Late Heavy Bombardment: Asteroid Storm on the Early Earth

The Late Heavy Bombardment is a proposed period, about 4.1 to 3.8 billion years ago, when the Moon, Earth and the other inner planets were battered by an unusually intense storm of asteroid and comet impacts. The idea arose in the 1970s from the ages of Moon rocks brought back by the Apollo missions and was later linked to the migration of the giant planets. It remains influential, but many scientists now doubt that a sudden “cataclysm” took place, rather than a gradual decline in impacts.

Background: a scarred Moon

The Moon’s surface is covered in craters, from tiny pits to huge basins hundreds of kilometres wide. Because the Moon has no plate tectonics, weather or liquid water, these scars have survived for billions of years. Earth must have been hit at least as often, but its active surface has erased almost all of the evidence.

Astronomers had long known that the bombardment rate must have been much higher in the early Solar System, when debris from planet formation was plentiful. The question was how that rate changed over time.

The Apollo evidence

Between 1969 and 1972 the Apollo missions collected rocks from six sites on the lunar near side. Many were impact melt rocks and breccias, rocks melted or shattered by collisions. Dating them with radioactive isotopes, especially the argon-argon and uranium-lead methods, gave a striking result: many impact ages clustered between about 3.8 and 4.0 billion years.

In 1974 Fouad Tera, Dimitri Papanastassiou and Gerald Wasserburg of the California Institute of Technology argued that this pointed to a “terminal lunar cataclysm”, a short burst of intense impacts around 3.9 billion years ago. The geologist Graham Ryder later became one of the hypothesis’s strongest advocates.

Support came from lunar meteorites, pieces of the Moon blasted off by impacts and found on Earth. Because they come from random places on the Moon, they offered an independent sample. In 2000 Barbara Cohen, Timothy Swindle and David Kring reported that impact melts in lunar meteorites also showed no ages older than about 3.9 billion years, which they took as support for a cataclysm.

The great lunar basins

Some of the Moon’s largest features may date from this period. The Imbrium basin, whose lava-filled floor forms the dark “Sea of Showers” visible from Earth, is usually dated to about 3.9 billion years ago. The Orientale basin, a spectacular multi-ringed structure on the Moon’s western edge, is one of the youngest large basins, at roughly 3.8 billion years.

The Moon has more than 40 recognised basins wider than about 300 kilometres. If many formed within a short window, the impact rate at that time would have been far higher than in the preceding few hundred million years.

Evidence beyond the Moon

Other worlds preserve signs of heavy early bombardment too. Mercury is covered in ancient craters, and its largest well-preserved basin, Caloris, is commonly dated to roughly 3.9 to 3.8 billion years ago from crater counts. The southern highlands of Mars are densely cratered and include giant basins such as Hellas, generally thought to be about 4 billion years old.

Some meteorites from the asteroid belt, including pieces thought to come from the large asteroid Vesta, carry impact-reset ages between about 4.1 and 3.5 billion years. These clues suggest that the whole inner Solar System experienced intense impacts in this era. However, ages from crater counts depend on assumptions about impact rates, so they cannot by themselves prove a sudden spike.

A cause: moving giant planets

For decades, the cataclysm lacked a clear cause. Why would impacts surge hundreds of millions of years after the planets formed?

In 2005 a team of astronomers, Rodney Gomes, Harold Levison, Kleomenis Tsiganis and Alessandro Morbidelli, published the “Nice model”, named after the French city where it was developed. In their simulations, the giant planets formed in a more compact arrangement. Slowly, interactions with a disc of icy bodies pushed Jupiter and Saturn into a configuration where Saturn completed one orbit for every two of Jupiter’s. This destabilised the system: Uranus and Neptune were flung outwards, and the icy disc and parts of the asteroid belt were scattered across the Solar System.

The model seemed to explain the timing of the bombardment as well as features of today’s Solar System, such as the orbits of the outer planets and the population of Jupiter’s Trojan asteroids. It became one of the most influential ideas in planetary science.

Doubts and revisions

Since the 2010s, the cataclysm has come under sustained criticism.

• Sampling bias. All six Apollo sites lie on the near side, many relatively close to the Imbrium basin. Debris from that single huge impact may have contaminated samples across the region, making one event look like many.
• Resetting of ages. In 2016 Patrick Boehnke and Mark Harrison argued that repeated heating by later impacts can partially reset argon ages, creating an artificial cluster around 3.9 billion years even if impacts declined steadily.
• Older basins. Some studies suggest basins such as Nectaris may be older than once thought, stretching the bombardment over a longer period.
• Earlier instability. Refinements of the Nice model suggest that the giant planet instability could have happened much earlier, perhaps within the first 100 million years of Solar System history, too early to cause a spike at 3.9 billion years.

Many scientists now favour a picture in which impacts declined from a high early rate, possibly with smaller bumps, rather than a single late cataclysm. Others think a modest late surge remains possible. Samples from older basins, such as the vast South Pole–Aitken basin on the far side, could help settle the question.

Effects on the early Earth

Whatever its exact shape, the bombardment would have had dramatic effects on Earth. The planet, being larger, would have received many more impacts than the Moon, including some by bodies tens to hundreds of kilometres across. The largest could have melted parts of the crust and boiled away the upper layers of the oceans.

Even so, studies such as one by Oleg Abramov and Stephen Mojzsis in 2009 concluded that the bombardment would not have sterilised the whole planet. Microbes living deep underground could have survived. Some researchers have even suggested that impacts created hydrothermal systems that might have favoured the origin of life on Earth. Layers of impact spherules, tiny glassy beads, in rocks from South Africa and Western Australia show that large impacts continued on Earth until at least about 3.2 to 2.5 billion years ago.

Timeline

• c. 4,510–4,450 million years ago: The giant impact that formed the Moon.
• c. 4,100–3,800 million years ago: Proposed window of the Late Heavy Bombardment.
• c. 3,900 million years ago: The Imbrium basin forms on the Moon.
• c. 3,800 million years ago: The Orientale basin forms; impact rates fall towards modern levels.
• AD 1969–1972: Apollo missions return lunar samples.
• AD 1974: Tera, Papanastassiou and Wasserburg propose a terminal lunar cataclysm.
• AD 2000: Lunar meteorite ages are reported in support of the cataclysm.
• AD 2005: The Nice model links the bombardment to giant planet migration.
• AD 2016: Boehnke and Harrison argue that the age cluster may be an artefact.

Why it matters

The Late Heavy Bombardment shaped the face of the Moon and set the conditions on the young Earth during the period when life may have begun. The debate over it also shows how scientific ideas change as new samples, better dating and improved computer models test earlier conclusions about the formation of the Solar System.