Our Moon’s Earliest Impact History Is Still A Puzzle

From Earth, our inner solar system seems to work like clockwork. Mercury, Venus, our Earth-Moon system, and Mars all orbit the Sun in very orderly fashion. But it wasn’t always so.

Our very early solar system was loaded with planetary building blocks (planetesimals) that ricocheted around like frenetic billiard balls. The Moon itself is the product of a Mars-sized object’s cataclysmic collision with Earth during the solar system’s first 100 million years. The resulting orbital debris coalesced into our anomalously large natural satellite, some 4.4 billion years ago. To say we’re lucky to be here to talk about it is an understatement.

Most early bombardment of the Earth, Moon, and inner solar system planets came from planetesimals that were left over from terrestrial planet formation, planetary scientist William Bottke, co-author of a comprehensive recent study of early planetary bombardment published in the journal Icarus, told me. The bombardment declined as time proceeded, with a long tail responsible for large relatively late impacts on the Earth and Moon, Bottke, a researcher at the Southwest Research Institute, told me.

Earth was hit by very large impacts for at least a billion years of its early history. Yet the early Moon is missing some predicted bombardment.

For this model to work, some process must have eliminated the earliest history of the Moon, says Bottke. That means the craters and basins there either do not go back to the Moon’s origin, or the Moon formed later than many argue, he says.

The team developed an accurate dynamical model that accounts for all major sources of impactors in the inner solar system (leftover planetesimals, asteroids and comets), the authors note. Each impactor population was calibrated from independent means (not using crater records), they note.

The leftover planetesimals produced most lunar impacts within the first 1.1 billion years of our solar system, with roughly 50 known lunar basins formed between 4.36 to 4.41 billion years ago, the authors note.

What’s Different About This Study?

We took one of the most advanced numerical models that simulate the formation of Earth and used it to infer what the impact history of the Earth-Moon would be, throughout the evolution of the solar system, Simone Marchi, a co-author and planetary scientist at the Southwest Research Institute, told me via email.

These levels of detailed calculations were never done before in a coherent, single simulation, says Marchi. Previous models did not adequately explain some important observational constraints, such as the number of large impact basins on the Moon, nor their approximate formation age, he says.

How did the team run their simulations?

To get enough statistics for our model runs, we used the NASA Pleiades Supercomputer, which really made the difference in being able to do this project, says Bottke.

Would other planetary systems have experienced the same type of early intense bombardment?

The question is timescale, says Bottke. Did early bombardment prevent the rise of complex life on Earth for an extended time, or he wonders if in some way if it helped by creating interesting niches for habitats?

But impacts produce reactive gases, and they consume the low levels of oxygen in the atmosphere that existed at early times, says Bottke. That means it was possible oxygen could not really rise on Earth until this early bombardment was more or less complete, he says.

Bottke Still Remains Optimistic About Life Arising Elsewhere

It seems likely that other solar systems had periods of early bombardment during and after their epochs of planet formation, says Bottke. The fact that our system has life after such a bombardment, however, means it is possible for life to emerge elsewhere, he says.

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