Moonquakes are more common than previously thought, a consequence of a shrinking and, at the same time, cracking Moon.
Scientists have uncovered more than a thousand previously unknown tectonic ridges across the Moon’s dark plains, showing the Moon is still contracting and reshaping itself. These features are among the youngest geological structures on the lunar surface.
Yet, since they form through the same forces linked to past moonquakes, they could signal new seismic hotspots.
Small mare ridges
The scientists at the National Air and Space Museum’s Center for Earth and Planetary Studies have created the first Moon map to provide detail of small mare ridges (SMRs), subtle geological features that signal tectonic activity on the Moon.
The analysis showed that SMRs are widespread across the lunar maria — the vast, dark plains on the Moon’s surface. The researchers’ discovery of how SMRs form introduces a new set of potential moonquake sources that could affect future site selections for lunar landings.
Lunar maria (‘seas’) are the dark topographical features that can be observed on the lunar surface, covering about 15% of the Moon’s crust.
Earth and Moon: Tectonically different
Both the Moon and Earth are tectonically active, yet this is in different ways. The Earth’s crust is divided into plates that slide past each other to produce mountain ridges, ocean trenches, and a ring of volcanoes around the Pacific Ocean.
In contrast, the Moon’s crust is formed of a series of stresses within the crust; these give rise to several distinct landforms. Included here are lobate scarps, which form when the crust compresses and the resulting forces push material up along a fault. This activity forms a ridge. The scarps, found in the lunar highlands, have formed within the last billion years, which is roughly the last 20 percent of the Moon’s history.
Recent mapping reveals 1,114 previously unrecognised ridges, bringing the total to 2,634. The average SMR is about 124 million years old. This closely matches the average age of lobate scarps (105 million years old). These comparable ages suggest that SMRs, like lobate scarps, rank among the Moon’s youngest geological features.
The study also shows that SMRs form along the same types of faults as lobate scarps. In some regions, scarps in the highlands transition into SMRs within the maria, reinforcing the idea that both structures share a common origin. When combined with existing data on lobate scarps, the new SMR catalogue offers a far more complete picture of the Moon’s recent contraction and tectonic evolution.
Explanation #1: Contraction
One explanation for the moonquakes is the contraction of the Moon. The Moon is contracting due to thermal contraction caused by its cooling interior. As the Moon formed about 4.5 billion years ago, it was largely molten. Over time, its interior cooled and solidified, causing the crust to contract. This contraction is significant for understanding lunar evolution. The primary driver of lunar contraction is thermal evolution, and it has implications for future lunar exploration, according to NASA.
Evidence suggests that the lunar maria have shrunk by 0.003 to 0.004 percent – a tiny fraction, yet significant enough to cause tremors.
As the interior cools, the surface contracts, creating the compressional forces that formed lobate scarps in the highlands.
However, lobate scarps do not explain all of the Moon’s relatively recent contraction features. Another class of landforms, small mare ridges, has also been identified.
Explanation #2: Location, location
SMRs form from the same compressional forces that create lobate scarps. The difference lies in location. Lobate scarps appear in the highlands, while SMRs are found only in the maria. The research team set out to systematically map these ridges across the lunar maria and investigate their role in recent tectonic activity.
It is reasoned that tectonic forces produce lobate scarps, where there are interconnected moonquakes. Because SMRs form through the same type of faulting, moonquakes also occur across the lunar maria wherever these ridges exist.
Research significance
Expanding the map of potential moonquake sources provides scientists with new opportunities to study the Moon’s interior and tectonic behavior. At the same time, it highlights possible seismic risks for astronauts who may one day explore or live on the lunar surface.
The study appears in The Planetary Science Journal, titled “A New Global Perspective on Recent Tectonism in the Lunar Maria.”
