Water was Earth's Silver Lining from the Start
Silver tells a volatile story of Earth's origin
The new study, published in the journal Science, indicates that water and other key volatiles may have been present in at least some of Earth's original building blocks, rather than acquired later from comets, as some scientists have suggested.
Compared to the solar system as a whole, Earth is depleted in volatile elements, such as hydrogen, carbon, and nitrogen, which likely never condensed on planets formed in the inner, hotter, part of the solar system. Earth is also depleted in moderately volatile elements, such as silver.
"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."
Silver has two stable isotopes, one of which, silver-107 was produced in the early solar system by the rapid radioactive decay of palladium-107. Palladium-107 is so unstable that virtually all of it decayed within the first 30 million years of the solar system's history.
"We found that the silver isotope ratios in mantle rocks from the Earth exactly matched those in primitive meteorites," says Carlson. "But these meteorites have compositions that are very volatile-rich, unlike the Earth, which is volatile-depleted."
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 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.
"Though I accept that about 85 per cent of the Earth's mass was built without volatile elements, the rest of it was- and that's quite an important difference in our understanding of the Earth's geological history," said Schönbächler. "We don't now need any theories about how water came to Earth - such as comets and asteroids - it was most likely here almost from the beginning. And water is what made Earth habitable for life."