[meteorite-list] Solar System Ice: Source of Earth's Water

[Ron Baalke]

http://carnegiescience.edu/news/solar_system_ice_source_earth%E2%80%99s_water  

Solar System Ice: Source of Earth's Water
Carnegie Institution of Science
July 12, 2012

Washington, DC  - Scientists have long believed that comets and, or a
type of very primitive meteorite called carbonaceous chondrites were the
sources of early Earth's volatile elements - which include hydrogen,
nitrogen, and carbon - and possibly organic material, too. Understanding
where these volatiles came from is crucial for determining the origins
of both water and life on the planet. New research led by Carnegie's
Conel Alexander focuses on frozen water that was distributed throughout
much of the early Solar System, but probably not in the materials that
aggregated to initially form Earth.

The evidence for this ice is now preserved in objects like comets
and water-bearing carbonaceous chondrites. The team's findings
contradict prevailing theories about the relationship between these two
types of bodies and suggest that meteorites, and their parent asteroids,
are the most-likely sources of the Earth's water. Their work is
published July 12 by Science Express.

Looking at the ratio of hydrogen to its heavy isotope deuterium in
frozen water (H2O), scientists can get an idea of the relative distance
from the Sun at which objects containing the water were formed. Objects
that formed farther out should generally have higher deuterium content
in their ice than objects that formed closer to the Sun, and objects
that formed in the same regions should have similar hydrogen isotopic
compositions. Therefore, by comparing the deuterium content of water in
carbonaceous chondrites to the deuterium content of comets, it is
possible to tell if they formed in similar reaches of the Solar System.

It has been suggested that both comets and carbonaceous chondrites
formed beyond the orbit of Jupiter, perhaps even at the edges of our
Solar System, and then moved inward, eventually bringing their bounty of
volatiles and organic material to Earth. If this were true, then the ice
found in comets and the remnants of ice preserved in carbonaceous
chondrites in the form of hydrated silicates, such as clays, would have
similar isotopic compositions.

Alexander's team included Carnegie's Larry Nitler, Marilyn Fogel,
and Roxane Bowden, as well as Kieren Howard from the Natural History
Museum in London and Kingsborough Community College of the City
University of New York and Christopher Herd of the University of
Alberta. They analyzed samples from 85 carbonaceous chondrites, and were
able to show that carbonaceous chondrites likely did not form in the
same regions of the Solar System as comets because they have much lower
deuterium content. If so, this result directly contradicts the two
most-prominent models for how the Solar System developed its current
architecture.

The team suggests that carbonaceous chondrites formed instead in the
asteroid belt that exists between the orbits of Mars and Jupiter. What's
more, they propose that most of the volatile elements on Earth arrived
from a variety of chondrites, not from comets.

"Our results provide important new constraints for the origin of
volatiles in the inner Solar System, including the Earth," Alexander
said. "And they have important implications for the current models of
the formation and orbital evolution of the planets and smaller objects
in our Solar System."
 
This work was partially funded by NASA Cosmochemistry, the NASA
Astrobiology Institute, Carnegie Institution of Canada, the Natural
Sciences and Engineering Research Council of Canada , the W.M. Keck
Foundation , and the UK Cosmochemical Analysis Network. For supplying
the many samples that were necessary for this work, the authors would
like to thank: the members of the Meteorite Working Group; Cecilia
Satterwhite and Kevin Righter (NASA, Johnson Space Center); Tim McCoy
and Linda Welzenbach (Smithsonian Museum for Natural History); Laurence
Garvie (Arizona State University); Sara Russell, Caroline Smith, and
Deborah Cassey (Natural History Museum, London).