All told, Apollo astronauts collected 842 pounds (382 kg) of rocks and dust during their six lunar landings. Three robotic Luna missions netted Soviet scientists another 10½ ounces (0.3 kg). Over the years hundreds, if not thousands of analyses on these precious samples have proved without doubt that, geologically speaking, the Moon is very old and very dead.
But it's a single pea-size nugget, collected during Apollo 16 and weighing only 1.88 grams, that has lunar geochemists in a tizzy right now. New results from Lars Borg (Lawrence Livermore National Laboratory) and three colleagues, published online last week in Nature, argue that the Moon has been lying a bit about its age. Instead of being at least 4.43 billion years old, as earlier assays had shown, the group determined an age of 4.36 billion years, give or take 3 million.
A difference of 70 million years might seem inconsequential — it's a change of less than 2% overall. But it makes a huge difference in the fast-paced environment of the primordial solar system. Borg and his team assert either that the Moon got splated into existence long after the inner solar system's chaotic collisional pinball had quieted down, or that all those anorthosite samples don't really represent the Moon's primordial crust. Either way, it's a potential game-changer for lunar science. (Provocatively, the researchers titled their paper "Chronological evidence that the Moon is either young or did not have a global magma ocean.")
Most every lunar researcher now accepts that the Moon formed after a Mars-size protoplanet careened into the just-formed Earth, jetting massive blobs of white-hot debris into orbit that rapidly coalesced into a sizable satellite. The Apollo 11 samples showed that much of the lunar crust consists of a rock "froth" that must have floated atop a deep, global ocean of magma as the Moon cooled and solidified. Geochemist G. Jeffrey Taylor (University of Hawaii) offers a clear explanation of this process here.
The Apollo samples show that the lunar crust is mostly ferroan anorthosite, or FAN, a low-density silicate mix that contains a little iron. Rare on Earth, anorthosite is notoriously difficult to date accurately. Modern age-dating techniques utilize the slow decay rates of uranium to lead and samarium to neodymium, isotopes hardly present in its crystals. Also, four eons of interplanetary pummeling can heat and fracture lunar rocks enough to alter the isotopic ratios.
Knowing all this, Borg and his team were extremely cautious in analyzing a carefully selected bit of lunar rock 60025. Their delicate work utilized improved lab techniques that eliminated potential sources or error, and three different isotopic "clocks" all yielded the same age to within a few million years.
So this new, younger-than-expected age seems (forgive me) rock solid. But is this single determination representative of the entire Moon? Lots of other careful isotopic dating have yielded an uncomfortably wide range of ages — many older, a few younger — for both the Moon's anorthositic crust and for the volcanic lavas that later formed the dark maria. "Our group is responsible for about half of the older FAN data," Borg told me, "and we do not like to cast doubt on our previous work."
Stephen Mojzsis, a lunar specialist at the University of Colorado, is skeptical. "To rubbish the whole geochronological history of FAN based on the analysis of one rock and then to apply that to the differentiation history of the Moon, is far too 'adventurous' a conclusion in my mind," he says.
There's always a catch, and in this case it's tiny bits of the mineral zircon gleaned from an Apollo 17 sample. In 2009 a team led by Alex Nemchin (Curtin university, Australia) used these tiny crystals to peg the lunar age at 4.42 billion years. "Zircon is a particularly useful mineral for age dating because its radiometric clock is not easy to reset by thermal events," explains Taylor. "It contains easily measurable amounts of uranium, making it ideally suited for uranium-lead dating."
So why is Borg's team getting a much-younger date? Perhaps, he suggests, the sample he studied didn't come from the Moon's original crust but from a version 1.1 created after some or all of the initial layer remelted and recrystallized. If that's the case, then early lunar history is far more complicated than most researchers are willing to accept.
But there is another way: force the Moon's magma ocean to cool very slowly over, say, 200 million years. Last year, MIT researchers Jennifer Meyer, Linda Elkins-Tanton, and Jack Wisdom proposed that the infant Moon, being much closer to Earth than it is now, would have experienced tidal stress strong enough to keep its just-formed crust flexing back and forth. This repeated distortion created heat that kept the underlying magma from completely solidifying for some 200 million years. (The gory details are here, but Elkins-Tanton provides a somewhat-simpler summary here.)
Bottom line: hold your bets for now. With luck, the twin spacecraft for NASA's Gravity Recovery and Interior Laboratory, or GRAIL, mission will reveal the structure of the lunar interior with enough fidelity to sort out the correct sequence of events. GRAIL's launch is planned for September 8th, just 2½ weeks from now.