The massive South Pole-Aitken (SPA) basin is one of the Moon’s dominant features, though it’s not visible from Earth. It’s on the lunar far side, and only visible to spacecraft. It’s one of the largest impact features in the Solar System, and there are many outstanding questions about it. What type of impactor created it? Where did the ejected material end up? Is it feasible or worthwhile to explore it?
But the biggest question could be: how old is it?
The SPA basin is about 2500 km (1600 mi) in diameter and between 6.2 and 8.2 km (3.9–5.1 mi) deep. Research shows that it’s the Moon’s oldest impact basin and likely formed between 4.2 and 4.3 billion years ago. That places it in the most intense period of bombardment in the inner Solar System. But there’s debate about the accuracy of that date. A more precise measurement would help scientists understand the history of the Solar System and the periods of bombardment that helped shape it.
Researchers at the University of Manchester and other institutions tackled the problem of the SPA’s age. Their results are in a paper in Nature Astronomy titled “Evidence of a 4.33 billion year age for the Moon’s South Pole–Aitken basin.” The lead author is Professor Katherine Joy from The University of Manchester.
“The implications of our findings reach far beyond the Moon. We know that the Earth and the Moon likely experienced similar impacts during their early history, but rock records from the Earth have been lost.”
Co-author Dr. Romain Tartese, University of Manchester
Whatever struck the Moon, the impact was catastrophic. Some estimates suggest the impactor was 200 km in diameter, far more massive than the 10 km Chicxulub impactor that ended the dinosaurs. This massive, energetic impact represents a key event in the inner Solar System’s history.
“Determining the timing of this catastrophic event is key to understanding the onset of the lunar basin-forming epoch, with implications for understanding the impact bombardment history of the inner Solar System,” the researchers write. “Despite this, the formation age of the SPA basin remains poorly constrained.”
The inner Solar System bodies have been pummelled by comets and asteroids. On Earth, the evidence of these impacts is mostly wiped away by billions of years of plate tectonics and weathering. There’s only faint evidence of most impacts. The Vredevort impact crater in South Africa was created by a massive impactor about two billion years ago. It’s so eroded that scientists aren’t certain how large the original impact structure was.
Since Earth’s impact features are incomplete, scientists study the lunar surface to understand both the Earth and the Moon’s bombardment history. Fortunately, some evidence from the lunar surface has made it to Earth in the form of samples collected by landers. Some serendipitous evidence also comes in the form of meteorites.
Study co-author Dr. Romain Tartese, Senior Lecturer at The University of Manchester, said, “The implications of our findings reach far beyond the Moon. We know that the Earth and the Moon likely experienced similar impacts during their early history, but rock records from the Earth have been lost. We can use what we have learnt about the Moon to provide us with clues about the conditions on Earth during the same period of time.”
When a large impactor travelling quickly strikes a rocky planet or moon, it releases a lot of energy. The impact can spread debris around the surface and even launch some into space. Scientists have studied multiple meteorites that came from lunar and Mars impacts, and they’ve learned a lot by studying them. In fact, there are so many of them that they’ve been able to categorize many meteorites according to their asteroidal parent bodies.
At least one piece of debris from the impact reached Earth: a lunar meteorite named Northwest Africa 2995.
Over the years, different researchers have examined NWA 2995. By comparing it to Apollo samples, they’ve found that it has the same oxygen isotope ratios, which points to a shared lunar origin. The meteorite’s minerals and texture are also very similar to crustal rocks from the lunar highlands.
The researchers write that the meteorite is in “good agreement with lithologies exposed within the southern region of the SPA basin.”
NWA 2995 was found in Algeria in 2005 and it hasn’t been on Earth for long. It’s only been here for a few thousand years, and by analyzing the concentration of certain cosmogenic nuclides, which are atoms produced by exposure to cosmic rays, scientists have determined that the rock has only been travelling in space for about 22 million years. So, though it was initially created in an ancient impact, it was only launched into space much later by a subsequent impact. MWA 2995 is relatively unchanged and can provide insights into the early Solar System.
NWA 2995 is what scientists call regolith breccia. Regolith is the layer of unconsolidated rocky material that covers bedrock. Breccia is a rock formed from angular fragments of rocks and minerals that are cemented together by fine-grained material. According to the authors, NWA 2995 represents an “ancient fused lunar soil, made up of many different rock and mineral components. ”
The researchers examined NWA 2995 to constrain the age of the SPA basin. They used radiometric dating on a range of mineral and rock components of the meteorite to find NWA 2995’s age.
The researchers also compared NWA 2995 with orbital data from NASA’s Lunar Prospector, which used a low polar orbit to map the Moon’s surface composition. They created a map showing the probabilities that the meteorite originated in different regions on the Moon.
They found that the meteorite most likely came from one of two locations, both inside the SPA. The nearby Cabannes craters are all the right size to eject a rock like NWA 2995.
The researchers analyzed the ages of uranium and lead in NWA 2995. Overall, the results indicate that the SPA basin formed about 4.32–4.33 billion years ago. That means that it formed about 120 million years before the main cluster of other lunar basins like the Serenitatis, Nectaris, and Crisium basins.
Dr Joshua Snape, Royal Society University Research Fellow at The University of Manchester, is one of the co-authors of the new research. “Over many years, scientists across the globe have been studying rocks collected during the Apollo, Luna, and Chang’e 5 missions, as well as lunar meteorites, and have built up a picture of when these impact events occurred,” Snape said.
“For several decades there has been general agreement that the most intense period of impact bombardment was concentrated between 4.2-3.8 billion years ago – in the first half a billion years of the Moon’s history,” said Snape. “But now, constraining the age of the South-Pole Aitken basin to 120 million years earlier weakens the argument for this narrow period of impact bombardment on the Moon and instead indicates there was a more gradual process of impacts over a longer period.”
These results will only grow stronger when future missions collect more samples from the area. “The proposed ancient 4.32 billion year old age of the South Pole-Aiken basin now needs to be tested by sample return missions collecting rocks from known localities within the crater itself,” said lead author Joy.
“Our proposed formation age for SPA will require confirmation from future radiometric dating of samples collected from the south of the Apollo basin area by the Chang’e–6 mission or from future proposed missions such as the Endurance-A rover concept that aims to collect 100?kg of samples from across the SPA basin floor,” the authors write in their conclusion.