A new analysis of a lunar rock brought back by the 1972 Apollo 16 mission suggests that the moon could be tens of millions of years younger than previously thought. Another possibility, scientists say, is that current models of how the moon cooled in its early years may be totally wrong.
The predominant theory of the moon's origin holds that a Mars-sized object slammed into Earth soon after the solar system formed about 4.56 billion years ago. After the impact, large volumes of melted material splashed into space, coalesced, and cooled into today's moon. Previous studies of lunar rocks suggest that the sea of molten rock covering the lunar surface began to solidify anywhere between 4.43 billion and 4.53 billion years ago. But those dates aren't very precise, largely because the concentrations of the trace elements used in the dating techniques are extremely low, says Lars Borg, a planetary scientist at Lawrence Livermore National Laboratory California. Now Borg and his colleagues have used several methods of radioactive dating to come up with a new—and surprising—date for when the moon's magma ocean cooled.
The team analyzed a 1.88-gram sample of a moon rock brought back to Earth by Apollo 16, a chunk of a magnesium- and iron-rich silicate mineral called ferroan anorthosite. Using three separate dating techniques that measure the ratios of lead, neodymium, and samarium isotopes, the researchers estimate that the rock had crystallized about 4.36 billion years ago, plus or minus 3 million years, they report online today in Nature. These analyses are the first to produce consistent ages from multiple dating techniques on the same moon rock, the scientists contend. "This is the first really reliable age for this suite of rocks," Borg says.
It's not likely that the moon rock the team analyzed is a bit of lunar crust that was melted and then recrystallized long after the moon formed due to the impact of a comet or asteroid, Borg says. That's because the mineral crystals in the rock are large, a sign the rock had cooled slowly at a depth several kilometers below the lunar surface. Also, he notes, the ratios of samarium and neodymium isotopes in the sample suggest that the rock isn't a remelted blend of previously separate rocks. So, the researchers claim, the extraordinarily young age for the lunar sample means that either the moon solidified significantly later than most previous estimates or current models of how the moon's crust formed are incorrect .
In the first case, the moon may have coalesced from the debris of an early impact more slowly than current models suggest it should have, or it may have retained more heat than expected, delaying the cooling that generated a veneer of crust. But in the second case, if samples such as the one analyzed for this study didn't solidify from a molten sea of rock soon after the moon formed, then the entire theory of how rocky bodies cool and solidify—including notions about the geochemical effects on the resulting rocks and their isotopic ratios—is on shaky ground. This, in turn, could upturn many if not all of the radio dating schemes used to estimate the ages of ancient rocks or significant events in planetary history.
Either of those options is very exciting, says Alex Halliday, an isotopic geochemist at the University of Oxford in the United Kingdom. In any case, Borg and his colleagues "have done a fantastic job of putting together a beautiful study of this rock, one of the most pristine samples of early lunar crust," he notes. "The findings suggest that the moon had a fiery start at an age much later than previously considered."
But Clive Neal, a planetary geologist at the University of Notre Dame in Indiana, suggests that there may be other explanations for the rock's apparent youth. In one possible scenario, the dense minerals that formed atop a relatively frothy bit of the moon's primordial crust, still floating on a sea of molten rock, could have rendered the island unstable. Then, like a top-heavy iceberg, that bit of crust could have flipped over, causing the minerals to melt and then recrystallize, in essence resetting the clock and giving a false impression of when the moon actually formed.
Regardless, Neal notes, "lunar samples are still giving us wonderful insights decades after they were brought back to Earth." The possibility that future analytical techniques can yield even more precise answers reinforces the notion that such samples must be preserved for posterity, he adds. "Moon rocks are the gifts that keep on giving."