Twin spacecraft orbited the Moon on a three-month mission in 2012. Together, the two satellites of NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission mapped the Moon’s gravitational field to better learn about its internal structure. The extraordinarily detailed map that the mission produced contained mostly what the GRAIL scientists expected to find: Gravity is stronger above the mountains and weaker above the craters. Yet when they removed the topographical contributions, what remained were strangely linear features1 stretching up to roughly 500 km, as seen by the blue lines in figure 1. At the time, it was unlike anything the team had seen.
Now a research team led by Jeff Andrews-Hanna (University of Arizona) has explained what was causing the patterns. Building on work by Nan Zhang (Peking University) and colleagues, Andrews-Hanna and his group have demonstrated that dense, titanium-rich materials below the surface could shape the gravitational...
