[meteorite-list] Water Was Present During Birth of Earth

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri, 14 May 2010 23:36:09 -0500
Message-ID: <D6A2AD87784A4D37A6A72C640E5CADCE_at_ATARIENGINE2>

http://www.sciencedaily.com/releases/2010/05/100513143457.htm

Water Was Present During Birth of Earth

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. 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.

Silver and palladium differ in their chemical properties.
Silver is the more volatile of the two, whereas palladium
is more likely to bond with iron. These differences allowed
the Carnegie researchers, which included Carlson, lead
author Maria Sch?nb?chler (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 to 100 million years after the origin of
the Solar System.

"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 group concludes that these contradictory observations
can be reconciled if Earth first accreted volatile-depleted material
until it reached about 85% 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.
Received on Sat 15 May 2010 12:36:09 AM PDT