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Gas Giant composition



 
 
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  #1  
Old July 2nd 03, 03:39 AM
Al Montestruc
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Default Gas Giant composition

A fellow was telling me that Jupiter and other gas giants are supposed
to be about 99.9% Hydrogen and Helium. I am suspicious of this for
the following reasons:

1. One cannot analize the spectra of the core of a planet, or
anything but the cloud top layer. Do tell me if I am wrong on this,
and how it could be done by us from earth or a human existing
spacecraft.

2. Denser materials will tend to sink to the core of a gas giant.

3. While we can know the mass of a gas giant from the period of
objects in orbit around it, the diameter will be more of a matter of
opinion, as atmopheres do not have very sharp boundaries.

4. We will also not know the temperature gradiant of the planet as a
function of distance from the center. While if we did I could buy
that a computer model could be made that would account for different
densities, not if you are not sure of the temperature gradiant.

5. Nuclear processes at the core (need not be fusion) can mess with
assumptions about temperature gradiants, and said nuclear processes
might involve large masses of Radium, Uranium and Thorium as in the
earth's core, which will all sink to the core of a gas giant as while
some ores of rock with Uranium ect are lighter than the mantle of the
earth, they are all a lot hevier than air or H2 gas.

Comments?
  #2  
Old July 2nd 03, 07:58 AM
David Knisely
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Default Gas Giant composition

Hi there. You posted:

A fellow was telling me that Jupiter and other gas giants are supposed
to be about 99.9% Hydrogen and Helium.


That is pretty close to the acutal figures for the composition of the
atmosphere. The most recent information I have puts the composition as
92.5% Hydrogen, 7.2% Helium, and 0.3% everything else. This means that
the atmosphere is 99.7% Hydrogen and Helium.

I am suspicious of this for the following reasons:

1. One cannot analize the spectra of the core of a planet, or
anything but the cloud top layer. Do tell me if I am wrong on this,
and how it could be done by us from earth or a human existing
spacecraft.


There is no need to be suspicious. The first hint that Hydrogen and
Helium might be present in large quantities came from the low mean
density of the planet (1.33 grams/cc) and large oblateness of Jupiter
due to its rotation. Earth-based spectroscopy detected the presence of
Methane and Ammonia, both of which contain Hydrogen. Various models
based on this information indicated that a large portion of Jupiter was
probably in the form of Hydrogen and possibly Helium. The Voyager
spacecraft's infrared spectrometers detected the signature of molecular
Hydrogen. Both Hydrogen and Helium emissions were also detected in the
Ultraviolet using Voyager's Ultraviolet spectrometer. As a direct
measurement, the entry probe from the Galileo spacecraft entered deep
into the atmosphere of Jupiter and sampled it with its Neutral Mass
Spectometer and Helium Abundance Detectors to determine its composition.

2. Denser materials will tend to sink to the core of a gas giant.


That is correct. It is thought that Jupiter has a small rocky solid
core perhaps several times as massive as the mass of the Earth,
surrounded by a thick shell of liquid metallic hydrogen and a thick
atmosphere rich in Hydrogen and Helium. This model is supported by the
strong magnetic field of Jupiter along with the way the planet's mass
distribution slightly perturbs the paths of spacecraft like Galileo.

3. While we can know the mass of a gas giant from the period of
objects in orbit around it, the diameter will be more of a matter of
opinion, as atmopheres do not have very sharp boundaries.


The diameter is known to a fairly high degree of accuracy, but that
doesn't make what you say exactly incorrect. However, the uncertainties
in where the atmosphere fades away does not significantly change the
density figure for the planet as a whole.

4. We will also not know the temperature gradiant of the planet as a
function of distance from the center. While if we did I could buy
that a computer model could be made that would account for different
densities, not if you are not sure of the temperature gradiant.


We do know the heat output from the planet, both from infrared
measurements from space and from the direct measurements of temperature
and pressure made during the Galileo probe's descent into Jupiter.
Again, the model of a mostly-Hydrogen planet is supported by this data.

5. Nuclear processes at the core (need not be fusion) can mess with
assumptions about temperature gradiants, and said nuclear processes
might involve large masses of Radium, Uranium and Thorium as in the
earth's core, which will all sink to the core of a gas giant as while
some ores of rock with Uranium ect are lighter than the mantle of the
earth, they are all a lot hevier than air or H2 gas.


Nuclear fission processes would supply a little heat, but otherwise,
they don't change the composition all that much. They would sink to the
core where they would contribute to some degree to the heat flow from
Jupiter, but again, the amounts present and their effects do not
contradict the idea that Jupiter is mostly Hydrogen and Helium.

Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club:
http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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* Attend the 10th Annual NEBRASKA STAR PARTY *
* July 27-Aug. 1st, 2003, Merritt Reservoir *
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  #3  
Old July 4th 03, 12:32 PM
G=EMC^2 Glazier
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Default Gas Giant composition

Hi David With all that hydrogen,and all those lightning strikes there
can't be any oxygen. However Jupiters moons are said to have
water(ice) surface,and that means they had oxygen. Do you think some day
Jupiter and its watery moons might be used where are deep space rocket
ships can fuel up? Bert.

 




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