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There’s still some geological activity on the moon, new images from NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft show.
The moon’s crust is being stretched, forming minute valleys in a few small areas on the lunar surface. And the team believes they were created less than 50 million years ago – pretty recent compared to the moon’s age of more than 4.5 billion years.
The images show small, narrow trenches, mostly much longer than they are wide, known as graben. Such valleys form when the crust stretches, breaks and drops down along two bounding faults.
“We think the moon is in a general state of global contraction because of cooling of a still hot interior,” says Thomas Watters of the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington.
“The graben tell us forces acting to shrink the moon were overcome in places by forces acting to pull it apart. This means the contractional forces shrinking the moon cannot be large, or the small graben might never form.”
The weak contraction suggests that the moon, unlike the terrestrial planets, didn’t completely melt in the very early stages of its evolution. Instead, it looks as if initially only the moon’s exterior melted, forming an ocean of molten rock.
Scientists are scratching their heads somewhat over the images, however.
In August 2010, the team spotted signs of contraction on the lunar surface, in the form of lobe-shaped cliffs known as lobate scarps – evidence that the moon shrank globally in the geologically recent past, and might still be shrinking today.
The team concluded the moon was shrinking as the interior slowly cooled – while the graben indicate, in contradiction, that regions of the lunar crust are also being pulled apart.
“It was a big surprise when I spotted graben in the far side highlands,” says Mark Robinson of the School of Earth and Space Exploration at Arizona State University.
“I immediately targeted the area for high-resolution stereo images so we could create a three-dimensional view of the graben. It’s exciting when you discover something totally unexpected and only about half the lunar surface has been imaged in high resolution. There is much more of the moon to be explored.”