National Science Foundation 10-084 - 13 May 2010
Silver Tells a Volatile Story of Earth’s OriginTiny variations in silver composition indicate water was in Earth's original building blocks
Tiny variations in the isotopic composition of silver in meteorites and Earth rocks are helping scientists put together a timetable of how our planet was assembled, beginning 4.568 billion years ago.
Results of a new study, funded by the National Science Foundation (NSF) and published this week in the journal Science, indicate 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.
The silver isotopes also presented another riddle, suggesting that Earth's core formed about five to 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's 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 Earth today.
Carnegie Institution of Science - 13 May 2010
Heterogeneous Accretion and the Moderately Volatile Element Budget of Earth
- Science 14 May 2010: Vol. 328. no. 5980, pp. 884 - 887, DOI: 10.1126/science.1186239