Scientists have found a volcanic 'hot spot' on the far side of the moon, indicating that the moon's been more geologically active, more recently, than previously believed.
The hot spot is a concentration of the radioactive element thorium, sitting between the Compton and Belkovich impact craters, and was first detected by Lunar Prospector’s gamma-ray spectrometer in 1998.
But new observations, made with the Lunar Reconnaissance Orbiter (LRO) optical cameras, indicate not just volcanism but also much rarer silicic volcanism.
"That’s very unusual,” says says Bradley Jolliff, of Washington University in St. Louis, who led the team that analyzed the LRO images.
"There are only about a half dozen other features on the Moon that are thought to be silica-rich, because the Moon, unlike the Earth, does not reprocess rock materials in a way that concentrates silica."
The volcanic province’s existence will force scientists to modify ideas about the moon’s volcanic history,
"To find evidence of this unusual composition located where it is, and appearing to be relatively recent volcanic activity is a fundamentally new result and will make us think again about the Moon’s thermal and volcanic evolution," says Jolliff.
Jolliff and his team suspect the newly discovered volcanic province might be much younger than most of the volcanic features in the Procellarum KREEP Terrane.
"Although we know from direct analysis of lunar rock samples that most of mare volcanism occurred three to four billion years ago, we can see from orbit some mare basalt flows that might have occurred as recently as one billion years ago," he says.
"If this volcanic province formed very late in the game, it couldn’t have been due to radioactive decay because those heat sources diminish with time and it gets harder and harder to get lavas to the surface."
However, says Jolliff, the moon may still have a molten outer core, generating pulses of heat like the Hawaiian volcanic chain. The GRAIL mission, due to launch later this year, could confirm this.