The Moon hosted volcanic activity until surprisingly recently.

This revelation has emerged from a study of the Moon rocks brought back by China’s Chang’e-5 lunar mission at the end of last year.

An international team of scientists has determined the age of these volcanic lunar rocks to be about 1.97 billion years old, suggesting that lava flowed on the surface of the Moon at a time in its history when, given its size, it had to have cooled off and gone inactive.

The study was published on 7 October in the journal Science.

More than 40 years after the American Apollo and Soviet Luna missions brought Moon samples back to Earth, China’s Chang’e-5 probe accomplished a grab-and-go mission in December 2020.

The setup comprised an orbiter, a lander, an ascent vehicle, and a return module, and all of it took off on 24 November atop a Long March 5 rocket from Wenchang Space Launch Center in Hainan province.

The goal of the Chang’e-5 was to collect and bring back rocks from some of the youngest volcanic surfaces on the Moon.

This is why it landed in the Oceanus Procellarum (Latin for “Ocean of Storms”) region — where evidence of recent volcanism could be scooped up, as Chang’e-5 did — before lifting up two days later with the samples to join its mother ship in lunar orbit and make the journey home.

The China National Space Administration (CNSA) robotic spacecraft returned with 1,731 grams (about 1.7 kg) of Moon samples after it landed on 17 December in the Inner Mongolia region. The rocks picked up were of the “basalt” type, which are formed from volcanic activity.

Months later, on 12 July this year, China delivered about 17 grams of this sample to 13 institutions that had written in to CNSA requesting to use the samples for the purpose of research.

Scientific analysis of the rocks to extract vital information about the geology and early history of Earth’s satellite has been underway ever since.

Now, scientists are learning that the age of the lunar samples is 2 billion years, with an uncertainty of 50 million years either way.

The analysis was carried out at the Beijing SHRIMP Center in the Chinese capital. SHRIMP is an abbreviation for “sensitive high-resolution ion microprobe” — it was the instrument of choice for probing the lunar samples.

“The process of determining the age of the rocks was complex, but in essence, we used a focused beam of charged particles to eject material from various mineral phases in the rocks and analysed the ejected material,” three co-authors of the study, Joshua Snape, Katherine Joy, and Romain Tartese, wrote in an article for The Conversation.

This led the larger group to an age estimate of 1.97 billion years, which, the three University of Manchester researchers write, is “a whole billion years younger than any previously dated basaltic lava from the Moon”.

The age of the lunar rocks collected by the National Aeronautics and Space Administration’s Apollo astronauts back in the 1970s was found to be between 3.1 billion years old and 4.4 billion years old.

It isn’t always possible to fetch rocks from the Moon to study here on Earth. So when that hasn’t been available for the most part of lunar scientific investigations, researchers have relied on the craters on the surface of the Moon to have a crack at figuring out the age of different regions.

The general idea is that the more craters there are on the surface, the older the planetary body — the Moon in this case — is.

This is because the Moon doesn’t have processes like erosion or mountain formation, which are observed on Earth, to sweep clean the evidence of craters on the surface. So, the craters remain and reflect the relative age.

But, for all its utility, the crater count doesn’t offer a precise age estimate. The laboratory analysis of the Chang’e-5 samples, though, has provided just that.

Knowledge of the age of a region with reasonable certainty along with the number of craters sprinkled over it enables scientists to make educated age estimates for other planetary objects of a comparable age, which is 4.5 billion years for the Moon, about the same as the Earth.

"It's a phenomenal result. In terms of planetary time, that's a very precise determination. And that's good enough to distinguish between the different formulations of the chronology,” Brad Jolliff told Washington University in St Louis, Missouri, United States (US), where he is Scott Rudolph Professor of Earth and Planetary Sciences. He is a co-author of the study.

Jolliff and others at Washington University have been collaborating with scientists from ShanDong University at WeiHai in China on lunar research for close to 15 years.

Besides China and the US, the international consortium participating in the analysis of Chang’e-5 Moon samples includes researchers from Australia, Sweden, and the United Kingdom.

Evidence of recent volcanism now raises the question — how was it around until till so late in the history of the Moon?

“We still need to explain what heat source is responsible for the comparatively recent melting of the interior on the Moon, which formed the internal “lake” of magma associated with the volcanic activity, and why it has become cool and inert today,” researchers Alexander Nemchin and Gretchen Benedix, both from the School of Earth and Planetary Sciences at Curtin University in Australia and part of the Chang’e-5 study, wrote in a separate article for The Conversation.

Scientific speculations present radioactive decay in the Moon’s interior and the tidal pull of the Earth’s gravity when the Moon was significantly closer as two possibilities.

If even more recent volcanic activity is observed on the Moon, as is expected and even indicated by remote observations, it will pose an interesting challenge for planetary scientists. As they begin to find answers, new insights on the Moon’s evolution will come to light.

More findings promise to be unearthed from the laboratory analysis of these lunar rocks, allowing scientists to learn more about not just the life and evolution of the Moon but also of other planetary occupants in the solar system.

Also Read: ISRO Calls For Proposals From India’s Researchers To Analyse Chandrayaan-2 Orbiter Data

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