Moon’s Crust as Fractured as can be

Researchers analyzed the gravity signatures of more than 1,200 craters (in yellow) on the far side of the moon. Credit: Courtesy of the researchers

The far side of the moon ie Lunar Highlands have been so heavily bombarded – particularly by small asteroids – that the impacts completely shattered the upper crust, leaving these regions essentially as fractured and porous as they could be. The scientists found that further impacts to these highly porous regions may have then had the opposite effect, sealing up cracks and decreasing porosity.

4 billion years ago, during the Late Heavy Bombardment, the moon took a severe beating, as an army of asteroids pelted its surface, carving out craters and opening deep fissures in its crust. Such sustained impacts increased the moon’s porosity, opening up a network of large seams beneath the lunar surface. The researchers observed this effect in the upper layer of the crust = megaregolith. This layer is dominated by relatively small craters, 30 kilometers or less in diameter. In contrast, it appears that deeper layers of crust, that are affected by larger craters, are not quite as battered, and are less fractured and porous.

MIT’s Jason Soderblom says the evolution of the moon’s porosity can give scientists clues to some of the earliest life-supporting processes taking place in the solar system. “The whole process of generating pore space within planetary crusts is critically important in understanding how water gets into the subsurface,” Soderblom says. “On Earth, we believe that life may have evolved somewhat in the subsurface, and this is a primary mechanism to create subsurface pockets and void spaces, and really drives a lot of the rates at which these processes happen. The moon is a really ideal place to study this.”

The team used data obtained by NASA’s Gravity Recovery and Interior Laboratory (GRAIL) – twin spacecraft that orbited the moon throughout 2012, each measuring the push and pull of the other as an indicator of the moon’s gravity. With GRAIL data, researchers mapped the gravity field in and around more than 1,200 craters on the far side of the moon. This region, the lunar highlands, makes up the moon’s most ancient, heavily cratered terrain. They then carried out Bouger correction to subtract the gravitational effect of mountains, valleys, and other topology from the total gravity field. What’s left is the gravity field beneath the surface, within the moon’s crust.

For craters smaller than 30 km in diameter, impacts both increased and decreased porosity in the upper layer of the moon’s crust. The researchers found that larger craters, which excavated much deeper into the moon’s crust, only increased porosity in the underlying crust ie these deeper layers have not reached a steady state in porosity, and are not as fractured as the megaregolith.

Gravity signatures of the larger craters in particular may provide insight into just how many impacts the moon, and other terrestrial bodies, sustained during the Late Heavy Bombardment. Ultimately, tracing the moon’s changing porosity may help track trajectory of the moon’s impactors 4 billion years ago.”This will help to understand the origin of the Late Heavy Bombardment, and whether it was disrupted material from the asteroid belt, or if it was further out.” http://news.mit.edu/2015/moon-crust-fractured-0910