The Chang’e 5 probe successfully landed in the desert of Mongolia on December 17, 2020, with on … [+] board the first lunar rocks returned to Earth in more than 40 years.
The famous ‘Man in the Moon’ is an optical illusion, based on patterns formed by dark and white terrain on the lunar surface. The light-colored highlands are composed of anorthosite, an igneous rock believed to be the remains of the primordial crust of the Moon, and are dated to 4.5 to 3.9 billion years. The lunar Maria, the dark regions of the Moon, were formed when the lunar basins were filled by basalt, a volcanic rock, some 3.9 to 3.2 billion years ago. There are no active volcanoes on the Moon to be found today.
But rock samples recently returned to Earth by China’s Chang’e-5 mission show that the Moon was far longer active than previously believed. Chang’e-5 is the first mission to retrieve lunar rocks and return them to Earth in 45 years.
In a study published in the journal Science, an international team of researchers found that the basalt rocks gathered by Chang’e-5 from a volcanic mound in the Oceanus Procellarum Maria are about 2 billion years old.
Jim Head, a research professor in Brown’s Department of Earth, Environmental and Planetary Sciences and co-author of the new study, says these samples fill critical gaps in scientists’ understanding of the Moon’s history.
Previous samples from the Apollo missions and the Soviet Luna missions all came from the central and eastern part of the Moon’s near side. But remote sensing data suggested that the most recent volcanism on the Moon was to be found in the western part, so that region became a prime target for sample collection.
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Knowing how long lunar mare volcanism lasted, is critically important to understand the thermal evolution of the Moon. The Moon is much smaller than Earth, losing its internal heat – driving volcanism on the surface – at a much faster rate. But somehow the Moon was able to keep pockets of molten rocks for much longer than previously believed, explains Professor Head:
“The region from which these samples were taken is a unique terrane on the Moon, which looks like it may have really high concentrations of radioactive elements—particularly thorium. So one idea for why volcanism lasted so much longer in this region compared to others was that you had all these radioactive elements concentrated together, which creates a lot of heat. That heat melts the mantle and you get volcanic flows.”
“However, in these samples we didn’t actually see an elevated radioactive element composition. If these radioactive elements are driving the volcanism in this region, we expect to see enhanced radioactivity in the samples. But we didn’t. Instead, the composition was similar to mare basalts from older deposits. So that casts some doubt on that hypothesis for long-lasting volcanism.”
According to Professor Head, the new timeframe is also significant for the further exploration of the solar system:
“When we look at a surface or a feature on the Moon from which we don’t have samples for radiometric dating, we try to estimate its age through the size-frequency-distribution of impact craters. Basically, as time goes by, larger impacts become more rare. So by counting craters of different sizes, we can establish a relative age of a surface. But between about one billion and three billion years ago, we don’t have many good data points to tell us what the impact flux looks like. So having an absolute radiometric date for this surface helps us to calibrate the flux curve, which helps us to date other surfaces. And that’s not true only for the Moon. This helps us calibrate ages for Mars, Venus and elsewhere.”
China has major ambitions in terms of its lunar exploration program. One potential mission is a robotic sample-return from the lunar far side—a region called the South Pole-Aitken Basin. One of the oldest and largest impact craters, parts of the lunar mantle could be exposed on the basin’s bottom. Recovered rock samples could also be radiometrically dated, giving an exact date for the impact.