The rock samples brought back from the Moon in December by China’s Chang’e-5 mission were really young.

It’s all relative, of course, but the analysis shows the basalt material – the solidified remnants of a lava flow – to be just two billion years old.

Compare this with the samples returned by the Apollo astronaut missions. They were all over three billion years of age.

The findings are reported in the journal Science.

China’s robotic Chang’e-5 mission was sent to a site on the lunar nearside called Oceanus Procellarum.

It was carefully chosen to add to the sum of knowledge gained from previous sample returns – the last of which was conducted by a Soviet probe in 1976.

Xiaochao Che and colleagues at the Sensitive High Resolution Ion MicroProbe (SHRIMP) Center in Beijing led the Chang’e-5 dating analysis, but worked with a broad international consortium.

The age data they’ve produced is fascinating because it proves volcanism continued on the Moon long after one might have expected such a small body to have cooled down and given up the activity.

Theorists will now be thinking through new ideas for what kind of heat source might have sustained the late-stage behaviour.

It doesn’t appear to have been driven by concentrated radioactive decay because the Chang’e-5 samples don’t contain a lot of the kind of chemical elements associated with this effect.

“One of the other options we discuss in the paper is maybe the Moon was able to stay active longer because of its orbital interactions with Earth,” speculated Dr Katherine Joy, a co-author from the University of Manchester, UK.

“Maybe the Moon wobbled back and forth on its orbit, resulting in what we call tidal heating. So, a bit like the Moon generates ocean tides on Earth, maybe the gravitational effect of the Earth could stretch and flex the Moon to generate frictional melting,” she told BBC News.