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Moon's Scarred Crust Hints at Recent Activity, Scientists Say
by Charles Q. Choi, SPACE.com ContributorDate: 19 February 2012 Time: 02:29 PM ET
These newly detected narrow linear troughs are known as graben, and they formed in highlands of the moon's far side. Forces acting to pull the lunar crust apart formed the Virtanen graben, informally named for a nearby crater. These graben are located on a rise with several hundred meters of relief revealed in topography derived from Lunar Reconnaissance Orbiter Camera. The rise is flanked by the rim of a ~2.5 km diameter degraded crater.
The moon's crust was apparently active far more recently than previously believed, scientists say.
These new findings raise questions about how the moon formed and evolved, researchers said.
Although the Earth's crust is still shifting, driven by the churning semimolten rock underneath it, researchers had thought the moon had cooled off much too long ago to still have any such tectonic activity. For instance, the youngest known tectonic features on the lunar landscape until now — small cliffs in the lunar highlands resulting from wrinkling of the surface as the moon's interior cooled and shrunk — are thought to be less than 1 billion years old, although by how much is uncertain.
Now, images collected by the Lunar Reconnaissance Orbiter hints the moon has probably seen tectonic activity within the last 50 million years.
"Overall on the moon, you have this contracting, shrinking environment, but in some places, apparently there's this stretching extension of the crust," said study lead author Thomas Watters, a planetary scientist at the Smithsonian Institution in Washington.
Locations of newly discovered young graben (red stars) — trenchlike scars — on the moon along with recently detected (white dots) and previously known (black dots) contractional lobate scarps. The newfound 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 the scientists detected, which reach up to about 1,640 feet (500 meters) wide and 1.1 miles (1.8 kilometers) long, appear relatively pristine. This suggests they formed recently — otherwise, they would be marred more often by craters from meteor impacts over time.
"We think they're less than 50 million years old, but they could be 10 million years old, could be 1 million years old, could have happened 40 years ago," Watters told SPACE.com. "The intriguing picture that's emerging of the moon is that there is recent geological activity going on."
Moonquakes detected by seismic sensors installed during the Apollo missions support the notion of recent activity on the moon, researchers added. All in all, the moon's interior may still be hot.
"The moon may not only have been tectonically active recently, but may still be tectonically active today," Watters said.
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.
Models of how the moon cooled over time suggest it was totally molten after its formation, and that it should now be contracting as it cools, forcing the surface to wrinkle. However, if this was true, such compression would have suppressed the creation of graben — these ditches typically form when the crust stretches, not crinkles.
Instead, these findings suggest the moon was not completely molten after it was formed. If this were the case, the moon would not contract strongly enough to suppress the emergence of graben.
"Currently, a popular idea for how the moon formed is that it was completely molten in the beginning— after a Mars-size object hit Earth very early in its history, the debris cloud from the surviving material formed the moon," Watters said. "This may lend support to alternative scenarios that the moon was not completely molten when it formed, that only part of it was, forming a magma ocean."
Future research can look for more graben in Lunar Reconnaissance Orbiter photos once the satellite finishes imaging the moon, Watters said. He and his colleagues detailed their findings online Feb. 19 in the journal Nature Geoscience.
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