Today, the Moon is a cold, dead world but it wasn’t always so. Early in its history, the Moon was host to volcanic activity.
Now, the latest results from the first ever samples returned from the far side of the Moon by China’s Chang’e 6 probe reveal that this volcanism could have happened much more recently than previously suspected. What remains unclear, however, is how these eruptions were able to keep going for so long.
The Moon is tidally locked with Earth, meaning the same side always faces towards us. Throughout human history, the dark lunar seas – known as mare – of the near-side have been clear to see.
But the lunar far-side, trapped away from our view, remained a mystery until the advent of the Space Age. In 1959, the Soviet Luna 3 satellite returned the first ever images of the Moon’s far side, revealing a lunar surface completely unlike the familiar near side. There are only a handful of small lunar seas. Instead, the vast majority of the far side is pockmarked with impact craters.
Why do we know so little about the far side of the Moon?
The Moon is dichotomous, meaning its two sides have a markedly different appearance. In recent years, experiments such as NASA’s GRAIL satellite revealed that this split personality extends beneath the surface too.
“There is a crustal dichotomy between the near and far sides, with the far-side crust being much thicker,” says Prof Clive Neal, a planetary geologist from the University of Notre Dame in the US.
The cause of this division is one of the biggest unanswered questions about the Moon. To get to the bottom of it, researchers must first look at what causes the two differing appearances in the first place.
In the 60s and 70s, the Luna and Apollo missions returned a huge hoard of Moon rocks, allowing geologists to confirm what they had long suspected – that the lunar maria were largely made of basalt, a mineral formed by cooled lava.
The Moon’s seas were in fact ancient volcanic floodplains laid down between 4.3 and 3.1 billion years ago. This proved once and for all that there had been volcanic activity on the near side.
While the absence of lunar maria might suggest there is no sign of volcanism on the far side, a closer examination of the far side’s craters shows this might not be the case. Over time, rocky worlds build up a patina of impact craters from meteor strikes.
If a planet is volcanically active, then lava flooding the surface fills these craters in, erasing them from the surface. This means the more craters a planetary surface has, the longer it’s been since it was volcanically active.
Using orbital images of the lunar surface, scientists have been able to count the Moon’s craters, finding the lunar far side does indeed seem to have been scrubbed clean by volcanism on roughly the same timescale as seen on the near side.
So what did the new mission find?
The only way to confirm this theory was to test samples from the far side for volcanic minerals. But, unfortunately, all the early lunar missions travelled to the easiest places to land – the Moon’s brightly lit equator on the near-side.
That changed on 1 June 2024, when China’s Chang’e 6 lander set down on the far-side in a region known as the South Pole-Aitken impact basin. It was the nation’s second venture to the far side, having previously landed a rover there in 2019. Chang’e 6’s main goal was to return a sample of the far side to Earth, finally revealing how geologically different the region is to the near side.
Shortly after landing, Chang’e 6 scooped up some of the lunar soil, known as regolith. It also used a 2m (6.5ft) long drill to collect a sample from underground, where the Moon rock is somewhat protected from the Sun’s radiation.
All in all, the mission collected 1,935g (4.2lb) of lunar material, which was parcelled up into an ascent vehicle and returned to Earth on 6 June.
The return capsule was swiftly transported to a special facility to be opened and given a preliminary examination, which revealed the sample did contain grains of basalt, proving the far side does indeed have a volcanic past.
To find out what this past looked like, over 100 basalt fragments were extracted and sent to two independent teams of researchers, who published their results in Science and Nature this November.
They revealed the basalts were around 2.8 billion years old, making them younger than the samples taken by Luna and Apollo.
How volcanism was possible is an “enigma”
The new samples were consistent with samples taken by China’s previous sample return mission, Chang’e 5, which were similarly young.
Both samples were devoid of a set of metals referred to as KREEP – potassium (which has the elemental symbol K), rare Earth metals and phosphorous – which the earlier samples had been rich in. There was also a distinct lack of radioactive metals in the Chang’e samples as well.
“The fact that the younger basalts – younger than 3 billion years – do not contain large quantities of KREEP radioactive elements on the near and far sides is the enigma,” says Neal, who was one of the few western scientists permitted to aid with the analysis at this time.
“That is a puzzle and is consistent with the young two-billion-year-old basalts from Chang'e-5.”
The heat from decaying radioactive metals is one of the main mechanisms keeping our own planet’s volcanism going, yet their apparent disappearance doesn’t seem to have stopped the Moon’s volcanism immediately. After all, the samples are, in many other ways, remarkably similar.
“They are similar in bulk compositions to previous samples, which adds to the puzzle – what was the heat source for generating such magmas?” says Neal.
Getting to the bottom of the mystery will almost certainly require not only more samples taken from a wide variety of locations across the Moon, but also a closer look at what’s happening under the surface.
“The lack of KREEP elements in the far-side basalts suggests the mantle of the Moon is also dichotomous. We really need to explore the interior of the Moon with a global geophysics network to understand the nature of the interior.”
It seems the far side still wants to keep some of its secrets hidden, at least for now.
About our expert
Prof Clive Neal is an expert in civil and environmental engineering and earth sciences at the University of Notre Dame in the US. His research has been published in Nature Geoscience, Science and Advances in Space Research.
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