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Was Proto-earth a Gas Giant ?
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Was Proto-earth a Gas Giant ? John Curtis 06-18-2006
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Posted by John Curtis on June 18, 2006, 6:21 pm
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 Perplexed in Peoria wrote:
> > Comparing the cummulative mass of satellites to the mass of their
> > central gas giants produces a constant ratio of 0.0001 which is a
> > hundred times smaller than Moon to Earth ratio of 0.012:
> > http://www.spaceref.com/news/viewpr.html?pid=20099
> > This discepancy in ratios becomes smaller when viewed from
> > planet-evaporation-model standpoint, where terrestrial planets
> > lost their hydrogen-helium atmospheres to UV stripping:
> > http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/08/text/
> > Reconstituting the lost H-He mass (98%) to original Earth
> > makes the Moon/Earth ratio 0.00025, which is more amenable to
> > reconciliation. John Curtis
>
> Isn't there some kind of evidence related to the scarcity of Neon
> and the isotopic ratio of of 22Ne to 20Ne? This supposedly shows
> that the Earth never had a gas-giant-like atmosphere, and that even
> if we had a somewhat lighter atmosphere to begin with, we definitely
> didn't lose it by simple evaporation. I seem to recall this as being
> evidence for a giant impact having created the moon and our current
> atmosphere being the result of outgassing (with minor contributions
> from cometary accretion). Sorry, I don't have links or details.
>
Solar (primordial) neon-20 , just like the solar hydrogen, helium and
carbon create a problem for the planetesimal model.
On the other hand, the presence of neon-20 and helium-3 in
archaean diamods supports the massive atmospheric pressure of
proto-earth:
http://www.ucolick.org/news/1999/99-09-29.html
>
> In any case, considering our moon to be just an exceptionally big
> gas-giant moon doesn't work for other reasons. Gas-giant moons are
> mostly ice, rather than rock. So you need to increase mass of both
> Moon and Earth to get to your hypothetical pre-evaporation
> proto-situation.
>
Had the proto-earth condensed on the other side of Jupiter, the Earth
would have remained three times as massive as Neptune and the
Moon would resemble an iceball. John Curtis


Posted by George on June 18, 2006, 6:55 pm
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>
>> Comparing the cummulative mass of satellites to the mass of their
>> central gas giants produces a constant ratio of 0.0001 which is a
>> hundred times smaller than Moon to Earth ratio of 0.012:
>> http://www.spaceref.com/news/viewpr.html?pid=20099
>> This discepancy in ratios becomes smaller when viewed from
>> planet-evaporation-model standpoint, where terrestrial planets
>> lost their hydrogen-helium atmospheres to UV stripping:
>> http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/08/text/
>> Reconstituting the lost H-He mass (98%) to original Earth
>> makes the Moon/Earth ratio 0.00025, which is more amenable to
>> reconciliation. John Curtis
>
> Isn't there some kind of evidence related to the scarcity of Neon
> and the isotopic ratio of of 22Ne to 20Ne? This supposedly shows
> that the Earth never had a gas-giant-like atmosphere, and that even
> if we had a somewhat lighter atmosphere to begin with, we definitely
> didn't lose it by simple evaporation. I seem to recall this as being
> evidence for a giant impact having created the moon and our current
> atmosphere being the result of outgassing (with minor contributions
> from cometary accretion). Sorry, I don't have links or details.
>
> In any case, considering our moon to be just an exceptionally big
> gas-giant moon doesn't work for other reasons. Gas-giant moons are
> mostly ice, rather than rock. So you need to increase mass of both
> Moon and Earth to get to your hypothetical pre-evaporation
> proto-situation.

It also doesn't sit well with the current configuration of the planets,
which have rocky inner worlds and gaseous outer ones. If the Earth was a
gaseous inner world, one might consider the possibility that the other
inner worlds were also gaseous. Then you have to explain why/how those
worlds also lost their hydrogen atmospheres.

George



Posted by John Curtis on June 19, 2006, 3:44 pm
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George wrote:
>
> It also doesn't sit well with the current configuration of the planets,
> which have rocky inner worlds and gaseous outer ones. If the Earth was a
> gaseous inner world, one might consider the possibility that the other
> inner worlds were also gaseous.
>
Agreed.
>
>Then you have to explain why/how those
> worlds also lost their hydrogen atmospheres.
>
Some planets lost their hydrogen because of their proximity to the Sun.
Others did not have enough gravity to hold on to hydrogen. John Curtis


Posted by George on June 19, 2006, 4:07 pm
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> George wrote:
>>
>> It also doesn't sit well with the current configuration of the planets,
>> which have rocky inner worlds and gaseous outer ones. If the Earth was
>> a
>> gaseous inner world, one might consider the possibility that the other
>> inner worlds were also gaseous.
>>
> Agreed.
>>
>>Then you have to explain why/how those
>> worlds also lost their hydrogen atmospheres.
>>
> Some planets lost their hydrogen because of their proximity to the Sun.
> Others did not have enough gravity to hold on to hydrogen. John Curtis

That doesn't explain why three of the four rocky planets were able to
retain substantial atmospheres. If the Earth ever had a significant
hydrogen atmosphere, it lost it very early in its history.

George



Posted by pete on June 22, 2006, 9:15 pm
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In sci.geo.geology, on Mon, 19 Jun 2006 16:07:46 -0400,

` > George wrote:
` >>
` >> It also doesn't sit well with the current configuration of the planets,
` >> which have rocky inner worlds and gaseous outer ones. If the Earth was
` >> a
` >> gaseous inner world, one might consider the possibility that the other
` >> inner worlds were also gaseous.
` >>
` > Agreed.
` >>
` >>Then you have to explain why/how those
` >> worlds also lost their hydrogen atmospheres.
` >>
` > Some planets lost their hydrogen because of their proximity to the Sun.
` > Others did not have enough gravity to hold on to hydrogen. John Curtis

` That doesn't explain why three of the four rocky planets were able to
` retain substantial atmospheres. If the Earth ever had a significant
` hydrogen atmosphere, it lost it very early in its history.

Yes, that's exactly what the math suggests. The velocity of a gas
at a given temperature is related to its mass (temp=energy; mv^2..).
As hydrogen and helium are so light, their velocities are high
enough that they approach escape velocity, and the top end of
the distribution curve gets stripped off the planet over a few
million years. At its temperature, Mars can only hold on to
gases as heavy as CO2, due to its lower gravity. The current
distribution of gases on the planets is exactly what you would
expect, doing the math, based on the mass and temperature of
each, with the exception of He, which has a higher concentration
as it is replenished from nuclear decay. H2 distribution is
complicated by the existence of CH4 and H2O, which should help
retain bound hydrogen, but that didn't happen on Venus, and I'm
not sure if the history of its atmospheric chemistry is fully
understood. It is however reasonable to assume that all the planets
started out with the same distribution of elemental components,
and as the sun switched on, sorting processes ensued.

This is all from memory of my astronomy undergrad major, several
decades ago.



--
==========================================================================
vincent@triumf[munge].ca Pete Vincent
Disclaimer: all I know I learned from reading Usenet.

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