A giant plain on the nearside of the moon is bordered by ancient rift valleys that acted as a “magma plumbing system” for the region’s volcanoes billions of years ago, scientists say.  Researchers had thought that a rocky ridge around the 3,200km-wide plain, named the Ocean of Storms, was the edge of an enormous impact basin created when an asteroid crashed into the moon.  But maps drawn up from measurements taken by Nasa’s Grail (Gravity Recovery and Interior Laboratory) mission orbiters found that the rocky features were lava-filled valleys that operated like a plumbing system.  Though volcanoes continue to erupt on Earth, most of the moon’s volcanic activity took place three to four billion years ago. Because the gravity is so weak on the moon, explosive eruptions blasted debris far further than they do on Earth.  Jeff Andrews-Hanna, a geophysicist at the Colorado School of Mines, used maps from the Grail mission to show that the rocky border around the lunar plain was not circular as would be expected from an asteroid impact. Instead, the rocky ridges were long rift valleys that had filled with lava and frozen over.  “This was a really surprising observation,” said Andrews-Hanna, whose study appears in the journal Nature. “It has really challenged our understanding of the evolution of the moon. There’s the very fact that we’re talking about rift valleys on the moon. Those are well known on Earth, and on Mars and Venus, but not on the moon.”  Soon after the moon’s formation, an abundance of radioactive elements made the Ocean of Storms warmer than the rest of the lunar surface. Andrews-Hanna said that the rifts might have formed as the plain cooled and contracted. “We think this whole province was pulling away from the moon around it and that would cause the extension to make the rift valleys,” he said.  The bright ancient highlands and dark blotches of basaltic lava make up the man-in-the-moon pattern of the lunar surface. The lava probably welled up through cracks in the rift valleys, which had thinned the crust of the fledgling moon.  Nasa’s Grail mission used two identical orbiters to create a detailed map of the moon’s gravitational field. The maps revealed hidden features beneath the surface, including regions called mass concentrations, or mascons, that generate a stronger-than-usual gravitational pull.  The probes, launched in 2011, mapped the moon’s gravity for nine months before reaching the end of their mission and signing off by flying into a mountain near the lunar north pole.
The discovery of rift valleys for the first time on the moon challenges conventional wisdom about our neighbour’s evolution