The Hadean is the earliest segment of geological time. In the conventional timescale it begins around 4.6 billion years (Ga) ago, the date of the oldest meteorites, which are assumed to be as old as anything in the solar system, and it ends around 3.9 billion years ago. As yet there is no agreed end-date, but 3.9 Ga is roughly the age of the Earth’s oldest rocks. By contrast, the age of the Moon’s oldest rocks is around 4.4 Ga, implying that something occurred towards the end of the Hadean to wipe out Earth’s earlier geological history.

We can get some idea as to what might have destroyed the original landmass by looking at our nearest neighbour. The highlands of the Moon are entirely disfigured by impact craters – the work of asteroids, not volcanoes. Over forty craters have diameters ranging from 300 to 1,200 km. The largest, similar in extent to western Europe, measures more than 2,500 km across, and nearly all of them date to the period 4.0-3.8 Ga. The largest asteroids hit predominantly the near side, suggesting that at its most intense the bombardment lasted only a few days, less than the time taken for the Moon to complete half a revolution round the Earth. The largest basins are also generally the youngest.

Conventional cosmology pictures the solar system as forming from a nebula of dust and gas, in a process where some of the aggregating bodies reached planet size and others did not. Embryonic planets increased in mass as smaller bodies collided with them; close to Jupiter’s orbit accretion was more difficult because of its enormous gravity. Asteroids, according to this view, are the remains of bodies that failed to reach planet size. Impacts from these bodies appear to have intensified around the end of the Hadean in an 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. Whether there were any impacts before 4.0 Ga is unknown. Around 3.8 Ga the bombardment abruptly stopped.

Earth’s experience was equally traumatic. More than 20,000 asteroids are estimated to have struck the planet. While dates further afield are not well constrained, it may be that ‘the Earth-Moon cataclysm was an event that resurfaced terrestrial planets throughout the inner solar system, producing most of the impact craters on Mercury, the Moon, and the ancient cratered highlands of Mars’ (Kring & Cohen 2002). The reason we cannot trace impact craters on our own planet is because its crust was destroyed and totally replaced.

The severity of the event is plain. What triggered the bombardment requires a lot of detective work. Meteorite analysis indicates that the oldest asteroids consist mostly of melt droplets, silicate minerals that were flash-heated to temperatures of 1800° C and vaporised. Material that might be interpreted as primeval ‘dust’ represents only a small proportion of the total contents. If we discount the radioisotope timescale, which tends to stretch even the briefest of events over millions of years, the composition of the impactors suggests a collision between two planets that perished in the encounter. Some of the material was vaporised and immediately recompacted to form droplet-bearing asteroids, other material scattered through the solar system like so much shrapnel. The asteroid belt between Mars and Jupiter, which we know was the source of the fragments, preserves the debris that remained.

Since Earth’s former landmass was annihilated, there is no telling whether it was inhabited or not. Given that the fossil record traces a progressive recovery from the cataclysm rather than an evolutionary tree, the implication is that the Earth was inhabited. Life cannot just conjure itself into being. The sequence of fossils, the complexity of organisms, and the instability which gave rise to successive igneous and sedimentary deposits all support the conclusion that the cataclysm at the end of the Hadean represents the key to understanding Earth’s troubled geological history.

Reviving a lost tradition

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 using such terms. Yet the possibility that, from their different points of view, science and historical tradition are converging upon one and the same event is just what is now suggesting itself.

Genesis is far from the only folk tradition that purports to remember such a crisis near the beginning of history. Similar traditions have been documented from pre-literate tribes throughout the world. Most refer simply to a global flood, but one or two give a hint that considerably more was involved. For example, take this excerpt from Mesopotamia’s Erra and Ishum, from the 9th century BC:

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 [the Underworld] quaked. …
The control of heaven was undone, the springs diminished, the flood-water receded. I went back, and looked and looked; it was very grievous.

(’Stars’ were any celestial points of light, including planets.) Genesis, similarly, characterises the event as an undoing of the creation. Nowhere does it use the term ‘flood’. Like the Mesopotamian texts, it chooses a word reserved exclusively for what happened in Noah’s day: in Hebrew, mabbul, in the Greek translation, kataclusmos, as also in the New Testament. The word was unique because the event was. In contrast to an inundation which submerged and then uncovered essentially the same surface, this was an upheaval in which water surged up from beneath the land and the land itself was shattered. Flooding was merely the consequence of that upheaval.

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.

The ‘deep’ was a great body of subterranean water which oozed up through springs to keep the land moist. (For details see The Cataclysm – more than a flood.) In the course of the cataclysm the primeval land was destroyed and the creatures that lived on it ‘blotted out’ – a fate precluding even fossilisation.

What the opening of ‘the windows of the heavens’ refers to we are not told, but we can get some clues from the occurrence of the same phrase in the book of Isaiah. In a vision of a future day of wrath the prophet saw the heavens trembling, the earth being broken and rocked to its foundations, the host of heaven falling like leaves from a fig tree. The Apocalypse is even more explicit. It visualises asteroids falling to the earth like figs shaken from their tree in a gale and causing the whole planet to shake. People hide in caves and among the rocks of the mountains, terrified of the wrath of their Maker.

There was only one moment in Earth history when the planet experienced a cataclysm that destroyed its very fabric; and there was only one juncture when the whole planet was under water. In both cases, this was the end of the Hadean and beginning of the Archaean.

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. Plants, animals and men were not fossilised because the land under their feet suddenly foundered, collapsing into the same interior magma from which, in due time, new land was to emerge. The ‘Hadean’ (from the Greek word Hades) is thus also aptly named, for it was then that the originally created world was buried and became a separate underworld beneath the world of the living.