Back then, when life on Earth was nothing more than algae living in the oceans, thick lava sprang from long, yawning cracks on the Moon’s surface and flooded the surrounding plains.

Last year, the Chinese Chang’e 5 lander scooped up some of the ancient lava flow – now solidified basalt – and brought it to Earth.

In a study published in Science, scientists dated two precious fragments of this basalt and fixed a date on one of the Moon’s last volcanic eruptions: 2 billion years ago.

Previous lunar missions retrieved lots of lunar land and rocks, but nothing was younger than about 3.1 billion years old, says Alex Nemchin, a geochemist at Curtin University and co-author of the study.

“Now it looks like we can at least start filling that gap with this young basalt.”

In doing so, the study will help planetary scientists more accurately calculate the age of surfaces on not only the Moon, but also moons and planets further away, 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 trial return mission – the fifth in the Chang’e series – exploded in late November 2020.

Its main task was to drill a 2-meter core of rock, scoop soil and rock up from the surface and then sling the lot to the ground in December.

This payload of rocks and debris was the first fresh lunar material to be brought to Earth since the American Apollo missions in the 1970s.

These earlier missions brought back basalt dating to about 3.6 and 3.1 billion years ago.

The problem, however, was that scientists did not have any “young” moonstones to date.

The Chang’e 5 mission was intended to change that.

Where the spacecraft touched was not accidental.

It was carefully guided to land on and collect specimens 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 gray stone called basalt.

It was produced back when the Moon was more dynamic, warmed by its tidal belly with Earth and was able to sustain volcanic eruptions.

Lunar regions with multiple craters are older

In general, planetary scientists can tell when a part of the moon was last flooded by lava – and thus its age – by counting its craters.

A strongly pockmarked area is older than a slippery area, simply because it has had more time to accumulate craters.

And even though planetary scientists knew that Oceanus Procellarum was younger than the Apollo landing sites because it had fewer craters, the question was still: how old was it really?

Various crater-based calculations ranged from 1.2 to 3.3 billion years old.

The same logic is used to date regions on other solids 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 calculate the actual age of the moonstone?

Professor Nemchin and his fellow researchers received two fragments of basalt scraped from the Moon’s surface.

Each small nugget of gold was only 3 or 4 millimeters wide, no larger than a piece of gravel.

Instruments in a laboratory in China measured small amounts of lead isotopes in the basalt and used the lead 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 that this technique, known as lead-lead dating, is the “gold standard” for items such as lead. The moonstone.

“We use lead-lead dating to date the formation of the solar system, the oldest meteorites we have 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 earlier on Earth. We can see major floods like the basalt of the Colombian River [of the north-west US] and parts of India.

A smaller version of the flood volcano is the currently erupting Cumbre Vieja volcano in the Canary Islands, where lava oozes cracks and fissures and floats above the ground.

“On the Moon, in this particular case, we can see these cracks, several hundred kilometers long, on the surface from images,” Professor Nemchin said.

“It is difficult to make an estimate, but we believe that in a few million years a huge amount of lava has been flooded. [Oceanus Procellarum]covering about 70,000 square kilometers to a thickness of about 50 meters. “

Meanwhile down here on Earth, things also looked very different.

The earth was only 2.5 billion years old.

Life had evolved to the point of single-celled organisms where the oceans were home to bacteria that looked very much like blue-green algae, but not much else, said Dr. McCoy-West.

“Modern plate tectonics would have been in operation at that time.

“But the oceans were largely barren, except for bacteria. It was still a very empty planet.”

Why some lunar samples will not be analyzed – yet

All samples that took a trip to Earth on Chang’e 5’s return module have so far been in China, Professor Nemchin said.

But in the coming months, he expects laboratories around the world will be able to apply for fragments to analyze themselves.

And a fair share of the Chang’e 5 samples will not be analyzed at all at present, but will be tucked away for future research when new and more sensitive techniques can retrieve even more information from them.

This is pretty standard practice going back to the first crew missions to the moon.

Despite bringing back hundreds of kilograms of lunar material, less than a third of the Apollo trait was analyzed at the time.

The rest was stored until a time when instruments and technology had been improved, and in 2019, NASA allowed some of the locked samples to be analyzed.

“Back in the 1970s, the people who run the Apollo missions could think ahead 50 years,” Professor Nemchin said.