February 20, 2012

Moon images show crust pulling apart

Posted: February 20, 2012
Farside Highlands Graben
Newly detected series of narrow linear troughs are known as graben, and they formed in highland materials on the lunar farside. Forces acting to pull the lunar crust apart formed the Virtanen graben, informally named for a nearby impact crater. These graben are located on a topographic rise with several hundred meters of relief revealed in topography derived from Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) stereo images (blues are lower elevations and reds are higher elevations). The rise is flanked by the rim of a ~2.5 km diameter degraded crater.
Photo by: NASA/GSFC/Arizona State University/Smithsonian Institution
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Graben Illustration
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.
Photo by: Arizona State University (Sue Selkirk)/Smithsonian Institution
3-Dimensional View of Farside Graben
Perspective views of the farside Virtanen graben created with topography derived from Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) stereo images. The graben (black arrows) extend along a shallowly sloping narrow rise in heavily cratered highland material. The 3-dimensional views were generated by draping a monochrome NAC mosaic over the topography. The bottom perspective view has no vertical exaggeration while in the top perspective view the vertical exaggeration has been increased by five times to bring out subtle changes in elevation and slope.
Photo by: NASA/GSFC/Arizona State University/Smithsonian Institution
Global Plot of Faults
Locations of newly discovered young graben (red stars) along with recently detected (white dots) and previously known (black dots) contractional lobate scarps. The newly discovered graben are widely distributed and located near lobate scarps. The locations of 26 shallow moonquakes recorded by the Apollo seismic network with stations at the Apollo 12, 14, 15, and 16 landing sites are shown by the (black triangles). The graben, lobate scarp, and moonquake locations are plotted on a shaded relief map merged with a global Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera LROC (WAC) stereo derived global topographic model of the Moon.
Photo by: NASA/GSFC/Arizona State University/DLR/Smithsonian Institution
Graben Crosscut Craters
A series of graben in a patch of mare basalts that occupy a valley south of Mare Humorum cut across and deformed several small diameter impact craters. The walls and floors of the graben crosscut a degraded 27 m diameter crater (inset, upper white arrow) and a 7 m diameter crater (inset, lower white arrow). Since small craters only have a limited lifetime before they are destroyed by other impacts, their deformation by graben indicates that these fault-bound troughs are relatively young.
Photo by: NASA/GSFC/Arizona State University/Smithsonian Institution

New images acquired by NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft show that the Moon’s crust is pulling apart – at least in some small areas. The high-resolution images from the Lunar Reconnaissance Orbiter Camera (LROC) provide evidence that the Moon has experienced relatively recent geologic activity.

In the new LROC images, a team of researchers discovered small, narrow trenches typically only hundreds to a few thousand meters (yards) long and tens to hundreds of meters wide, indicating the lunar crust is being pulled apart at these locations. These linear troughs or valleys, known as graben, are formed when crust is stretched, breaks and drops down along two bounding faults. A handful of these graben systems have been found across the lunar surface. The team proposes that the geologic activity that created the graben occurred less than 50 million years ago (very recent compared to the Moon’s current age of over 4.5 billion years).

In August, 2010, the team identified physical signs of contraction on the lunar surface, in the form of lobe-shaped ridges or scarps (known as lobate scarps), using LROC images. They suggest that these scarps indicate 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 that it was shrinking as the interior slowly cooled. The new images of graben therefore present a contradiction – regions of the lunar crust that are being pulled apart as the Moon shrinks.

“We think the Moon is in a general state of global contraction due to cooling of a still hot interior. The graben tell us that forces acting to shrink the Moon were overcome in places by forces acting to pull it apart,” says 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. “This means the contractional forces shrinking the Moon cannot be large, or the small graben might never form.”

The small graben indicate that contractional forces in the lunar crust are relatively weak. The weak contraction indicates that unlike the terrestrial planets – Mercury, Venus, Earth, and Mars – the Moon did not completely melt in the very early stages of its evolution. Instead, an alternative scenario better fits the observations: only the Moon’s exterior initially melted forming a magma ocean.

“It was a big surprise when I spotted graben in the farside highlands,” says Mark Robinson of the School of Earth and Space Exploration at Arizona State University, coauthor and principal investigator of LROC. “I immediately targeted the area for high resolution stereo images so we could create a 3-dimensional view of the graben.  It’s exciting when you discover something total unexpected. Only about half the lunar surface has been imaged in high resolution. There is much more of the Moon to be explored.”

As the LRO mission progresses and coverage increases, scientists will have a better picture of how common these young graben are, and more importantly, what other types of tectonic features are nearby. The number of graben systems the team finds may help scientists refine the state of stress in the lunar crust.

Nikki Cassis, ncassis@asu.edu
School of Earth and Space Exploration