Researchers using NASA's Lunar Reconnaissance Orbiter have discovered new evidence of relatively fresh geologic activity on the moon, raising questions about its formation and the arc of its 4.5 billion-year history.

This shows the largest of the newly detected graben found in highlands of the lunar farside. The broadest graben is about 500 meters (1,640 feet) wide and topography derived from Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) stereo images indicates they are almost 20 meters (almost 66 feet) deep. Credit: NASA/Goddard/Arizona State University/Smithsonian Institution
 
The latest announcement is another sign the moon is not geologically dead. Earlier research using LRO's sharp-eyed camera found cliffs and staircase-like terrain formed by thrust faults, leading scientists to conclude the moon is still cooling billions of years after it coalesced.

High-resolution images from the Lunar Reconnaissance Orbiter Camera, or LROC, reveal long, narrow trenches that appear to be flanked by two parallel fault systems. The troughs, known as graben, form when the lunar crust is pulled apart, fractures and drops down, according to a NASA statement.

The findings appear in a paper published in the March issue of the journal Nature Geoscience.

The discovery follows a revelation announced in August 2010 that the moon shrunk in its recent past, and may still be contracting today. Scientists say the graben features are evidence that forces tugging the lunar crust apart overcame the forces shrinking the moon.

Graben are troughs formed when the lunar crust was stretched and pulled apart. This stretching causes the near-surface materials to break along two parallel normal faults, the terrain in between the twin faults drops down forming a valley. Credit: Arizona State University/Smithsonian Institution
 
"We think the moon is in a general state of global contraction because of cooling of a still hot interior," said Thomas Watters, lead author of the paper and a researcher at 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."

Scientists believe the graben formed less than 50 million years ago. The graben provide evidence that not only is the moon shrinking, but there may still be other geologic forces at work on the surface to pull apart the lunar crust.

"This pulling apart tells us the moon is still active," said Richard Vondrak, LRO project scientist. "LRO gives us a detailed look at that process."

The thought of lunar shrinkage is not new; scientists have long predicted the moon contracted as it cooled from its hot and violent genesis four billion years ago.

Artist's concept of the LRO spacecraft in orbit around the moon. Credit: NASA
 
But LRO's findings indicate there could still be some leftover dynamism inside the moon driving geologic activity at the surface. If it's not still going on today, scientists know it only stopped in the recent past.

Mark Robinson, LROC principal investigator from Arizona State University, said only about half of the moon's surface has been covered by the orbiter's high-resolution camera, which can spot features as small as 1 meter, or about 3.3 feet.

"It was a big surprise when I spotted graben in the far side highlands," Robinson said. "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."