Giant Asteroid Impacts Formed, As Well As Destroyed, The Moon’s Crust

Fifty years after the Apollo missions, we are still learning from the material they brought back. The latest discovery suggests one Moon rock was formed at temperatures above 2,300ºC (4,300ºF), a temperature almost certainly produced by a large asteroid impact. The discovery marks the first time we have seen evidence that asteroids helped crusts form on the Moon and inner planets, rather than being purely destructive.

Humans walked on the Moon six times, partly because NASA hoped that by visiting more locations they would find different geology from that at the first site in the Sea of Tranquility. Lunar troctolite 76535, returned by the final Apollo mission, justifies this decision by containing a tiny grain of a mineral known as baddeleyite.

“By first looking at this rock, I was amazed by how differently the minerals look compared to other Apollo 17 samples,” said Dr Ana Cernok, from the Royal Ontario Museum (ROM), in a statement. “Although smaller than a millimetre, the baddeleyite grain that caught our attention was the largest one I have ever seen in Apollo samples.”

More importantly, Cemok was able to identify structures within the baddelyeite that reveal it was once cubic zirconia. Best known for its resemblance to diamonds, cubic zirconia requires immense temperatures to form. In Nature Astronomy, Cemok and colleagues propose an asteroid impact occurred.

The grain was measured to be 4.328 billion years old (with a remarkable precision of plus or minus 8 million years), indicating it dates back almost to the Moon’s beginnings. The authors think it was formed 10-30 kilometers (6-18 miles) beneath the lunar surface near the South Pole and then thrown more than half way across the satellite to land at the Apollo 17 site.

The Moon’s surface bears testimony to the more recent asteroid strikes it has experienced, and we know such events were more common, and often larger, in the Solar System’s first few hundred million years. Previously, however, there was no evidence to suggest such impacts helped build lunar crust, rather than simply destroying what formed out of the magma ocean.

“Rocks on Earth are constantly being recycled, but the Moon doesn’t exhibit plate tectonics or volcanism, allowing older rocks to be preserved,” the ROM’s Dr Lee White said. “By studying the Moon, we can better understand the earliest history of our planet. If large, super-heated impacts were creating rocks on the Moon, the same process was probably happening here on Earth.”

The paper notes that Lunar troctolite 7653 is “one of the most intensively studied samples from the Moon.” Yet, Cemok was the first to notice the grain and recognize its significance.