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An unassuming rock collected from the surface of the moon over 50 years ago by the Apollo 17 astronauts Gene Cernan and Harrison Schmitt could completely alter what we thought we knew about the early days of the moon and, by extension, the solar system.
The rock sample, catalogued simply as 76535, has a chemistry and texture that indicates it formed deep down in the moon's crust, nearly 31 miles ( 50 kilometers) underground. Moreover, radioisotope dating places it as having been on the lunar surface for 4.25 billion years.
Liberating rocks so deep takes the most enormous of impacts. The assumption had been that the impact that gouged out the South Pole–Aitken Basin, which is the largest impact site on the moon, had dug out rock 76535, especially since their ages approximately match.
However, new simulations show that rock 76535 probably formed beneath the ground where it was found at Apollo 17's landing site in the Taurus–Littrow Valley on the eastern flank of Mare Serenitatis (Sea of Serenity).
There has always been an air of doubt over the claim that the South Pole–Aitken basin was the point of origin for rock 76535. After all, the rock displays no evidence for a violent past, yet the South Pole–Aitken basin and Mare Serenitatis are practically on opposing sides of the moon. It seems inconceivable that a rock could be gouged out of the ground and sent from the southern far-side to the northern near-side without the rock displaying evidence of shock heating, scarring and other symptoms of having a giant asteroid crash on its head, ferociously excavate it from the ground and fling it into a different hemisphere.
Now, detailed computer simulations of giant impacts on the moon, led by Evan Bjonnes of the Lawrence Livermore National Laboratory in California, show exactly how rock 76535 could have literally arisen in Mare Serenitatis.
"We sought a simpler, local explanation," said Bjonnes in a statement. "And the models kept showing the same thing — big impacts can lift deep rocks to the surface without over-shocking them."
The simulations indicate that during the latter stages of an impact like the one that formed Mare Serenitatis, a newly formed crater floor can undergo collapse as the super-heated crust allows material to flow more freely. In the simulations, as the floor collapses, to make room for it up to 33,588 cubic miles (140,000 cubic kilometers) of material can be pushed up to the surface more gently than if it were simply gouged out. This would explain the lack of scarring or shock-heating on rock 76535 — it had simply risen to the surface through the liquified crust in the immediate aftermath of the impact that formed Mare Serenitatis.
That's a neat discovery, but the repercussions could span the solar system. As Bjonnes puts it: "This rock may be small, but it carries a huge story about the moon's early history."
If rock 76535 was excavated 4.25 billion years ago, that means that Mare Serenitatis must have formed 4.25 billion years ago — but that is 300 million years earlier than what lunar geologists had thought based on other lines of evidence such as crater counts.
If the Mare Serenitatis basin is older, then perhaps other lunar impact basins are also older than we had calculated. Because the airless surface of the moon is often used to calibrate impact rates in the early solar system — because similar impacts on Venus, Earth and even Mars have weathered away — any change in the timeline of impact events on the moon will affect the timeline in the rest of the solar system too.
"By pushing Serenitatis back in time, we're shifting the entire timeline of when big impacts happened across the solar system," said Bjonnes. "That has a ripple effect for understanding Earth’s early environment too."
With astronauts soon to be heading back to the moon there is an ideal opportunity to prove these findings, since the same processes must have happened to other lunar maria, and they too might have rocks on the surface like 76535 that astronauts could bring back to Earth for more detailed investigation.
The findings were published on Sept. 18 in the journal Geophysical Research Letters.
