August 19, 2010 | 2
It’s an unfortunate fact of life that people often shrink a bit as they age. But we can at least take solace in the fact that the moon, too, seems to be have gotten a bit smaller of late.
New imagery from a NASA spacecraft suggests that the moon may have contracted somewhat in relatively recent geologic time. Photographs from the Lunar Reconnaissance Orbiter, or LRO, which launched in 2009 to map the moon in detail, have revealed more than a dozen previously unknown tectonic features called lobate scarps—landforms that are thought to have formed as the moon cooled and contracted after its formation.
"The best way to describe them is if you were on the surface of the moon, a lobate scarp looks like a stair-step in the landscape," says Thomas Watters, a geologist at the Smithsonian National Air and Space Museum in Washington, D.C., and lead author of a study on the landforms in the August 20 issue of Science. "This is because a lobate scarp is formed by a thrust fault—a fault formed when crustal materials are pushed together causing them to break," Watters adds. "Crustal material is thrust up along the fault forming a cliff or scarp."
The newfound scarps are relatively small, extending only a few meters above the surrounding surface in some cases, and had not been identified previously in lower-resolution imagery. Some previously known scarps had been spotted in photographs taken of the moon during the Apollo missions of the 1960s and 1970s, but those scarps were all relatively close to the lunar equator, where Apollo operations were clustered. The landforms are relatively unscarred by meteorite impacts, implying that they appeared less than a billion years ago. (The moon is about 4.5 billion years old.) But equatorial evidence alone could mean a number of things, such as a slight change in the moon’s overall shape that produced localized contraction accompanied by stretching elsewhere.
By virtue of its polar orbit about 50 kilometers above the surface of the moon, LRO covers the entire lunar sphere. Images from LRO’s cameras have revealed 14 previously unknown lobate scarps across the lunar surface.
The freshness and global distribution of the scarps, Watters and his colleagues report, may indicate a late-stage, all-over contraction of Earth’s only natural satellite. A rough estimate based on the size of the scarps shows that the moon’s radius may have decreased by 100 meters or so as a result. Even neglecting the recent episode of possible shrinkage, the moon is getting smaller all the time from Earth’s vantage point. As the moon drives ocean tides on Earth, its orbit grows ever wider, increasing the distance between the moon and Earth by nearly four centimeters a year.
Photograph of lunar scarp courtesy of NASA/GSFC/Arizona State University/Smithsonian