[meteorite-list] What Do We Know About The Origin of the Earth's Oceans?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 3 Oct 2012 11:53:33 -0700 (PDT)
Message-ID: <201210031853.q93IrXOZ009041_at_zagami.jpl.nasa.gov>

http://www.scientificamerican.com/article.cfm?id=what-do-we-know-about-the

What do we know about the origin of the earth's oceans? Is it more
likely that they derive from icy comets that struck the young earth or
from material released from the earth's interior during volcanic
activity?

Scientifc American


Tobias C. Owen of the Institute for Astronomy in Honolulu, Hawaii,
offers this overview:

"This is a very good question, because we do not yet have an answer that
everyone accepts.

"The origin of the oceans goes back to the time of the earth's formation
4. 6 billion years ago, when our planet was forming through the
accumulation of smaller objects, called planetesimals. There are
basically three possible sources for the water. It
could have (1) separated out from the rocks that make up the bulk of the
earth; (2) arrived as part of a late-accreting veneer of water-rich
meteorites, similar to the carbonaceous chondrites that we see today; or
(3) arrived as part of a late-accreting veneer of icy planetesimals,
that is, comets.

"The composition of the ocean offers some clues as to its origin. If all
the comets contain the same kind of water ice that we have examined in
Comets Halley and Hyakutake- -the only ones whose water molecules we've
been able to study in detail-- then comets cannot have delivered all the
water in the earth's oceans. We know this because the ice in the comets
contains twice as many atoms of deuterium (a heavy isotope of hydrogen)
to each atom of ordinary hydrogen as we find in seawater.

"At the same time, we know that the meteorites could not have delivered
all of the water, because then the earth's atmosphere would contain
nearly 10 times as much xenon (an inert gas) as it actually does.
Meteorites all carry this excess xenon. Nobody has yet measured the
concentration of xenon in comets, but recent laboratory experiments on
the trapping of gases by ice forming at low temperatures suggest that
comets do not contain high concentrations of the xenon. A mixture of
meteoritic water and cometary water would not work either, because this
combination would still contain a higher concentration of deuterium than
is found in the oceans.

"Hence, the best model for the source of the oceans at the moment is a
combination of water derived from comets and water that was caught up in
the rocky body of the earth as it formed. This mixture satisfies the
xenon problem. It also appears to solve the deuterium problem--but only
if the rocky material out near the earth's present orbit picked up some
local water from the solar nebula (the cloud of gas and dust surrounding
the young sun) before they accreted to form the earth. Some new
laboratory studies of the manner in which deuterium gets exchanged
between hydrogen gas and water vapor have indicated that the water vapor
in the local region of the solar nebula would have had about the right
(low) proportion of deuterium to balance the excess deuterium seen in
comets.

"The point to emphasize here is that this is a model, a working
hypothesis that must be rigorously tested by many additional
measurements. We need to study more comets. We also need to learn more
about the water on Mars, where we have another
chance to investigate the sources described above. On the earth, plate
tectonics has caused oceanic water to mix considerably with material
from the planet's interior; such contamination probably did not occur on
Mars, where plate tectonics does not seem to occur. These investigations
(and other related studies) are currently under way. This is an active
area of research!" **

James C. G. Walker of the University of Michigan confirms that
conclusion, adding his perspective:

"The best current thinking is that volatiles (elements and compounds,
including water, that vaporize at low temperatures) were released from
the solid phase as the earth accreted. Thus, the earth and its oceans
and atmosphere grew together.

"During accretion, the kinetic energy of the colliding planetesimals was
converted into thermal energy, so the earth grew extremely hot as it
came together. The material forming the earth was probably too hot for
ice to have been a major carrier of water. Most of the water was
probably present originally as water trapped in clay minerals or as
separate hydrogen (in hydrocarbons) and oxygen (in iron oxides), rather
than as ice.

"Since the end of the period of accretion, more than four billion years
ago, there has been a continual exchange of volatile material--including
water--between the surface of the earth and the planet's interior
(that is, between the crust and the mantle). Volcanoes release water
and carbon dioxide to the atmosphere and ocean. Subduction of sediments
rich in volatiles takes place at deep ocean trenches. The sinking of
oceanic crust at subduction zones carries water and carbon dioxide
back into the mantle. These processes can all be seen at work today.

"In short, icy cometary material probably has not been important in
providing water for the earth's oceans, but there is little sure
knowledge in this field."
Received on Wed 03 Oct 2012 02:53:33 PM PDT