The Moon Is Shrinking! The Moon Is Shrinking!
Like the cry from a celestial ‘Chicken Little’, NASA’s Lunar Reconnaissance Orbiter (LRO) has called to attention astronomers and geologists alike with its information indicating that the Moon is shrinking. The Moon, created as a result of a massive collision between a large body and the early Earth, was always thought to have shrunk as it cooled over the eons. The data from LRO indicates that this process may still be going on in the recent past—possibly as recently as a hundred million years ago, a relatively short time geologically speaking.
In the image below one can see a fault running through several small craters marked by arrows. These faults thrust through these younger craters, but more importantly they thrust surface material up and over the craters, forming a scarp. Looking closely at the enlarged image in the box one can see part of the crater in the center has been destroyed by the fault. Note that the smaller craters are thought to be younger because the constant bombardment of the Lunar surface by meteors tends to wipe out smaller craters leaving the larger ones mostly intact. Based on this information astronomers conclude that these faults occurred after the craters were formed.
There is also seismic evidence of geological activity on the Moon. The Apollo missions left behind seismographs, which show activity that can be attributed to meteor impacts as well as geological activity in the Moon’s interior. Some of the events recorded by these instruments may be associated with some of the faults LRO has documented.
So, what is going on? Well, it is thought that the Moon is still cooling internally. Its core may still hold a lot of heat from the initial formation of the Moon and possibly from the late heavy bombardment of the inner solar system about 4 billion years ago. As the hot interior of the Moon cools, it contracts and causes the surface to collapse, fracturing and forming the scarps that we see.
These scarps were known from the Apollo days, but they were only documented around the Moon’s equatorial region where the Apollo missions were focused. LRO has documented scarps over a much larger portion of the Moon’s surface and the most likely explanation for them is that the Moon is shrinking as its interior cools.
How much has it shrunk? Dr. Thomas Watters of the Center for Earth and Planetary Studies at the Smithsonian Air and Space Museum has estimated that the Moon’s radius has shrunk by about 300 feet, or 100 meters over the last billion years.
So, don’t worry, and enjoy the Moon as it lights our night sky. It’s going to be around for a long time!