Carnegie Institution for Science | 2011 Aug 17
Earth’s Moon could be younger than previously thought, according to new research from a team that includes Carnegie’s Richard Carlson and former-Carnegie fellow Maud Boyet. Their work will be published online in Nature on August 17.
The prevailing theory of our Moon’s origin is that it was created by a giant impact between a large planet-like object and the proto-Earth. The energy of this impact was sufficiently high that the Moon formed from melted material that was ejected into space. As the Moon cooled, this magma solidified into different mineral components.
Analysis of lunar rock samples thought to have been derived from the original magma has given scientists a new estimate of the Moon’s age.
According to this theory for lunar formation, a rock type called ferroan anorthosite, or FAN, is the oldest of the Moon’s crustal rocks, but scientists have had difficulty dating FAN samples. The research team, led by Lars E. Borg of the Lawrence Livermore National Laboratory, included Carlson of Carnegie’s Department of Terrestrial Magnetism, Boyet-- now at Université Blaise Pascal--and James N. Connelly of the University of Copenhagen. They used newly refined techniques to determine the age of a sample of FAN from the lunar rock collection at the NASA Johnson Space Center.
The team analyzed the isotopes of the elements lead and neodymium to place the FAN sample’s age at 4.36 billion years. This figure is significantly younger than earlier estimates of the Moon’s age that range as old as the age of the solar system at 4.568 billion years. The new, younger age obtained for the oldest lunar crust is similar to ages obtained for the oldest terrestrial minerals--zircons from western Australia--suggesting that the oldest crusts on both Earth and Moon formed at approximately the same time, and that this time dates from shortly after the giant impact.
This study is the first in which a single sample of FAN yielded consistent ages from multiple isotope dating techniques. This result strongly suggests that these ages pinpoint the time at which the sample crystallized.
“The extraordinarily young age of this lunar sample either means that the Moon solidified significantly later than previous estimates, or that we need to change our entire understanding of the Moon’s geochemical history,” Carlson said.
Moon younger than previously thought
University of Copenhagen | 2011 Aug 17
Analysis of a piece of lunar rock brought back to Earth by the Apollo 16 mission in 1972 has shown that the Moon may be much younger than previously believed. This is concluded in new research conducted by an international team of scientists that includes James Connelly from the Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen. Their work has just been published in Nature.
The prevailing theory of our Moon’s origin is that it was created by a giant impact between a large planet-like object and the proto-Earth very early in the evolution of our solar system. The energy of this impact was sufficiently high that the Moon formed from melted material that began with a deep liquid magma ocean.
As the Moon cooled, this magma ocean solidified into different mineral components, the lightest of which floated upwards to form the oldest crust. Analysis of a lunar rock sample of this presumed ancient crust has given scientists new insights into the formation of the Moon.
Luna rock from Apollo 16
“We have analysed a piece of lunar rock that was brought back to Earth by the Apollo 16 mission in 1972. Although the samples have been carefully stored at NASA Johnson Space Center since their return to Earth, we had to extensively pre-clean the samples using a new method to remove terrestrial lead contamination. Once we removed the contamination, we found that this sample is almost 100 million years younger than we expected," says researcher James Connelly of the Centre for Star and Planet Formation.
According to the existing theory for lunar formation, a rock type called ferroan anorthosite, also known as FAN, is the oldest of the Moon’s crustal rocks, but scientists have had difficulty dating samples of this crust.
Newly-refined techniques help determine age of sample
The research team, which includes scientists from the Natural History Museum of Denmark, Lawrence Livermore National Laboratory, Carnegie Institute’s Department of Terrestrial Magnetism and Université Blaise Pascal, used newly-refined techniques to determine the age of the sample of a FAN that was returned by the Apollo 16 mission and has been stored at the lunar rock collection at the NASA Johnson Space Center.
The team analysed the isotopes of the elements lead and neodymium to place the age of a sample of a FAN at 4.36 billion years. This figure is significantly younger than earlier estimates of the Moon’s age that range to nearly as old as the age of the solar system itself at 4.567 billion years. The new, younger age obtained for the oldest lunar crust is similar to ages obtained for the oldest terrestrial minerals - zircons from Western Australia - suggesting that the oldest crust on both Earth and the Moon formed at approximately the same time.
This study is the first in which a single sample of FAN yielded consistent ages from multiple isotope dating techniques. This result strongly suggests that these ages pinpoint the time at which this sample crystallised. The extraordinarily young age of this lunar sample either means that the Moon solidified significantly later than previous estimates – and therefore the moon itself is much younger than previously believed - or that this sample does not represent a crystallisation product of the original magma ocean. Either scenario requires major revision to previous models for the formation of the Moon.
Goodnight, Old Moon
Science NOW | Sid Perkins | 2011 Aug 17
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."
Chronological evidence that the Moon is either young or did not have a global magma ocean - LE Borg et al
- Nature (online 17 Aug 2011) DOI: 10.1038/nature10328
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