The Moon's pitted surface is a story of repeated impacts over a long time. Although Earth's active geology can erase most evidence of impacts over time, the Moon does not have any mechanism that can. It is a stark reminder of an impact-rich history.
Because both Earth and Moon would be affected equally by frequent impacts, the visible record of lunar-cratering cratering can be used to help us understand Earth's history and formation. There is something wrong with our knowledge of the Moon's history. There is some evidence missing from the Moon's early impacts, according to lunar evolution modelling, asteroid dynamics and lunar samples.
Recent research suggests that the impact on basin formation was even greater than we thought. Scientists believe that some of these impacts left crater imprints almost invisible.
This research focuses on the Moon's ancient magma phases. The Moon's early history was marked by a large, global ocean of magma. The magma cooling over millions of years left traces from large impacts. They wouldn't look like imprints left by impacts that occurred when the Moon was still solid.
The paper, Large impact cratering in lunar magma ocean sedimentification is published in Nature Communications. Associate Professor Katarina Miljkovic from Curtins School of Earth and Planetary Science and Space Science and Technology Centre is the lead author.
The lunar magma ocean was formed over four billion years ago. As the Moon was cooling, impacts caused by that time left crater imprints almost invisible.
Miljkovic, the lead author, stated in a press release that these large impact craters (often referred to simply as impact basins) formed during the lunar magma sea solidification over four billion years ago. They should have produced craters with a different appearance to those created later in geologic time.
The cooling down of the young, molten Moon took many millions of years. The Moon's surface was very soft during that period. Impacts would have left different imprints on it than we see today.
Miljkovic stated that the Moon was formed from a very young Moon with a global magma sea. This ocean cooled over millions years to create the Moon we see today. Asteroids and other bodies would not have left so many imprints on a soft surface. This means that there would have been little geophysical or geologic evidence of impact.
One of the brightest moons is Tycho crater. It is only 108 million years young. Tycho is the centre of a ray system that radially streaks material. It would be very different to see the impacts that took place when the Moon was a cooling Magma Ocean. Image Credit: CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=51289
Miljkovic said that although the time required for solidification of lunar magma ocean differs between studies, it could have been extended enough to witness some of the impact bombardment history typical of the early periods of solar system evolution.
Researchers don't know when the lunar magma sea cooled or solidified. Different studies yielded different results. It cooled in 10 million years, according to some studies. Other studies claim it took 200 million years. Other research has shown that certain regions cool much slower, and can take up to 500 millions years to solidify. Radiogenic lunar crustal ages range from 4.47Ga up to 4.31Ga. This is broadly in line with the range that the authors describe in their paper.
Researchers believe that the semi-solidified Moon had a low viscosity layer between crust and mantle. This layer would have been similar to a melt layer. A large enough asteroid to cause an impact basin on the Moon would have caused it to be vulnerable to extreme crustal relaxation, which could explain why other evidence has shown that the Moon was more frequently subject to impacts earlier in its Earth-Moon evolution.
The study's average profiles of the crust-mantle and surface relief for two types of impact basins are shown in this figure. The top panel displays profiles of three impact basins that were formed while the Moon was still partially solid. Two younger impact basins are shown at the bottom, which were formed when the Moon was solid. The bottom panel shows two younger impact basins that were formed when the Moon was solid. Image Credit: Milkjovic et al 2021.
The Apollo program collected Moon rocks that suggest large impact-basin forming impacts may have occurred in the Moon's first 200 million year. There is evidence that the cratering records from this period are incomplete. Recent research has shown that there may have been 200 basin-forming impacts that occurred before 4.35 Ga, which are not included in the crater records.
The study's figure shows the results from simulations by the teams. The top panel shows a 60km diameter impactor hitting the Moon at 17 km/s. The lower panel shows a 120km diameter impactor striking at the Moon at the same speed. The top panel is comparable to the impact that created the Orientale and Nectaris basins. While the bottom panel is similar to that of the South Pole Aitken Basin impact. The panels to the left show profiles when the Moon does not have a melt layer. While the panels to the right show profiles when there is a melt layer between the crust, mantle and the crust. These results are valid for three hours after impacts. Image Credit: Milkjovic et al 2021.
Study shows that many of the Moon's ancient impact basins are almost unrecognizable. However, it is crucial to understand the Moon's history and, by extension, Earth's and other planets' history. It also shows how many impact basins, such as the South Pole Aitken Basin were formed while the Moon was still in its liquid state.
Researchers write that these basins could have had a different topographic or crustal signature than younger basins as long as the melt layer was at least 10 km thick.
These basins, which were formed before the Moon was solid, would show less prominent crustal thickness signatures than younger impact basins. The topographic signature would also not have prominent concentric rings. The likelihood that the basin will not be recognized in any cratering records is higher if the melt layer is thicker and the crust is thinner.
The authors conclude their paper by stating that it is hard to determine the number of ancient impacts basins. Their work also supports recent predictions of greater impact fluxes during the Pre-Nectarian period than can be inferred from the lunar cratering records.
The study's figure shows the radial profiles for the impacts at the simulation's end. The 60 km impactor is at the top and 120 km impactor at the bottom. Clear differences exist between simulations that include a melt layer and those without. Image Credit: Milkjovic et al 2021.
Understanding the Moon's early impacts is an important part of understanding how the Solar System formed. When it comes to the formation of the Moon and lunar cratering, there are many differences between theories and evidence. We set out to investigate the discrepancy in theory and observation of the lunar crating records, Associate Professor Miljkovic stated.
Miljkovic stated that translating this finding will allow future researchers to understand the effects of the early Earth's impact and how they would have affected the evolution of our planets.
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