Earth's moon might not be as old as widely believed, as emerging new techniques date moon rocks to a more recent time.
The moon is thought to have formed from debris after a planet-sized body collided with the Earth 4.5 billion years ago. Many researchers have suggested that the new moon, formed freshly after collision, took tens of thousands of years to cool, but not everyone believes it cooled that rapidly.
An analysis on the moon rock has scientists thinking that the moon may be 200 million years younger than widely believed and it never had the magma ocean that scientists think covered its surface soon after it was formed.
The study was published in the Aug. 17 issue of Nature. But other scientists disagree with the new study conclusions and believe that the moon is 4.5 billion years old, as is originally believed.
Scientists tested moon rock samples collected by NASA's 1972 Apollo 16 mission landings - paying particular attention to ferroan anorthosites (FANs), which are thought to have been one of the last things to crystallize out of the magma oceans, forming the first lunar crust.
But FANs have been difficult to date as the rocks contain only tiny amounts of the lead isotopes normally used to date rocks and are contaminated with lead dust from Earth.
The researchers used three methods to calculate the ages of FANs with unprecedented precision by analyzing isotopes of lead, samarium and neodymium within carefully purified samples of these rocks.
All isotopes of an element have the same number of protons, but it is the number of neutrons that make them different from each other.
The researchers found that the three calculations resulted in similar ages, which is an average of about 4.36 billion years.
"After many years of trying, we have found a way to reliably date the ages of lunar crustal rocks with high precision. We can apply this technique to address many questions regarding the timing of ancient events on the moon," said the study lead author, Lars Borg, a planetary scientist at the Lawrence Livermore National Laboratory in California.
The researchers concluded that the formation of the rock was about 200 million years after the solar system's first solid materials came into existence, suggesting a much younger age of the moon than previously thought.
Another radical explanation that Borg proposed for the younger moon rock is that the analyzed crustal rock didn't form after the magma cooled down. Perhaps the moon never even had a magma ocean and the rocks were formed some other way.
Borg's discovery comes on the heels of a proposal that Earth once had two moons that later merged in a slow-motion collision.
This could explain the varying ages that have been reported for lunar FAN samples, but Borg is cautious: "The ages of most FANs are not determined with enough confidence or with enough temporal resolution to rigorously assess this possibility," he told Nature.
"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," said team member Richard Carlson of the Carnegie Institution for Science in Washington.
Borg's dating of the moon rock drew praise from other scientists but his analysis garnered criticism. Alex Halliday, an isotope geochemist from the Oxford University, believes that there are less dramatic explanations for the age of the moon rock. The crustal rock could simply not represent the oldest rocks on the surface.
"I hope it's going to cause a real stir," said Clive Neal, a planetary geologist at the University of Notre Dame, who was not involved in the study. But he added, the researchers need much more evidence that other rocks have been inaccurately dated before they jump to radically different theories about the moon's formation.