Internal geological processes on the moon are almost non-existent. However, when it gets smacked by a space rock, its surface can change dramatically. Debris from that impact can also travel over large distances, transplanting material from one impact site hundreds of kilometers away, where it can remain untouched in its inert environment for billions of years.
So when Apollo 17 astronauts took regolith samples at their landing site near Serenitatis Basin, they collected not only rocks from the basin itself, but from other impacts that had happened billions of years ago. Differentiating material that actually formed part of the Basin from material that landed their after an impact has proven difficult.
One nearby impact in particular caused problems – material from the impact that created the Imbrium basin made up the majority of samples taken by the Apollo 17 astronauts. Located slightly to the northwest of Serenitatis, this basin was caused by a much larger impact, which also happened much more recently than the one that created Serenitatis.
Despite that age difference, it is hard to differentiate rocks from one basin or another just by looking at them. A particular rock did stand out though – known as the Station 8 boulder after the geological station it was found next to, it did form as part of the Serenitatis basin rather than its younger neighbor. It also surprised scientists with its age.
Previous estimates of the age of the basin put it at 3.8-3.9 billion years. However, analysis of the phosphate materials in the sample returned from the Station 8 boulder show its age to be closer to 4.2 billion years. That would make it one of the oldest craters on the moon, having formed only approximately 300 million years after the moon itself.
With plenty of manned moon missions on the horizon, this certainly won’t be the last time samples will be gathered from the basin. And the techniques the scientists, led by a team at the Open University, used are applicable to other missions such as the sample return mission currently on its way back from Bennu. Maybe in the future a crater will be found that’s even older than Serenitatis – but for now, it looks like we already have a sample of some of the oldest rocks possible from the moon.
The Open University – Lunar samples record impact 4.2 billion years ago that may have formed one of the oldest craters on the Moon
NASA – NASA Opens Previously Unopened Apollo Sample Ahead of Artemis Missions
UT – NASA Has a New Challenge to Bring Frozen Samples of the Moon Back to Earth
Image of the moon highlighted with the two basins mentioned in the article. Serenitatis is shown with the Apollo 17 landing site demarcated.