The Moon was Pummeled Even Harder by Asteroids Than it Looks

The Moon’s pitted surface tells a tale of repeated impacts over a long period of time. While Earth’s active geology erases most evidence of impacts, the Moon has no mechanism that can do the same. So there it sits, stark evidence of an impact-rich past.

The visible record of lunar cratering is used to understand Earth’s formation and history since periods of frequent impacts would affect both bodies similarly. But something’s wrong in our understanding of the Moon’s history. Impact crater dating, asteroid dynamics, lunar samples, impact basin-forming simulations, and lunar evolution modelling all suggest there’s some missing evidence from the Moon’s earliest impacts.

New research says that there were even more large, basin-forming impacts than we think. Scientists think that some of those impacts left crater imprints that are nearly invisible.

The research delves into the Moon’s ancient magma phase. Early in its history, the Moon was a vast, global ocean of magma. Large impacts that occurred as the magma cooled over millions of years may have left their mark. But they wouldn’t resemble imprints from impacts when the Moon was solid.

The new paper is titled “Large impact cratering during lunar magma ocean solidification.” It’s published in the journal Nature Communications. The lead author is Associate Professor Katarina Miljkovic, from Curtin’s School of Earth and Planetary Science and the Space Science and Technology Centre.

Over four billion years ago the lunar magma ocean solidified. Impacts that occurred during that time, as the Moon was cooling, left crater imprints that are nearly invisible.

In a press release, lead author Miljkovic said, “These large impact craters, often referred to as impact basins, formed during the lunar magma ocean solidification more than four billion years ago, should have produced different looking craters, in comparison to those formed later in geologic history.”

It took millions of years for the young molten Moon to cool down. During that time the surface was soft, and obviously, impacts would leave very different imprints than what we see on the surface of the Moon now.

“A very young Moon had formed with a global magma ocean that cooled over millions of years, to form the Moon we see today,” Miljkovic said. “So when asteroids and other bodies hit a softer surface, it wouldn’t have left such severe imprints, meaning there would be little geologic or geophysical evidence that impact had occurred.”

The Tycho crater is one of the Moon’s brightest. It’s relatively young, at about 108 million years old. A ray system of radial streaks of material is visible centred on Tycho. The impacts that occurred when the Moon was a cooling magma ocean would look very different. Image Credit: CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=51289

“The timeframe for the solidification of the lunar magma ocean varies significantly between different studies, but it could have been prolonged enough to experience some of the large impact bombardment history typical for the earliest periods of the solar system evolution,” said Miljkovic.

Researchers aren’t certain when the lunar magma ocean cooled and solidified. Different studies have produced different results. Some studies suggest it cooled within about 10 million years after it formed, some studies say much longer, up to 200 million years. And other research shows that some regions cooled much more slowly, taking up to 500 million years to solidify. “Radiogenic lunar crustal ages span from 4.47~Ga to 4.31~Ga, which falls broadly within this range…” the authors write in their paper.

The researchers think that the partially-solidified Moon would have had a low-viscosity layer between the crust and mantle, kind of like a melt layer. When an asteroid large enough to create an impact basin struck the Moon, the basin would “…be susceptible to immediate and extreme crustal relaxation forming almost unidentifiable topographic and crustal thickness signatures.” Evidence for these impacts may not be detectable, which fits with other evidence showing that the Moon was subjected to more impacts early in the Earth-Moon evolution.

This figure from the study shows averaged profiles of the surface relief and crust-mantle interface for two groups of impact basins. The top panel shows profiles for three ancient impact basins formed when the Moon was still partially molten. The bottom shows two younger impact basins from when the Moon was solid. Note the obvious crater rim profiles in the lower panel, which are missing in the top panel. Image Credit: Milkjovic et al 2021.

Moon rocks gathered during the Apollo program suggest that a number of large, impact-basin forming impacts should have occurred during the Moon’s first 200 million years. Evidence suggests that the cratering record from that period is incomplete. Recent research shows that there could’ve been as many as 200 basin-forming impacts before 4.35 Ga that aren’t accounted for in the crater record.

This figure from the study shows the results of the team’s simulations. The top panel is for a 60 km diameter impactor striking the Moon at 17 km/s and the lower panel is for a 120 km diameter impactor striking the Moon at the same speed. The top is similar to the size of the impact that formed the Orientale or Nectaris basins, while the bottom is similar to the size of the impact that formed the South Pole Aitken Basin. Panels on the left show the profiles created when the Moon has no melt layer, and the panels on the right show profiles created when a melt layer sits between the crust and the mantle. Results are for 3 hours after impacts. Image Credit: Milkjovic et al 2021.

The study shows that many ancient impact basins would be nearly unrecognizable on the Moon. But finding them is important to understanding the Moon’s history, and by extension, the history of Earth’s formation and of the other planets, too. It also shows that many impact basins, including the South Pole Aitken Basin, were formed when the Moon wasn’t fully solidified, and still had a melt layer between the crust and mantle.

“Those basins would have formed with a different topographic and crustal signature in comparison to younger basins, as long as the melt layer was >10?km thick,” the researchers write in their paper.

When compared with younger impact basins formed when the Moon was solid, these ancient basins would have less prominent crustal thickness signatures and “…the topographic signature would not exhibit prominent concentric rings. In fact, the thicker the melt layer and the thinner the crust, the higher the probability that the basin would not even be recognizable in the cratering record at all…” they write.

They end their paper by saying that the number of ancient impact basins is difficult to constrain. They also point out that their work is “…consistent with recent predictions of higher impact fluxes in the Pre-Nectarian epoch than are inferred from the observable lunar cratering record.”

This figure from the study shows radial profiles of the impacts at the end of the simulation. The top is from a 60 km diameter impactor and the bottom is from a 120 km diameter impactor. The differences between simulations with a melt layer and without a melt layer are clear. Image Credit: Milkjovic et al 2021.

Understanding early impacts on the Moon is part of understanding the earliest epochs in the Solar System, and how the planets and the Moon formed. There are differences between theory and evidence when it comes to lunar cratering and the formation of the Moon. “In this research, we set out to explain the discrepancy between theory and observations of the lunar crating record,” Associate Professor Miljkovic said.

“Translating this finding will help future researchers understand the impact that the early Earth could have experienced and how it would have affected our planet’s evolution,” Miljkovic said.

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Evan Gough

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