The moon has grown smaller in the recent past, and may still be shrinking, new images from NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft reveal.
The images show a series of modest cliffs – up to 300 meters high – and indicate relatively recent tectonic activity.
As the moon cooled and contracted, the mantle and surface crust were forced to respond, forming faults, or cliffs. Many have a semi-circular or lobe-shaped appearance.
“We estimate these cliffs, called lobate scarps, formed less than a billion years ago, and they could be as young as a hundred million years,” said Dr Thomas Watters of the Smithsonian’s National Air and Space Museum.
“Based on the size of the scarps, we estimate the distance between the moon’s center and its surface shrank by about 300 feet.”
The team believes the scarps are some of the newest features on the moon. They cut across small craters, which are generally themselves young as they are quickly eroded by other impacts.
In addition, large craters, which are likely to be old, don’t appear on top any of the scarps, and the scarps look crisp and new.
Lobate scarps on the moon were first discovered during the Apollo missions. But researchers couldn’t be sure they weren’t just the result of local activity around the equator.
However, the LRO images show found 14 previously undetected scarps, seven of which are at high latitudes, confirming that they’re a global phenomenon and making a shrinking moon the most likely explanation.
The youth of the scarps implies that the moon could have been cooling and shrinking very recently. Seismometers have recorded moonquakes, and while most can be attributed to things like meteorite strikes, the Earth’s gravitational tides, and day/night temperature changes, it’s justpossible that some may be associated with ongoing scarp formation.
The team plans to compare photographs of scarps by the Apollo Panoramic Cameras to new LRO images to see if any have changed over the decades.
While Earth’s tides probably aren’t strong enough to create the scarps, they could contribute to their appearance, perhaps influencing their orientation, according to Watters.
During the next few years, the team hopes to use LRO’s high-resolution Narrow Angle Cameras (NACs) to see if the scarps have a preferred orientation or other features that might be associated with Earth’s gravitational pull.