[meteorite-list] Critical Assessment of the Comet as Provider of Earth's Waters

From: MstrEman <mstreman_at_meteoritecentral.com>
Date: Wed, 3 Oct 2012 02:53:35 -0400
Message-ID: <CAPwdm9ERey0ZNoVAwMPMKh8BC+3pkGnG_yj9DiJb1KheYT-ekQ_at_mail.gmail.com>

 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?

Full Article at
<http://www.scientificamerican.com/article.cfm?id=what-do-we-know-about-the>

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. "

Elton
Received on Wed 03 Oct 2012 02:53:35 AM PDT