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 4.0 Ga ago, roughly the age of the Earth’s oldest preserved rocks. By contrast, the Moon’s oldest rocks date to around 4.5 Ga, implying that something occurred around the end of the Hadean to wipe out Earth’s earlier history. The Moon tells us what that something was. Since there is no atmospheric weathering on the Moon and no plate tectonics, its early geological record remains well preserved. Even today, its entire surface is disfigured by impact craters. The 70 or so craters with diameters over 300 km (called ‘basins’) are all older than 3.9 Ga. A few have diameters in excess of 1000 km. The very biggest, the South Pole-Aitken basin, measures 2400 km across, similar in area to the whole of Europe.
Atheistic cosmology pictures the solar system as having formed from a nebula. In the course of aggregation, some bodies reached planet size, others did not. Embryonic planets grew as smaller bodies collided with them, while bodies failing to reach planet size ended up as asteroids. Hence the the fact that asteroid impacts were most intense early on in the Moon’s history (not to mention Mercury’s and Mars’s history) has always been understood as reflecting a period when the solar system was still assembling. Although the Moon and the planets had all reached full size by 4.5 Ga, there were then many more asteroids in space than now and these dwindled in total mass gradually, as either they collided with the planets or plunged into the Sun.
The chronology of the lunar impacts remains to be firmed up. Most of the rocks recovered by the Apollo astronauts date to around 3.92 Ga. In the 1970s, this prompted the idea of a ‘late heavy bombardment’ or ‘lunar cataclysm’ about that time – ‘late’ because that date was long after the time when the frequency of impacts should have been greatest. However, the evidence was never unequivocal and further analysis suggested that the samples were dominated by far-flung ejecta from just one impact, the impact that produced the huge Imbrium basin (1321 km across). Samples of widely separated provenance were returning the same age because Imbrium was one of the last basins to be excavated and its ejecta lay on top of those from earlier impacts (Nemchin et al. 2021).
Very recently the attribution to Imbrium too has been questioned. It now appears that, at the Apollo 14 landing site at least, the ejecta originated from the Orientale basin (960 km across, also huge), which on the basis of crater counts is thought to have formed some 0.2 Ga after Imbrium (Werner et al. 2022). Prior to 3.9 Ga, most dates cluster around 4.2 Ga (Vanderliek et al. 2021). Since an indeterminate time gap separates Imbrium from all older basins the new work in effect resurrects the heavy bombardment idea but shifts it back 300 million years. After Orientale, large impacts abruptly stopped.
Earth’s experience must have been similarly traumatic, for the planet was never more than 400,000 km from its satellite; sooner or later it too must have been hit. The only reason we cannot trace obvious impact craters on Earth from this time is that the original landmass was destroyed and replaced, as upwelling magma generated new crust above its ruins, mostly oceanic crust. Infrequently and with less catastrophic effect large asteroids continued to impact the Earth into the Proterozoic and beyond. The largest crater on the present continental crust is the 2.02 Ga Vredefort Crater on South Africa’s Kaapvaal craton. The outer rim is indistinct, but when produced would have been around 250 km wide. On impact the asteroid excavated the crust to a depth of 40 km or more before rebound and immediately filled the cavity with melt, now largely weathered away.
What could have caused the bombardment? We get some clues by analysing meteorites. Meteorites are the remains of meteoroids, which derive from asteroids. Most of the inner solar system’s asteroids lie in a belt between Mars and Jupiter, but some orbit the Sun at distances much nearer the Earth. If the fragments get drawn into Earth’s gravitational field, the bigger ones will survive the atmospheric burning that turns them into shooting stars and become available for geological investigation. What we have found is that they 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. And 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 material.
If we discount the radiometric timescale, which elongates even brief events over millions of years, the meteorite evidence (further discussed here) suggests that this side of the Kuiper Belt there once existed two more planets than the eight known today. These exploded, either in a collision or as a result of thermonuclear heating within the core and mantle. The outer parts shattered; the middle parts vaporised and then re-compacted to form small droplet-bearing bodies; the refractory core simply melted. Some of this 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 succeeding 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 discontinuities between the major groups of organisms and the instability which gave rise to successive igneous and sedimentary deposits support the idea that the cataclysm in the late Hadean represents the key to understanding Earth’s troubled history.
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, and apertures of the heaven opened, and rain fell upon the earth 40 days and 40 nights.
There is only one moment in history that corresponds to the cataclysm in Genesis and that is the late Hadean, just before the point where (traced back in time) the geological record disappears. No Hadean rocks exist on Earth because the eruption of the deep, coupled with the onslaught from outer space, shattered the thin, dry lithosphere above the deep 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. The term ‘Hadean’ (from the Greek Hades) is well chosen. The original world became a separate underworld beneath the land of the living.
Since the rocks collected by the Apollo missions cannot be attributed with certainty to any specific basin apart from Orientale, the absolute chronology of the impact sequence remains unknown. However, basins can be ordered relative to each other, partly by considering evidence that the ejecta from one impact overlie the basin of another and partly by counting the number of smaller craters (though still large – at least 20 km across) within the basins. If there was a constant flux of smaller asteroids, and if the flux tailed off after the largest asteroids tailed off, more craters should overprint the older basins than the younger basins. Naturally, the method only provides an approximate chronology. Sometimes the (not always clear) ejecta evidence conflicts with the crater count evidence. Also, many of the basins, particularly on the nearside, were subsequently filled with lava, completely drowning some of the smaller craters. There is a trade-off: on the one hand, the larger, more prominent craters in the basins may be more representative, being less likely to have got submerged; on the other, there is a loss of statistical power, since there are many more craters in the 20+ km range than, say, the 64+ km range.
For a long time the most authoritative analysis of the basin sequence was that by Don Wilhelms, published in 1987. He divided the basins into two major groups: those that formed before Nectaris and those that formed after. Distinct from the second group was a third, comprising Imbrium, Schrödinger and Orientale. In the early 21st century satellites with the ability to map surface topography and areal variations in gravity enabled many more craters to be identified, including buried ones. As a result, some workers have questioned the dating of Crisium after Humboldtianum and have also argued that Serenitatis is pre-Nectarian. The count of craters in the 64+ and 90+ km range, however, tends to support Wilhelms’ analysis (Evans et al. 2018). One major revision that does seem to stand up, based on crater count, is that Hertzsprung, despite its fresh appearance, is older than previously proposed (Orgel et al. 2018). Serenitatis is now dated to 4.2 Ga (Černok et al. 2021) and possibly Nectaris should be dated to 4.21 Ga, consistent with the whole sequence being geologically instantaneous. The new crater counts suggest that Freundlich-Sharanov, Grimaldi (?), Apollo and Planck, should also be regarded as Nectarian.
Thus analysed, there are fifteen Nectarian basins (Table 9). Remarkably, when we plot their relative order against longitude and subtract 360 degrees from the older basins (since, for example, 145° is the same as -215°), they follow a consistent trend (Fig. 48). Even allowing for some uncertainty in the age assignments, what the plot reveals is that around 4.2 Ga the Earth-Moon system passed through a swarm of large asteroids in the space of just a few weeks. Only Humorum’s crater count is anomalous, for whatever reason. Possibly Apollo 14 samples that date to 3.94 Ga derive from Humorum (Snape et al. 2016, Kruijer et al. 2023), in which case Humorum would be later than Serenitatis and the bombardment series ending with Bailly. The initial midpoint of the lunar hemisphere facing the bolides was around 0º (-360), that is, the onslaught first impacted the hemisphere that precisely coincided with the farside. The progression of impacts then continued in the same direction as the Moon’s anticlockwise orbit round the Earth. It stopped about 50 days later, when the midpoint of the facing hemisphere was around 250º – a close match for Genesis’s 40 days with respect to asteroids hitting the Earth. There really was a ‘heavy bombardment’.
As a consequence, the year increased in length by 1.5%, from 360 days to 365, while the lunar month decreased by 1.6%, from 30 days to 29.5.
Some time after the terrestrial mabbul ‘the rain from the heaven was restrained’ – it did not immediately cease. On the Moon, a brief hiatus was followed by another, smaller swarm which excavated the Imbrium, Schrödinger and Orientale basins, again ending abruptly. Dating to c. 4.1 Ga, these basins represent stragglers after the main event. From time to time large asteroids were to hit both the Moon and the Earth again in succeeding ages, but never large enough to excavate basin-size craters, and never again in large numbers.