The Moon Is Still Wrinkling
The Moon presents a convincing impression of completion. Its great dark plains hardened billions of years ago. Its most obvious features are impact scars. With no wind, rain, rivers, or moving tectonic plates to remake the scenery, the lunar surface can seem less like a landscape than a record placed under glass.
But the Moon is not finished. It is still losing heat, still contracting, and still gathering new wrinkles.
Those wrinkles are called lobate scarps: low, curving cliffs that can run for kilometers across the surface. They form where the crust is compressed and one block of rock is pushed up over another along a thrust fault. Seen from orbit in low sunlight, a scarp casts a narrow shadow like a welt across the ground. On the surface, it would look more like a steep step or rounded ridge than a canyon.
The basic mechanism begins deep below. The Moon formed hot, and its interior has been cooling ever since. Cooling material generally contracts. As the volume inside decreases, the rigid outer crust must accommodate a slightly smaller world. It cannot shrink smoothly like a balloon losing air. It breaks, overlaps, and buckles.
The change is modest on a planetary scale. Research based on the accumulated shortening across lunar scarps has estimated a decrease of roughly 100 meters in the Moon’s radius over hundreds of millions of years. Set against a radius of about 1,737 kilometers, that is almost nothing. Geology, however, is very good at turning almost nothing into a cliff.
Apollo astronauts gave scientists the first close views of some scarps, but the Lunar Reconnaissance Orbiter transformed the census after it began mapping the Moon in 2009. High-resolution images revealed a global population of thousands of these landforms. Their distribution is one reason researchers connect them to contraction of the whole Moon rather than to isolated local events.
Their appearance also suggests surprising youth. Small impact craters crosscut by faults, crisp topography, and displaced rocks indicate that some scarps formed within the geologically recent past. “Recent” has a generous meaning in planetary science: many are thought to be younger than a billion years, and evidence at some sites points to activity within the last tens of millions of years. That is young enough to complicate the picture of a world that went quiet near the beginning of solar-system history.
Then there are the moonquakes. Seismometers deployed during the Apollo missions operated as a network from 1969 to 1977. They detected several kinds of seismic events, including vibrations from impacts, deep moonquakes associated with Earth’s tidal pull, and 28 shallow moonquakes. The shallow events were comparatively strong and remain the most relevant to the young faults near the surface.
Connecting old seismic recordings to particular faults is difficult. Four stations on the near side of the Moon make a sparse network, and the estimated epicenters have large uncertainties. A 2019 reanalysis relocated the shallow events and found eight estimated epicenters within 30 kilometers of a young fault scarp, a distance consistent with strong shaking. The timing of several events also matched periods of high compressional tidal stress. Together with signs of recent rockfalls and disturbed soil near scarps, the pattern supports an active Moon—but it does not turn every visible scarp into a confirmed earthquake source.
Earth contributes to the stress. Its gravity continually deforms the Moon by a small amount as the distance between the two bodies changes during the lunar orbit. These tidal forces do not explain the global contraction, but they can add to the pressure already stored in the crust. A fault close to slipping may receive a final nudge from the monthly rhythm of the orbit.
A moonquake is not simply an earthquake in gray surroundings. Earth’s fractured, fluid-altered crust tends to damp seismic energy. The Moon is colder, drier, and more rigid, with intensely broken near-surface material created by impacts. Seismic waves can scatter and reverberate there for much longer. Apollo instruments recorded some signals fading only gradually, which is the sober science behind the theatrical claim that the Moon “rang like a bell.” It did not sound through the vacuum; instruments measured prolonged vibrations in rock.
That persistence matters if people intend to place long-lived equipment, landing pads, habitats, or tall structures on the surface. A 2024 study modeled a shallow quake in the south polar region and found that strong ground motion and quake-triggered landslides are plausible near some candidate exploration areas. The practical lesson is not that the lunar south pole is about to split open. It is that site selection and engineering should treat seismic hazards as real local variables, not as impossibilities on a supposedly dead world.
The uncertainties remain large because the Moon’s seismic record is both precious and geographically narrow. Apollo’s instruments stopped transmitting in 1977, and scientists have been extracting new information from that limited archive ever since. A broader, longer-lived network would locate events more precisely, distinguish fault activity from other sources, and reveal how often meaningful shaking occurs.
For now, the scarps supply a visible clue. They are not towering mountain chains or dramatic rifts. They are small accommodations to an almost imperceptible change: an old world cooling by degrees, its circumference tightening, one fault slipping past another.
The Moon still deserves its reputation for preserving the past. Footprints, craters, lava plains, and ancient debris can remain recognizable for immense spans of time. Yet preservation is not the same as stasis. Even under glass, the exhibit is settling.