Silver reveals volatile story of Earth's origin

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Washington, May 14 (ANI): Miniscule variations in the isotopic composition of silver found in meteorites and Earth rocks is giving scientists clues about how Earth began assembling 4.568 billion years ago.

The study suggests that water and other key volatiles may have been already present on the Earth rather than deposited on it by meteors.

Earth has fewer reserves of volatile material like hydrogen, carbon, and nitrogen as compared to other planets in the solar system that lie in its inner, hotter parts.

"A big question in the formation of the Earth is when this depletion occurred," says co-author Richard Carlson of the Carnegie Institution for Science's Department of Terrestrial Magnetism. "That's where silver isotopes can really help."

The differences in the two isotopes - silver and palladium, allowed the Carnegie researchers, which included Carlson lead author Maria Schvnbdchler (a former Carnegie Institution postdoctoral scientist now at the University of Manchester) Erik Hauri, Mary Horan, and Tim Mock to use the isotopic ratios in primitive meteorites and rocks from Earth's mantle to determine the history of Earth's volatiles relative to the formation of Earth's iron core. Other evidence, specifically from hafnium and tungsten isotopes, indicates that the core formed between 30 and 100 million years after the origin of the Solar System.

The silver isotopes also presented another riddle, suggesting that the Earth's core formed about 5-10 million years after the origin of the Solar System, much earlier than the date from the hafnium-tungsten results.

The group concludes that these contradictory observations can be reconciled if Earth first accreted volatile-depleted material until it reached about 85 percent of its final mass and then accreted volatile-rich material in the late stages of its formation, about 26 million years after the Solar System's origin. The addition of volatile-rich material could have occurred in a single event, perhaps the giant collision between the proto-Earth and a Mars-sized object thought to have ejected enough material into Earth orbit to form the Moon.

The results of the study support a 30-year old model of planetary growth called "heterogeneous accretion," which proposes that the Earth's building blocks changed in composition as the planet accreted. Carlson adds that it would have taken just a small amount of volatile-rich material similar to primitive meteorites added during the late stages of Earth's accretion to account for all the volatiles, including water, on the Earth today.

The study has been published in the journal Science. (ANI)

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