The moon, Earth’s constant companion in the cosmos, is gradually getting smaller. While imperceptible to the casual observer, the moon’s diameter has contracted by about 150 feet over the last few hundred million years. This slow shrinkage is primarily driven by the cooling of the moon’s interior, causing its surface to wrinkle and shake with moonquakes. As NASA prepares for future lunar missions, understanding the implications of the shrinking moon is becoming increasingly important.
The Science Behind the Shrinking Moon
The moon’s formation, resulting from a cataclysmic collision between early Earth and another protoplanet around 4.5 billion years ago, left it with a molten interior. As the moon’s core has cooled over time, it has caused the lunar body to contract. The moon’s solid crust has had to adapt to this change in volume, leading to the formation of thrust faults – cliff-like ridges created by the crust being pushed together and forced upward.
NASA’s Lunar Reconnaissance Orbiter, in orbit around the moon since 2009, has captured images of these thrust faults across the lunar surface, providing strong evidence of the ongoing contraction. Estimates suggest the moon has shrunk by about 150 feet in diameter over the last few hundred million years, and the process is still ongoing.
Moonquakes: The Seismic Activity of a Shrinking Moon
The active shrinking of the moon is thought to be the cause of seismic activity known as moonquakes. During the Apollo missions between 1969 and 1977, astronauts placed seismometers on the lunar surface, which recorded 28 moonquakes ranging from 2 to 5 on the Richter scale. More recently, scientists have reanalyzed this Apollo-era data using modern algorithms, linking the largest recorded moonquake, a magnitude 5 event near the south pole, to a cluster of young thrust faults.
Moonquakes differ from earthquakes in several ways. They can last much longer, sometimes for hours, compared to Earth’s quakes, which typically last only seconds or minutes. Additionally, due to the moon’s weaker gravity, a quake of the same magnitude will feel much stronger on the moon than on Earth.
Implications for Future Lunar Missions
As NASA prepares for future crewed missions to the moon, such as the Artemis program, the potential impact of moonquakes on lunar infrastructure is a growing concern. The lunar south pole region, which contains permanently shadowed areas that could potentially harbor water ice, is of particular interest for exploration and establishing long-term outposts.
However, this region is also home to a number of thrust faults and has been the location of powerful moonquakes in the past. A recent study published January 2024 in The Planetary Science Journal, available for download below, found that the continuing lunar shrinkage has led to significant surface warping in the south polar region, including areas proposed for the Artemis III mission.
Modeling of the south polar region has shown that some areas, such as the slopes in the permanently shadowed Shackleton Crater, are particularly susceptible to landslides triggered by even light seismic activity. While the risk posed by moonquakes to short-term missions is considered low, the establishment of long-term lunar outposts will need to take into account the potential hazards associated with the moon’s seismic activity.
The Future of Lunar Exploration
Despite the challenges posed by the shrinking moon and its associated moonquakes, the scientific community remains committed to further lunar exploration. By understanding the processes driving the moon’s contraction and the resulting seismic activity, mission planners can better prepare for the establishment of a sustainable human presence on our celestial neighbor.
The shrinking moon serves as a reminder that even seemingly inactive worlds can hold surprises and challenges for those who seek to explore them. As we continue to unravel the mysteries of the moon and plan for its future exploration, the knowledge gained from studying its slow contraction and seismic activity will prove invaluable.
