Perspective view looking to the northeast of the moon reveals a small volcanic province.
Human beings have been staring at the moon since long before we were human beings at all. Far back in biological history, some light-sensitive eyespot on some prehuman thing must have first registered the shimmer of moonlight, and the lunar love affair began.
For most of the eons that have passed since then, we only looked at part of the moon — the half that is eternally pointed toward Earth. It wasn't until 1968, during the flight of Apollo 8, that the first modern human eyes got an in-person look at the mysterious far side.
The differences between the two hemispheres could not be clearer. While the facing side of the moon features vast, dark plains of cooled lava — which the ancients assumed were seas — the far side is mostly an expanse of tens of thousands of impact craters. It is the tug of the Earth, astronomers believe, that is responsible for the different topography. Earthly gravity pulls with greater force on the dense, iron-and-magnesium interior of the moon than on the lighter upper layers. This causes the core to shift slightly earthward, thinning out the crust on that half of the moon. Volcanoes or meteor impacts on the near side could thus cause more copious lava bleeds, which spread out across the surface and form plains. The far side had a tougher hide and was thus less easily damaged.
That, in any case, is part of the story. But when the Lunar Prospector spacecraft orbited the moon in 1998, it found something curious: a bright bull's-eye of radioactive thorium on the far side of the moon between the craters Compton and Belkovich — a formation that seemed suspiciously volcanic. Now the next-generation Lunar Reconnaissance Orbiter (LRO) has turned its optical cameras on the site and has indeed discovered a vented mountain in the center of the thorium field, suggesting that not only is volcanism responsible but a particularly rare type of volcanism — at least on the moon — that produces lighter silicas instead of heavier basalts. What's more, while all lunar volcanoes were assumed to have last stirred 3 billion to 4 billion years ago, this one appears much fresher — just a billion or so years old.
"To find evidence of this unusual composition located where it is and appearing to be relatively recent ... is a fundamentally new result and will make us think again about the moon's volcanic and thermal evolution," says planetary scientist Bradley Jolliff of Washington University, who led the LRO research team.
From its earliest history, the moon was never going to be as geologically active a place as Earth. It was just too small and thus cooled fast, shutting down its thermal engines when it was still relatively young. Without that internal heat and the fluidity that results, the moon could not develop Earth's complex system of plate tectonics that keeps rocks recycling and allows shallow deposits of lava to crystallize into lighter silicates.
All this should have ruled out the phenomenon the Lunar Prospector team spotted and the LRO team confirmed. But what the LRO's cameras recorded was unmistakable: a volcanic field up to 35 km (22 miles) across, with a peak at the center defined by a distinctive volcanic vent. The presence of thorium, which is often found mixed with silicates, was the telltale marker of a particularly shallow volcano. And the freshness of the field — comparatively free of meteor craters — put its formation at just a billion years ago.
"We see small-scale features that haven't been completely beaten up and obliterated by the impact process," says Jolliff.
Jolliff and his colleagues can't say with certainty how the volcano formed, but they are reasonably certain it can't be due to heat generated by radioactive deposits in the moon's core, since they would have long ago decayed. Instead, the outer core of the moon might not be cold and solidified, as geologists assumed, but rather may still be molten. "A pulse of heat from deep in the mantle might melt a pocket of ... rock at the base of the crust," Jolliff says. "As this lava began to crystallize, it would have differentiated to produce a more silicic melt that was enriched in thorium."
O.K., it might take a geologist to get even remotely excited by a phrase like "a more silicic melt that was enriched in thorium," and at some point most people lose the thread of why such arcane rock science matters. But this rock science might challenge some of our most basic assumptions about how Earth's closest cosmic companion formed — and how active it remains today. Greater insight could come later this year when NASA launches the GRAIL mission (an acronym for gravity recovery and interior laboratory), in which a pair of satellites will orbit the moon in tandem and study lunar gravity by measuring tiny fluctuations in the distance between the two craft. This, in turn, will yield more data about the moon's internal anatomy.
For now, robots will be the only way to study these questions, since NASA's most recent manned moon program has been scrapped. "What we really need to test this and other new ideas about the moon," says Jolliff, "is sustained human exploration of our nearest and geologically very interesting neighbor." That's not going to happen anytime soon, but for now, the machines are doing an impressive job in our stead.