Geological Activity on the Moon

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, 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 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 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 shrank by approximately 300 
feet. The graben were an unexpected discovery and the images provide 
contradictory 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." 

As the LRO mission progresses and coverage increases, scientists will 

have a better picture of how common these young graben are and what other types of tectonic features are nearby. The graben systems the team finds may help scientists refine the state of stress in the lunar crust. 
"It was a big surprise when I spotted graben in the far side highlands," said co-author Mark Robinson of the School of Earth and Space Exploration at Arizona State University, principal investigator of LROC. "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." 
The research was funded by the LRO mission, currently under NASA's Science Mission Directorate at NASA Headquarters in Washington. LRO 

is managed by NASA's Goddard Space Flight Center in Greenbelt, Md.