The Hadean is the earliest segment of geological time, beginning, in the conventional timescale, around 4.6 billion years (Ga) ago, the age of the oldest meteorites. It ends around 3.9 billion years ago, the age of the Earth’s oldest preserved rocks. By contrast, the Moon’s oldest rocks date to around 4.4 Ga, implying that something occurred towards the end of the Hadean to wipe out Earth’s earlier geological history. The Moon tells us what that something was, for its entire surface is disfigured by impact craters – the work of asteroids. Over 70 craters have diameters ranging from 300 to 1200 km. The very biggest, similar in extent to western Europe, measures 2500 km across.
Atheistic cosmology pictures the solar system as having formed from a nebula of dust and gas. In the process of aggregation, some bodies reached planet size, others did not. Embryonic planets increased in mass as smaller bodies collided with them, while bodies failing to reach planet size ended up as asteroids. Impacts from these bodies intensified around the end of the Hadean in a puzzling episode called the ‘lunar cataclysm’ or ‘late heavy bombardment’ – late because, for some reason, it did not occur until more than 500 million years after the period of planet formation. To what degree there were impacts before 4.0 Ga is unknown – several impact melts and breccias on the Moon date to around 4.2 Ga, so there may have been some. While the majority date around 3.9 Ga, they are likely to be dominated by ejecta from one of the last basin-forming impacts on the nearside, the one that produced the huge Imbrium crater, so the possibility that the bombardment was a drawn-out affair cannot be excluded. After 3.8 Ga large impacts abruptly stopped.
Sooner or later, Earth’s experience must have been no less traumatic, for the Earth was at the centre of the Moon’s orbit and whatever swarm of asteroids hit the one would have hit the other. The reason we cannot trace obvious impact craters on Earth from this time is that the original landmass was destroyed and, in time, totally replaced by plate tectonics.
While the severity of the bombardment is clear, the cause is not. But we can get some clues by analysing meteorites. Meteorites are stray fragments of asteroids that have survived being burnt in the atmosphere and have reached the Earth. Most of the inner solar system’s asteroids lie in a belt between Mars and Jupiter. Meteorites, and therefore asteroids, consist of minerals typical of rocky planets – predominantly iron, as in the core of a planet, and silicates, as in the enveloping crust and mantle. They all have ages greater than the age of Earth’s oldest rocks. Many contain mysterious droplets that condensed from older rock flash-heated to temperatures of 1800 °C and vaporised, only later aggregating with other floating material. Thus an alternative to seeing them as leftovers from a proto-planetary nebula would be to interpret them as debris from a planetary explosion.
If we discount the radiometric timescale, which elongates even brief events over millions of years, the meteorite evidence (further discussed here) suggests that there once existed more than eight planets, and at least one of them exploded, either in a collision (as cosmologists have been postulating) or as a result of thermonuclear heating within the core and mantle. Some of the material was vaporised and then re-compacted to form droplet-bearing asteroids; other material scattered through the solar system like so much shrapnel. Today’s asteroids, comets and rocky moons preserve what was left after most of the debris had careered into the Sun and into, or onto, other planets.
There is no telling whether Earth’s former landmass was inhabited. The fossil record implies that it was, since it appears to trace a progressive recovery from the cataclysm. Life cannot just conjure itself into being. Taken together, the sequence of fossils, the complexity of fossilised organisms and the instability which gave rise to successive igneous and sedimentary deposits support the idea that the cataclysm at the end of the Hadean may represent the key to understanding Earth’s troubled history.
The terms ‘cataclysm’, describing the end-Hadean bombardment, and ‘Noachian’, describing the earliest period in Mars’s preserved history, were chosen by scientists without any idea that they might refer to the same event as Genesis describes. Perhaps providentially, in the last few decades science and historical tradition have been converging on one and the same event.
The very heavens I made to tremble, the positions of the stars of heaven changed, and I did not return them to their places.
Even Erkalla [Hades, the Underworld] quaked.
The control of heaven was undone, the springs diminished, the flood-water receded. I went back, and looked; it was very grievous.
On that day all the springs of the great deep burst forth, and the windows of the heavens were opened, and rain fell upon the earth 40 days and 40 nights.
There was just one time in history when Earth experienced a cataclysm of this ferocity. This was at the end of the Hadean, at the very point where its geological record disappears. The beginning of the following aeon, the Archaean, was the only time when Earth was wholly under water. Fossils thereafter trace the process of ecological recovery.
No Hadean rocks exist on the Earth because the eruption of the deep, coupled with the onslaught from outer space, shattered its crust and consumed it. As the land foundered under their feet, plants, animals and men were obliterated. Eventually the land itself was subducted into the mantle. Like the words ‘cataclysm’ and ‘Noachian’, the term ‘Hadean’ (from the Greek Hades) is well chosen. The original world became a separate underworld beneath the land of the living.