Image: Illustration created with AI
The Moon has shrunk by about 50 metres over the last few hundred million years. Here is how scientists know, why it causes moonquakes and what it means for the astronauts heading back.
It looks as permanent as anything in the sky, but the Moon is slowly getting smaller. Over the last few hundred million years it has shrunk by about 50 metres (around 150 feet) across its radius, and the evidence is written across its surface in thousands of small cliffs and ridges. The shrinking has not stopped, and it may still be shaking the ground today.
A cooling interior
The Moon formed hot, around 4.5 billion years ago, and it has been losing that heat ever since. As its interior cools, it contracts. A common comparison is a grape drying into a raisin, but there is one important difference. A grape has a soft, flexible skin that wrinkles. The Moon's crust is cold and brittle, so instead of wrinkling it cracks.
As the interior pulls inwards, sections of crust are pushed together and forced up over one another along what geologists call thrust faults. The result is a scattering of step-like cliffs known as lobate scarps.
Cliffs seen from orbit
Some of these scarps were first spotted in photographs taken during the Apollo missions, but only near the equator, where the spacecraft flew. That changed after NASA's Lunar Reconnaissance Orbiter (LRO) arrived in 2009. Its cameras have since mapped the whole Moon in fine detail and found thousands of scarps spread across the globe.
Their even spread matters. If the Moon were being squeezed by one particular force, such as the pull of the Earth, the faults would line up in a pattern. Instead they are scattered almost everywhere, which points to the Moon shrinking as a whole.
The scarps are also young by lunar standards. Many are crisp and sharp, and they cut across small craters that would have been worn away long ago if the faults were ancient. Some appear to be less than 50 million years old, and a number may still be moving.
Moonquakes from the Apollo era
Between 1969 and 1972 the Apollo astronauts left seismometers on the Moon. The instruments from Apollo 12, 14, 15 and 16 kept sending data back to Earth until they were switched off in 1977, and in that time they recorded 28 shallow moonquakes.
In 2019 a team led by Thomas Watters of the Smithsonian's National Air and Space Museum reanalysed those recordings and compared them with the LRO maps. They found that at least eight of the quakes happened close to known scarps, which suggested that the faults are still active and the Moon is still contracting. Some of these quakes were strong enough to rate around 5 on the scale used for earthquakes, and unlike earthquakes they could carry on shaking for hours.
A challenge for Artemis
The finding has very practical consequences. In 2024 Watters and colleagues looked at the region around the lunar south pole, where NASA plans to land astronauts as part of its Artemis programme. They linked a group of faults there to the most powerful shallow moonquake the Apollo instruments ever recorded, on 13 March 1973.
Their models showed that a quake like that could shake the ground strongly for tens of kilometres around the fault. On the steep slopes of the south polar craters, it could also set off landslides. Some of the areas being considered for crewed landings sit close to these faults, so the team argued that any long-term base should be designed and placed with moonquakes in mind.
Even the smooth seas are wrinkling
The latest work shows the shrinking reaches the dark, flat lunar seas too. In a study published at the end of 2025, Smithsonian researchers mapped small ridges in the maria on the near side of the Moon. They identified 1,114 new ridge segments, bringing the total known to 2,634, with an average age of about 124 million years.
These small mare ridges appear to form in the same way as the scarps in the highlands, through the same contraction of the Moon. That means the possible sources of moonquakes are more widespread than once thought, and are not limited to the highlands and the poles.
Will the Moon disappear?
Not in the slightest. A shrink of around 50 metres is tiny compared with the Moon's radius of about 1,737 kilometres. It is roughly like a football losing the thickness of a sheet of paper. Your piece of the Moon is not going anywhere.
What it does mean is that the Moon is not quite the dead, unchanging world it is often taken for. It is still cooling, still cracking and still, now and again, shaking. As astronauts return over the coming years, new seismometers placed on the surface should tell us far more about how often it happens and how strong those moonquakes can be.
