Researchers believe they have pinpointed the age of the South Pole Aitken Basin to 4.32 billion years ago.
The Moon, like the Earth, has been bombarded by asteroids and comets since its formation, leaving behind craters of varying sizes. The largest ones are known as basins.
The exact timing and intensity of most of these events, notably the oldest and largest basin on the Moon, have remained unclear to scientists-until now.
A team of scientists, including from the University of Portsmouth, have investigated the age of the formation of the massive South Pole-Aitken (SPA) basin, which is located on the far side of the Moon and stretches more than 2,000 kilometres.
To do this, they analysed a lunar meteorite known as Northwest Africa 2995. It was found in Algeria in 2005 and is what geologists refer to as a regolith breccia, which means it contains fragments of different rock types that were once a lunar soil and have been fused together by the heat and pressure involved in an impact event.
James Darling , Professor of Earth and Planetary Science at Portsmouth's School of the Environment and Life Sciences , said: "By analysing the isotopes of uranium and lead found in a range of mineral and rock fragments within the meteorite, we were able to determine the materials dated back to between 4.32 and 4.33 billion years ago.
"Importantly, evidence from the internal structure of minerals suggests that this is the timing of an extreme event that reset the ages of the sample."
The proposed date it was created is around 120 million years earlier than what is believed to be the most intense period of impact bombardment on the Moon.
The finding, published in Nature Astronomy , provides a clearer picture of the Moon's early impact history.
Dr Joshua Snape , Royal Society University Research Fellow at The University of Manchester, explained: "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.
"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. 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."
The researchers compared their results to data collected by NASA's Lunar Prospector mission, which orbited the Moon studying its surface composition between 1998 and 1999. The comparison revealed many chemical similarities between the meteorite and the rocks within the SPA basin, confirming their link and enabling the new age estimate.
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."
This new understanding opens new avenues for future lunar exploration.
Professor Katherine Joy from The University of Manchester, said: "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."
The research team included The University of Manchester, the Institute of Geology and Geophysics - Chinese Academy of Sciences in Beijing, the Swedish Museum of Natural History in Stockholm, and the University of Portsmouth.
Earlier this year, Professor James Darling from Portsmouth also worked on a study which identified the specific locations from which most of Mars's meteorites originate.
The study, published in Science Advances , traced the 200 martian meteorites to five impact craters within two volcanic regions on the red planet called Tharsis and Elysium. This allows the rocks to be placed into context on the surface of Mars, greatly enhancing understanding of the chronology and volcanism of the planet.
"Both studies have shed new light on the Moon and Mars' evolution", said Professor Darling. "Understanding the origins and ages of meteorites continues to provide exciting insights into the Solar System's past, and unique insights into the early history of the Earth."
