Evidence collected by Chinese spacecraft suggests that volcanic eruptions on the Moon occurred as early as 120 million years ago. Until the past few years, scientists believed that volcanic activity on the Moon ended about 2 billion years ago.
These findings, published in Science, come from an analysis of lunar rock and soil brought to Earth by China’s Chang’e 5 spacecraft in 2020. While these results are difficult to reconcile with the accepted history of lunar volcanism, it is possible that some regions of the moon’s interior were overly enriched with radioactive elements, which generate the heat that drives volcanic activity.
The area where Chang’e 5 landed, called Oceanus Procellarum, is likely one such region where rocks were rich in these heat-producing elements.
Volcanic eruptions are a major way that all rocky planetary bodies lose their heat. The rocky bodies in our solar system are Earth, Venus, Mars, Mercury, Jupiter’s satellite Io, and Earth’s satellite Moon.
All available evidence suggests that volcanism is currently active on Venus. On Mars, we can infer the age of formation of large lava flows by counting the number of impact craters on these flows.
This crater-counting technique relies on the fact that craters form randomly and uniformly across planetary surfaces, so highly cratered areas are assumed to be older. The results suggest that Mars, which is half the size of Earth, is volcanically active every few million years.
This is expected, since larger bodies conserve heat better than smaller ones. On this basis Mercury, which is a third of Earth’s size, and our Moon, which is a quarter of Earth’s size, should have been volcanically dead for about 2 billion years.
The same is true of Io, which is similar in size to our Moon. However, tidal forces generated by gravitational interactions with Jupiter provide Io with an additional, strong heat source. As a result Io is very volcanically active.
dark areas of the moon
Most eruptions on the Moon occurred near the edges of vast depressions formed early in the Moon’s history by asteroid impacts. Lava filled the interiors of these basins, forming dark regions on the Moon’s near side. These regions are called maria (singular mare), meaning sea in Latin, because early observers mistook the flat sheets of lava for expanses of water.
Analysis of the composition and age of samples returned from these oceanic regions by six Apollo missions and three Soviet robotic probes was consistent with the belief that there had been no geologically recent volcanic activity on the Moon.
This understanding persisted until very high-resolution images of the lunar surface became available from the US Lunar Reconnaissance Orbiter mission after the mission’s launch in 2009. Counting the number of very small impact craters revealed a lack of craters in some volcanic regions with unusual surface textures, called irregular mare patches (IMPs).
The simplest explanation was that these IMPs were young, typically around 100 million years old. This is 20 times younger than the expected minimum age of 2 billion years.
In an attempt to reconcile these observations with the accepted history of lunar volcanism, it was pointed out that the lack of any atmosphere on the Moon would cause eruptions there to differ considerably from those on Earth. The lack of confining pressure would cause nearly all of the gaseous compounds dissolved in the erupted lava to escape, causing some lava flows to contain very large numbers of gas bubbles – to the point of foaming.
When meteorites strike this soft foam, they create craters that are much smaller than they would be on solid rock, which is why the crater-based method gives much lower ages.
There has been much debate over this issue, and the best way to resolve it is to bring samples back to Earth for detailed laboratory analysis. Chang’e 5 brought back samples from a very large lava flow that, as was already known from crater-counting, was relatively young in geological terms.
Initial analysis of many of the lava fragments was consistent with the long-accepted theory that volcanic activity on the moon ceased 2 billion years ago. However, closer examination of the Chinese samples, as described in the new Science paper, focused on some of the smallest fragments — mostly rocks broken up by the meteorite impact and melted into droplets.
Three of these 3,000 droplets have been identified as being of volcanic origin based on their detailed chemistry, and are only about 120 million years old – similar to the younger age estimates for IMPs found elsewhere on the Moon.
Lunar explosion
For example, lunar eruptions should have included tall lava fountains, like those commonly seen erupting in Hawaii. While most of these droplets would have been deposited in lava flows, some would have been ejected tens of kilometers to other parts of the moon’s surface.
The three “volcanic droplets” identified in the Chang’e 5 sample probably did not originate from the same vent that produced the bulk of the rock and soil that landed on Earth. This explains why these droplets are much smaller than the lava flows at Chang’e 5’s landing site.
These three glass-like droplets are the first physical evidence we have for unusually recent volcanic activity on the Moon. The new results indicate that for volcanic activity to have occurred so recently, the concentration of heat-producing radioactive elements in some regions would have to have been much higher than in others. Therefore, these findings could lead to a major revision of our understanding of the evolution of the Moon.
Lionel Wilson, Emeritus Professor, Lancaster Environment Centre, Lancaster University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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