Four billion years ago, over 500 million years after planets had formed, things were looking tidy. Planets, moons, and the Sun itself had largely cleared the solar system of planetesimal debris, things should have been quiet. But this was when the inner solar system apparently experienced what scientists call the Late Heavy Bombardment. But how do we know that this happened?
Heavy Bombardment in the Inner Solar System
Four billion years ago, the inner solar system appears to have been bombarded with an unexpectedly large number of meteor collisions. And big ones, too. It was a “heavy bombardment” that created the largest impact basins on the Moon, Mars, and Mercury.
The mass of impacting material increased by a factor of about 1000 compared to times shortly before. It was “late” because it happened hundreds of millions of years after the solar system had completed most of its formation through gravitational accretion.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Evidence from Moon Rocks
The first hints of the Late Heavy Bombardment were from the Apollo missions bringing back lunar samples for study on Earth. Many of the samples were of rocks that had been deformed or created by melting during large impacts. These crystalline melt rocks required impacts that were tens to hundreds of kilometers in extent.
The ages of those melt rocks all cluster between about 3.75 and 3.95 billion years ago. This suggested that several large impacts occurred on the Moon over a concentrated time period of 200 million years—geologically, a short amount of time.
Other lunar samples provided ages for some of the large impact basins we see on the near side, like the Imbrium, Nectaris, and Serenitatis basins. The ages of the large impact basins were somewhat more spread out, from about 4.2 billion to about 3.7 billion years old. So, there’s some uncertainty about how to describe the duration of bombardment.
But the evidence is compelling that there were a lot more impacts, and much larger impacts, around this time, compared to earlier or later times.
Evidence from Mars, Mercury, and the Asteroid Belt
We also see evidence for the same period of bombardment on other bodies. On Mars, the giant impact craters Hellas and Argyre in the southern hemisphere are determined to have formed between 3.8 and 4.1 billion years ago.
On Mercury, there’s the Caloris impact basin. Crater counting, where we use the density of craters in a region to determine its age, suggests that Caloris is about 3.9 billion years old as well.
There’s even evidence from the asteroid belt. Some meteorites we have on Earth that came from asteroid parent bodies have features that only form when an impact creates a shockwave. These shockwave features imply a fast relative velocity between the impactor and impactee.
What about the ages of these meteorites with shock features? Some of them have really old ages, between 4.55 to 4.4 billion years old. That’s around the time planets were forming. After 4.4 billion years ago, there seems to have been a lull. But then, at 4 billion years ago, something must have changed, because we have numerous samples that are between 4.0 and 3.4 billion years old. So, the second wave of shocked meteors found on Earth overlaps with other evidence of the Late Heavy Bombardment.
Learn more about a solar system time machine and meteorites.
Nothing on Earth, but All Over the Moon
Unfortunately, Earth and Venus can’t really add much more to this evidence. Earth’s surface is very young because of plate tectonics. Earth has very little crust older than about 3.8 billion years to study for impacts, and most is much younger. Venus experienced a massive resurfacing event sometime between 200 million and 1 billion years ago, leaving no older crust around to study for impacts.
But the evidence for a period of heavy bombardment is strong: large impacts on the terrestrial planets and our Moon, signs of shock impacts in the asteroid belt, and the age distribution of meteors found on Earth. These all suggest that the inner solar system was bombarded with some large meteors around 4 billion years ago.
We don’t know exactly when it started or ended, maybe as early as 4.2 billion years ago, perhaps continuing until 3.5 billion years ago. But a lot of evidence points to a spike in the number of impacts around 4 billion years ago.
Learn more about exploring the Earth-Moon system.
An Imbrium Bias in Lunar Samples?
But this apparent spike could also be a result of a sampling bias. For instance, it is possible that the lunar samples collected during the Apollo missions are all just from one large impact crater on the Moon: Imbrium.
That’s not because all samples were from Imbrium, but because the Imbrium impact was so large, it would have spread ejecta over large distances. And since Imbrium is the youngest of the large impact basins, the ejecta would be on top of everything.
Could it be possible that samples we collected from other locations on the Moon all happen to be ejecta from Imbrium? If so, that would explain why all the ages in the samples cluster around the age of Imbrium.
There may not have been a dramatic increase in impactors. It might have been a more steady stream of impactors from the beginning of the solar system up to about 3.5 billion years ago, rather than a spike centered at 4 billion years ago. The fact that we don’t see a lot of impact melt crystals older than about 4.2 billion years may be simply because older material was more likely to be pulverized by later impacts. Perhaps, virtually all the older evidence has been destroyed.
So, there’s more to learn. The most important thing are more Moon rocks, from a wider variety of locations and different impact basins. If more samples from the Moon resulted in a correction to the ages previously determined for lunar features, it would result in a shift in estimated ages throughout the inner solar system.
The timing of events early in the solar system may be fundamentally changed by better knowledge of lunar impact basins.
Common Questions about the Late Heavy Bombardment
The Late Heavy Bombardment is a term used by scientists to refer to the unusual spike in large meteor impacts in the inner solar system about 4 billion years ago.
The first hints of the Late Heavy Bombardment were from lunar samples brought back by the Apollo missions. The ages of those rocks all cluster between about 3.75 and 3.95 billion years ago. This suggested that several large impacts occurred on the Moon over a concentrated time period of 200 million years.
On Mars, the giant impact craters Hellas and Argyre are the evidence of the Late Heavy Bombardment. It is believed that these craters in the southern hemisphere of the planet were formed between 3.8 and 4.1 billion years ago.