According to the Giant Impact Hypothesis, the Moon formed from a massive impact between a primordial Earth and a Mars-sized object (Theia) roughly 4.5 billion years ago. This is largely based on the study of sample rocks retrieved by the Apollo missions and seismic studies, which revealed that the Earth and Moon are similar in composition and structure. Further studies of the surface have revealed features that suggest the planet was once volcanically active, including lunar maria (dark, flat areas filled with solidified lava).
In the past, researchers suspected that these maria were formed by contractions in the interior that occurred billions of years ago and that the Moon has remained dormant ever since. However, a new study conducted by researchers from the National Air and Space Museum (NASM) and the University of Maryland (UMD) revealed small ridges on the Moon’s far side that are younger than those on the near side. Their findings constitute another line of evidence that the Moon still experiences geological activity billions of years after it formed.
The research was conducted by Cole Nypaver and Thomas R. Watters, a postdoctoral student and Senior Geologist with the NASM’s Center for Earth and Planetary Studies at the Smithsonian Institute. They were joined by Jackie Clark, an Assistant Research Scientist with UMD’s Department of Geology. The paper detailing their findings, “Recent Tectonic Deformation of the Lunar Farside Mare and South Pole–Aitken Basin,” recently appeared in the Planetary Science Journal.
Based on previous research, scientists have determined that the Moon once had a magnetic field. Like Earth’s, this field was powered by a dynamo in the Moon’s interior caused by a liquid outer core (surrounding a solid inner core) that rotated opposite to its axial rotation. However, crystallization began in the Moon’s core about 4 billion years ago, causing this dynamo to disappear between 2.5 and 1 billion years ago. This led to the disappearance of its magnetosphere and volcanic activity, ceasing about 3 billion years ago. As Clark summarized in a recent UMD press release:
“Many scientists believe that most of the Moon’s geological movements happened two and a half, maybe three billion years ago. But we’re seeing that these tectonic landforms have been recently active in the last billion years and may still be active today. These small mare ridges seem to have formed within the last 200 million years or so, which is relatively recent considering the moon’s timescale.”
Using advanced mapping and modeling, Nypang, Watters, and Clark found 266 previously unknown small ridges on the Moon’s far side. These were largely arranged in groups of 10 to 40 ridges that likely formed in narrow areas 3.2 to 3.6 billion years ago where underlying weaknesses in the lunar crust may have existed. Based on a technique known as “crater counting,” the team found that these ridges were notably younger than other features in their surroundings.
“Essentially, the more craters a surface has, the older it is; the surface has more time to accumulate more craters,” said Clark. “After counting the craters around these small ridges and seeing that some of the ridges cut through existing impact craters, we believe these landforms were tectonically active in the last 160 million years.”
The team also noted that the ridges observed on the far side of the Moon were similar in structure to ones found on the near side. This suggests both were created by the same forces, possibly by shallow moonquakes first detected by the Apollo missions. Scientists have since deduced that these are caused by a combination of shifts in the Moon’s orbit and its gradual shrinking – which explains why the Moon still experiences landslides. Understanding the factors that shape the lunar surface is of immense importance to future lunar missions.
As Clark indicated, this presents opportunities for further studies of lunar evolution:
“We hope that future missions to the moon will include tools like ground penetrating radar so researchers can better understand the structures beneath the lunar surface. Knowing that the moon is still geologically dynamic has very real implications for where we’re planning to put our astronauts, equipment and infrastructure on the moon.”
Further Reading: University of Maryland, The Planetary Science Journal
