New images from NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft show the moon's crust（外壳） is being stretched, forming minute valleys in a few small areas on the lunar surface. Scientists propose this geologic（地质的） activity occurred less than 50 million years ago, which is considered recent compared to the moon's age of more than 4.5 billion years. A team of researchers analyzing high-resolution images obtained by the Lunar Reconnaissance Orbiter Camera (LROC) show small, narrow trenches typically much longer than they are wide. This indicates the lunar crust is being pulled apart at these locations. These linear valleys, known as graben（地堑） , form when the moon's crust stretches, breaks and drops down along two bounding faults. A handful of these graben systems have been found across the lunar surface.

"We think the moon is in a general state of global contraction because of cooling of a still hot interior," said Thomas Watters of the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum in Washington, and lead author of a paper on this research appearing in the March issue of the journal Nature Geoscience. "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（收缩） suggests that the moon, unlike the terrestrial planets, did not completely melt in the very early stages of its evolution. Rather, observations support an alternative view that only the moon's exterior initially melted forming an ocean of molten rock.

In August 2010, the team used LROC images to identify physical signs of contraction on the lunar surface, in the form of lobe-shaped cliffs known as lobate（叶状的） scarps. The scarps are evidence the moon shrank globally in the geologically recent past and might still be shrinking today. The team saw these scarps widely distributed across the moon and concluded it was shrinking as the interior slowly cooled.

Based on the size of the scarps, it is estimated that the distance between the moon's center and its surface shank by approximately 300 feet. The graben were an unexpected discovery and the images provide contradictory（矛盾的） evidence that the regions of the lunar crust are also being pulled apart.

"This pulling apart tells us the moon is still active," said Richard Vondrak, LRO Project Scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "LRO gives us a detailed look at that process."