Our shrinking moon

Astronomers from NASA, the SETI Institute, the Smithsonian Institution and various universities have discovered evidence that the moon is contracting. The researchers used images from NASA’s Lunar Reconnaissance Orbiter (LRO) to draw their conclusions.

The images revealed a series of lunar cliffs, known as lobate scarps (lobe-shaped cliffs). Although some of these scarps were first discovered during the Apollo missions in the 70’s, at that time, instrumentation only allowed for photographing near the lunar equator. In contrast, the LRO has found the scarps scattered across the lunar surface, indicating that they are a moonwide phenomenon.

Just as mountains on Earth are caused by the movements of tectonic plates, so too are lunar cliffs caused by the contraction of the lunar surface. This isn’t so surprising. Scientists have long thought that the moon, which was created as the red-hot result of a massive collision, would have shrunk as it cooled. And indeed, estimates are that over the past four billion years, the moon has shrunk by about 600 feet in diameter. The interesting thing is that many of the scarps created by the contracting process cut across small, and thus relatively short-lived, craters. This means that some of the scarps may only be a hundred million years old or younger, evidence that the shrinking process is still underway.

A fault cuts across and deforms several small diameter (~40-m diameter) impact craters (arrows). About half of the rim and floor of a 20 m-in-diameter crater shown in the box has been lost. Since small craters only have a limited lifetime before they are destroyed by newer impacts, their deformation by the fault shows the fault to be relatively young.

Credit: NASA/Goddard/Arizona State University/Smithsonian