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

From: Galactic Stone & Ironworks <*meteoritemike_at_meteoritecentral.com*> Date: Wed, 3 Oct 2012 19:56:26 -0400 Message-ID: <CAKBPJW8Hpsn0Gc6TxF_uwaKfF+wHW7B1LR3hb+qedCt9h7Uvzg_at_mail.gmail.com> · meteoritemike_at_meteoritecentral.com

-- 
-------------------------------------------------------------
Web - http://www.galactic-stone.com
Facebook - http://www.facebook.com/galacticstone
Twitter - http://twitter.com/GalacticStone
Pinterest - http://pinterest.com/galacticstone
RSS - http://www.galactic-stone.com/rss/126516
-------------------------------------------------------------
On 10/3/12, Steve Dunklee <steve.dunklee at yahoo.com> wrote:
> If the oceans were from comet impacts both the moon and mars would have
> similar amounts of water. Or shall we delve into the atmosphere of Venus?
> Things at the beginning of the system were most probably interesting. why
> not just accept it was  random bombardment and outgassing from solar
> radiation which caused a variety of  planets and planetismals.
>
> --- On Wed, 10/3/12, MstrEman <mstreman at gmail.com> wrote:
>
>> From: MstrEman <mstreman at gmail.com>
>> Subject: [meteorite-list] Critical Assessment of the Comet as Provider of
>> Earth's Waters
>> To: "Meteorite-list" <meteorite-list at meteoritecentral.com>
>> Date: Wednesday, October 3, 2012, 6:53 AM
>>  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
>> ______________________________________________
>>
>> Visit the Archives at
>> http://www.meteoritecentral.com/mailing-list-archives.html
>> Meteorite-list mailing list
>> Meteorite-list at meteoritecentral.com
>> http://six.pairlist.net/mailman/listinfo/meteorite-list
>>
>
> ______________________________________________
>
> Visit the Archives at
> http://www.meteoritecentral.com/mailing-list-archives.html
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
>