Further evidence of recent lunar geologic activity

Close-up of the "Virtanen graben" field, near the 18.29°N, 180.79°E, on the central meridian of the lunar far side. From LROC Narrow Angle Camera observation M136355592RE (LRO orbit 5228, August 13, 2010; resolution 0.66 meters from 59.85 km). This LROC NAC frame, along with M136362376, were used by Mark Robinson and colleagues at Arizona State University to create a Digital Terrain Model of the Virtanen graben in November 2010. That DTM can be explored HERE.

Images and elevation models from NASA's Lunar Reconnaissance Orbiter (LRO) appear to show the Moon's crust is being stretched, forming miniature valleys in a few small places on the lunar surface. A team of investigators will present their findings at the upcoming Lunar and Planetary Science Conference as evidence that this geologic activity occurred less than 50 million years ago, a very recent time in relation to the Moon's estimated age of roughly 4.575 billion years.

Researchers analyzing high-resolution images obtained by the Lunar Reconnaissance Orbiter Camera (LROC) have shown many small, narrow trenches typically much longer than they are wide, indicating 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 already been identified across the lunar surface and are cited as evidence the Moon may be shrinking.

"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, 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."

Full width (about 5 km wide) of LROC NAC DTM"Virtanen
graben 1;" the small rectangle is the field of view seen in the
image above [NASA/GSFC/Arizona State University].

The weak contraction suggests that the moon, unlike 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, Maryland. "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."

DERIVATION OF ABSOLUTE MODEL AGES FOR LUNAR LOBATE SCARPS
van der Bogert, Hiesinger, Banks, Watters and Robinson, LPSC #1847

Lobate Scarp or Fluidized Ejecta (November 10, 2011)

LROC: Lunar Landslides! (October 15, 2011)

LROC: Tectonics at the edge of Procellarum (October 13, 2011)

Scarps in Schrödinger (September 28, 2011)

LROC: lobate scarp in Xenophanes (September 14, 2011)

Wrinkled Planet (May 3, 2011)

Too brief an expedition to a lobate scarp (August 24, 2010)

Moon geologically active, cooling and shrinking (August 19, 2010)

Updated map of lunar graben, lobate scarps and further more recent topographic features broadly hinting Earth's Moon is not "dead," as once assumed, but geologically active [NASA/GSFC/DLR/Smithsonian CEPS/Arizona State University].