via Nature Geoscience, DOI: 10.1038/ngeo1387
Recent stretch marks have been found on the moon, a surprising find given previous evidence that the moon is shrinking.
In 2010, researchers discovered that the moon has been contracting over time. The shrinkage is making the moon shrivel like a raisin, creating features called wrinkle ridges all over its surface. That shrivelling makes sense, because the moon's interior should be gradually contracting as it loses the heat from its violent formation more than 4 billion years ago. Similar wrinkles have been found on Mercury.
Despite the overall contraction, it now appears that patches of the moon's surface have recently experienced stretching, says a team led by Thomas Watters of the Smithsonian Institution in Washington DC.
NASA's Lunar Reconnaissance Orbiter revealed troughs called graben, which form when a body's surface is pulled apart (see this illustration).
Very ancient stretch marks had been found on the moon before. But the newly discovered graben must be less than 50 million years old, given how few craters have formed on them. The unexpected stretch marks may be due to magma pooling in pockets beneath the moon's surface, causing the ground above it to swell, the researchers say.
"It's exciting when you discover something totally unexpected," says team member Mark Robinson of Arizona State University in a NASA press release. "And only about half the lunar surface has been imaged in high resolution. There is much more of the moon to be explored."