Back when life on Earth was no more than algae living in the oceans, thick lava gushed from long, gaping fissures on the Moon’s surface, flooding the surrounding plains.
- The Chinese Chang’e 5 mission brought Moon rocks and soil to Earth in December
- Geochemists analysed two fragments and calculated they were formed from lava 2 billion years ago
- The study will help calibrate a crater-counting technique used for dating planetary surfaces across the solar system
Last year, the Chinese Chang’e 5 lander scooped up some of that ancient lava flow — now solidified basalt — and brought it to Earth.
In a study published in Science, researchers dated two precious fragments of that basalt and pinned a date on one of the Moon’s last volcanic eruptions: 2 billion years ago.
Previous Moon missions retrieved plenty of lunar soil and rocks, but nothing was younger than around 3.1 billion years old, said Alex Nemchin, a geochemist at Curtin University and a co-author of the study.
“Now it seems we can at least start to fill in that gap with this young basalt.”
In doing so, the study will help planetary scientists more accurately calculate the age of surfaces on not just the Moon, but also moons and planets further afield, said Alex McCoy-West, a geochemist at James Cook University who was not involved in the study.
Why we needed fresh Moon rocks
China’s sample return mission — the fifth in the Chang’e series — blasted off in late November 2020.
Its main task was to drill a 2-metre core of rock, scoop up soil and rock from the surface, then sling the lot to Earth in December.
That payload of rock and debris was the first fresh lunar material brought to Earth since the US Apollo missions of the 1970s.
Those earlier missions brought back basalt that was dated to around 3.6 and 3.1 billion years ago.
The problem, though, was researchers didn’t have any “young” lunar rock to directly date.
The Chang’e 5 mission aimed to change that.
Where the spacecraft touched down was no accident.
It was carefully guided to land on and collect samples from a young, smooth plain called Oceanus Procellarum, or “Ocean of Storms”.
Oceanus Procellarum — and the rest of the Moon’s surface — is made of grey rock called basalt.
It was produced back when the Moon was more dynamic, heated by its tidal tug with Earth, and was able to sustain volcanic eruptions.
Moon regions with more craters are older
Generally, planetary scientists can tell when a patch of the Moon was last flooded by lava — and, thus, its age — by counting its craters.
A heavily pockmarked area is older than a smooth one, simply because it’s had more time to accumulate craters.
And while planetary scientists knew Oceanus Procellarum was younger than the Apollo landing sites, because it had fewer craters, the question remained: how old was it exactly?
Different crater-based calculations varied from 1.2 to 3.3 billion years old.
The same logic is used to date regions on other solid bodies in the solar system, Professor Nemchin said.
“Then, of course, we can apply that idea to everything else, like Mars and Mercury and so on.”
How do we work out the actual age of Moon rocks?
Professor Nemchin and his fellow researchers received two fragments of basalt that were scraped from the Moon’s surface.
Each tiny nugget was only 3 or 4 millimetres wide, no bigger than a piece of gravel.
Instruments in a lab in China measured minute amounts of lead isotopes in the basalt, and used lead’s rate of radioactive decay to calculate how old basalt samples were.
The answer? 1.96 billion years, give or take 57 million years.
Dr McCoy-West said this technique — known as lead-lead dating — is the “gold standard” for objects such as Moon rocks.
“We use lead-lead dating to date the formation of the solar system, the oldest meteorites we’ve ever found, the oldest zircons on Earth, the oldest minerals on Earth,” he said.
What was the Moon like 2 billion years ago?
So what did Oceanus Procellarum look like?
Instead of mega explosive eruptions, the volcanic activity would have been more like a flood, Professor Nemchin said.
“It happened on Earth in the past. We can see big floods like Colombia River basalts [of the north-west US] and parts of India.
“On the Moon, in this particular case, we can see these cracks, several hundred kilometres long, on the surface from images,” Professor Nemchin said.
“It’s difficult to make an estimate, but we think within a few million years, a huge amount of lava flooded [Oceanus Procellarum], covering around 70,000 square kilometres to a thickness of about 50 metres.”
Meanwhile, down here on Earth, things looked a lot different too.
Earth was just 2.5 billion years old.
Life had evolved to the point of single-celled organisms, with the oceans home to bacteria that looked a lot like blue-green algae, but not much else, Dr McCoy-West said.
“Modern-day plate tectonics would have been operating by then.
“But the oceans were pretty much barren, apart from bacteria. It was still a very empty planet.”
Why some Moon samples won’t be analysed — yet
All samples that hitched a ride to Earth on Chang’e 5’s return module have, so far, remained in China, Professor Nemchin said.
But in the coming months, he expects labs around the world will be able to apply for fragments to analyse themselves.
And a fair portion of the Chang’e 5 samples won’t be analysed at all for now, but will be tucked away for future research, when new and more sensitive techniques can glean even more information from them.
This is pretty standard practice, dating back to the first crewed missions to the Moon.
Despite bringing back hundreds of kilograms of Moon material, less than a third of the Apollo haul was analysed at the time.
The rest was kept in storage until a time when instruments and technology had improved, and in 2019, NASA allowed some of the locked-away samples to be analysed.
“Back in the 1970s, the people running the Apollo missions could think ahead 50 years,” Professor Nemchin said.