Suspended in the sky, the moon has stared unblinking at the Earth for billions of years. But new work suggests the placid sphere’s two faces may belie a violent childhood — one that involved the death of a small celestial companion. The moon may also be lying about when it was born, by millions of years.
Together, the studies, published in August in Nature, lend a somewhat shady character to this perennial source of both fear and inspiration. With the Lunar Reconnaissance Orbiter already keeping a close eye on the moon (SN Online: 9/18/09) and the recently launched twin GRAIL spacecraft (SN Online: 9/20/11) tasked with mapping its interior, scientists hope to uncover more clues to the satellite’s past.
Not since the Apollo era has Earth’s nearest neighbor attracted such attention and resources.
“There’s definitely a resurgence in lunar science,” says planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colo. “The moon will always be a singularly important object in many ways. It’s our moon. Its history is tied to us. It’s not just proximity; it’s what we think of as a shared history of origin.”
Most scientists believe that the shared history dates to just over 4.5 billion years ago, when the Earth and a Mars-sized object collided. The resulting impact blasted molten debris into space, where it formed a disk around the planet. Eventually, the fiery material coalesced and became the early moon — a big ball of crackling rock and the host of a global magma ocean. As that ocean solidified and cooled, it differentiated into the lunar crust and mantle, forming the rocks later plucked from the surface by those legendary space cowboys, the Apollo astronauts.
But when planetary geologist Lars Borg recently investigated one of these moon rocks in a mass spectrometer, the ingredients hinted at a revision to the classic tale.
New lunar years
Borg, of the Lawrence Livermore National Laboratory in California, and his colleagues set out to determine the rock’s age by comparing levels of different isotopes, versions of an element differing in the number of neutrons in their atomic nuclei.
The rock would reveal when minerals floated atop the lunar magma ocean and solidified into crust, marking the moon’s formation. After looking at the levels of decaying lead and neodymium isotopes, though, Borg’s team found that this particular rock was much younger than expected, and not just by a bit. It was only 4.36 billion years old, about 150 million years younger than theory would suggest.
The result, published in the Aug. 18 Nature, is surprising, but it’s hard to argue with. Scientists agree that the team’s methods were solid, the analyses careful.
“Everybody’s question, including our own, is ‘What does this mean?’ ” Borg says.
One possibility is that the moon cooled slowly. “But that gets to be problematic, because there’s good evidence that cooling happened rapidly,” Borg says. Another option is that the rock came from a regional magma sea or some other process, rather than the global magma ocean believed to be the source of such rocks.
Linda Elkins-Tanton, a planetary scientist at the Carnegie Institution for Science in Washington, D.C., who studies magma oceans, says she’s not yet ready to rewrite the global magma ocean story. “I would be very cautious about changing our understanding of lunar solidification based on one age,” she says.
She suggests that scientists go back and redate older moon rocks — including some as old as 4.45 billion years — using the improved analysis.
Clive Neal of the University of Notre Dame in Indiana agrees. “I don’t think you can throw the baby out with the bathwater here. Go back and look at some of the samples with older ages…. Do the same careful work that Borg et al did.”
Redating these samples could reveal that the moon is in fact younger than previously thought — a third explanation for the age of Borg’s rock — though scientists aren’t ready to support such a conclusion yet.
Companion moonlet
For now, the mystery of the moon’s age remains unsolved, joining another lingering puzzle regarding the satellite’s past: why the hemispheres are so different. The nearside is smoother, with large volcanic basins. The farside is mountainous and cratered, with elevations exceeding 8 kilometers.
Planetary scientist Erik Asphaug of the University of California, Santa Cruz and Martin Jutzi of the University of Bern in Switzerland have a new explanation for this disparity. The moon might not have always been a companionless, lonely crescent, they suggested in the Aug. 4 Nature. Once, it might have had a friend sharing its orbit, a smaller moonlet that adorned Earth’s skies with another shifting face for tens of millions of years. But, in this team’s story, that friendship had a sad ending.
One day, in a slow-motion collision, the moonlet smacked into the moon’s farside. Because the splat happened slowly, it didn’t produce a crater. Instead, the moonlet pancaked, covering the moon’s backside with rocks.
The extra material deposited on the lunar backside would explain the asymmetrical thickness of the lunar crust, which varies by as much as 50 kilometers between the hemispheres. Such a smack could have also pushed a residual magma ocean and subsurface metallic layer to the moon’s nearside, explaining the elemental composition beneath that portion of the satellite’s skin.
“I’m pretty cautious about running head over heels about a hypothesis,” Asphaug says, “but of the models I’ve put together involving collisions, this one has the most pieces that go click.”
Canup likes the new idea. “It’s definitely out of the box compared to the prior explanations,” she says, referring to existing theories that link internal lunar processes to the moon’s two different faces. She has produced several simulations of the original moon-forming giant impact — which Asphaug and Jutzi based their work on — and her models have included short-lived smaller moons.
Since they are littler, these moonlets would solidify more quickly and be made of rocks with older ages than the moon’s.
Borg says he would expect some small fraction of the moonlet’s shattered remains to litter the nearside. Perhaps some of them were even swiped by the Apollo astronauts, explaining the range of ages now measured from moon rocks.
“One interpretation is that this age range really reflects the older ages from the backside mixing with younger ages on the nearside,” Borg says. But he thinks it is more likely that the older ages on some of the rocks are unreliable.
By mapping the moon’s gravity field, the GRAIL mission might uncover new clues about the moon’s interior structure, such as how thick the crust is and whether a global magma ocean ever existed. But the best way to address the new hypotheses presented by Borg’s and Asphaug’s teams is to collect more moon rocks, including from the farside highlands.
Such rocks are more than souvenirs brought back from Earth’s companion, and they’ll do more than help scientists unlock the mysteries of the moon. The extraterrestrial chunks may also hold clues to how terrestrial bodies form, including the watery sphere people call home.
“Once every couple of months, we get out the telescope, we get out the kids, and we look up at the moon in the backyard. And I explain how it might be relevant for the ground they’re standing on,” Borg says. “We look at the Apollo landing site. I’m just left with a sense of awe.”
