July 12, 2010

Earth Younger Than Previously Thought

Earth could be as much as 70 million years younger than scientists originally believed, according to a new study which claims that it could have taken the planet 100 million years to reach its current size instead of the widely believed 30 million years.

Dr. John Rudge, a research fellow at the University of Cambridge's Trinity College, along with an international team of experts, compared geochemical information originally found in the Earth's mantle and compared it to the same material from meteorites from about the same era.

According to their findings, which were published in Nature Geoscience, accretion--the Earth's developmental process--slowed considerably once the planet reach 60 percent of its present size. As a result, Dr. Rudge and his associates claim that the formation of the Earth took more than three times as long as previously thought, and as a result, the planet as we know it has been in existence for less time than had been once accepted.

"The whole issue hinges on working out how long it took for the core of the Earth to form, which is one of the big unknowns in this area of science," said Dr. Rudge, the co-author of the study, according to the University of Cambridge web site.

As a result of their research, Dr. Rudge and his colleagues believe that Earth is approximately 4.467 billion years old. Previously, scientists believed the planet was 4.537 billion years old.

"One of the problems has been that scientists usually presume Earth's accretion happened at an exponentially decreasing rate," the Cambridge university fellow added. "We believe that the process may not have been that simple and that it could well have been a much more staggered, stop-start affair."

The speed of the accretion is the only issue being tackled by the international team of researchers, who support the longstanding theory that the Earth was originally formed when dozens of smaller planetary bodies (also known as planetary embryos) collided and blended together due to the heat caused by the impact. Some experts believe that the process was completed after a Mars-sized planetary embryo crashed into the forming Earth, causing part of it to break off and form the Moon.

"The collisions caused part of the planet to melt, and allow metal to segregate to the centre of the Earth to form the core," Dr. Rudge told BBC News science reporter Victoria Gill on Monday, adding that during the process, "the planet differentiated into its molten metal core and outer-lying mantle."

According to a Cambridge University press release, Rudge and his team studied isotopes from various elements that would have undergone radioactive decay during the accretion process. Assuming that certain elements had an affinity for the silicate mantle and others were naturally attracted to metal in the center of the Earth, the scientists created several different, new models that showed how the accretion process could have occurred.

"While a wide variety of options emerged, the modeling process showed that the Earth almost certainly could not have formed within 30 million years," the university's Sunday news report claimed. "Instead, the results suggested that the planet initially grew very quickly, reaching two-thirds of its size within about 10 to 40 million years. Accretion then slowed down, however, and took perhaps another 70 million years to complete."

The study, which is entitled, "Broad bounds on Earth's accretion and core formation constrained by geochemical models," was published online May 23.

Also credited as co-author on the study are Thorsten Kleine and Bernard Bourdon. All three researchers are credited as being members of the Institute of Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland, while Kleine is also credited as representing the Institut fr Planetologie, Westfälische Wilhelms-Universität Mnster, Mnster, Germany.

In the abstract of their report, Rudge and his associates write the following: "The Earth formed through the accretion of numerous planetary embryos that were already differentiated into a metallic core and silicate mantle. Prevailing models of Earth's formation, constrained by the observed abundances of metal-loving siderophile elements in Earth's mantle, assume full metal--silicate equilibrium, whereby all memory of the planetary embryos' earlier differentiation is lost."

"Using the hafnium--tungsten (Hf"“W) and uranium--lead (U"“Pb) isotopic dating systems, these models suggest rapid accretion of Earth's main mass within about 10 million years (Myr) of the formation of the Solar System. Accretion terminated about 30 or 100 Myr after formation of the Solar System, owing to a giant impact that formed the Moon. Here we present geochemical models of Earth's accretion that preserve some memory of the embryos' original differentiation."

"These disequilibrium models allow some fraction of the embryos' metallic cores to directly enter the Earth's core, without equilibrating with Earth's mantle," they added. "We show that disequilibrium models are as compatible with the geochemical observations as equilibrium models, yet still provide bounds on Earth's accretion and core formation"¦ Our results indicate that only 36-percent of the Earth's core must have formed in equilibrium with Earth's mantle. This low degree of equilibration is consistent with the siderophile element abundances in Earth's mantle."


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