Jan. 26, 2009 -- Dating of the oldest-known piece of lunar zircon, brought back from our nearest neighbor in 1972, has pushed back the time when the moon's surface first formed.
The landmark find, published online today in Nature Geoscience, has allowed the team of German, Australian and U.S. scientists to give a "precise younger age limit" for the solidification of the moon's surface.
Lead author Alexander Nemchin, of Curtin University of Technology, said the moon is generally believed to have formed from the debris of a collision between the earth and a Mars-sized body more than 4.5 billion years ago.
Nemchin, a researcher within the Department of Applied Geology, said the heat generated by the coalescing of debris that formed the moon led to the creation of a magma ocean about 500 to 800 kilometers thick.
The cooling, or crystallization, of the lunar magma ocean resulted in the moon as we observe it today, he said.
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Nemchin said there has been considerable debate over the precise timing of this crystallization of the magma ocean.
"The timing of lunar magma ocean crystallization remains loosely constrained to the first 250 million years of lunar history," he said.
However he said analysis of the zircon grains can give a precise timing to this process because zircon is formed during the very last stages of solidification of the magma ocean.
Zircon is used to precisely date geological processes in rocks because it contains uranium which decays into lead at a known rate, Nemchin said.
By analyzing the ratio of lead and uranium isotopes in the lunar zircon, Nemchin and his colleagues, dated the zircon grain as being 4.42 billion years old.
Nemchin said this shows the moon's crust was almost completely formed within about 100 million years. He admits the finding is "only as good as the oldest zircon."
"If someone finds older material we can push it further," he said. "But we know [the cooling] is not going to take any longer."
Scientists have long known there was zircon on the moon. But after an initial focus on the mineral in the early days of lunar exploration, interest in zircon faded over the past 10-20 years.
Following his team's discovery last year of the world's oldest diamond trapped inside zircon crystals in the Jack Hills region of Western Australia, Nemchin applied to NASA to use the same dating techniques on moon rock.
"For us the projects are related," Nemchin said. "At this stage we believe a similar magma ocean once covered Earth."
"There is a common point in time when the moon was formed in a collision with Earth. The question then is whether the two magma oceans were the result of this collision or the terrestrial magma ocean was before this collision."
Nemchin said it is important to understand the details of the Earth's evolution as it "defines the whole history of the planet."
"If we understand what is going on [back] then, we will understand much better what we see now," he said.
The zircon grain supplied by NASA came from the Apollo 17 mission which landed at Taurus Litro on Dec. 11, 1972.
Nemchin said it appears this landing site is the "most promising" in terms of looking at the early history of the moon.
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