What is the Late Heavy Bombardment?

A pale planet seen from a distance is bombarded by meteorites, causing explosions on its surface. Other rocky objects float by the planet.

Asteroids sent hurling at the inner solar system by a shift in the orbits of the giant planets strike a young planet in this artist's concept.

NASA's Goddard Space Flight Center Conceptual Image Lab

Around 4 billion years ago, our young inner solar system underwent a cataclysmic pummeling by asteroids that carved huge basins into Earth’s Moon. That’s the theory of the Late Heavy Bombardment, which posits that a sudden change in the orbits of the giant planets ― Jupiter, Saturn, Uranus, and Neptune ― threw the asteroid belt into disarray and sent those leftover pieces of the solar system’s formation crashing into the inner planets.

Among the evidence for this period, thought to have lasted anywhere between 20 and 200 million years, are the scarred surfaces of the Moon, Mercury and Venus, the distribution of asteroids in the asteroid belt, and the dates of impact melt in Moon rocks from the Apollo missions.

When astronauts visited the Moon, they brought back rock samples containing impact melt, a type of rock that only forms when material melts instantaneously upon being struck by an impactor. Scientists found that many of the samples, despite coming from different locations on the near side of the Moon, had similar ages of roughly 4 billion years, with very few samples showing older impacts. This potentially indicated a period of increased and dramatic collisions in the solar system.

Lunar probes have discovered that the Moon's gravitational field is uneven. A handful of impact basins exhibit unexpectedly strong gravitational pull, possibly due to an excess concentration of mass resulting from dense material being drawn from the lunar mantle toward the surface after an impact. These mass concentration areas or "mascons" are thought to have formed during the Late Heavy Bombardment. This image shows variations in lunar gravity (measured in the unit of acceleration called miliGals [mGal]). Red corresponds to mass excesses and blue corresponds to mass deficiencies.

NASA/JPL-Caltech/MIT/GSFC

Craters of various sizes dot the landscape of Mercury

Like the Moon, Mercury's surface is dotted with craters.

NASA/JPL-Caltech/University of Arizona

The idea was and remains fairly controversial. Some argued that the samples came from too small an area (only about 4 percent of the Moon) to be conclusive. Perhaps they were all affected by the same impact or a few impacts. But samples of lunar meteorites that fell to Earth were found to follow the same pattern.

On Earth, most evidence of even such an intense bombardment would have been destroyed by erosion and plate tectonics, but signs like geological layers of small beads called spherules, created when droplets of molten and vaporized rock condensed and fell back to Earth, have been discovered that date back to about 3.5 billion years.

The Late Heavy Bombardment isn’t a done deal. As more studies were done, scientists realized that lunar basins were extremely difficult to date and could be contaminated with material from the impacts that formed other basins. More precise measurements of the Apollo samples over time found tiny portions of rock that were older than 4 billion years. Scientists argued that the timelines for the way the bombardment would have affected Earth didn’t match up with other studies and theories about when water and life took hold on the planet.

The debate continues today, with scientists proposing that the Late Heavy Bombardment was more gradual, broken into segments, or that such a spike didn’t occur at all. A decisive answer could be provided by future lunar missions that return samples from other locations on the Moon, especially the large basins such as the South-Pole Aitken basin. Such samples could show that the evidence of impacts after 4 billion years ― but not before ― is consistent over the lunar surface. Or they could show that the Late Heavy Bombardment was no more than an illusion caused by incomplete data.

A mostly round asteroid Vesta with large craters visible.

Researchers are divided over whether the asteroid Vesta, a remnant from the early solar system, shows evidence for or against the Late Heavy Bombardment.

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

An illustration of meteorites bombarding a landscape. The ground is bare rock, molten in places and smoking. Pools of molten rock and fires dot the terrain. Three meteorites streak through a cloudy, smoky sky.

Meteorites bombard a molten landscape in this illustration of the Late Heavy Bombardment.

NASA's Goddard Space Flight Center Conceptual Image Lab

Writer: Tracy Vogel; Science Advisors: Daniel P. Moriarty (University of Maryland at College Park), Natalie M. Curran (NASA's Goddard Space Flight Center)

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