a former planet. That history might well have included a period when the planet’s interior heated up and underwent mineral segregation as a result of radioisotope decay. ‘Growing evidence exists that chondritic meteorites represent the products of a complex, multi-stage history of accretion, parent body modification, disruption and re-accretion’ (Sokol et al. 2007). Far from dating the origin of the solar system, CAIs and chondrules are simply the oldest materials in the solar system to have survived from a period of thorough-going destruction.

Moreover, the evidence from the Hadean suggests that the rates of radioactive decay were extremely fast. For example, while some chondrules date to 2-3 million years after the CAIs (assuming rates of decay the same as now), others appear to have formed at the same time as them: occasionally indeed there are inclusions within chondrules, and chondrules within inclusions. One study (Bizarro et al. 2004) even suggests that the period in which chondrules from within the same meteorite formed was at least 1.4 million years. Yet neither the ‘rapid heating event’ associated with the chondrules nor the supernova blast associated with the CAIs could have gone on for anything like this time.

We know, in fact, that CAIs all formed within a very brief period: less than 500,000 years and possibly as little as 100,000 years in radioisotope terms. On the other hand, if CAIs originated from a supernova, these ejecta would have been passing through the solar system at speeds of kilometres per second. Such speeds imply a much shorter formation time: possibly as little as a few weeks. Why the apparent overall formation time of chondrules in that case was more protracted is unclear. Explanations in the conventional scenario have included inhomogeneous distribution of the radioisotopes and resetting of the radioisotope clock by secondary disturbance.

If we reject the idea that ‘4.568 billion years ago’ – give or take 2-3 million years – represents actual time, then the lateness of the ‘late heavy bombardment’, which appears to have been a direct consequence of the planetary explosions, ceases to be problematic. The actual interval between the two events was not 600 million years but the time taken for asteroids to reach the Moon, from whose rocks we get the later date. That could have been as little as a hundred years. One of the clearest implications of the asteroid data is that radioisotope dates cannot be taken at face value.

Another source of knowledge?

Knowledge of the solar system has progressed by leaps and bounds over the last few decades and opens up the possibility that there may originally have been twelve planets rather than the present eight, within a firmament that was bounded in every direction by a cocoon of water. Surprisingly, the Genesis tradition describes just the same arrangement. The solar system was created with waters ‘under the firmament’, on Earth, and waters ‘above the firmament’, enclosing the Sun, the Moon and the planets. At one time there were twelve planets (Gen 37:9). The Apocalypse also refers to twelve, four of which were subsequently destroyed so that their fragments swept through space and hit the Earth (Rev 12:4). Four from twelve leaves eight. It was only in 2006 that, from their very different point of view, astronomers came to the conclusion that the present solar system comprises only eight planets, not nine as previously believed.

In a work which visualises asteroids falling to the earth like figs from a tree (Rev 6:13) the look- back to an earlier such occasion is not gratuitous. One such body plunges into the ocean and kills a third of all marine creatures. Another leaves debris which poisons rivers and springs. It is as if the scientific discoveries of modern times are unsealing a message that has lain hidden until precisely this age. If that is so, we ignore that message at our peril.

See also:
The six days of creation
The created world no longer exists
The origin of the solar system

References