From: Andy Soos, ENN
Published August 19, 2011 05:02 PM

The Age of the Moon and the Earth

Man lives the traditional 3 score and ten. How old the Moon and Earth? New research using a technique that measures the isotopes of lead and neodymium in lunar crustal rocks shows that the moon and Earth may be millions of years younger than originally thought. The common estimate of the moon's age is as old as 4.5 billion years old (roughly the same age as the solar system) as determined by mineralogy and chemical analysis of moon rocks gathered during the Apollo missions. However, Lawrence Livermore National Laboratory scientist Lars Borg and international collaborators have analyzed three isotopic systems, including the elements lead, samarium and neodymium found in ancient lunar rocks, and determined that the moon could be much younger than originally estimated. In fact, its age may be 4.36 billion years old.

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The age of the Earth is traditionally thought of as 4.54 billion years (4.54 × 109 years ± 1%). This age is based on evidence from radiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples.

The ages of Earth and Moon rocks and of meteorites are measured by the decay of long-lived radioactive isotopes of elements that occur naturally in rocks and minerals and that decay with half lives of 700 million to more than 100 billion years to stable isotopes of other elements. These dating techniques, which are firmly grounded in physics and are known collectively as radiometric dating, are used to measure the last time that the rock being dated was either melted or disturbed sufficiently to rehomogenize its radioactive elements.

Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also found in the Minnesota River Valley and northern Michigan (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years).  They have been dated by a number of radiometric dating methods and the consistency of the results give scientists confidence that the ages are correct to within a few percent.  

The oldest dated moon rocks, however, have ages between 4.4 and 4.5 billion years and provide a minimum age for the formation of our nearest planetary neighbor. Thousands of meteorites, which are fragments of asteroids that fall to Earth, have been recovered. These primitive objects provide the best ages for the time of formation of the Solar System.  The results show that the meteorites, and therefore the Solar System, formed between 4.53 and 4.58 billion years ago.       

The new Moon research has implications for the age of Earth as well. The common belief is that the moon formed from a giant impact into the Earth and then solidified from an ocean of molten rock (magma).

"If our analysis represents the age of the moon, then the Earth must be fairly young as well," said Borg, a chemist. "This is in stark contrast to a planet like Mars, which is argued to have formed around 4.53 billion years ago. If the age we report is from one of the first formed lunar rocks, then the moon is about 165 million years younger than Mars and about 200 million years younger than large asteroids."

Chemical evolution of planetary bodies ranging from asteroids to large rocky planets is thought to begin with differentiation through solidification of magma oceans hundreds of kilometers in depth. The Earth's moon is the typical example of this type of differentiation. However, one interpretation of Borg's findings is that this may not have occurred on the moon.

"The moon is supposed to be old and have a lunar magma ocean, but our new measurements show the moon is young and did not have a magma ocean," Borg said.

The research appears in the August 17 online edition of the journal, Nature.

For further information:  https://www.llnl.gov/news/newsreleases/2011/Aug/NR-11-08-02.html

Photo:  NASA

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