Lunar volcanoes suggest the moon may still be warm
The man in the moon may still have some fire in his belly. A new study argues that magma erupted onto the lunar surface less than 100 million years ago – nearly a billion years later than previously thought. If confirmed, the finding suggests that radioactive elements may be keeping the moon's innards toasty even today.
The moon is thought to have formed from the debris of a collision between Earth and a Mars-sized body about 4.5 billion years ago. Its fiery birth kept its surface molten for a few hundred million years. But even after its crust solidified, magma regularly erupted onto the moon's surface until about 3 billion years ago, creating vast basaltic plains known as maria. After that, the eruptions largely stopped, with the most recent volcanic features dating to about a billion years ago.
Now Sarah Braden at Arizona State University in Tempe and colleagues say that dozens of small rocky formations spotted by NASA's eagle-eyed Lunar Reconnaissance Orbiter were laid down by lava no more than 100 million years ago – a geological eye-blink.
The spacecraft, which has been orbiting the moon since 2009, can make outdetails as small as 50 centimetres across, providing the best orbital view yet of the moon's surface. Scouring the images, Braden and her team found 70 regions that stood out from their surroundings, most of which were new to science.
Called irregular mare patches, or IMPs, the features measure less than 5 kilometres across, and are sprinkled over the moon's near side within the larger maria. They have two-toned textures, with smooth, usually dark, rock lying over rougher, blockier rock. That may be because the first lava to emerge during an eruption formed a rough crust that was then overlaid by smoother flows, Braden says.
The IMPs appear relatively fresh-faced compared with their surroundings, suggesting they have experienced no more than 100 million years of impacts from space rocks, the team say. If so, "the moon was more active in recent history than previously thought possible", says Braden.
"Young volcanism indicates possibly more magma, or magma at higher temperatures, or magma at shallower depths, or all of the above," she says. The heat powering this activity may come from gravitational tugs from Earth or the decay of radioactive elements beneath the moon's surface.
"This paper demonstrates how much we don't know about the moon," saysPeter Schultz at Brown University in Providence, Rhode Island, who wasn't involved with the work.
But he has another explanation: he believes magma lying deep within the moon produces gas that seeps up through cracks and occasionally bursts through the surface. In 2006, he suggested such bursts could explain the few IMPs known at the time. He also believes that this "degassing" has occurred even more recently than Braden's estimate for volcanism, with a 3-kilometre-wide IMP named Ina forming no more than 10 million years ago.
Both explanations suggest that "the moon isn't dead", he says. "We need to visit such sites to understand what happened – or could still be happening."